More Important Topics of Cylinder

The Cambrian is a major division of the geologic timescale that begins about 542 million years before the present (BP) at the end of the Proterozoic eon and ended about 490 million years BP with the beginning of the Ordovician period.

It is the first period of the Paleozoic era of the Phanerozoic eon. The Cambrian is the earliest period in whose rocks are found numerous large, distinctly-fossilizable multicellular organisms that are more complex than sponges or

medusoids. During this time, roughly fifty separate major groups of organisms or "phyla", including almost all the basic body plans of modern animals, emerged suddenly, in most cases without evident precursors. This radiation of animal phyla is referred to as the Cambrian explosion.

By nearly any measure, the most successful animals on the planet are the arthropods. They have conquered land, sea and air, and make up over three-fourths of all currently known living and fossil organisms, or over one million species in all.

Since many arthropod species remain undocumented or undiscovered, especially in tropical rain forests, the true number of living arthropod species is probably in the tens of millions. One recent conservative estimate puts the number of arthropod species in tropical forests at 6 to 9 million species (Thomas, 1990).

Arthropods range in distribution from the deep sea to mountain peaks, in size from the king crab with its 12-foot armspan to microscopic insects and crustaceans, and in taste from chocolate covered ants to crawfish jambalaya and lobster Newburg. Despite this unbelievable diversity, the basic body plan of arthropods is fairly constant. Arthropods have a stiff cuticle made largely of chitin and proteins, forming an exoskeleton that may or may not be further stiffened with calcium carbonate.

They have segmented bodies and show various patterns of segment fusion (tagmosis) to form integrated units (heads, abdomens, and so on). The phylum takes its name from its distinctive jointed appendages, which may be modified in a number of ways to form antennae, mouthparts, and reproductive organs.

Soft-bodied relatives of the arthropods, as well as trace fossils that were made by some arthropod-like organisms, appear in the Vendian.

However, arthropods underwent rapid evolution in the Cambrian Period; Cambrian localities like the world-famous Burgess Shale in British Columbia are rich in unusual athropods, many of which are not obviously related to the traditional classes of living arthropods. Trilobites were a dominant marine group in the early Paleozoic. The earliest arachnids turn up in the Silurian, and move onto land shortly before the insects appeared in the middle Devonian Period, about 385 million years ago; over the next hundred or so million years both groups radiated into several diverse lineages.

The annelid fossil record is sparse, but a few definite forms are known as early as the Cambrian, and there are some signs they were around in the later Precambrian.

A few small groups have been treated as separate phyla: the Pogonophora and Vestimentifera, now included in the family Siboglinidae, and the Echiura.

The annelids, collectively called Annelida (from Latin annellus "little ring), are a large phylum of animals, comprising the segmented worms, with about 15 000 modern species including the well-known earthworms and leeches.

They are found in most wet environments, and include many terrestrial, freshwater, and especially marine species, as well as some which are parasitic or mutualistic.

They range in length from under a millimetre to over 3 metres.

Anatomy

Annelids are triploblastic protostomes.

The body cavity is a coelom, a fluid-filled cavity in which the gut and other organs are suspended. Oligochaetes and polychaetes typically have spacious coeloms; in leeches, the coelom is largely filled in with tissue and reduced to a system of narrow canals; archiannelids may lack the coelom entirely.

The coleom is divided into a sequence of compartments by walls called septa. In the most general forms each compartment corresponds to a single segment of the body, which also includes a portion of the nervous and (closed) circulatory systems, allowing it to function relatively independently. Each segment is marked externally by one or more rings, called annuli. Each segment also has an outer layer of circular muscle underneath a thin cuticle and epidermis, and a

system of longitudinal muscles. In earthworms, the longitudinal muscles are strengthened by collagenous lamellae; the leeches have a double layer of muscles between the outer circulars and inner longitudinals. In most forms they also carry a varying number of bristles, called setae, and among the polychaetes a pair of appendages, called parapodia.

Anterior to the true segments lies the prostomium and peristomium, which carries the mouth, and posterior to them lies the pygidium, where the anus is located. The digestive tract is usually specialized. Different species of annelids have a wide variety of diets, including active and passive hunters, scavengers, filter feeders, direct deposit feeders which simply ingest the sediments, and blood-suckers.

The vascular system and the nervous system are separate from the digestive tract. The vascular system includes a dorsal vessel conveying the blood toward the front of the worm, and a ventral longitudinal vessel which conveys the blood in the opposite direction. The two systems are connected by a vascular sinus and by lateral vessels of various kinds, including in the true earthworms, capillaries on the body wall.

The nervous system has a solid, ventral nerve cord from which lateral nerves arise in each segment. Every segment has an autonomy; however, they unite to perform as a single body for functions such as locomotion. Growth in many groups occurs by replication of individual segmental units, in others the number of segments is fixed in early development.

Reproduction

Depending upon species, annelids can reproduce both sexually and asexually.

Asexual reproduction

Asexual reproduction by fission is a method used by some annelids and allows them to reproduce quickly. The posterior part of the body breaks off and forms a new individual. The position of the break is usually determined by an epidermal growth. Lumbriculus and Aulophorus, for example, are known to reproduce by the body breaking into such fragments. Many other taxa (such as most earthworms) cannot

reproduce this way, though they can regrow the posteriormost segments in most instances.

Sexual reproduction

Sexual reproduction allows a species to better adapt to its environment. Some annelid species are hermaphroditic, while others have distinct genders.

Hermaphrodite annelids like earthworms mate periodically throughout the year in favored environmental conditions. Earthworms mate by copulation. Two worms which are attracted by each other's secretions lay their bodies together with their heads pointing opposite directions. The fluid is transferred from the male pore to the other worm. Different methods of sperm tranference have been observed in different genera, and may involve internal spermathecae (sperm storing chambers) or spermatophores that are attached to the outside of the other worm's body.

Most polychaete worms have separate males and females and external fertilization. The earliest larval stage, which is lost in some groups, is a ciliated trocophore, similar to those found in other phyla. The animal then begins to develop its segments, one after another, until it reaches its adult size. The oligochaetes and leeches tend to be hermaphroditic and lack free-living larvae of this sort. While annelids have some regenerative abilities, sometimes to the point where each half of an adult divided cross-wise will survive, this is not universal, and especially does not occur among the earthworms as folklore would suggest.

Relationships

The arthropods and their kin have long been considered the closest relatives of the annelids, on account of their common segmented structure, but a number of differences between the two groups suggest this may be convergent evolution. The other major phylum which is of definite relation to the annelids are the molluscs, which share with them the presence of trochophore larvae. These groups are united as the Trochozoa, and when the arthropods are included, they and the annelids are treated in a subgroup called the Articulata.

Classes and subclasses of Annelida

Clitellata

Oligochaeta - The class Oligochaeta includes the megadriles (earthworms), which are both aquatic and terrestrial, and the microdrile families such as tubificids, which include many marine members as well.

Leeches (Hirudinea) - These include both bloodsucking external parasites and predators of small invertebrates.

Polychaeta - This is the largest group of the Annelids and majority are marine. All segments are identical each consisting a parapodia. The parapodia are used for swimming, burrowing and feeding current.

Most trilobites lived in fairly shallow water and were benthic; they walked on the bottom, and probably fed on detritus.

A few, like the agnostids, may have been pelagic, floating in the water column. Cambrian and Ordovician trilobites generally lived in shallow water.

After the Ordovician, when many trilobite groups declined or went extinct, the survivors tended to be restricted to deeper water.

Food particles were stirred up by the legs and passed forward to the mouth.

Since the mouth had no large mandibles, we infer that trilobites were not usually predatory and were restricted to soft food. Some trilobites had long spines on the first leg segment, the gnathobase; these may have been able to tear up larger pieces of food, and probably scavenged for a living.

Fossil burrows and tracks have been found that match trilobite bodies very precisely; these show that trilobites could burrow into sediment to feed or to avoid predators.

Many trilobites living after the Cambrian developed the ability to roll up, also probably as a defense against predators.

This specimen of Pliomera fischeri, from the Ordovician of the St. Petersburg region of Russia, is doing just that.

Notice the notches in the pygidium just where it contacts the glabella.

In the living trilobite, these would have allowed water to enter and flow over the gills, so that the trilobite could breathe while enrolled.

The Ordovician period is the second of the six (seven in North America) periods of the Paleozoic era. The Ordovician follows the Cambrian period and is followed by the Silurian period.

The Ordovician, named after the Welsh tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a situation where followers of Adam Sedgwick and Roderick Murchison were placing the same rock beds in the Cambrian and Silurian periods respectively. Charles Lapworth simply took all the conflicting strata and placed them in the new Ordovician period.

Brachiopod

The earliest known brachiopods are found in the late Neoproterozoic. The first brachiopods were inarticulate (hingeless), but articulate (hinged) brachiopods appeared soon thereafter, in the Lower Cambrian. Brachiopods are extremely common fossils throughout the Paleozoic.

The major shift came with the Permian extinction. Before this extinction event, brachiopods were more numerous and diverse than bivalve mollusks.

Afterwards, in the Mesozoic, their diversity and numbers were drastically reduced, and they were largely replaced by bivalve mollusks. Mollusks continue to dominate today, and the remaining orders of brachiopods survive in fringe environments of more extreme cold and depth. Brachiopods - both articulate and inarticulate - are still present in modern oceans. The most abundant are the terebratulids (class Terebratulida).

The perceived resemblance of terebratulid shells to ancient oil lamps gave the brachiopods their common name "lamp shell". The phylum most closely related to Brachiopoda is probably the small phylum Phoronida (known as "horseshoe worms").

The inarticulate brachiopod genus Lingula has the distinction of being the oldest, relatively evolutionarily unchanged animal known. The oldest Lingula occur in the very early Cambrian, roughly 550 million years ago. The origin of brachiopods is unknown.

A possible ancestor is a sort of ancient "armored slug" known as Halkeria that was recently been found to have had small brachiopod-like shields on its head and tail.

During the Ordovician and Silurian periods brachiopods became adapted to life in most marine environments and became particularly numerous in shallow water habitats, in some cases forming whole banks in much the same way as bivalves (such as mussels) do today. In some places, large sections of limestone strata and reef deposits are composed largely of their shells.

Throughout their long geological history the brachiopods have gone through several major proliferations and diversifications, and have also suffered from major extinctions as well.

It has been suggested that the slow decline of the brachiopods over the last 100 million years or so is a direct result or the rise in diversity of filter feeding bivalves, which have ousted the brachiopods from their former habitats. However, it should be noted that the greatest successes for the bivalves have been in habitats which have never been adopted by the brachiopods, such as burrowing and free swimming.

The abundance, diversity, and rapid evolution of brachiopods during the Paleozoic make them useful as index fossils when correlating strata across large areas.

Jellyfish or Medusa (also known as a hydromedusa) is a form of cnidarian in which the body is shortened on its principal axis and broadened, sometimes greatly, in contrast with the hydroid or polyp.

Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below.The upper or aboral surface is called the exumbrella and the lower surface is called the subumbrella; the mouth is located on the lower surface, which may be partially closed

The margin of the disk bears sensory organs and tentacles. In the class Hydrozoa medusae are the sexual individuals of many species, alternating in the life cycle with asexual polyps, but in Scyphozoa (or jellyfishes proper) and Cubozoa the medusa alone is well developed. Except the freshwater jellyfish, these are the only classes in which medusae appear.

collectively called Annelida (from Latin annellus "little ring"), are a large phylum of animals, comprising the segmented worms, with about 15,000 modern species including the well-known earthworms and leeches. They are found in most wet environments, and include many terrestrial, freshwater, and especially marine species (such

as the polychaetes), as well as some which are parasitic or mutualistic. They range in length from under a millimeter to over 3 meters (the seep tube worm Lamellibrachia luymesi).

Anatomy

Annelids are triploblastic protostomes with a coelom, closed circulatory system and true segmentation. Oligochaetes and polychaetes typically have spacious coeloms; in leeches, the coelom is largely filled in with tissue and reduced to a system of narrow canals; archiannelids may lack the coelom entirely. The coleom is divided into a sequence of compartments by walls called septa. In the most general forms each compartment corresponds to a single segment of the body, which also includes a portion of the nervous and (closed) circulatory systems, allowing it to function relatively independently.

Each segment is marked externally by one or more rings, called annuli. Each segment also has an outer layer of circular muscle underneath a thin cuticle and epidermis, and a system of longitudinal muscles. In earthworms, the longitudinal muscles are strengthened by collagenous lamellae; the leeches have a double layer of muscles between the outer circulars and inner longitudinals. In most forms they also carry a varying number of bristles, called setae, and among the polychaetes a pair of appendages, called parapodia.

Anterior to the true segments lies the prostomium and peristomium, which carries the mouth, and posterior to them lies the pygidium, where the anus is located. The digestive tract is quite variable but is usually specialized. For example, in some groups (notably most earthworms) it has a typhlosole (to increase surface area) along much of its length. Different species of annelids have a wide variety of diets, including active and passive hunters, scavengers, filter feeders, direct deposit feeders which simply ingest the sediments, and blood-suckers.

The vascular system and the nervous system are separate from the digestive tract. The vascular system includes a dorsal vessel conveying the blood toward the front of the worm, and a ventral longitudinal vessel which conveys the blood in the opposite direction. The two systems are connected by a vascular sinus and by lateral vessels of various kinds, including in the true earthworms, capillaries on the body wall.

The nervous system has a solid, ventral nerve cord from which lateral nerves arise in each segment. Every segment has an autonomy; however, they unite to perform as a single body for functions such as locomotion. Growth in many groups occurs by replication of individual segmental units, in others the number of segments is fixed in early development.

Reproduction

Depending upon the species, annelids can reproduce both sexually and asexually.

Asexual reproduction

Asexual reproduction by fission is a method used by some annelids and allows them to reproduce quickly. The posterior part of the body breaks off and forms a new individual. The position of the break is usually determined by an epidermal growth. Lumbriculus and Aulophorus, for example, are known to reproduce by the body breaking into such fragments. Many other taxa (such as most earthworms) cannot reproduce this way, though they have varying abilities to regrow amputated segments.

Sexual reproduction

Sexual reproduction allows a species to better adapt to its environment. Some annelida species are hermaphroditic, while others have distinct sexes.

Most polychaete worms have separate males and females and external fertilization. The earliest larval stage, which is lost in some groups, is a ciliated trochophore, similar to those found in other phyla. The animal then begins to develop its segments, one after another, until it reaches its adult size.

Earthworms and other oligochaetes, as well as the leeches, are hermaphroditic and mate periodically throughout the year in favored environmental conditions. They mate by copulation. Two worms which are attracted by each other's secretions lay their bodies together with their heads pointing opposite directions. The fluid is transferred from the male pore to the other worm.

Different methods of sperm transference have been observed in different genera, and may involve internal spermathecae (sperm storing chambers) or spermatophores that are attached to the outside of the other worm's body. The clitellata lack the free-living ciliated trochophore larvae present in the polychaetes, the embryonic worms developing in a fluid-filled "cocoon" secreted by the clitellum.

Fossil record

The annelid fossil record is sparse, but a few definite forms are known as early as the Cambrian, and there are some signs they were around in the later Precambrian, but the earliest unequivocal annelid fossils are only known from the Cambrian. Because the creatures have soft bodies, fossilization is an especially rare event.

The best-preserved and oldest annelid fossils come from Cambrian Lagerstätten such as the Burgess Shale of Canada, and the Middle Cambrian strata of the House Range in Utah. The Annelids are also diversely represented in the Pennsylvanian-age Mazon Creek fauna of Illinois. A few small groups have been treated as separate phyla: the Pogonophora and Vestimentifera, now included in the family Siboglinidae, and the Echiura.

Their skeletons tend to have arm-like extensions that resemble spikes, which are used both to increase surface area for buoyancy and to capture prey. Most radiolarians are planktonic, and get around by coasting along ocean currents.

Most are somewhat spherical, but there exist a wide variety of shapes, including cone-like and tetrahedral forms (see the image above). Besides their diversity of form, radiolarians also exhibit a wide variety of behaviors.

They can reproduce sexually or asexually; they may be filter feders or predators; and may even participate in symbiotic relations with unicellular algae.

Though their silica skeletons have allowed us to find numerous fossils, scientists still have not been able to successfully develop a complete classification scheme for them. The evolution of the Radiolaria can be easily traced on the broad scale, with major transitions in the global fauna, but a concise taxomony reflecting the evolutionary relationships of major groups is still elusive.

Until comparatively recently, radiolarians were primarily studied by micropaleontologists, and only at the end of the 20th century have scientists from other fields begun to study these fascinating protists as well.

The largest, such as Pterygotus, reached 2 meters or more in length, but most species were less than 20 cm. They were formidable predators among the coral reefs that thrived in the warm, shallow seas of the Silurian period,

around 410 million years ago. Eurypterids were the most fearsome swimming predators of the Palaeozoic. Although called "sea scorpions", only the earliest ones were marine. While the earliest eurypterids may have lived in the sea, it is thought that most swam in small pools of fresh water. The move from the sea to fresh water probably occurred by the Pennsylvanian period. Eurypterus is perhaps the most well-known genus of eurypterid, of which 200 fossil species are known.

The genus Eurypterus was created in 1825 by James E. DeKay, a zoologist. He recognized the arthropod nature of the first ever described eurypterid specimen found by Dr. S. L. Mitchell. In 1984, Eurypterus remipes was named the State Fossil of New York.Body Structure

Though fossils are a bit unclear, the typical eurypterid had a large, flat, semicircular carapace, followed by a jointed section, and finally a tapering, flexible tail, with a long spine at the end. Under the head of the eurypterids were twelve body segments known as tergites. The tail, which is spiked and may have been poisonous, is known as the telson.

Some eurypterids have paddles, which were used to propel themselves through water. Some argue that the paddles were also used for digging. Underneath, the creature had 8 pairs of jointed legs for walking, two small scorpion-like claws at the front (pedipalps). Other features, common to ancient and modern arthropods of this type, include ocelli, compound eyes, and chelicerae

Although many eurypterids had legs too tiny to do more than allow them to crawl over the sea bottom, a number of forms had large stout legs, and were clearly capable of terrestrial locomotion (like land crabs today).

While functional studies suggest that eurypterids used out-of-phase walking techniques, their trackways indicate that they used in-phase, hexapodous (six-legged) and octopodous (eight-legged) gaits. Some species may have been amphibious, emerging onto land for at least part of their life cycle. They may have been capable of breathing both in water and in air.

Eurypterid Fossils

Eurypterid fossils have been found on nearly every continent. Locations currently producing excellent fossils include western New York State and southern Ontario, Canada in Silurian rock. Although relatively rare, the fossils are famous for excellent preservation. People seeking eurypterid fossils commonly search at Ridgemount Quarry, in Fort Erie, Ontario Canada.

Eurypterids are related to the modern horseshoe crab and sea scorpion. About two dozen families of eurypterids are known. They went extinct in the Permian-Triassic extinction event.

During the later Ordovician, the continents were in very different positions than you find them today. The continents have only assumed their present locations on the globe in the last few ten's of million years. The process

which moves the continents around the surface of the Earth is called "Plate Tectonics," and is a refinement of the earlier ideas of continental drift. North America roughly straddled the equator. It was rotated about 45 degrees clockwise from its presentorientation.

Much, if not all, was covered by ocean. The map shows North America separated from other continents; an alternative theory places it very near the west coast of South America.

(The positions of the plates during the Ordovician has been determined by the remnant paleomagnetism in the rocks deposited at the time. Paleomagnetism can only determine the paleolatitude, and paleolongitude must be determined from other, indirect evidence.)

Southern Europe, Africa, South America, Antarctica and Australia were bonded together into a supercontinent of Gondwana, and was in position over the South Pole. Western and Central Europe were separate from the rest of Eurasia, and were rotated about 90 degrees counterclockwise from their present orientation, and was in the southern tropics.

Ordovican communities typically displayed a higher ecological complexity than Cambrian communities due to

the greater diversity of organisms.

However, as in the Cambrian, life in the Ordovician continued to be restricted to the seas. Species Affected: The Ordovician extinction occurred at the end of the Ordovician period, about 440-450 million years ago.

This extinction, cited as the second most devastating extinction to marine communities in earth history, caused the disappearance of one third of all brachiopod and bryozoan families, as well as numerous groups of conodonts, trilobites, and graptolites. Much of the reef- building fauna was also decimated. In total, more than one hundred families of marine invertebrates perished in this extinction.

The Silurian is a major division of the geologic timescale that extends from the end of the Ordovician period, about 439 million years ago (mega years ago, mya), to the beginning of the Devonian period, about 408.5 mya. As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by 5-10 million years. The base of the

Silurian is set at a major extinction event where 60% of marine species were wiped out. See Ordovician-Silurian extinction events.

Superfamily Eurypteracea

Ordovician to Permian

This superfamily includes the "typical" (unmodified) Eurypterids, in which the last prosomal appendages developed as swimming legs that carry paddles formed by expansion of the two penultimate joints. They can be considered the ancestral lineage from which the other groups evolved.

Eurypterus remipes Dekay

length about 20 cm

Ludlow of Western Euramerica (New York)

illustration from Moore, Lalicker and Fischer, Invertebrate Fossils family Eurypteridae

time range: Llandeilo to Frasnian

known distribution: Euramerica , Asia habitat: Marginal marine and fresh water representative taxa:

Baltoeurypterus tetragonophthalmus (= Eurypterus fischeri), Eurypterus lacustris, Eurypterus remipes

notes: typical Eurypterids. The body is fairly elongate; the largest species attaining a length of about a meter but most much smaller (average about 20 cm). The prosoma is squared off, the compound eyes kidney-shaped and placed towards the middle of the head, so they look upwards. Between them are two ocelli or simple eyes.

Lepidoderma mansfieldi

Late Carboniferous - length about 12 cm

illustration from Treatise on Invertebrate Paleontology

family Adelophthalmidae

time range: Llandovery to Artinskian

known distribution: Euramerica , Asia

habitat: Marginal marine, brackish and fresh water; amphibious representative taxa: Lepidoderma mansfieldi, Lepidoderma mazonense (both late Carboniferous), Adelophthalmus sellardsi (early Permian), notes: Mostly small forms, similar to Eurypterus but spiny, the outer surfaces with pointed scales and striae; the elongate body equipped with spurs.

The postabdomen is narrow and the telson very long

(styliform). The compound eyes are located somewhat towards the center of the head. The walking legs are mostly devoid of spines. In Adelophthalmus the genital appendage of the male is long, of the type "B" form short, with spatulate lateral lobes. These creatures seem to have been semi-aquatic swamp dwellers. They were among the last of the Eurypterids.

The ancient (cohort Belemnoidea) and modern Coleoidea (cohort Neocoleoidea) diverged from the external shelled Nautiloidea around 425 million years ago. Unlike most modern cephalopods, ancient varieties had protective shells.

These shells at first were conical but later developed into curved nautiloid shapes seen in modern nautilus species. Internal shells still exist in many non-shelled living cephalopod groups but most truly shelled cephalopods, such as the ammonites, became extinct at the end of the Cretaceous.

The Baragwanathia plant formed scrubby cover over tidal flats in the latter part of the Silurian Period, a time of earth history almost inconceivably long ago (400 million years).

At the time Central Victoria was part of a submarine trench bounding the east of the continent.

According to the plate tectonics theory this was itself part of an enormous land mass called Pangaea, comprised of the continental masses prior to separation into their present day entities.

The rocks at the Yea site are sandstone and shale, alternating in very clearly defined beds that have been deformed and significantly tilted from their original horizontal aspect.

The Baragwanathian fossils are present only in some of the beds, generally as pieces of stem 10 to 50cm in length with attached long narrow leaves.

Deep burial for millions of years has resulted in drastic alteration of the stem, which is now evident as impressions, often made clearly visible by iron staining on the more weathered surfaces.

As mentioned above, the rocks in the Yea area were deposited under an ancient sea now long departed. This means of course that the Baragwanathia plant must have either grown on the shore line or has been washed out to sea by stream or other action, to ultimately attain the fossil condition.

Large vascular land plants (plants with woody tissue) like Baragwanathia have not been found elsewhere in rocks of Silurian age.

The vegetation at this time principally existed in the sea as algae like the kelp

and seaweed common in the oceans of today.

Land plants are known from overseas beds of similar age to those of this Yea site, but they were small erect naked stems a few centimetres in height (often with a fertile extremity), quite unlike the large vegetative body of Baragwanathia.

Baragwanathia is not the only fossil plant found at Limestone Road, about a dozen others have been recognised, but less than half of these have so far been described and named.

One or two are possibly relatives of Baragwanathia but most are not vascular land plants.

There are also animal fossils at the site. A mollusc fossil, called Orthoceras is even more common than Baragwanathia, and divalve shells are occasionally found. At least one fish, affiliation not yet determined, has also been collected.

More important from the point of view of establishing the age of the beds are members of an extinct Order, the Graptoelites. These are important because the species present are used as the criterion for the confirmation of the Silurian age. Baragwanathia is found at several other parts of Central Victoria but these localities are all in younger ( Devonian ) rocks.

Registration in the National Estate (after application by interested parties) is approved only after close scrutiny into the merit of the application. This Yea locality is one of only two fossil localities so far accorded this status in Victoria.

The other fossil locality is the Koonwarra Fish Bed locality in South Gippsland, of Cretaceous age. It contains a renowned fish, insect, crustacean, bird feather, and plan fossil assemblage.-W Baragwanath was Director of the Geological Survey of Victoria from 1920 to 1943.

Sea stars do not have movable skeletons, but instead possess a hydraulic water vascular system. The water

vascular system has many projections called tube feet, on the ventral face of the sea star's arms, which function in locomotion and feeding.

Digestion

Sea star digestion is carried out in two separate stomachs, the cardiac stomach and the pyloric stomach. The cardiac stomach, which is a sacklike stomach located at the center of the body may be everted - pushed out of the organism's

body and used to engulf and digest food. Some species take advantage of the great endurance of their water vascular systems to open the shells of molluscs (clams, mussels and the like), and inject their stomachs into the shells. Once the stomach is inserted inside the shell it digests the mollusk in place. Because of this ability to digest food outside of its body, the sea star is able to hunt prey that are much larger than its mouth would otherwise allow (including mollusks, arthropods, and even small fish). Partially-digested food is passed to the inside of the sea star where digestion continues in the pyloric stomach. Due to all of this digestive demand, the sea star's arms are filled with digestive glands called pyloric caeca or hepatic caeca.

External Anatomy

The tube feet can be clearly seen on this sea starSea stars are composed of a central disc with five arms exhibiting pentaradial symmetry. The mouth is located underneath (oral or ventral) the sea star, and the spiney surface covering the species is called the aboral or dorsal surface. On the aboral surface there is a structure called the madreporite which acts as a water filter and supplies their water vascular system with water to move (Gilbertson, 1999). Sea stars have a simple eye at the end of each arm. The eye is able to "see" only differences of light and dark; useful in detecting movement.

A sea star on the sea floor.On the surface of the sea star surrounding the spines are small white objects known as pedicellariae. There consists thousands and thousands of these pedicellariae on the external body. Inside the sea star underneath all the hepatic caeca consist the gonads which are involved in reproduction. The radial canal which is across each arm of the sea star has what are called ampullae which surround the radial canal.

The ampullae are teeth-like structures.

Sea stars are developmentally (embryologically) known as deuterostomes. Since echinoderms and chordates share this same embryological pattern, they are thought to be closely related. Nevertheless, as these creatures are invertebrates and not actually fish, most marine biologists are pushing to completely replace the term starfish with sea star.

Regeneration

Some species of sea stars have the ability to regenerate lost arms. When some sea stars are cut into pieces, each part that includes a portion of the central disk may grow into a whole other organism.

One genus particularly noted for its regeneration ability is Linckia, named for naturalist J.H. Linck. These sea stars can cast off an arm that regrows into an entire organism, as a means of asexual reproduction.

Geological history

Fossil sea stars (Asteroidea) have been found in rocks as old as the middle Ordovician period. Brittle stars (Ophiuroidea) are first recorded as fossils in rocks from the Carboniferous period. Complete fossil sea stars are very rare, but where they do occur they may be abundant. Most fossil sea stars consist of scattered individual plates or segments of arms.

This is because the skeleton is not rigid, as in the case of echinoids (sea urchins), but is composed of many small plates (or ossicles) which quickly fall apart and are scattered after death and the decay of the soft parts of the creature. Scattered sea star ossicles are reasonably common in the Cretaceous Chalk Formation of England.

Three famous localities where complete fossil sea stars are found are the Devonian Bundenbach slates of Bundenbach in Germany, the Jurassic lithographic Solnhofen limestone of Solnhofen in Germany, and the Jurassic 'Sea star bed' of the Middle Lias formation near Bridport, Dorset in England.

Although the basic echinoderm pattern of five-fold symmetry can be recognized, most crinoids have many more than five arms.

Crinoids usually have a stem used to attach themselves to a substrate, but many live attached only as juveniles and become free-swimming as adults.

There are only a few hundred known modern forms, but crinoids were much more numerous both in species and numbers in the past. Some thick limestone beds dating to

the mid- to late-Paleozoic are entirely made up of disarticulated crinoid fragments. Button-like, fossilized pieces of crinoid stem from Ordovician limestone of Batavia, Ohio (USA)The earliest known crinoids come from the Ordovician. They are thought to have evolved from primitive echinoderms known as Eocystoids.

Confusingly, another early group of echinoderms were also the Eocrinoids, but that group is currently thought to be an ancestor of blastoids rather than of crinoids.

Some fossil crinoids, such as Pentacrinites, seem to have lived attached to floating driftwood and complete colonies are often found. Sometimes this driftwood would become waterlogged and sink to the bottom, taking the attached crinoids with it.

The stem of Pentacrinites can be several metres long. Modern relatives of Pentacrinites live in gentle currents attached to rocks by the end of their stem, which is fairly short.

Drawing of the calyx of the crinoid PentacrinitesMost modern crinoids are free-swimming and lack a stem. Examples of free-swimming crinoid fossils include Marsupitsa, Saccocoma and Uintacrinus.

Many fossils of free-swimming crinoids (such as Pterocoma) are found in the Jurassic-dated Solnhofen limestone of Solnhofen in Germany, and the Cretaceous-dated Niobrara chalk of Kansas contains large numbers of Uintacrinus.

The crinoids have had an eventful geologic history. Once evolved, they soon spread to a variety of marine habitats. The group as a whole suffered a major crisis during the Permian period when most of the crinoid forms of the Palaeozoic era died out, with a few surviving into the Triassic period. During the Mesozoic era there was another great radiation of the crinoids with more modern forms possessing flexible arms becoming widespread.

Fossil crinoid stems from Arkansas.The long and varied geological history of the crinoids demonstrates how well the echinoderms have adapted to filter-feeding. The fossils of other stalked filter-feeding echinoderms, such as blastoids, are also found in the rocks of the Palaeozoic era. These extinct groups can exceed the crinoids in both numbers and variety in certain horizons. They were evidently competing with the crinoids on an equal basis. However, none of these others survived the crisis at the end of the Permian period.

An abundance of (stemmed) crinoids occurs in the rocks of the Silurian period in the United Kingdom and the eastern United States, the Devonian period of Kentucky, Michigan, New York state and the Eifel region of Germany, the Carboniferous period of the United Kingdom, Belgium and Russia, the Mississippian period of Iowa and Indiana, the Pennsylvanian period of the mid-continental United States, the Permian period of the island of Timor, and the Triassic period of Germany.

Their closest living relative is probably the modern nautilus, whom they resemble. Their fossil shells have the form of flat spirals (though there are some rarer non-spiraled forms, called heteromorphs) and are responsible for the animals' name as they somewhat resemble a tightly coiled ram's horn (the god Ammon was commonly depicted as a

man with ram's horns).

Plinius the Elder (died 79 AD near Pompeii) called fossils of these animals ammonis cornua, "horn of Ammon." Often the name of ammonite species ends in ceras, Greek (???a?) for "horn" (e.g. Pleuroceras).

Jeletzkytes, a Cretaceous ammonite from the USA. Because ammonites and their close relatives are extinct, little is known about their way of life. Their soft body parts are practically never preserved in any detail. Nonetheless, a lot has been worked out by examining ammonite shells and by using models of these shells in water tanks.

Most ammonites probably lived at the surface of ancient seas; this is suggested by the fact that their fossils are often found in rocks that were laid down under conditions where no benthic (bottom-dwelling) life is found. Many of them are

thought to have been good swimmers with flattened, discus-shaped, streamlined shells (such as Oxynoticeras), although some were less effective swimmers and were likely to have been bottom dwellers. Ammonites probably preyed on fishes and crustaceans and were themselves preyed upon by marine reptiles (such as Mosasaurs). Fossilized shells are sometimes found showing teeth marks from such attacks.

Basic shell anatomy

The chambered part of the ammonite shell is called a phragmocone. The phragmocone contains a series of progressively larger chambers, called camerae (sing. camera) that are divided by thin walls called septa (sing. septum). Only the last and largest chamber, the body chamber, was occupied by the living animal. As it grew, it added newer and larger chambers to the open end of the coil.

A thin living tube called a siphuncle passed through the septa, extending from the ammonite's body into the empty shell chambers. Through an osmotic process, the ammonite emptied water out of and secreted gas into these shell chambers. This enabled it to control the buoyancy of the shell.

A primary difference between ammonites and nautiloids is that the siphuncle of ammonites runs along the inside of the outer axis of the shell, while the siphuncle of nautiloids runs through the center of the septa and camerae.

Sexual dimorphism

Ammonite species, Jurassic eraOne feature found in shells of the modern nautilus is the variation in the size of the shell according to the gender of the animal, the shell of the male being slightly smaller than that of the female. This sexual dimorphism is thought to be an explanation to the variation in size of certain ammonite shells of the same species, the larger shell (called a macroconch) being female, and the smaller shell (called a microconch) being male. This is thought to be because the female required a larger body size for egg production. A good example of this sexual variation is found in Bifericeras from the early part of the Jurassic period of Europe.

It is only in relatively recent years that the sexual variation in the shells of ammonites has been recognized. The macroconch and microconch of one species were often previously mistaken for two closely related but different species occurring in the same rocks. However, these "pairs" were so consistently found together that it became apparent that they were in fact sexual forms of the same species.

Variations in shape

The majority of ammonites have a shell that is a planispiral flat coil, but some have a shell that is partially uncoiled, partially coiled and partially straight (as in Australiceras), nearly straight (as in baculites and belemnites), or coiled helically - superficially like that of a large gastropod (as in Turrilites and Bostrychoceras).

These partially uncoiled and totally uncoiled forms began to appear during the early part of the Cretaceous and are known as heteromorphs.

Perhaps the most extreme and bizarre looking example of a heteromorph is Nipponites, which appears to be a tangle of irregular whorls lacking any obvious symmetrical coiling.

However, upon closer inspection the shell proves to be a three-dimensional network of connected "U" shapes. Nipponites occurs in rocks of the upper part of the Cretaceous in Japan and the USA.

Ammonites vary greatly in the ornamentation of their shells. Some may be smooth and relatively featureless, except for growth lines, and resemble that of the modern nautilus. In others various patterns of spiral ridges and ribs or even spines are shown.

This type of ornamentation of the shell is especially evident in the later ammonites of the Cretaceous.

The aptychus

Like the modern nautilus, many ammonites were probably able to withdraw their body into the living chamber of the shell and developed either a single horny plate or a pair of calcitic plates with which they were able to close the opening of the shell. The opening of the shell is called the aperture. The plates are collectively termed the aptychus or aptychii in the case of a pair of plates, and anaptychus in the case of a single plate. The aptychus were identical and equal in size.

Asteroceras, a Jurassic ammonite from EnglandAnaptychi are rare as fossils. They are found in ammonites from the Devonian period through those of the Cretaceous period.

Aptychi only occur in ammonites from the Mesozoic era and are normally found detached the shell and are rarely preserved in place. Still, sufficient numbers have been found closing the apertures of fossil ammonite shells as to leave no doubt as to their intended purpose. (This long standing and wide-spread interpretation of the function of the aptychus has recently been disputed. The latest studies suggest that the apatychus may have in fact formed part of a special jaw apparatus).

Large numbers of detached aptychi occur in certain beds of rock (such as those from the Mesozoic in the Alps). These rocks are usually accumulated at great depths. The modern nautilus lacks any calcitic plate for closing its shell, and none has been found in any of the extinct nautiloids. It does, however, have a leathery head shield it uses to cover the opening when it retreats inside.

There are many forms of aptychus, varying in shape and the sculpture of the inner and outer surfaces, but because they are so rarely found in position within the shell of the ammonite it is unclear as to which species of ammonite many aptychi belong.

Size

Few of the ammonites occurring in the lower and middle part of the Jurassic period reach a size exceeding 23 centimetres (9 inches) in diameter. Much larger forms are found in the later rocks of the upper part of the Jurassic and the lower part of the Cretaceous, such as Titanites from the Portland Stone of Jurassic of southern England, which is often 53 centimetres (2 feet) in diameter, and Pachydiscus seppenradensis of the Cretaceous period of Germany, which is the largest known ammonite sometimes reaching 2 metres (6.5 feet) in diameter. The largest known North American ammonite is Parapuzosia bradyi from the Cretaceous with specimens measuring 137 centimetres (4.5 feet) in diameter.

Ammonite distribution

A specimen of Hoploscaphites from the Pierre Shale of South Dakota. Much of the original shell has survived.Starting from the late Silurian, ammonites were extremely abundant, especially in the Mesozoic seas. Many genera evolved and ran their course quickly, becoming extinct in a few million years. Due to their rapid evolution and widespread distribution, ammonites are useful for geologists and paleontologists for biostratigraphy. They are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods.

An iridescent ammonite from Madagascar.When ammonites are found in clays their original mother-of-pearl coating is often preserved. This type of preservation is found in ammonites such as Hoplites from the Cretaceous Gault clay of Folkestone in Kent, England.

The Cretaceous Pierre Shale formation of the United States and Canada is well known for the abundant ammonite fauna it yields, including Baculites, Placenticeras, Scaphites, Hoploscaphites, and Jeletzkytes, as well as many uncoiled forms. Many of these also have much or all of the original shell still intact.

Other fossils, such as many found in Madagascar, display iridescence. These iridescent ammonites are often of gem quality when polished.

The ammonites survived several major extinction events, with often only a few species surviving. But each time, this handful would diversify into a multitude of forms. Ammonites fossils become less abundant during the upper part of the Cretaceous, and none survive into the Cenozoic era. The last surviving lines died out about 65 million years ago along with the dinosaurs in the Cretaceous-Tertiary extinction event.

During the Devonian Period the first fish evolved legs and started to walk on land as amphibians, and the first arthropods like insects and spiders also started to colonize terrestrial habitats. The first seed-bearing plants spread across dry land, forming huge forests. In the oceans, fish diversified into the first sharks, and the first lobe-finned and bony fish. The first ammonite mollusks appeared, and trilobites, the mollusk-like brachiopods, as well as great coral reefs were still common. The Late Devonian extinction severely impacted marine life. The paleogeography was

dominated by the supercontinent of Gondwana to the south, the continent of Siberia to the north, and the early formation of the small supercontinent of Euramerica in the middle.

Corals

Although corals first appeared in the Cambrian period, some 570 million years ago, they are extremely rare as fossils until the Ordovician period, when Rugose and Tabulate corals became widespread.

Fossil coral Heliophyllum halli from the Devonian of Canada.Tabulate corals occur in the limestones and calcareous shales of the Ordovician and Silurian periods, and often form low cushions or branching masses alongside Rugose corals. Their numbers began to decline during the middle of the Silurian period and they finally became extinct at the end of the Permian period. The skeletons of Tabulate corals are composed of a form of calcium carbonate known as calcite. Rugose corals became dominant by the middle of the Silurian period, and became extinct early in the Triassic period. The Rugose corals may be either solitary or colonial, and like the Tabulate corals their skeletons are also composed of calcite. The finest details of their skeletal structures are often well preserved, and such fossils may be cut and polished.

Scleractinian corals diversified during the Mesozoic and Cenozoic eras and are at the height of their development today. Their fossils may be found in small numbers in rocks from the Triassic period, and they are relatively common fossils in rocks from the Jurassic and Cretaceous periods as well as the Caenozoic era. The skeletons of Scleractinian corals are composed of a form of calcium carbonate known as aragonite. Although they are geologically younger than the Tabulate and Rugose corals, the aragonite skeleton Scleractinian corals does not tend to preserve well, so it is often easier to find fossils of the more ancient Tabulate and Rugose corals.

At certain times in the geological past corals were very abundant, just as modern corals are in the warm clear tropical waters of certain parts of the world today. And like modern corals their fossil ancestors built reefs beneath the ancient seas. Some of these reefs now lie as great structures in the midst of sedimentary rocks.

Such reefs can be found in the rocks of many parts of the world including those of the Ordovician period of Vermont, the Silurian period

of the Michigan Basin and in many parts of Europe, the Devonian period of Canada and the Ardennes in Belgium, and the Cretaceous period of South America and Denmark. Reefs from both the Silurian and Carboniferous periods have been recorded as far north as Siberia, and as far south as Australia.

However, these ancient reefs are not composed entirely of corals. Algae and sponges, as well as the fossilized remains of many echinoids, brachiopods, bivalves, gastropods, and trilobites that lived on the reefs help to build them.

These fossil reefs are prime locations to look for fossils of many different types, besides the corals themselves.

Corals are not restricted to just reefs, many solitary corals may be found in rocks where reefs are not present (such as Cyclocyathus which occurs in the Cretaceous period Gault clay formation of England).

As well as being important rock builders, some corals are useful as zone (or index) fossils, enabling geologists to date the age the rocks in which they are found, particularly those found in the limestones of the Carboniferous period.

Corals and past climate

Some coral species exhibit banding in their skeletons resulting from annual variations in their growth rate.

In fossil and modern corals these bands allow geologists to construct year-by-year chronologies, a kind of incremental dating, which combined with geochemical analysis of each band, can provide high-resolution records of paleoclimatic and paleoenvironamental change.

The tusk shells are a class Scaphopoda of marine mollusks distinguished by curved tubular shells open at both ends, resembling a elephant's tusk (thus the name). They are mostly small, with some species reaching 15 centimeters long, and live in the bottom sediment where they feed on microscopic detritus and organisms such as foraminifera.

The several hundred known species are found worldwide. The mantle is entirely within the shell. The foot extends from the larger end of the shell, and is used to burrow

through the substrate. A number of minute tentacles around the foot, called captacula, sift through the sediment and latch onto bits of food which they then convey to the mouth. The mouth has grinding teeth that break the bit into smaller pieces for digestion.

The scaphopid vascular system is rudimentary, lacking both heart and blood vessels; the blood is held in sinuses throughout the body cavity, and pumped by the rhythmic action of the foot. Tusk shells are well-known in the fossil record, first appearing in the Ordovician (last of all molluscan groups). The shells were used by the natives of the Pacific Northwest as wampum.

So pronounced is this evolutionary radiation that the Devonian has been called "the age of Fish".

The Carboniferous is a major division of the geologic timescale that extends from the end of the Devonian period, about 340 million years ago (mya), to the beginning of the Permian period, about 280 mya.

As with most older geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by 5–10 million years. The Carboniferous is named for the extensive coal beds of that age found in England and Western Europe. In North

America, the first third of the Carboniferous is called the Mississippian period, and the remainder is called the Pennsylvanian.

Eogyrinus

Eogyrinus was a long-bodied (15 foot) aquatic predator. It lived much like a an alligator in the swamps and deltas. The dorsal along the back would have stabilized it while swimming after prey. During the Carboniferous period (which began 360 million years ago), vast extensions of land were covered by dense forests of plants representing groups that are minoritary today, like the already mentioned ferns. Some land vertebrates began reproducing themselves through amniotic eggs, which prevented that the animals became dry in their interiors. These animals, the reptiles, started to put eggs on land, overcoming the amphibians in what concerns to their capability for adapting to a non-aquatic environment and, therefore, they achieved to expand into the interior of the continents

Hylominus: The most ancient reptile known, which lived during the Carboniferous period.

While everybody thought it was extincted since 65 millions of years, in december 22th 1938, Hendrick Goosen, captain of the Nerine took found in his net a strange fish, from a 70 meters depth.

There was an upwelling that day (cold water that going up)

and some other abyssal sharks were caught that day. The fish in question was from an electric blue, and had a 1,50 meters lenght. Since 1952, up to 200 specimens were caught, from a 100 to 300 meters depth. Even if it was a big discovery for the Occidental world, the Comores' fishers knows this fish since a long time.

They call it Kombessa and sometime eat it, and they also use it's rough skin to put on their bicycle's tire when they have a flat before to put something more appropriated! The scientifical name of the animal is Lattimeria chalumnae, in honor to Marjorie Courtenay-Latimer and in reference to the place where it was found (Chalumna river).The fish is caracterised by its pawlike fins. Its tail's also caracteristic to the specie. Africa is full of mysteries, and this one is about to be solved. But is there are other prehistoric animals living in Africa? Perhaps yes...

The Permian is a geologic period that extends from about 280 to 248 million years before the present (mya). As with most older geologic periods, the strata that define the Permian are well identified, but the exact date of the period's start is uncertain by a few million years.

The Permian follows the Carboniferous (Pennsylvanian in North America) and is followed by the Triassic. The end of the period is marked by a major extinction event, called the Permian-Triassic extinction event, that is more tightly dated. The Permian is named from the extensive exposures

in the region around the city of Perm in Russia. Permian exposures consist largely of continental redbeds and shallow water marine exposures.

Plesiosaurs

Were large, carnivorous aquatic reptiles. They are somewhat fancifully said to look like "a turtle with a string running through it", though they lacked a shell.

They first appeared in the late Triassic period and thrived until the K-T extinction at the end of the Cretaceous. Despite being large Mesozoic reptiles, they were not a type of dinosaur. Modern reports of plesiosaurs are usually explained as basking shark carcasses or hoaxes.

Description

The typical plesiosaur had a broad body and a short tail. They retained their ancestral two pairs of limbs, which evolved into large flippers. Plesiosaurs evolved from the earlier nothosaurs, who had a more crocodile-like body; major types of plesiosaur are primarily distinguished by head and neck size.

As a group, the plesiosaurs were the largest aquatic animals of their time, and even the smallest were about 2 m (6.5 ft) long. They grew to be considerably

larger than the largest giant crocodiles, and were bigger than their successors, the mosasaurs. However, their predecessors as rulers of the sea, the dolphin-like ichthyosaurs, are known to have reached 23 m in length, and the modern whale shark (18 m), sperm whale (20 m), and especially the blue whale (30 m) are known from considerably larger specimens.

Behavior

Plesiosaurs have been discovered with fossils of belemnites (squid-like animals), and ammonites (giant nautilus-like molluscs) associated with their stomachs. They had powerful jaws, probably strong enough to bite through the hard shells of their prey. The bony fish (Osteichthyes), started to spread in the Jurassic, and were likely prey as well.

No eggs or evidence of live birth has been discovered, but it has been theorized that smaller plesiosaurs may have crawled up on a beach to lay their eggs, like the modern leatherback turtle.

Another curiosity is their four-flippered design. No modern animals have this swimming adaptation, so there is considerable speculation about what kind of stroke they used. While the short-necked pliosaurs may have been fast swimmers, the long-necked varieties were built more for maneuverability than for speed. Skeletons have also been discovered with gastroliths in their stomachs, probably to help with buoyancy.

Families

The earliest group of plesiosaurs, the plesiosaurids, had small heads and long necks. They evolved about 220 million years ago in the late Triassic, and were the first major group of plesiosaurs to became extinct, about 175 million years ago in the early Jurassic.

The next group of plesiosaurs was characterized by a large head and a short neck, and are collectively known as pliosaurs. The largest pliosaurs, such as Kronosaurus, had jaws 3 m (10 ft) long, and may have reached up to 12–15 m (40–50 ft) in length, and weighed more than 10,000 kg (11 tons). Isolated vertebrae and teeth from England may belong to specimens up to 20 m (65 ft) long, and weighing perhaps 20,000 kg (22 tons).

Pliosaurs had thick, conical teeth, and were the top carnivores of their domain. They fed on other marine reptiles, including their relatives — pliosaur teeth marks have been discovered on other plesiosaurs, like the cryptoclidids. The pliosaurs evolved about 200 million years ago, in the early Jurassic, and became extinct about 80 million years ago in the Cretaceous.

The third group also had a long neck and a tiny head, and were known as the cryptoclidids. Overall, they were shorter and more gracile than the plesiosaurids, but had longer necks in proportion to their body length. Their teeth were also small and slender, and may have been used to filter food from the sediment in shallow coastal waters. They first appeared about 160 million years ago at the end of the Jurassic period, and lasted until the extinction event at the end of the Cretaceous, about 65 million years ago.

The last group, the elasmosaurs, took the long-neck and tiny head tendency to extremes. They were the longest, reaching from 13–17 m (42–56 ft) in length, but most of that was neck; they weighed much less than the more massive pliosaurs. They had up to 72 bones in their neck (vertebrae), more than any other animal. They lived at the same time as the cryptolidids, so they must have filled a different ecological niche. See Elasmosaurus for the typical genus.

Classification and history

The plesiosaur is one of the earliest fossils identified by paleontologists, along with the Mosasaurus, and the dinosaur Iguanodon. The first specimen, belonging to the Plesiosaurus genus, was found in 1821 by Mary Anning, in the Oxford Clay deposits near Lyme Regis, England, and she found the first good specimen just three years later.

The species was formally described and named by Henry de la Beche and William Daniel Conybeare in 1821. The name they chose means "near lizard", dervived from the Greek plesios ("near") and sauros ("lizard"). The Plesiosauria taxon was named by Henri Marie Ducrotay de Blainville in 1835.

Most of the plesiosaur material discovered in the 19th century was from the same deposits. Sir Richard Owen alone named almost a hundred new species. Despite this, however, plesiosaurs are very poorly known.

Most of the new "species" were described based on a few isolated bones, without sufficient diagnostic characteristics to separate them from any of the other species that had previously been described.

Many of these species have since been invalidated, but insufficent work has been done in the past century to clean up this taxonomic mess. The genera Plesiosaurus in particular is problematic. Most of the new species were placed there, so the taxon is a dumping ground for an almost random collection

of bones.

Two other factors make it difficult to classify plesiosaurs. While they have been found on every continent, including Antarctica, almost all specimens are known from either the late Jurassic Oxford Clay in England where the first specimen was found, or from the middle Cretaceous Niobrara Chalk in Kansas, in the United States. Since only two links in a large evolutionary chain are well known, it is hard to extrapolate the large stretch in between.

Plesiosaurs also have another problem. The traditional way to classify plesiosaurs is by their gross body shape, but it appears the same body shape evolved multiple times in an example of convergent evolution. Recent analysis shows that the extremely long-necked elasmosaurs are actually descended from at least three unrelated lineages, making the taxon polyphyletic. Some pliosaurs may also be more closely related to long-necked species than other short-necked species. The four major groupings, while convenient, do not appear to be based on actual evolutionary relationships.

Recent discoveries

In 2002, the "Monster of Aramberri" was announced to the press. Discovered in 1982 at the village of Aramberri, in the Mexican state of Nuevo León, it was originally classified as a dinosaur. The specimen is actually a very large pliosaur, possibly reaching 15 m (50 ft) in length. The media published exaggerated reports claiming it was 25 m (80 ft) long, and weighed up to 150,000 kg, which made it the largest predator of all time. This error was perpetuated in BBC's documentary series Walking with Dinosaurs, which also prematurely classified it as a Liopleurodon ferox.

In 2004, what appears to be a 100 percent intact juvenile plesiosaur was discovered at Bridgwater Bay National Nature Reserve in the United Kingdom, by a local fisherman. The fossil measures 1.5 m (5 ft) in length, and may be related to the Rhomaleosaurus. It is probably the best preserved specimen of a plesiosaur ever discovered.

In fiction

Spoiler warning: Plot or ending details follow.

The plesiosaur is popular among children and cryptozoologists, and appears in a number of children's books, and several films. It has appeared in films about lake monsters, including Magic in the Water (1995), and movies about the Loch Ness Monster, such as Loch Ness (1996). In both films, the creature primarily serves as a symbol of a lost, child-like sense of wonder.

Contrary to reports, the long-necked, sharp-toothed creature in the classic film King Kong (1933) — which flips a raft full of rescuers on their way to save Fay Wray, and then munches on the swimmers — is not a plesiosaur. Despite striking a profile in the mist very similar to the famous "Surgeon's Photo" of the Loch Ness Monster, it then chases the routed heroes onto dry land, where it is clearly intended to be a sauropod, like the Brontosaurus (now Apatosaurus).

Loch Ness and lake monsters

The "Surgeon's Photo" of the Loch Ness Monster. In November 1993, Christian Spurling confessed on his deathbed that he made it from a toy submarine and putty.Main articles: sea monster, lake monster, Loch Ness Monster

Lake or sea monsters sightings are occasionally explained as plesiosaurs. While the survival of a small, unrecorded breeding colony of plesiosaurs for the 65,000,000 years since their apparent extinction is unlikely, the discovery of real and even more ancient living fossils like the Coelacanth, and previously unknown but enormous deep-sea animals like the colossal squid have have fueled imaginations. However, none of these reports have stood up to scientific scrutiny.

The 1977 discovery of a carcass with flippers and what appeared to be a long neck and head by the Japanese fishing trawler Zuiyo Maru off New Zealand created a plesiosaur craze in Japan, but later analysis suggested it was actually a decayed basking shark.

The Loch Ness Monster is commonly reported to resemble a plesiosaur, though just as frequently the creature described bears little or no resemblance.

In addition, the lake is too cold for a cold-blooded animal to easily survive, air-breathing animals like plesiosaurs would be easily spotted when they surface to breathe, the lake is too small to support a breeding colony, and the loch itself was only formed 10,000 years ago during the last ice age. The sightings can be explained as some combination of waves, floating debris, mist-mirages, swimming animals (like the otter, which can reach 6 ft in length), and hoaxes.

The National Museum of Scotland confirmed that vertebrae discovered on the shores of Loch Ness in 2003 belong to a plesiosaur, though there are some questions about whether the fossils were planted. In any case, the 150,000,000 year-old fossils vastly predate the formation of the loch.

Amphibians developed with the characteristics of pharyngeal slits/gills, a dorsal nerve cord, a notochord, and a post-anal tail at different stages of their life. They have persisted since the dawn of tetrapods 390 million years ago in the Devonian period, when they were the first four-legged animals to develop lungs.

During the following Carboniferous period they also developed the ability to walk on land to avoid aquatic competition and predation while allowing them to travel from water source to water source. As a group they maintained the status of the dominant animal for nearly 75

million years. Throughout their history they have ranged in size from the 15 foot long Devonian Ichthyostega, to the slightly smaller 6 foot long Eryops, and down to the tiny 1 centimeter long Psyllophryne didactyla, commonly known as the Brazilian Gold Frog. Amphibians have mastered almost every climate on earth from the hottest deserts to the frozen arctic, and have adapted to climatic change with ease.

Eryops ("big eye")

Is a genus of extinct, semi-aquatic amphibian found primarily in the Permian-aged Admiral Formation of Archer County, Texas, but fossils are also found in New Mexico and parts of the eastern United States. Eryops averaged a little over 5 feet (1.5 m) long, making it one of the largest land animals of its time.

Several complete skeletons of Eryops have been found in the Lower Permian, but skull plates and teeth are the most common fossils. Although it had no direct descendants, it is the best-known Permian amphibian and a remarkable example of natural engineering.

Eryops is an example of an animal that made successful adaptations in the movement from a water environment to a terrestrial one. It retained, and refined, most of the traits found in its fish ancestors. Sturdy limbs supported and transported its body while out of water. A thicker, stronger backbone prevented its body from sagging under its own weight. Also, by utilizing vestigial fish jaw bones, a rudimentary ear was developed, allowing Eryops to hear airborne sound.

Anatomy

The skull of Eryops is proportionately large, being broad and flat and reaching lengths of 2 feet. It had an enormous mouth with many sharp teeth in strong jaws. Its teeth had enamel with a folded pattern, hence its classification with the Labyrinthodonts ("maze toothed").

Within the wide, gaping jaw, the fang-like palatal teeth, when coupled with the gape, suggest an inertial feeding habit. This is when the amphibian would grasp its prey and, lacking any chewing mechanism, toss its head up and backwards, throwing the prey farther back into its mouth. Such feeding is seen today in the crocodile and alligator. It is taken that Eryops was not very active, thus a predatory lifestyle, while possible, was probably not the norm. It is more likely that it fed on fish either in the water or

on those that became stranded at the margins of lakes and swamps. A large supply of terrestrial invertebrates were also abundant at the time, and this may have provided a fairly adequate food supply in itself.

Eryops’ eye sockets were large and directed upward. The body was low to the ground and supported by short, massive limbs. The tail was short, suggesting the animal was not a fast or powerful swimmer. The flat skull with the large eyes and nostrils placed on the top of the head are suggestive that Eryops used stealth for hunting, much like a modern crocodile, and sat quietly in the water waiting for prey with only its eyes and nostrils visible above the water.

The pectoral girdle of Eryops was highly developed, with a larger size for both increased muscle attachment to both it and the limbs. Most notably, the shoulder girdle was disconnected from the skull, resulting in improved terrestrial locomotion. The crossopterygian cleithrum was retained as the clavicle, and the interclavicle was well-developed, lying on the underside of the chest.

In primitive forms, the two clavicles and the interclavicle could have grown ventrally in such a way as to form a broad chest plate, although such was not the case in Eryops.

The upper portion of the girdle had a flat, scapular blade, with the glenoid cavity situated below performing as the articulation surface for the humerus, while ventrally there was a large, flat coracoid plate turning in toward the midline.

The pelvic girdle also was much larger than the simple plate found in fishes, accommodating more muscles.

It extended far dorsally and was joined to the backbone by one or more specialized sacral ribs. The hind legs were somewhat specialized in that they not only supported weight, but also provided propulsion. The dorsal extension of the pelvis was the ilium, while the broad ventral plate was comprised of the pubis in front and the ischium behind. The three bones met at a single point in the center of the pelvic triangle, called the acetabulum, providing a surface of articulation for the femur.

The main strength of the ilio-sacral attachment of Eryops was by ligaments, a condition structurally, but not phylogenetically, intermediate between that of the most primitive embolomerous amphibians and early reptiles. The condition that is more usually found in later vertebrates is that cartilage and fusion of the sacral ribs to the blade of the ilium are utilized in addition to ligamentous attachments.

Respiration

Modern amphibians breathe by inhaling air into lungs, where oxygen is absorbed. They also breathe through the moist lining of the mouth and skin. So too did Eryops, but its ribs were too closely spaced to suggest that it simply expanded the rib cage. More likely, it depressed the hyoid apparatus to expand the oral cavity and elevated the floor of the mouth while it and the nostrils were closed. This forced air back into the lungs. Air could then be forced back out by contraction of the elastic tissue in the lung walls.

Locomotion

Eryops had typical amphibian posture exhibited by the upper arm and upper leg extending nearly straight out from its body, while the forearm and the lower leg extended downward from the upper segment at a near right angle. The body weight was not centered over the limbs, but was rather transferred 90 degrees outward and down through the lower limbs, which contacted the ground.

Most of the animal's strength was used to just elevate its body off the ground for walking, which was probably slow and difficult. With this sort of posture, only short, broad strides could be achieved. This has been confirmed by fossilized footprints found in Carboniferous rocks.

Ligamentous attachments within the limbs were present in Eryops, being important because they were the precursor to bony and cartilagenous variations seen in modern terrestrial animals that use their limbs for locomotion.

The primary species of Eryops has been named Eryops megacephalus (“big head”).

are an extinct group of marine animals (subclass Ammonoidea) in the phylum Mollusca and class Cephalopoda. Their closest living relative is probably the modern nautilus, whom they resemble.

Their fossil shells have the form of flat spirals (though there are some rarer non-spiraled forms, called heteromorphs) and are responsible for the animals' name as they somewhat resemble a tightly coiled ram's horn (the god Ammon was commonly depicted as a man with ram's horns).

Plinius the Elder (died 79 AD near Pompeii) called fossils of these animals ammonis cornua, "horn of Ammon." Often the name of ammonite species ends in ceras, Greek for "horn" (e.g. Pleuroceras).

Location: South Africa and Tanzania.

Size: 1.2m (4 ft).

Unusually for a herbivore , this therapsid had a largepair of

canine teeth in its upper jaw.These may have been used to dig up roots. There were numerous anomodont therapsids of the genus Dicynodon in land environments. These bulky and clumsy animals made their way through thickset coastal vegetation, feeding on soft and juicy underground parts of large horstails. Dicynodon possessed nothing but a pair of exaggerated fangs in the upper jaw and the lost teeth were replaced by horn plates.

The fangs became heavily worn when digging. With the exception of its

prominent tusks, this animal was toothless. Much like a turtle, it cropped vegetation with a horny beak. Dicynodonts were one of the more successful of the later therapsids, persisting until the very end of the Triassic.

The genus Cynognathus had a more-or-less worldwide distribution. Fossils have so far been recovered from South Africa, South America, China and Antarctica. The genus Cynognathus ("Dog jaw") has been given several different names over the years.

It has also been known as Cistecynodon, Cynidiognathus, Cynogomphius, Karoomys, Lycaenognathus, Lycochampsa, Lycognathus, and Nythosaurus. In addition, according to the records of the Peabody Museum of Yale University,

Richard Owen used the name Nythosaurus for this animal in 1876.

This usage seems to be unconnected with Cynognathus. Cynognathus is presently the only recognized member of family Cynognathidae. Opinions vary as to whether all remains belong to the same species.

The species Cynognathus crateronotus is also known as Cistecynodon parvus, Cynidiognathus broomi, Cynidiognathus longiceps, Cynidiognathus merenskyi, Cynognathus beeryi, Cynognathus minor, Cynognathus platyceps, Cynogomphius berryi, Karoomys browni, Lycaenognathus platyceps, Lycochampsa ferox, Lycognathus ferox, Nythosaurus browni.

Fifteen different names for one Mesozoic creature might be regarded as excessive, but it's by no means a record. The dinosaur Plateosaurus engelhardti, has been named well over 20 times.

The genera Karoomys, Cistecynodon and Nythosaurus are known only from tiny juveniles, while Lycognathus cucullatus seems to be a misidentified snake from the Balearic Islands, although confirmation is elusive.

Fossils have been found in Karoo; the Puesto Viejo Formation; Fremouw Formation, in South Africa/Lesotho; Argentina; Antarctica; and China.

Age: Spathian (Lower Triassic) - Anisian (Middle Triassic)

The dentary was equipped with differentiated teeth that show this animal could effectively process its food before swallowing. The presence of a secondary palate in the mouth indicates that Cynognathus would have been able to breathe and swallow simultaneously. The lack of ribs in the stomach region suggests the presence of an efficient diaphragm: an important muscle for mammalian breathing. Pits and canals on the bone of the snout indicate concentrations of nerves and blood cells. In mammals, such structures allow hairs (whiskers) to be used as sensory organs. All of these features indicate that Cynognathus was an endothermic animal: a "warm blooded" creature with a relatively high metabolic rate, which needed to be able to process food and oxygen quickly.

Its unusual wide, flat skull has attracted much conjecture;

one suggestion is that it prevented potential predators from easily swallowing the animal. A series of fossils shows that the skull shape of Diplocaulus's ancestors gradually widened with time.

Over four out of five extant (living today) animal species are arthropods, with over a million modern species described and a fossil record reaching back to the early Cambrian.

Arthropods are common throughout marine, freshwater, terrestrial, and even aerial environments, as well as including various symbiotic and parasitic forms. They range in size from microscopic plankton (~0.25 mm) up to forms several metres long.

The Arthropoda make up a very successful phylum— 75%

of all animals on Earth are arthropods.The Greek word Arthropoda means "jointed feet." The arthropods have a segmented body with appendages on each segment. They have a dorsal heart and a nervous system on the ventral side of their bodies. All arthropods are covered by a hard exoskeleton that is made out of chitin, a polysaccharide. Periodically, an arthropod sheds this covering when it molts. This covering prevents the arthropod from drying out, but also prevents arthropods from growing too big.

The arthropod group identified with the subphylum Chelicerata is the class Arachnida. The most familiar arachnid is the spider. These organisms have two body regions, six jointed appendages, simple eyes, and often carry on respiration by means of book lungs. Their chelicerae are hollow fangs that pierce prey. The second appendages, the pedipalps, contain sensory receptors. They also have four pairs of jointed legs.

On the tip of the abdomen of many spiders there are spinnerets, which they use to make silk for their web. Other arachnids include the scorpions with its pedipalps shaped like pincers, and the mites and ticks, which can be destructive to both plants and animals.

Lobsters, crabs, shrimp, and barnacles belong to the class Crustacea. Their bodies are divided into three parts: abdomen, thorax, and head. Most are aquatic and use gills for respiration. The young stage is a nauplius larva. The number and type of head appendages helps to determine the crustaceans. One typical crustacean which looks like a lobster is a crayfish. It has four pairs of antennae on its head. Large eyes are attached on the head.

Behind those are its mandibles or jaws, which are used to chew food. They are helped by the two pairs of maxillae right behind them. The crayfish also has walking legs and claws on its thorax region. On the abdomen are appendages called swimmerets that females use to hold their eggs. Other groups of arthropods include the Diplopoda, commonly known as millipedes, and the Chilopoda, or the centipedes. A major difference between these groups is the number of legs on each segment.

Basic arthropod structure

The success of the arthropods is related to their hard exoskeleton, segmentation, and jointed appendages. The appendages are used for feeding, sensory reception, defense, and locomotion.

Arthropods respire (breathe) through a tracheal system; a potential difficulty considering that the skeletal structure is external and covers nearly all of the body. Aquatic arthropods use gills to exchange gases. These gills are specialized with an extensive surface area in contact with the surrounding water. Terrestrial arthropods have internal surfaces that are specialized for gas exchange. The insects have tracheal systems: air sacs leading into the body from pores, called spiracles, in the cuticle.

Arthropods have an open circulatory system. Hemolymph, a copper-based blood analogue, is propelled by a series of hearts into the body cavity where it comes in direct contact with the tissues. Arthropods are protostomes.

There is a coelom, but it is reduced to a tiny cavity around the reproductive and excretory organs, and the dominant body cavity is a hemocoel, filled with hemolymph which bathes the organs directly. The arthropod body is divided into a series of distinct segments, plus a presegmental acron

which usually supports compound and simple eyes and a postsegmental telson. These are grouped into distinct, specialized body regions called tagmata. Each segment at least primitively supports a pair of appendages.

The cuticle in arthropods forms a rigid exoskeleton, composed mainly of chitin, which is periodically shed as the animal grows. They contain a inner zone (procuticle) which is made of protein and chitin (a polysaccharide) and is responsible for the strength of the exoskeleton. The outer zone (epicuticle) lies on the surface of the procuticle. It is nonchitinous and is a complex of proteins and lipids.

It provides the moisture proofing and protection to the procuticle. The exoskeleton takes the form of plates called sclerites on the segments, plus rings on the appendages that divide them into segments separated by joints.

This is in fact what gives arthropods their name—joint feet—and separates them from their very close relatives, the Onychophora and Tardigrada. The skeletons of arthropods strengthen them against attack by predators and are impermeable to water. In order to grow, an arthropod must shed its old exoskeleton and secrete a new one.

This process, molting, is expensive in energy consumption. During the molting period, an arthropod is vulnerable. Once their cuticle hardens they are fully developed and can never grow again. Their cuticles slowly expand as they increase in mass. They breakdown (digest) their cuticle every now and then when they need to grow. Their cuticle hardens at their adult size and they slowly grow to fill it up.

Arthropod relationships

At one point it was considered that the different subphyla of arthropods had separate origins from segmented worms, and in particular that the Uniramia were closer to the Onychophora than to other arthropods. However, this is rejected by most workers, and is contradicted by genetic studies.

Traditionally the Annelida have been considered the closest relatives of these three phyla, on account of their common segmentation. More recently, however, this has been considered convergent evolution, and the arthropods and allies may be closer related to certain pseudocoelomates such as roundworms that share with them growth by molting, or ecdysis. These two possible lineages have been termed the Articulata and Ecdysozoa.

The classification of the arthropods varies somewhat from source to source. There are five main subgroups: the Trilobita, Chelicerata, Myriapoda, Hexapoda, and Crustacea, which may be variously ranked from subphyla to classes, with various other taxa introduced above or below them and corresponding changes in the ranks of their subgroups.

Here we have followed a "splitting" taxonomy, containing only generally accepted groups and assigning them higher ranks.

Aside from these major groups, there are also a number of fossil forms, mostly from the lower Cambrian, which are difficult to place, either from lack of obvious affinity to any of the main groups or from clear affinity to several of them.

The Paleozoic is bracketed by two of the most important events in the history of animal life. At its beginning, multicelled animals underwent a dramatic "explosion" in diversity, and almost all living animal phyla appeared within a few millions of years. At the other end of the Paleozoic, the largest mass extinction in history wiped out approximately 90% of all marine animal species. The causes of both these events are still not fully understood and the subject of much research and controversy. Roughly halfway in between, animals, fungi, and plants alike

colonized the land, the insects took to the air, and the limestone shown in this picture was deposited near Burlington, Missouri.

The Paleozoic took up over half of the Phanerozoic, approximately 300 million years. During the Paleozoic there were six major continental land masses; each of these consisted of different parts of the modern continents. For instance, at the beginning of the Paleozoic, today's western coast of North America ran east-west along the equator, while Africa was at the South Pole.

These Paleozoic continents experienced tremendous mountain building along their margins, and numerous incursions and retreats of shallow seas across their interiors. Large limestone outcrops, like the one shown above, are evidence of these periodic incursions of continental seas.

Many Paleozoic rocks are economically important. For example, much of the limestone quarried for building and industrial purposes, as well as the coal deposits of western Europe and the eastern United States, were formed during the Paleozoic

Periods

The Cambrian

It marks an important point in the history of life on earth; it is the time when most of the major groups of animals first appear in the fossil record. This event is sometimes called the "Cambrian Explosion", because of the relatively short time over which this diversity of forms appears. It was once thought that the Cambrian rocks contained the first and oldest fossil animals, but these are now to be found in the earlier Vendian strata.

Subdivisions of the Cambrian:

There are 5 major subdivisions of the Cambrian Period for North America (Laurentia during the Cambrian). International ages (subdivisions) have not been established. The oldest unnamed age is 543 to 520 million years ago, while the remaining six ages are from 520 to 490 million years ago, each approximately 5-6 million years long.

Tomotian Age, Montezuman Period, Dyeran Period, Marjuman Period, Steptoan Period and Sunwaptan Period

The Tommotian Age: 530 Million Years Ago

The Tommotian Age, which began about 530 million years ago, is a subdivision of the early Cambrian. Named for rock exposures in Siberia, the Tommotian saw the first major radiation of the animals, or metazoans, including the first appearance of a great many mineralized taxa such as brachiopods, trilobites, archaeocyathids, molluscs, echinoderms, and more problematic forms. Soft-bodied members of many other phyla were also appearing and diversifying at this time.

A few million years before the Tommotian, in the Vendian, the continents had been joined in a single supercontinent called Rodinia (from the Russian word for "homeland", rodina.) As the Cambrian began, Rodinia began to fragment into smaller continents which did not always correspond to the ones we see today. Much later, in the Permian, the continents came back together to form a new supercontinent, called Pangaea.

World climates were mild; there was no glaciation. Most of North America lay in warm southern tropical and temperate latitudes, which supported the growth of extensive shallow-water archaeocyathid reefs all through the Lower Cambrian. Siberia, which also supported abundant reefs, was a separate continent due east of North America. Baltica -- what is now Scandinavia, eastern Europe, and European Russia -- lay to the south.

Most of the rest of the continents were joined in a supercontinent known as proto-Gondwana, depicted on the right side of the map; South America, Africa, Antarctica, India, and Australia are all visible. What is now China and east Asia was fragmented at the time, with the fragments visible north and west of Australia. Western Europe was also in pieces, with most of the pieces lying northwest of what is now the north African coastline. The present-day southeastern United States are visible wedged between South America and Africa; they did not become part of North America for another 300 million years.

Life in the Cambrian

Other than in most other periods of earth history, the Cambrian fossil record indicates a distinct development from simple organisms to organisms comparable in morphology and organization to the present-day animals. This rapid phylogenetic development started in the latest Proterozoic and was more-or-less finished at the end of the Early Cambrian. The development is documented by faunal assemblages represented by

(1) the Ediacara fauna,

(2) the first complex trace fossils,

(3) the earliest shelly faunas, and

(4) the onset of the typical Cambrian macrofaunas.

Additional important data come from fossil archives. It is amazing that this rapid evolution took place in an interval of less than 25 m.y., and the evolution from the first hard-part animals to the presence of most of the present-day phyla was restricted to an interval of probably less than 10 m.y. Multicellular life evolved at an incredible supersonic speed, and for this reason this part of organismal evolution is termed the "Cambrian Explosion", or "Evolution's Big Bang."

Ediacara “fauna”

The oldest fossils of metazoan aspect are remains that belong to the so-called “Ediacara fauna”. The peculiar character of this fauna was first recognized in the Pound Quartzite, Ediacaran Hills, South Australia, although the same type of fauna was earlier discovered in the present-day southern Namibia. Ediacaran-type fossils are now known from numerous sites worldwide (such as Namibia, Ireland, England, northwestern Russia, South Australia, Newfoundland and the Canadian Northwest Territories) in rocks dated between 610 and 510 m.y., thus ranging from the Late Proterozoic (Vendian or late Neoproterozoic) to the Middle Cambrian.

The typical Ediacaran fauna of Late Proterozoic age includes organismal remains that look like feathery fronds, pouches or disk. Frond-like remains usually show delicate branches, and none of these organisms had heads or obvious circulatory, nervous or digestive systems. Earlier suggested affinities include sea pens, molluscs, jellyfish, or worms, but a new interpretation by A. Seilacher now assigns them as single-celled organisms with hydraulic architecture. This previously unknown kingdom Vendobionta of organisms is characterized by flattened, quilt-like anatomy, and is seemingly an experiment in life with fluid-filled, air-mattress-type bauplans.

The first metazoans

Coexisting with latest Neoproterozoic Ediacaran faunas are simple trace fossils ("worm burrows") that were created by multicellular animals, the first evident Metazoa. Indeed, the stratigraphical occurrence of trace-fossils depicts an evolution to more complicated traces, which, in turn, proves the progressive evolution to more anatomically complicated animals that were able to perform a progressively complex behaviour. The first trace with a somewhat complicate pattern is Trichophycus pedum (formerly known as "Phycodes pedum").

It occurs nearly worldwide, and its first occurrence is with late fossils of typical Ediacaran aspect or, usually, in strata above them, whereas the first shelly fossils appeared clearly later. Hence, the ichnofossil assemblage with Trichophycus pedum marks the first occurrence of well-developed, fairly complex metazoan animals, and this is today regarded as the most useful landmark to characterize the boundary between the Precambrian and the Phanerozoic and, synchronously, the Proterozoic and the Cambrian.

Accordingly, the International Subcommission on Cambrian Stratigraphy (through its Working Group on the Precambrian-Cambrian Boundary) made the official decision in 1991 to draw the base on the Cambrian at the first appearance date (FAD) of Trichophycus pedum in the reference section at Fortune Head, southeastern Newfoundland. Other characterisitic assemblages with more and more complicate trace fossils occur later that than Trichophycus pedum but still before the first hard-part fossils.

The first shelly faunas

No animals are known from the very base of Cambrian that had hard parts, either as an external skeleton or simply spicules. The first shelled metazoas which are then characteristic for the Cambrian occur well above the earliest complex trace fossils. This suggests that hard part production evolved later, and the typical Cambrian faunas (such as trilobites, archaeocyaths, and small shelly fossils) are unknown before the middle part of the Early Cambrian.

The evolution of shelled metazoan is reflected by the appearence of successively more advanced shelly fossils. The typical small shelly fossils (SSFs, or early shelly fossils, ESFs) are tiny (generally 1 to 5 mm) tubes, spines, cones and plates that are not clearly allied with modern groups. Many of these organisms were recognized either as of unknown affinity or as representatives or groups that became extinct before the end of the Cambrian. The most "primitive" stage is marked by characteristic elements, such as anabaritids, tommotiids, and hyolithellids, known as the "Tommotian fauna." Later SSFs have been identified as sclerites of worm-like animals or as early representatives of the major fossil groups.

Cambrian macrofauna

Skeletonized organisms and organisms with distinct limbs became more and more abundant as the Early Cambrian progressed. The first macroscopic faunas occur at the end of the Siberian Tommotian Stage, when major reefal complexes were formed by archaeocyaths. Archaeocyaths are sponges with a simple morphology. Their calcareous skeleton consists of an inner and an outer wall that are variably connected.

The major Cambrian animal group in the fossil record are the trilobites. Trilobites are arthropods with a characteristic longitudinal and transverse tripartition that was name giving. They are not only abundant in various shelf deposits but are helpful as index fossils and to characterize biofacies and paleobiogeography. Despite the relatively strong faunal provincialism, Cambrian trilobites constitute the biostratigraphical framework that allows to compare rock successions from regions on different Cambrian continents.

Arthropods are apparently the most diverse of the Cambrian animal groups. Numerous enigmatic forms with strongly differing cephalic appendages are known, especially from the various fossil archives. Bivalved arthropods of ostracod aspect but with phosphatic shell are important fossils in several regions.

Brachiopods are relatively frequent but rarely abundant fossils. The first articulate brachiopod groups occur at the end of the Early Cambrian. Conodonts are important are biostratigraphic tools in the Upper Cambrian. Other characteristic Cambrian invertebrates include early mollusc groups (helcionellids, pelagiellids), hyoliths, and echinderms (e.g., helicoplacoids, eocrinoids, cinctans, edrioasteroids).

The fossil archives

The Cambrian period is surprisingly rich in beds that yield well-preserved fossils. Due to general decay, fossils are usually remains of only hard parts that have enough potential to be preserved. However, unusual circumstances may serve for a preservation of soft-parts. This does not only give much more insight into the morphology and anatomy of the hard-part animals but usually enlarges the spectrum of the known animals because only between 5 and 10 percent of the fossils in those fossil archives have a skeleton that would be preserved under normal circumstances.

Burgess Shale

The most prolific example for such a fossil archive is the Middle Cambrian Burgess Shale from the Burgess Pass, Yoho National Park, British Columbia. It represents a so-called Lagerstätten deposit, where shallow dwelling marine animals were swept to a deeper site of the shelf and rapidly buried by slumps. The Burgess Shale fauna was discovered in 1909 by Charles Walcott, who interpreted the fauna as belonging entirely to modern phyla. However, recent studies have shown that many of the organisms had strange and previously unknown morphologies and have no modern analogues. They are now recognized to represent a large number of higher taxa that are only known from the Cambrian fossil archives and probably became extinct before the end of the Cambrian.

Chengjiang

Even more beautifully preserved are faunas from the Early Cambrian of Chengjiang, Yunnan Province, on the Yangtze Platform, South China. The Chengjiang locality yielded a number of fossils that were only known from the Burgess Shale, but also another number of strange creatures. Description of the Chengjiang fossils is in progress.

Orsten

Another type of fossil archive is found in the Upper Cambrian of Sweden. Typical deposits there are smelly alum shales, which have fossil-rich calcareous nodules ("orsten"). Dissolution of these nodules by organic acids revealed a marvellous fauna of microscopic arthropods in an amazingly good, three-dimensional preservation. Most of these fossils represent early and often previously unknown arthropod groups, but probably also the earliest known tardigrades and pentastomids (which are extant groups but otherwise unknown from the fossil record).

Other fossil lagerstätten

Additional fossil archives with soft-part preservation are known from the Early Cambrian Sirius Passet, Greenland, the Early Cambrian Mount Cap Formation of the Canadian Northwest Territories, the Early Cambrian Emu Bay Shale, South Australia, and the Middle Cambrian of Siberia, and we can be ceratin that there are many more to be discovered...

Once more: The Cambrian Explosion

The latest Neoproterozoic to Early Cambrian fossil record indicates that multicellular life evolved into a large number of possible bauplans as soon as it got a foothold. These bauplans, or types of organization, characterize high-ranked taxa such as phyla.

Although life developed to a huge diversity as seen today, probably no new phyla developed in post-Cambrian times and the number of phyla has actually decreased since. The Middle Cambrian may thus represent the time with the organizational diversity at a maximum. What are the reasons of the Cambrian Explosion? This is a question that nobody can answer with enough certainty in the moment.

Physical examination of latest Proterozoic and Cambrian rocks indicate that there was

(1) a distinct fluctuation of carbon isotopes around the Proterozoic-Cambrian,

(2) a dramatic increase of the d34S curve,

(3) an increase of the global sea-level,

(4) a distinct rise of the phosphorite production, and

(5) a slow increase of oxygen in the atmosphere from late Proterozoic to early Phanerozoic times.

These facts form the frame of a probably complex scenario, which ecologically equals the filling of an ecological barrel. However, we only hypothesize factors that may be responsible for a dramatic increase of phylogenetic development, such as possibly simpler Cambrian genomes or a more direct translation of gene to product, which may have enabled early diversification.

Other hypotheses are needed to explain the rapid evolution and diversification of hard parts. Most of those hypothesis focus on changes in the physico-chemical environment and ecological stimuli (such as the evolution of the first predators). Regardless of the reasons, the novelty of hard parts led to more efficiency and improvements in the performance of animals and so is directly related to "advanced" animal groups such as arthropods and the group to which we belong, the chordates.

The Ordovician: 490 to 443 Million Years Ago

The Ordovician period began approximately 510 million years ago, with the end of the Cambrian, and ended around 445 million years ago, with the beginning of the Silurian. At this time, the area north of the tropics was almost entirely ocean, and most of the world's land was collected into the southern super-continent Gondwana. Throughout the Ordovician, Gondwana shifted towards the South Pole and much of it was submerged underwater.

The Ordovician is best known for the presence of its diverse marine invertebrates, including graptolites, trilobites, brachiopods, and the conodonts (early vertebrates). A typical marine community consisted of these animals, plus red and green algae, primitive fish, cephalopods, corals, crinoids, and gastropods. More recently, there has been found evidence of tetrahedral spores that are similar to those of primitive land plants, suggesting that plants invaded the land at this time.

From the Early to Middle Ordovician, the earth experienced a milder climate in which the weather was warm and the atmosphere contained a lot of moisture. However, when Gondwana finally settled on the South Pole during the Late Ordovician, massive glaciers formed causing shallow seas to drain and sea levels to drop. This likely caused the mass extinctions that characterize the end of the Ordovician, in which 60% of all marine invertebrate genera and 25% of all families went extinct.

Subdivisions of the Ordovician:

Tremadocian, Arenigian, Llanvirnian, Llandeilian, Caradocian, Ashgillian.

In addition to having the ideal conditions for the beginnings ofcoal, several major biological, geological, and climatic events occurred during this time. One of the greatest evolutionary innovations of the Carboniferous was the amniote egg, which allowed for the further exploitation of the land by certain tetrapods.

The amniote egg allowed the ancestors of birds, mammals, and reptiles to reproduce on land by preventing the desiccation of the embryo inside. There was also a trend towards mild temperatures during theCarboniferous, as evidenced by the decrease in lycopods and large insects and an increase in the number of tree ferns.

Geologically, the Late Carboniferous collision of Laurussia (present-day Europe and North America) into Godwanaland (present-day Africa and South America) produced the Appalachian mountain belt of eastern North America and the Hercynian Mountains in the United Kingdom. A further collision of Siberia and eastern Europe created the Ural Mountains.

The stratigraphy of the Lower Carboniferous can be easilydistinguished from that of the Upper Carboniferous. The environment of the Lower Carboniferous in North America was heavily marine, when seas covered parts of the continents. As a result, most of the mineral found in Lower Carboniferous is limestone, which are composed of the remains of crinoids, lime-encrusted green algae, or calcium carbonate shaped by waves.

The North American Upper Carboniferous environment was alternately terrestrialand marine, with the transgression and regression of the seas caused by glaciation. These environmental conditions, with the vast amount of plant material provided by the extensive coal forests, allowed for the production of coal. Plant material did not decay when the seas covered them and pressure and heat eventually built up over the millions of years to transform the plant material to coal.

Early Carboniferous Tournaislam Late Carboniferous Vashkirian

Visean Moscovian

Serk Ukhovian Kasimovian

Gzelian

The vegetation of the early Devonian consisted primarily of small plants, the tallest being only a meter tall. By the end of the Devonian, ferns, horsetails and seed plants had also appeared, producing the first trees and the first forests. Archaeopteris, shown below left, is one of these first trees.

Also during the Devonian, two major animal groups colonized the land. The first tetrapods, or land-living vertebrates, appeared during the Devonian, as did the first terrestrial arthropods, including wingless insects and the earliest arachnids. In the oceans, brachiopods flourished, like the beautifully pyritized brachiopod Paraspirifer bownockeri from Ohio, pictured above and to the right. Crinoids and other echinoderms, tabulate and rugose corals, and ammonites were also common. Many new kinds of fish appeared.

During the Devonian, there were three major continental masses: North America and Europe sat together near the equator, much of their current land underneath seas. To the north lay a portion of modern Siberia. A composite continent of South America, Africa, Antarctica, India, and Australia dominated the southern hemisphere.

Subdivisions of the Devonian:

Early Devonian Lockhovian Middle Devonian Eifelian Late Devonian Frasnian

Praghian Givetian Fammenian

Emsian

The global geography of the Permian included massive areas of land and water. By the beginning of the Permian, the motion of the Earth's crustal plates had brought much of the total land together, fused in a supercontinent known as Pangea. Many of the continents of today in somewhat intact form met in Pangea (only Asia was broken up at the time), which stretched from the northern to the southern pole. Most of the rest of the surface area of the Earth was occupied by a corresponding single ocean, known as Panthalassa, with a smaller sea to the east of Pangea known as Tethys.

Models indicate that the interior regions of this vast continent were probably dry, with great seasonal fluctuations, because of the lack of the moderating effect of nearby bodies of water, and that only portions received rainfall throughout the year. The ocean itself still has little known about it. There are indications that the climate of the Earth shifted at this time, and that glaciation decreased, as the interiors of continents became drier.

Subdivisions of the Permian:

Early Permian Aseliam Late Permian Kungurian

Sakmarian Kazanian

Artinskian Tatarian

Coral reefs made their first appearance during this time, and the Silurian was also a remarkable time in the evolution of fishes. Not only does this time period mark the wide and rapid spread of jawless fish, but also the highly significant appearances of both the first known freshwater fish as well as the first fish with jaws. It is also at this time that our first good evidence of life on land is preserved, including relatives of spiders and centipedes, and also the earliest fossils of vascular plants.

Llandoverian, Wenlockian, Ludlovian, Pridolian