Review of Paleozoic Invertebrate lecture Major theme of ESCI 102: plate tectonics drives biology Oceans open and close,changing climate, creating & destroying ecological niches Paleozoic starts with Cambrian period –Cambrian explosion –appearance of multicelled organisms/Homeobox/hox genes

Mass extinctions are followed by adaptive radiations Big three Cambrian organisms: –Trilobites, brachiopods, archaeocyathids After Cambrian comes Ordovician –transgression -> adaptive radiation Ordovician ends with a mass extinction Silurian, Devonian –Ordovician, Devonian extinctions followed by adaptive radiation

The situation was bad as the Permian ended, and then it got worse.

Tetrapod trackway at Valentia Island Ireland These fossilized fooprints –365 million years old –evidence of one of the earliest four-legged animals on land Paleozoic Life History — Vertebrates and Plants Photo courtesy of Ken Higgs, U. College Cork, Ireland

12% of geologic time Cenozoic Era 65-0 – Recent Life Mesozoic Era – Middle Life Paleozoic Era – Ancient life – better resolution reflects fossil preservation Phanerozoic Eon

Vertebrates: chordates whose notochord is a spinal column Phyllum Chordata: –notochord physical rod supporting nerve cord –dorsal hollow nerve cord bundled nerve fibers connect brain to muscles –gill slits (pharyngeal slits) Openings connecting inside throat to outside neck –tail

The most primitive vertebrates are fish –oldest fish remains are in Upper Cambrian rocks All known Cambrian and Ordovician fossil fish –found in shallow nearshore marine deposits –earliest nonmarine fish remains in Silurian strata suggests saltwater origins Fish started in saltwater – fragment of a plate from Anatolepis cf. A. Heintzi, Upper Cambrian marine Deadwood Formation of Wyoming: a primitive member of the class Agnatha (jawless fish)

Fish range from the Late Cambrian to the present The oldest and most primitive of the class Agnatha: the ostracoderms, “bony skin” These are armored jawless fish that first evolved during the Late Cambrian –reached their zenith during the Silurian and Devonian –and then became extinct Ostracoderms — “Bony Skinned” Fish

Devonian Seafloor ostracoderm (Hemicyclaspis) placoderm (Bothriolepis) acanthodian (Parexus) ray-finned fish (Cheirolepis)

Major evolutionary advantage –jawless ancestors could only feed on detritus –jawed fish could chew food and become active predators, thus opening many new ecological niches The vertebrate jaw is an excellent example of evolutionary opportunism –the jaw probably evolved from the first three gill arches of jawless fish Evolution of Jaws

The fossil remains of the first jawed fish are found in Lower Silurian rocks and belong to the acanthodians: large spines scales covering much of the body jaws teeth and reduced body armor Acanthodians

Geologic Ranges of Major Fish Groups

The other jawed fish that evolved during the Late Silurian were the placoderms, “plate-skinned” Placoderms were heavily armored jawed fish –lived in both freshwater and the ocean –like the acanthodians, reached their peak of abundance and diversity during the Devonian Other Jawed Fish

Geologic Ranges of Major Fish Groups

A Late Devonian marine scene from the midcontinent of North America Late Devonian Marine Scene

Many fish evolved during the Devonian Period including –the abundant acanthodians –placoderms –ostracoderms –other fish groups Age of Fish

Class Chrondrichthyes, –represented today by sharks, rays, and skates, –first evolved during the Middle Devonian Cartilaginous fish have never been as numerous nor as diverse as their cousins, the bony fish, –but they are important members of the marine vertebrate fauna Cartilaginous Fish

Because bony fish are the most varied and numerous of all the fishes –and because the amphibians evolved from them, –their evolutionary history is particularly important There are two groups of bony fish –the common ray-finned fish –and the less familiar lobe-fined fish Bony Fish

Arrangement of fin bones for (a) a ray-finned fish (b) a lobe-finned fish –muscles extend into the fin allowing greater flexibility Ray-Finned and Lobe-Finned Fish

Began in freshwater in the Devonian predicessors of familiary fish like trout, bass, perch, salmon, and tuna –rapidly diversified to dominate the Mesozoic and Cenozoic Seas Ray-Finned Fish Rapidly Diversified es/may00/images/trout.jpg

Geologic Ranges of Major Fish Groups

The crossopterygians are an important group of lobe- finned freshwater fish because amphibians evolved from them During the Devonian, two separate branches of crossopterygians evolved –one led to the amphibians –while the other invaded the sea Amphibians Evolved from Crossopterygians

The crossopterygians that invaded the sea –called the coelacanths –were thought to have become extinct at the end of the Cretaceous In 1938, however, a fisherman caught a coelacanth off Madagascar –since then several dozen more have been caught both there and in Indonesia Coelacanths science/coelacanth_ html

Eusthenopteron, –a member of the rhipidistian crossopterygians –had an elongate body –and paired fins –that it could use to move about on land The crossopterygians are thought to be amphibian ancestors The crossopterygians that became amphibians were the Rhipidistians.

Similarities between the crossopterygian lobe-finned fish and the labyrinthodont amphibians Fish/Amphibian Comparison Their skeletons were similar

Comparison of the limb bones of a crossopterygian (left) and an amphibian (right) Color identifies the bones that the two groups have in common Comparison of Limbs

Although amphibians were the first vertebrates to live on land, they were not the first land-living organisms Land plants, which probably evolved from green algae, first evolved during the Ordovician Furthermore, insects, millipedes, spiders, and snails invaded the land before amphibians Amphibians— Vertebrates Invade the Land