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 245-65– Middle Life

Paleozoic Era 544-245– 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

http://www.iopus.com/iim/demo/slideshow.htm

• 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

http://www.ariverneversleeps.com/backissues/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

http://www.calacademy.org/science_now/archive/headline_science/coelacanth_010601.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