Outline 15:Paleozoic Life

The Evolution of Vertebrates: Fish and

Amphibians

Phylum Chordata

• All chordates have a dorsal nerve cord.

• Chordates with vertebrae are the vertebrates. The vertebrae surround the spinal cord.

• Primitive chordates (invertebrates) include sea squirts and arrow worms.

Your spinal cord is a dorsal nerve cord

Tunicates – most primitive chordates

mobile larva

sessile adult

The Lancelet, a living primitive chordate

Vertebrate Phylogeny

Chordate Fossils

• The oldest known chordates are Pikaia from the Burgess Shale and the older Yunnanozoon from China.

• Both have a dorsal nerve cord, but no vertebrae.

Pikaia, an early chordate

Yunnanozoan, a chordate from the early Cambrian of China

Fossil Fish

• Fish are vertebrates.• Oldest fish in the Ordovician.• They are the Agnatha, or jawless

fish.• Teeth on their lips and tongues.

What is a vertebrate?

The simplest vertebrate: a

toothless hagfish

A living jawless fish, the lamprey, Class Agnatha

A fossil jawless fish, Class Agnatha

Major Fish Groups• Agnatha: jawless fish; still living• Jawed groups:

–Placoderms: extinct armored fish with cartilaginous internal skeleton

–Chondrichthyes: cartilaginous fish (sharks and rays); no float bladder

–Osteichthyes: bony fish; float bladder

40,000 spp.

Tetrapod ancestor

Origin of Jaws

• Jaws evolved by modification of the gill arches, tiny bones used to hold open the gill slits.

• Jaws may have evolved to help with respiration by pumping water over the gills when the jaws opened and closed.

• Biting came later.

Evolution of jaws

Jaws evolved by modification of the gill arch bones.

Gill Arches are visible during human embryonic development

The extinct placoderms were the armored fish of the Paleozoic

Fossil tooth of a Great White shark

Megalodon

A modern Great White Shark

The Osteichthyes: Bony Fish

• 2 main groups–Ray-finned fish–Lobe-finned fish

• 40,000 species of living ray-finned fish -the teleosts, a great evolutionary success!

• 2 kinds of living lobe-finned fish, a great evolutionary failure?

Fossil bony fish, Class Osteichthyes

Fossil bony fish, Class Osteichthyes

Lobe-Finned Fish

• Include the living lungfish and coelocanth, plus the extinct rhipidistians.

• Bones and muscles of lobe fins could be used to walk on land.

• The rhipidistians walked out of the water in the late Devonian to become the first amphibians.

Bone structure in fins of ray-finned and lobe-finned fish

Comparison of Ray Fins and Lobe Fins

Coelocanth, a living lobe-finned fish,

Class Osteichthyes

Coelocanth, a living lobe-finned fish, Class Osteichthyes

Skeleton of the coelocanth

Fish-amphibian phylogeny

South American lungfish in its burrow

An Australian lungfish

Mudskippers, ray-finned fish acting like amphibians

A Frog Fish, a modern ray-finned fish with

“fingers”

The evolutionary

step from fish to amphibian

was not difficult.

Old Theory: Looking for water in the Devonian led to evolution of lungs.

New Theory: Lungs evolved earlier for rapid swimming by predators in the open ocean

Evolution of the tetrapod walking leg from the lobe fin

Hindlimb of Ichthyostega

Ichthyostega: Photographs of part and counterpart superimposed to show seven digits

Rhipidistian fish

Rhipidistian fish

HumanTetrapod

Lobe-fin fish

Early Tetrapod Feet5, 6, or 7 toes

Tiktaalik roseafrom the Late Devonian of

Ellsmere Island,

Canada, 2006

http://thecolbertreport.cc.com/videos/xgsf42/neil-shubin

http://thecolbertreport.cc.com/videos/xgsf42/neil-shubin

Tiktaalik rosea

Tiktaalik rosea

Amphibians: Fish out of water• Reproduction: same as fish• Breathing: same as lungfish• Fluid retention: mucous glands of

fish, later became oil glands in skin.• Locomotion: lobe fins modified into

walking legs.• Conclusion: not a big evolutionary

jump

Skull roof of Ichthyostega

Acanthostega andIchthyostega, with 7 to 8 digits

Summary of the Mid Paleozoic terrestrial invasion

Guinness “Evolution”

http://www.youtube.com/watch?v=1t4sdgvy-pk

Permian amphibian with 5 digits

Life at the water hole, late Paleozoic

Early and large amphibians

Outline 15:�Paleozoic LifePhylum ChordataSlide Number 3Tunicates – most primitive chordatesSlide Number 5Slide Number 6Chordate FossilsSlide Number 8Slide Number 9Fossil FishSlide Number 11Slide Number 12Slide Number 13Slide Number 14Major Fish GroupsSlide Number 16Origin of JawsEvolution of jawsSlide Number 19Slide Number 20Slide Number 21Slide Number 22MegalodonSlide Number 24The Osteichthyes: Bony FishSlide Number 26Slide Number 27Lobe-Finned FishBone structure in fins of ray-finned and lobe-finned fishSlide Number 30Slide Number 31Slide Number 32Slide Number 33Slide Number 34Slide Number 35Slide Number 36Slide Number 37Slide Number 38Slide Number 39Slide Number 40Slide Number 41Slide Number 42Slide Number 43Rhipidistian fishSlide Number 45Slide Number 46Slide Number 47Slide Number 48Slide Number 49Slide Number 50Slide Number 51Slide Number 52Amphibians: Fish out of waterSlide Number 54Slide Number 55Summary of the Mid Paleozoic terrestrial invasionSlide Number 57Slide Number 58Slide Number 59Early and large amphibiansSlide Number 61Slide Number 62