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.
Paleozoic Life History — Vertebrates and
Plants
Tetrapod trackway at Valentia
Island Ireland
These fossilized fooprints
– 365 million years old
– evidence of one of the
earliest four-legged animals
on land
Photo courtesy of Ken Higgs, U. College
Cork, Ireland
Phanerozoic Eon
• 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
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
Fish started in saltwater
• 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
– 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)
Ostracoderms —
“Bony Skinned” 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
Devonian Seafloor
acanthodian
(Parexus)
ray-finned fish
(Cheirolepis)
placoderm (Bothriolepis)
ostracoderm (Hemicyclaspis)
Evolution of Jaws
• 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
Acanthodians
• 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
Geologic Ranges of
Major Fish Groups
Other Jawed Fish
• 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
Geologic Ranges of
Major Fish Groups
Late Devonian Marine Scene
• A Late Devonian marine scene from the midcontinent of
North America
Age of Fish
• Many fish evolved during the Devonian Period
including
–
–
–
–
the abundant acanthodians
placoderms
ostracoderms
other fish groups
Cartilaginous 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
http://www.iopus.com/iim/demo/slideshow.htm
Bony 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
Ray-Finned and Lobe-Finned 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 Fish Rapidly Diversified
• 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
http://www.ariverneversleeps.com/backissu
es/may00/images/trout.jpg
Geologic Ranges of
Major Fish Groups
Amphibians Evolved from
Crossopterygians
• The crossopterygians are an important group of lobefinned 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
Coelacanths
• 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
http://www.calacademy.org/science_now/archive/headline_
science/coelacanth_010601.html
The crossopterygians that became amphibians were
the Rhipidistians.
• 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
Fish/Amphibian Comparison
• Similarities between the crossopterygian lobe-finned fish
and the labyrinthodont amphibians
• Their
skeletons were
similar
Comparison of Limbs
• Comparison of the limb bones of a crossopterygian (left)
and an amphibian (right)
• Color identifies the bones that the two groups have in
common
Amphibians—
Vertebrates Invade the Land
• 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