Historical Geology 102
Life in the Paleozoic II


Invertebrates and reef organisms continue to flourish.
Coiled cephalopods evolve - Nautiloids are modern examples.
The Devonian is often referred to as the Age of Fish.
Fish underwent major evolutionary developments.
Increased numbers of fish.
Agnathids (jawless fish) evolved during the Ordivician and jawed fish appeared by the late Silurian.


Plate-skinned Fish
Armored Agnathids went extinct in the Devonian
Placoderms reached a peak of deveolpment in the Middle Devonian but declined throughout the remainder of the Paleozoic Placoderms gave rise to two important groups of fish - Chrondrichthyes and Osteichthyes

Cartilage vs. Bone

Chrondrichthyes are fish with skeltons composed of cartilage
Modern examples include sharks and rays
Osteichthyes are fish with bony skeletons


One line of fish, the Rhipidistians, evolved into amphibians
This required a number of major evolutionary developments

Land Organisms

Seeds first evolve in the Devonian
Forests delevop - composed of horsetails and fern trees
Insects are becoming wide spread

Devonian Extinction

The end of the Devonian is marked by another mass extinction
Many of the reef building invertebrates went extinct at this time
Cause of this extinction - ???

Mississippian Period

Conifers evolve - "pine trees"
Seed ferns evolve
Scaled trees evolve
Within the oceans crinoids become very abundant - this Period is often called the Age of Crinoids

Pennsylvanian Period

Cyclothems are occurring rapidly
Coal swamps were extensive during this time
Amphibians and insects were the common animals living in these swamps

Pennsylvanian Insects

Insects during the Pennsylvanian diversified and grew to large sizes.


In the Pennsylvanian the first reptiles evolved from amphilians
Evolutionary developments in reptiles

Permian Period

Two major events during this Period
1) Evolution of two important reptile groups - Pelycosaurs and Therapsids
2) Mass extinction at the end of the Period


Also called the sail-back reptiles.
Best known Pelycosaurs is Dimetrodon (this reptile is often include in books on dinosaurs, but is not a dinosaur)
The function of the sail?


Therapsids are mammal-like reptiles that may have evolved from the Pelycosaurs
Two main groups of therapsids evolved at this time - carnivore therapsids and herbivore therapsids
This group of reptiles is believed to have given rise to mammals

Permian Mass Extinction

The end of the Paleozoic is marked by the largest mass extinction in history
50% of all Families went extinct
Included rugose corals, blastoids, several groups of brachiopods and crinoids, and trilobites
Several groups saw reduction in numbers including fish, amphibians and reptiles

Permian Mass Extinction

A number of theories have been proposed to account for this mass extinction.