Historical Geology 102
Life in the Late Paleozoic I

Early Paleozoic Life

Primarily confined to shallow-water marine environments.
Animals with hard body parts appear at the beginning of the Cambrian - this defines the beginning of the Phanerozoic.


Two major evolutionary events at the beginning of the Cambrian.
A) Rapid increase in the number and types of Phyla. - Brachiopoda, Mollusca, Arthropoda, Echinodermata, Vertebrata
Most major Phyla found today evolved during the Cambrian?
Burgess Shale Fossils

Phylum Increase

Some argue that Phyla had evolved prior to the Cambrian.
Reason for apparent rapid evolution:
1) Preserved in the rock record due to development of hard body parts.
2) Development of hard body parts allowed new forms to evolve.

Hard Body Parts

B) Development of skeletons and hard body parts.


First group to increase and become dominant (i.e. abundant) - trilobites (Example #1, Example #2).
Trilobite means three lobe fossil.
Approx. 50% of Cambrian fossils are trilobites.
Lived by crawling along the ocean floor, taking in sediment and digesting the organic material.

Brachiopods - Mollusks

Brachiopods evolved at this time - did not become dominant until the Ordovician-Silurian-Devonian Periods.
Mollusks - three major groups evolved during the Cambrian.
Gastropods (snails are in this group)
Bivalves (clams and oysters)
Cephalopods (nautiloids and squids)


Vertebrates are any animal having a segmented vertebral column.
Humans are vertebrates.
First vertebrates were jawless fish - Agnathids.
Covered with bony plates - described as "armored tanks moving slowly along the bottom".

Ordovician Period

Regression of the Sauk Sea and other regressions caused a restriction of living space on the continental margin.
Broad epicontinental seas are no longer present.
This results in greater competition for space and resources.
Several new groups evolve.

Corals and Graptolites

Corals evolve - colonial and solitary corals (animals).
Graptolites evolve - Colonial animals similar to corals - thought to be extinct until recent discovery of living graptolites off New Caledonia coast.
Graptolites are important index fossils.
Index fossils are fossils which are wide spread and change rapidly over time.

Invasion of the Land

Life was restricted to the oceans.
Ordovician to Silurian Periods - plants moved onto the land.
First undisputed land plant fossils are from the Middle Silurian.
Fossil spores have been found in Ordovician aged rocks suggesting that land plants were present.

Land Plant Evolution

Marine plants faced 3 major evolutionary problems before living on land was possible.
1) Desiccation
2) Fluid Transport System
3) Reproduction

Land Plant Implications

1) Ground cover and root systems reduce the amount of sediment removed by erosion.
2) Chemical weathering of rocks changed due to plants taking in certain nutrients and leaving others.
3) Soil developed - sediment + organic matter.
4) Atmospheric oxygen levels increased.

Ordovician Extinction

End of the Ordovician marked by the extinction of many marine organisms.
More than 100 Families of invertebrates became extinct.
~50% of brachiopods and byrozoan Families died off.
Cause of the extinction - ???

Silurian Period

Corals evolved during Ordovician - first coral reefs develop in Silurian.
Reefs had existed prior to this time but were small and made from algae and sponges.
Coral reefs are larger - stronger
Block and dissipate strong wave action creating sheltered areas - barrier reefs

Arthropod Invasion!!!

Earliest indication of arthropods moving onto land - possible millipede track from the Ordovician - first land animals.
Devonian Period - wingless insects including mites, spiders and centipedes.
Arthropods land invasion may have begun in Ordovician. - was occurring during Silurian. - well established by Devonian.
Oldest known land arthropod is from the Late Silurian - 420 m.y.
Fossils of spiders and centipedes.
Early land arthropods were detritovores - ate decaying animals and decaying animal waste.
No evidence is present for herbivore arthropods from this time.


Jawed fish evolved during Silurian
Jaws evolved from a modification of the gill structure.
Ability to bite and grasp prey.
This leads to a different feeding style and diet.