GEOL 102 Historical Geology

Spring Semester 2012
The Early Paleozoic Era II: When Trilobites Ruled the Earth

Marine life of the early Paleozoic
Based on statistical work by Jack Sepkoski, marine invertebrate communities are often broken down into three separate "evolutionary faunas":

All three categories exist in the Cambrian, and persist until the present (even if some component members have died off). However, these "packages" of distantly related groups tend to be common at the same time, or rare at the same time.

The Cambrian fauna dominates during the Cambrian, remains common in the Ordovician, and became progressively rarer in the Silurian and later. The Paleozoic fauna is rare in the Cambrian, becomes more common in the Ordovician, and dominates the rest of the Paleozoic: it remains an important part of the Mesozoic and Cenozoic seas. The Modern fauna is very rare in the Cambro-Ordovician, but continues a stead rise throughout the Phanerozoic: in the post-Paleozoic it is the most abundant fauna.

Life in the Cambrian:
Very different from present seas, or even post-Ordovician seas:

Some important groups:

Other Cambrian life includes first appearances of:

Most Cambrian organisms are only known from their hard parts, but the Early Cambrian Chengjiang site in China and the Middle Cambrian Burgess Shale in British Columbia preserve soft-tissue impressions.

Terminal Cambrian Extinctions:

Life of the Ordovician:
Cambrian fauna still common, but Paleozoic fauna on the rise.

The Ordovician Radiation (also called the Great Ordovician Biodiversification Event or GOBE): Life moves away from substrate-water interface. Appearance of deep burrows (worms, clams, etc.), of tall attached epifauna (bryozoans, crinoids, blastoids, etc.), and some diversification of nektonic forms.

Some important groups:

Radiations of articulate brachiopods, gastropods (snails), echinoderms (especially stalked crinoids and blastoids).

Decline of stromatolites: Probably due to more specialized grazers (gastropods, echinoids, etc.).

1rst tabulate-stromatoporoid reefs (more important in middle Paleozoic). Fish diversity increases, but still jawless. The bony-armored jawless fish are sometimes called "ostracoderms": this is a paraphyletic grade rather than a clade. Also, oldest good evidence of terrestrial plants.

Terminal Ordovician Extinctions:

Silurian marine life:

New taxa:

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Last modified: 19 January 2012