GEOL 102 Historical Geology

Spring Semester 2018
The Early Paleozoic: The Cambrian & Ordovician

Detail of reconstruction of global paleogeography at 470 Ma (Early/Middle Ordovician boundary) by paleogeographer Ron Blakey
So now we are at the Phanerozoic:

Global and Regional Geology of the Cambrian and Ordovician

Starting with the Paleozoic:

Phanerozoic Eon: 541 - 0 Ma

Base of Cambrian (and thus of the Paleozoic, and Phanerozoic):

The "Long Fuse" of the Cambrian Explosion:

More on Cambrian life later.

The Cambrian Substrate Revolution
Evidence for microbial matgrounds in shallow marine sediments (siliciclastic & carbonate) during Neoproterozoic:

During Cambrian, rise of some bioturbators, with limited vertical mining abilities.

Presence of surviving mat scratchers (diverse ancestral mollusks, esp. diverse monoplacophorans and polyplacophorans), and some probable mat-sticking echinoderms (helioplacoids). As rise of deeper burrowers and grazing increases in late Cambrian, increasingly Phanerozoic-style substrates:

Paleogeography and Tectonics of the Cambrian
Pannotia (supercontinent) breaks apart at or near the base of the Cambrian. Breaks up into:

Global (eustatic) transgressions in Middle & Late Cambrian:

Paleogeography and Geology of the Ordovician:
Gondwana moves poleward

During Late Ordovician, major (but brief) ice age.

In eastern Laurentia, Early & Middle Ordovician huge stable carbonate platform (continuation of Cambrian).

During Middle-Late Ordovician, the Taconic Orogeny (first of the Appalachian orogenies):

Collision and suturing between Avalonia and Laurentia possible in northern part of Appalachian region; however, still seem to have been separate further south.

Glaciers at end of Ordovician produce pulses of extinction.

Evolution of Life in the Cambrian and Ordovician

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:

Some relevant videos:
What caused the Cambrian Explosion?

An overview of the entire Paleozoic:

A look at trilobites:

And at conodonts:

To Lecture Notes.

Last modified: 27 March 2018

Detail from Carel Brest van Kempen's "Burgess Shale Fauna" (1989)