The Cambrian Radiation

How did we get so many animals so quickly?

Mistaken Point, Nfld

Subdivisions of the Precambrian

Ediacara, Aust

Vendian age rockscontain a diverse fauna

The Ediacaran fauna: a variety of soft-bodied metazoans;named after the region in Australia where Vendian agerocks are exposed

Ediacaran fossils can be lumped into 4 groups

Jellyfish impressions

tracks

fronds

Segmented animals

So, by the late Precambrianlife was already highly evolved

The Precambrian/Cambrian boundary in Nfld

If complex life forms evolved in the Precambrian, what is so special

about the Precambrian/Cambrian transition?

• “The Cambrian radiation” really consists of 3 phases: (1) Early Cambrian small, soft-bodied organisms; (2) the “Tommotian fauna” of small animals with skeletons and (3) the Middle Cambrian appearance of a diverse fauna bearing “hard parts” or exoskeletons.

So, it’s not really a “big bang” of evolution, but a series of events

The Early Cambrian “small shelly fauna” of the Tommotianstage.

Each “shell” is only a few mm in size. Clearly marineinvertebrate animals were beginning to evolve the ability to extract CaCO3 from seawater to precipitate shells. Theevolution of shells enabled organisms to occupy shallowwater (= fill new niche)

Mt Burgess in the Canadian Rockies, is a location whereMiddle Cambrian rocks containing a diverse invertebrate

fauna containing representatives of everyextant invertebrate phylumexcept the bryozoa.

This fauna is termed the“Burgess Shale fauna”

The Burgess Shale fauna

Artist’s rendition of the weird organismssometimes called “evolutionary experiments”

photographs of some Burgess Shale fossils

The firsttrilobite

Following the evolution of the Burgess Shale fauna many of these organisms- some of which comprised completely

new phyla- went extinctThe trilobites were one group that diversified, as

did the brachiopods and molluscs.It’s important to note that with one exception

(the bryozoa) all the marine invertebrate phyla evolved during this 3rd phase of Cambrian radiation; including the first primitive vertebrate! Tetrapod evolution marks the only other major body plan to evolve since that time!!!