The Cambrian Radiation How did we get so many animals so quickly?

Mistaken Point, Nfld Subdivisions of the Precambrian

Ediacara, Aust Vendian age rocks contain a diverse fauna

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

Ediacaran fossils can be lumped into 4 groups Jellyfish impressions tracks fronds Segmented animals So, by the late Precambrian life 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 Tommotian stage. Each “shell” is only a few mm in size. Clearly marine invertebrate animals were beginning to evolve the ability to extract CaCO 3 from seawater to precipitate shells. The evolution of shells enabled organisms to occupy shallow water (= fill new niche)

Mt Burgess in the Canadian Rockies, is a location where Middle Cambrian rocks containing a diverse invertebrate fauna containing representatives of every extant invertebrate phylum except the bryozoa. This fauna is termed the “Burgess Shale fauna”

The Burgess Shale fauna Artist’s rendition of the weird organisms sometimes called “evolutionary experiments”

photographs of some Burgess Shale fossils The first trilobite

Following the evolution of the Burgess Shale fauna many of these organisms- some of which comprised completely new phyla- went extinct The 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!!!