The Radiation of the First Animals
by Dr. Jerry Lipps
Department of Integrative Biology and The Museum
University of California, Berkeley
To begin Dr. Lipps' talk you can click here or read this brief overview, below, that provides links to the best places in the talk for specific topics.
Animals first appear in the fossil record around 580 million
years ago as frond-like forms, jellyfish-like imprints,
and trace fossils. These fossils appear simultaneously
on all continents, except Antarctica, and each assemblage contains
roughly the same kinds. This appearance is still about 700-300
million years later than the molecular data suggest that
animals originated, leaving an enormous period of time without
a fossil record. Perhaps, the first animals were small, unskeletonized,
or destroyed by geologic processes.
At the base of the Cambrian period about 545 million years
ago, all modern phyla of animals and many algae and protists appear
or radiated--including phytoplankton, forminifera,
archeocyaphids, trilobites. Most explanations for
this radiation involve properties of animals themselves (intrinsic
causes), but the coincident events in algae and protozoans
suggest perhaps a more ubiquitous, ecologic trigger (extrinsic
causes). One possibility is oceanographic changes that
increased nutrient supplies to the shallow waters, hence a radiation
of trophic links in the food chains of that time.