Cambrian Explosion Evolutionary Diversification 543-506 mya

Cambrian Explosion: All major body plans first made an appearance in the fossil record during a 40 my period

Ediacaran Fuanas entirely soft-bodied organisms from 565 mya Brachina delicata Spriggina floundersi (sponges, jellyfish, comb jellies)

New Fossil Finds are Pushing Back Estimates of Divergence Times Fossil embryos suggest precambrian diversification of bilateralians (Xiao et al. 1998) Possible flatworm or arthropod zygotes and embryos

Burgess Shale Faunas 520 mya (trilobites, segmented worms, molluscs, chordates)

New Fossil Finds are Pushing Back Estimates Of Animal Divergence Times (Shu et al. 1999) 530 my Cambrian vertebrate: Haikouichthys eraicunensis

Small subunit RNA most basal earliest fossils

Cambrian: Diversification of Animal Body Plans Symmetry a. Radial or asymmetrical: Diploblast (endoderm and ectoderm) b. Bilateral: Triploblast (endo, ecto, and mesoderm) Coelomate i. Protostomes ii.Deuterostomes Also: segmented body plans, shells, exoskeletons, appendages, notochords

Was the Cambrian Explosion Explosive? Molecular clock estimates suggest 900-1200 my divergence times for the major animal groups (Wray et al., 1996). i.e. Major animal lineages were established pre - Cambrian. if so There should be fossil evidence!

What Caused the Cambrian Explosion? Environmental change: Higher oxygen may have allowed for larger, energetically costly morphologies. Diversification of phytoplankton may have spurred the evolution of herbivores and predators. Genetic changes? Cloudina

Stasis Is Evolution Too! Darwin’s View Punctuated Equilibrium (Gould and Eldridge, 1972)

Jackson and Cheetham, 1994

Why Does Stasis Occur? not for lack of genetic variation dynamic stasis in pliocene bivalves

Extinction Mass extinctions account for 4% of all extinctions The big 5 of the phanerozoic.

Iridium concentration in clay layer at KT Boundary Other evidence: Chicxulub crater Microtektites Soot deposits Evidence of tsunami

Habitat Destruction Human Population by 2050 = 13 billion Current extinctions are occurring at 100 - 1000 times the normal or background rate. May et al. 1995, Pimm et al., 1995 Human Population by 2050 = 13 billion