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7 4 4913 age of metazoa fossil record age of metazoa

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Unformatted text preview: 575 mya – independent ? •  Sponges, cnidarians earliest –  confirmed by biomarker molecules for sponges •  Earliest bilaterian animals –  Cambrian Explosion, ~520 Mya –  Burgess Shale  ­ sudden appearance of modern animal body plans –  Generally believed that animal life appeared very suddenly at this @me Proterozoic Life Ediacaran Fauna (~640 MYA) •  Oldest [email protected] (~575 MYA book) • Windermeria & Ediacaraia • Cnidarian (Mawsonite) & wormlike (Dickinsonia) (fig 5.7) 4 4/9/13 Age of Metazoa – fossil record Age of Metazoa – molecular clocks PNAS 1996 Used well calibrated dates from certain groups (molluscs, vertebrates, echinoderms) and calculated rates of substitution—rates were similar for different groups. Did relative rate tests and found that rates were generally similar across phyla of animals. Then used these rates to estimate the age of divergence of the different phyla —the oldest shared ancestor. Obtained a much older date, about 1 Billion years! Similar results for 7 different genes. 5 4/9/13 What explains the discrepancy between fossils and molecular clocks? •  Gaps in fossil record –  but there is a rich fossil record [email protected] in the Cambrian, so not clear why this would change so [email protected] •  Rates of [email protected] slowed down in all lineages •  But why? •  Other...
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