Estimating Metazoan Divergence Times with a Molecular Clock
Extended Abstract – Prato 2004

K.J. Peterson
Dept of Biological Sciences, Dartmouth College, Hanover NH 03755, U.S.A.
(email: Kevin.J.Peterson@Dartmouth.edu)

Number of Words: 333
Number of References: 0 
Number of Figures: 0 
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Running header text: Metazoan Divergence Times

Accurately dating when the first bilaterally symmetrical animals arose is crucial to our understanding of early animal evolution. The earliest unequivocally bilaterian fossils are -555 million years old. In contrast, molecular-clock analyses calibrated using the fossil record of vertebrates estimate that vertebrates split from dipterans (Drosophila) -900 million years ago. Nonetheless, comparative genomic analyses suggest that a significant rate difference exists between vertebrates and dipterans because the percentage difference between the genomes of mosquito and fly is greater than that between fish and mouse, even though the vertebrate divergence is almost twice that of the dipteran divergence. Here we show that the dipteran rate of molecular evolution is similar to other invertebrate taxa (echinoderms and bivalve molluscs), but not to vertebrates, which significantly decreased their rate of molecular evolution with respect to invertebrates. Using a data set consisting of the concatenation of seven different amino acid sequences from 32 in-group taxa (giving a total of 11 different invertebrate calibration points scattered throughout the bilaterian tree and across the Phanerozoic), we estimate that the last common ancestor of bilaterians arose -570 million years ago. In addition, we date the appearance of both the trochophore and dipleurula larval forms to -550-525 million years ago, but the evolution of both the pilidium and the cyphonautes were post-Cambrian events, evolving sometime between the Ordovician and the Devonian. Interestingly, it now appears that five different feeding larval forms all evolved in the Ordovician, a time characterized by the rapid rise in species diversity of epifaunal suspension feeders. We argue that the rise of larvae is due principally to the evolution of benthic macrophagy, and the subsequent evolution of planktotrophy and set-aside cells are responses to the evolution of suspension feeding in the Late Cambrian and Ordovician. Because it is the evolution of mesozooplankton that links the eukaryotic plankton to the eukaryotic benthos, and effectively establishes the ”modern" or Phanerozoic ocean, the evolution of complex life cycles is one of the principle causal factors underlying the Cambrian explosion itself.