ORIGIN OF THE EUMETAZOA - TESTING ECOLOGICAL PREDICTIONS OF MOLECULAR CLOCKS AGAINST THE PROTEROZOIC FOSSIL RECORD

Molecular clocks have the potential to shed light on the timing of early metazoan divergences, but differing algorithms and calibration points yield conspicuously discordant results. We argue that competing molecular clock hypotheses should be testable in the fossil record, on the principle that fundamentally new grades of animal organization will have ecosystem-wide impacts. Using a set of seven nuclear-encoded protein sequences, we demonstrate the paraphyly of Porifera and calculate sponge/eumetazoan and cnidarian/bilaterian divergence times using both distance (ME) and likelihood (ML) molecular clocks: Minimum Evolution brackets the appearance of Eumetazoa between 634 Ma and 604 Ma, whereas Maximum Likelihood suggests it was between 867 Ma and 748 Ma. Significantly, the ME, but not the ML, estimate is coincident with a major regime change in the Proterozoic acritarch record, including: 1) the disappearance of low-diversity, evolutionarily static, pre-Ediacaran acanthomorphs; 2) the radiation of a high-diversity, short-lived Doushantuo-Pertatataka microbiota (DPM); and 3) an order-of-magnitude increase in evolutionary turnover rate. We interpret this regime change as a consequence of the novel ecological challenges accompanying the evolution of the eumetazoan nervous system and gut. Thus, the more readily preserved microfossil record provides positive evidence for the absence of pre-Ediacaran eumetazoans, and strongly supports the veracity - and therefore more general application - of the ME molecular clock.