ON THE ORIGIN OF MORPHOSPECIES – USING BAYESIAN PHYLOGENETICS TO QUANTIFY THE MODE OF ANCESTOR DESCENDANT RELATIONSHIPS IN PALEOZOIC INVERTEBRATES

If a fossil record is well-sampled, first principles predict that there is then a reasonable expectation that some observed morpho-species might be ancestral to other taxa. Modern tip-dating Bayesian phylogenetics employ explicit models of sampling in the fossil record, thus providing a quantitative framework for estimating sampled ancestors. Furthermore, the processes by which new morphologically-distinguishable chrono-species arise can be distinguished by comparing estimated divergence times against the stratigraphic ranges of their inferred ancestors. Ancestor-descendant relationships are often categorized into three possible modes, with morphotaxa originating (1) via ‘anagenetic’ changes independent of branching, (2) by symmetrically ‘bifurcating’ into two new morphotaxa, or (3) by morphologically distinct descendants ‘budding’ off their morphologically-persistent ancestor. Specific cases have been attributed to each pattern, but there is little consensus on which mode is most common, or whether these dynamics vary across and within major fossil-rich groups. Our assumptions about the mode of ancestor-descendant relationships have broad impacts on our interpretation of evolutionary history, from whether we expect morphological change to correlate with speciation, or whether taxonomic turnover in the fossil record might be artificially inflated by the pseudo-extinction implicit to anagenesis and bifurcation. Here, we assemble morphological matrices and stratigraphic range data from more than thirty studies of various invertebrate groups with well-sampled fossil records, predominantly Paleozoic groups, such as brachiopods, graptolites, trilobites, crinoids and other echinoderms. These datasets differ not only in the particular clades assessed, but also the taxonomic resolution and sampling schema. Using tip-dating Bayesian analyses, we address the extent to which sampled ancestors are inferred to occur in these fossil records, as well as the degree to which these records provide support for particular modes of morphological differentiation among taxa.