Bayesian Tip-dated Phylogenetics: Topological Effects, Stratigraphic Fit and the Early Evolution of Mammals

ABSTRACT The incorporation of stratigraphic data into phylogenetic analysis has a long history of debate, but is not currently standard practice for palaeontologists. Bayesian tip-dating (or morphological clock) phylogenetic methods have returned these arguments to the spotlight, but how tip-dating affects the recovery of evolutionary relationships has yet to be fully explored. Here we show, through analysis of several datasets with multiple phylogenetic methods, that topologies produced by tip-dating are outliers when compared to topologies produced by parsimony and undated Bayesian methods, which retrieve broadly similar trees. Unsurprisingly, trees recovered by tip-dating have better fit to stratigraphy than trees recovered by other methods, due to trees with better stratigraphic fit being assigned a higher prior probability. Differences in stratigraphic fit and tree topology between tip-dating and other methods appear to be concentrated in parts of the tree with weaker character signal and a stronger influence of the prior, as shown by successive deletion of the most incomplete taxa from a sauropod dataset. Tip-dating applied to Mesozoic mammals firmly rejects a monophyletic Allotheria, and strongly supports diphyly of haramiyidans, with the late Triassic Haramiyavia and Thomasia forming a clade with tritylodontids, which is distant from the middle Jurassic euharamiyidans. This result is not sensitive to the controversial age of the eutherian Juramaia . A Test of the age of Juramaia using a less restrictive prior reveals strong support from the data for an Early Cretaceous age. Our results suggest that tip-dating incorporates stratigraphic data in an intuitive way, with good stratigraphic fit a prior expectation that can be overturned by strong evidence from character data.