A Total-Evidence Dated Phylogeny of Echinoids and the Evolution of Body Size across Adaptive Landscape

Abstract

Several unique properties of echinoids (sea urchins) make them useful for exploring macroevolutionary dynamics, including their remarkable fossil record that can be incorporated into explicit phylogenetic hypotheses. However, this potential cannot be exploited without a robust resolution of the echinoid tree of life. We revisit the phylogeny of crown group Echinoidea using both the largest phylogenomic dataset compiled for the clade, as well as a large-scale morphological matrix with a dense fossil sampling. We also gather a new compendium of both tip and node age constraints, allowing us to combine phylogenomic, morphological and stratigraphic data using a total-evidence dating approach. For this, we develop a novel method for subsampling phylogenomic datasets that selects loci with high phylogenetic signal, low systematic biases and enhanced clock-like behavior. Our approach restructure much of the higher-level phylogeny of echinoids, and demonstrates that combining different data sources increases topological accuracy. We are able to resolve multiple alleged conflicts between molecular and morphological datasets, such as the position of Echinothurioida and Echinoneoida, as well as unravelling the relationships between sand dollars and their closest relatives. We then use this topology to trace the evolutionary history of echinoid body size through more than 270 million years, revealing a complex pattern of convergent evolution to stable peaks in macroevolutionary adaptive landscape. Our efforts show how combining phylogenomic and paleontological evidence offers new ways of exploring evolutionary forces operating across deep timescales.