BAYESIAN TIP-DATING AND TREE-BASED ANALYSES OF MORPHOLOGIC EVOLUTION: AN EMPIRICAL ASSESSMENT OF RATE HETEROGENEITY AND MORPHOSPACE OCCUPATION AMONG PALEOZOIC CRINOIDS

The application of model-based approaches to phylogeny estimation and the development of tree-based statistical methods have greatly expanded paleobiological research programs investigating the tempo and mode of morphologic evolution. For example, the direct study of character changes along branches of time-calibrated phylogenetic trees provides high-resolution insight into macroevolutionary dynamics and complements analyses of morphospace occupation through time.

Despite obvious advantages, tree-based analyses of evolution are subject to numerous biases that may affect the outcome of comparative methods. Potential biasing factors include incorrect tree topologies and branch lengths, the presence of ancestor-descendant relationships, and incomplete sampling and/or fossil preservation. To help overcome these challenges, phylogenetic paleobiologists are increasingly 1) incorporating more realistic models of diversification and sampling when inferring phylogenetic relationships and 2) applying comparative analyses to many alternative time-calibrated phylogenies.

Here, I present an empirical study estimating tree-based rates of morphologic trait evolution using a new species-level discrete character matrix of Paleozoic cladid crinoids. To estimate morphologic evolution along branches, I first infer phylogenetic relationships using a Bayesian tip-dating approach incorporating the time-varying ‘skyline’ implementation of the sampled-ancestor fossilized birth-death (SA-FBD) model. This approach accounts for temporal variation in diversification and fossil preservation, as well as the potential for sampling ancestors. Next, I estimate rates of morphologic evolution across a broad sample of time-calibrated trees from the Bayesian posterior distribution using a maximum-likelihood approach correcting for character completeness.

Results indicate significant heterogeneity in rates through time and among clades, with instances of elevated rates linked to major transitions in crinoid ecologic communities. In particular, a major shift in rates is associated with the transition to the Late Paleozoic Crinoid Macroevolutionary Fauna when ‘poteriocrine’ cladids became ecologically dominant.