TESTING THE TAXONOMIC STRUCTURE OF PALEOZOIC PAN-CLADID CRINOIDS: A STATISTICAL APPROACH USING THE FOSSILIZED BIRTH-DEATH PROCESS AND BAYESIAN PHYLOGENETIC INFERENCE

Knowledge of phylogenetic relationships among fossil species is fundamental to understanding geologic patterns of taxonomic diversification, morphologic and ecologic trait evolution, and the evolutionary origin of crown group taxa. However, most methods of phylogenetic inference previously used by paleo-systematists do not accommodate idiosyncrasies unique to paleobiological data, such as (incompletely) sampling taxa over time, and therefore do not take full advantage of the information provided by the fossil record.

The advent of Bayesian “tip-dating” approaches to inferring phylogenies is especially promising for paleo-systematists because prior distributions can be tailored to accommodate the study of fossil taxa. For example, the Fossilized Birth-Death (FBD) process (Stadler 2010; Heath et al. 2014) provides a model for setting prior distributions on parameters describing general diversification dynamics, tree topology, and the temporal distribution of sampled lineages. Moreover, Bayesian phylogenetic inference can also accommodate rate heterogeneity among characters and lineages.

In this study, I conducted phylogenetic analyses using Bayesian tip-dating in an attempt to bring phylogenetic order to a geologically long-lived taxon characterized by a contentious classification scheme adrift in taxonomic anarchy: pan-cladid crinoids. Spanning the Ordovician—Recent, the Pan-Cladida includes crown group crinoids as well as Paleozoic stem groups. Although the Cladida comprises the greatest number of genera among major crinoid lineages, it is the least understood in terms of evolutionary relationships. For this analysis, more than 100 representative Paleozoic species were sampled for phylogenetic character coding across a broad swath of both taxonomic and morphologic gradients to resolve general relationships among crinoid higher taxa and constrain the branching positions of nominal cladids previously hypothesized to represent stem-group articulates. Thus, this analysis provides a step towards building a phylogeny linking fossil stem and crown group crinoids. In addition, the phylogenetic analysis described here provides an evolutionary template for dissecting patterns of adaptive radiation and eco-morphologic trait evolution in Paleozoic pan-cladid crinoids.