DIVERSIFICATION DYNAMICS OF CROWN GROUP CRINOIDEA (ECHINODERMATA): TESTING ADAPTIVE RADIATION IN COMATULIDS USING BAYESIAN ANALYSIS OF MACROEVOLUTIONARY MIXTURES

The rich, geologic record of fossil crinoids reveals a diverse and prodigious history of adaptive radiation, morphologic innovation, ecologic turnover, and mass extinction represented by more than 8,000 fossil species (Lane 1990). In contrast, the ~600 or so species of extant crinoids have received relatively little attention from a macroevolutionary perspective. This may be explained, in part, by low preservation rates among the free-swimming, stalkless Comatulida (feather stars), which comprises ~85% of all living crinoid species. Further, most post-Paleogene stalked crinoids migrated into deeper, offshore habitats characterized by low preservation potential. Thus, paleontologic approaches to studying crinoid macroevolution cannot easily be extended to quantify diversification patterns among crown group crinoids.

Nevertheless, insight into the macroevolutionary history of crinoids can be extracted by examining variation in species richness among extant clades. In their pioneering paper, Meyer and Macurda (1977) noted that comatulids differed from other crinoids in numerous morphologic and ecologic traits and explained their present-day richness as a result of adaptive radiation. Unfortunately, their adaptive radiation hypothesis has been difficult to test quantitatively because when taken at face value, the fossil record of Recent crinoids is largely inadequate to address this question.

Phylogenetic methods offer an alternative approach to studying macroevolutionary patterns because time-calibrated phylogenies of extant species contain information regarding their underlying diversification dynamics, such as speciation rates. Using the molecular phylogeny of Rouse et al. (2013), I examine diversification dynamics among crown group crinoids in a Bayesian framework using BAMM (Bayesian Analysis of Macroevolutionary Mixtures) (Rabosky 2014). BAMM enables the construction of branch-specific rates of diversification and detects the phylogenetic location of rate shifts while accounting for rate variation both through time and among lineages using Markov-chain Monte-Carlo. Results largely support Meyer and Macurda’s (1977) hypothesis of adaptive radiation in comatulids and indicate a significant rate shift occurred along the branch leading to the Comasteridae.