EVALUATING MORPHOLOGICAL CONVERGENCE VERSUS CONSERVATISM ACROSS THE DIPLOBATHRID CRINOID PHYLOGENY

Patterns of morphologic evolution among crinoids have primarily been established based on temporal rather than phylogenetic trends across the group. Although broad patterns of crinoid skeletal construction and body organization are understood, numerous questions regarding the origin and evolution of morphologic variation can only be answered by statistically comparing traits between taxa across time-scaled phylogenetic trees. Robust phylogenies of crinoids have, until recently, remained elusive, which has precluded the study of morphologic patterns within a phylogenetic framework. Advances in resolving the crinoid tree of life, however, provide new opportunities for addressing evolutionary questions by studying the timing, frequency, and patterns of character transformations within a phylogenetic context.

It has been recently established that the subclass Diplobathrida, a long-lived (Ordovician-Mississippian) group of camerate crinoids, is a monophyletic group, and a robust phylogeny for the clade has been recovered. This makes diplobathrid crinoids a useful model group for exploring patterns of morphologic evolution and quantitatively assessing hypotheses of evolutionary trends. For example, it has been suggested that evolutionary convergence and/or parallelism in overall calyx design was widespread among crinoids, although this has not been tested within a phylogenetic framework. Similarly, patterns of convergence versus conservatism of individual calyx characters are poorly understood.

Here, I revisit long-standing questions regarding the evolutionary origin of calyx designs using diplobathrid crinoids. The diplobathrid tree topology is time-scaled using the cal3 method of Bapst (2013). A combination of parsimony-based and maximum likelihood-based ancestral state reconstruction along with Bayesian stochastic character mapping are applied to time-scaled trees to determine the ancestral states of characters, the frequency of character transformations, and whether characters related to calyx design are labile or conserved.