A PHYLOGENETIC TEST OF THE SUPRAGENERIC CLASSIFICATION OF DIPLOBATHRID CRINOIDS

Order Diplobathrida (Subclass Camerata, Class Crinoidea) is a long-lived clade that persisted from the Lower Ordovician (Tremadocian) to the Upper Mississippian (Serpukhovian). Diplobathrids are taxonomically diverse, with 100 recognized genera and more than 360 named species and are prominent constituents of Lower Paleozoic faunas. Despite their diversity and abundance, evolutionary relationships within the Diplobathrida remain obscure. Quantitative phylogenetic analyses have only been performed on subsets of diplobathrid taxa that were limited stratigraphically or taxonomically; a comprehensive analysis has never been conducted. In addition, much of the traditional suprageneric classification of Diplobathrida has been questioned but remains unresolved.

This study undertook a quantitative genus-level phylogenetic analysis of the Diplobathrida to infer evolutionary relationships within the group and to evaluate the congruence of the traditional classification scheme with the phylogeny. All 100 of the currently recognized diplobathrid genera were considered, and more than 120 morphological characters were included. Because of the extensive stratigraphic range of the Diplobathrida and the relatively low character to taxon ratio, multiple quantitative phylogenetic methods were employed. Following recovery of a well-supported tree, the R package, STRAP, was used to time-scale the resulting tree.

Ideally, classification schemes reflect evolutionary relationships where named groups of taxa represent true clades, that is, monophyletic groups. The phylogeny presented here provides an evolutionary framework that is used to test the monophyly of named higher taxa. Because the phylogeny is based on numerous taxonomically informative characters, character mapping on the recovered tree was utilized to identify which characters are diagnostic for suprageneric clades and which should be removed from diagnoses. Preliminary results indicate that although some named higher taxa are consistent with the recovered tree topology, others represent non-monophyletic groups. This confirms the need for a re-evaluation of the classification of higher taxa within the Diplobathrida and highlights one of the useful applications of phylogenetic analyses.