PHYLOGENY OF DISPARID CRINOIDS

Disparids are a morphologically diverse crinoid clade (Phylum Echinodermata) that evolved by the earliest Ordovician and persisted until the late Permian. Disparids were of prime importance during the Ordovician when they were one of the dominant clades of the Early Paleozoic Crinoid Evolutionary Fauna. Regardless of their diminished relative importance after end-Ordovician extinctions, post-Ordovician disparids did include genera with very high local abundance; striking morphological modifications; and the Calceocrinidae, which is the longest ranging crinoid family (Middle Ordovician to Lower Permian). Using computational phylogenetic methods, it is demonstrated that the fundamental basis of the disparid phylogenetic tree during the Ordovician was controlled by aboral cup symmetries formed by varying numbers and placement of compound radial plates. After the Ordovician, modification of other aspects of disparid morphology yielded major branches in their phylogenetic tree. These include, among others, the planispirally coiled column in the Myelodactylidae, multiple armlets articulated on a single radial plate in the Calceocrinidae and other families, three-fold crown symmetry in the Holynocrinidae, five long unbranched arms in the Synbathocrinidae, five short arms that form an interlocking cover over the oral surface in the Pygmaeocrinidae, and the total loss of arms in the Zophocrinidae and some Allagecrinidae. Family-level subdivision of the Disparida is not finalized; however, as presently understood, as many as 13 disparid families arose during the Ordovician, 5 families arose during the Silurian, 7 families arose during the Devonian, and 1 family arose during the Permian.