PATTERNS OF MORPHOLOGICAL DIVERSIFICATION WITHIN THE SUBORDER AGNOSTINA (TRILOBITA)

Members of the Order Agnostida (Salter 1864) have a long and perplexing history of uncertain systematics. On all taxonomic levels, the extremely conserved morphologies of many agnostine arthropods make them the subject of ongoing systematic revision. Agnostina, the subclade of Agnostida that excludes Eodiscina, is a moderately diverse group comprised of 13 recognized families and over 100 genera. Taxonomic diversification of this clade is “bottom-heavy” in terms of generic origination rates, with a peak in the middle Cambrian and subsequent decline until its extinction in the Late Ordovician. It is expected that morphological diversity of this clade demonstrates a similar “bottom-heavy” pattern.

Specimens from 13 families of Agnostina plus additional taxa classified as “Uncertain” and “Unassigned” were coded for a set of characters that describe cephalic, thoracic, and pygidial traits. A preliminary phylogenetic tree of Agnostina was constructed using parsimony-based methods, rooted on the stratigraphically oldest species, Eoagnostus roddyi. Cephalic sclerites from representative specimens were selected and subject to geometric morphometric analysis. Specimens were chosen based on the ability to apply a common landmark configuration that includes coverage of the cephalic margin and border, anterior glabella, posterior glabella, and basal lobes. For this reason, effaced genera were excluded. The resultant morphospace was then overlain with the phylogenetic tree in order to produce a phylomorphospace. This study is the first attempt to explore disparity of this enigmatic clade in a phylogenetic context. It serves as a foundation for ongoing research into developmental constraints that may shape patterns of morphological evolution in this very conservative group of arthropods.