Phylogenetic and Biogeographic Analysis of Cretaceous Spatangoid Echinoids from the Western Interior Seaway

Abstract

Members of the echinoid order Spatangoida, a highly diverse and abundant marine invertebrate clade, were important denizens of the Cretaceous Western Interior Seaway (WIS), an epicontinental seaway that divided North America in two during an interval of greenhouse conditions between roughly 100 and 65 million years ago. Despite being well represented in the Cretaceous rocks of Texas and surrounding areas, the phylogenetic patterns of spatangoid echinoids of the WIS, especially at the species level, have yet to be fully resolved. Further, because of their complex morphologies, they represent one of the few marine invertebrate groups from this time period that are amenable to phylogenetic study. Another interesting question pertaining to taxa found in the WIS is their biogeographic origins and whether they represent an endemic radiation or are instead derived from one or more invasions from other regions. In order to reconstruct a hypothesis of spatangoid phylogenetic relationships, a parsimony analysis was conducted using a character matrix of 32 characters collected from 20 species. Species that occur in the WIS were considered comprehensively; species from other regions such as South America, Europe, and North Africa were additionally incorporated into the analysis. Phylogenetic biogeographic analysis was then conducted on the spatangoid phylogeny using a modified version of Fitch Parsimony Analysis, and several episodes of vicariance and range expansion were identified across the phylogeny. These were possibly further related to some of the various major episodes of sea-level rise and fall during the Cretaceous.