PHYLOGENETIC AND BIOGEOGRAPHIC PATTERNS IN SPATANGOID ECHINOIDS FROM THE CRETACEOUS WESTERN INTERIOR SEAWAY

Members of the successful 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 the time period that are amenable to phylogenetic study. In order to reconstruct spatangoid phylogeny, a parsimony analysis was conducted using a character matrix of 30 characters derived from 21 species. Species that occur in the WIS were considered comprehensively; species from other regions such as South America, Europe, and North Africa were incorporated into the analysis as well. The resultant phylogeny indicates that the toxasterids Heteraster and Washitaster are sister groups. Further, the Hemiasteridae, as typically construed, appears to be paraphyletic.

Another interesting question pertaining to taxa found in the WIS are their biogeographic origins and whether they represent endemic radiations or are instead derived from one or more invasions from other regions. Phylogenetic biogeographic analysis was conducted on the spatangoid phylogeny using a modified version of Brooks Parsimony Analysis (mBPA). mBPA provides information on congruent patterns of both vicariance and geodispersal, which are key biogeographic processes, and also indicates the relative role sea-level rise and fall played in influencing evolution; further, it also is informative regarding the prevalence of invasions between different biogeographic regions. In the case of spatangoids, it appears that there were several episodes of invasion from various regions into the WIS during the Cretaceous; moreover, there appears to be a relationship between the geographic source of the invasions and the environmental habit of the spatangoids that occur there.