EVOLUTIONARY BIOGEOGRAPHY AND PALEOECOLOGY OF DIPLOPORAN ECHINODERMS ACROSS THE ORDOVICIAN-SILURIAN BOUNDARY

The Late Ordovician Mass Extinction (LOME) non-randomly altered echinoderm communities in Laurentia resulting in empty niche space. New echinoderm communities did not establish themselves until the middle Silurian, which primarily migrated into this region from Baltica. However, relatively little else is known about the details of the evolutionary dynamics across the Late Ordovician–Silurian boundary, especially in noncrinoid echinoderm groups. In this study, we examine the evolutionary dynamics of a particular clade of extinct blastozoan echinoderms, the sphaeronitid diploporans. Using phylogenetic, morphologic, and biogeographic data, we analyze how sphaeronitids evolved across the LOME, dispersed to North America, and filled unoccupied niches in the Silurian. Specifically, we look at the enigmatic Holocystites Fauna, predominantly found in central North America. The Holocystites Fauna has been a focus of interest in echinoderm evolution because of their unique distribution patterns and morphological features.

Analyses indicate that two dispersal events occurred from Baltica into Laurentia, which took place first during the Middle to Late Ordovician, and then during the Late Ordovician into the early Silurian, leading to the Holocystites Fauna populating central North America. There was no significant expansion of morphological change once these echinoderms spread into Laurentia (during either dispersal event), which could relate to the narrow and previously established niche or possibly due to developmental constraints within the clade. Even though morphological innovation is constrained during this ecological and biogeographic transition, there are significant changes in community structure (i.e. abundance) and body size (approximately doubling in size in North American taxa) indicating the importance of competitive release and dispersal events in understanding evolutionary dynamics of fossil taxa.