Morphological Dynamics and Response Following the Dispersal of Ordovician–Silurian Diploporan Echinoderms to Laurentia

The Late Ordovician Mass Extinction (LOME) left vacated niche space and brought about significant changes in echinoderm community structures across Laurentia. New echinoderm communities, having migrated into Laurentia from Baltica, did not fully establish themselves until the middle Silurian. However, the details of the evolutionary dynamics of non-crinoid echinoderms across the Ordovician–Silurian boundary is understudied. Herein, we examine the evolutionary dynamics of a clade of extinct echinoderms, the sphaeronitid diploporans. Using a combination of phylogenetic, morphologic, and biogeographic data, we analyze how sphaeronitids evolved and dispersed across the LOME and filled unoccupied niches during the Silurian in Laurentia. Analyses indicate that one dispersal event occurred from Baltica into Laurentia, during the Middle to Late Ordovician, leading to the enigmatic and well-known Holocystites Fauna populating central North America. As the holocystitids filled the unoccupied niches from the LOME, there was no significant expansion of morphological forms, which could be related to the narrow, previously established niches that crinoids vacated during the LOME, or possibly due to developmental constraints within the clade. Although morphological change is constrained during this event, there are some significant changes in community structure (i.e., certain species of diploporans became unusually abundant) and body size (i.e., Laurentian specimens approximately doubled in size compared to Baltic taxa). These changes indicate the importance of competitive release and dispersal events in understanding evolutionary dynamics of fossil taxa.