MORPHOLOGICAL EVOLUTION WITHIN HOLOCYSTITID DIPLOPORITANS

Biogeography is an important driver of evolutionary trends, such that the morphological consequences of migrations and faunal invasions are vital in understanding large-scale evolutionary patterns. The holocystitids are diploporitan echinoderms typified by having food grooves that lack floor plates, which end with a single, large brachiole facet. The holocystitids are a locally abundant and largely North American clade of diploporitans that became established, likely following a migration from Europe. Following the Late Ordovician Mass Extinction, few lower tiered echinoderms with broad feeding structures survived such that the Holocystites fauna, particularly Holocystites, experienced a competitive release following the invasion.

To explore the evolutionary patterns that occurred during this migration, we constructed a novel morphological character suite. We characterized 24 individuals, from holocystitids and closely related diploporitan taxa, from seven genera and combined the morphological patterns with a recently published phylogeny to construct a phylomorphospace. This methodology allows for the visualization of morphology within an evolutionary framework.

Holocystitids are most commonly preserved without complete holdfasts, feeding structures, or oral plates and taphonomic differences appear to play a significant role in the resulting morphospace. Holocystitid theca are robust and could likely remain intact on the seafloor for significant periods following death. Therefore, the loss of plates around the mouth and abrasion of surface features can result in taphonomic biases between individuals with complete theca. Overall, the more abundant taxa (Holocystities) also contain the best-preserved individuals such that taphonomy varies in concert with the ecological patterns being explored. Preliminary results show that Holocystites occupied a fairly constricted area in morphospace compared with more basal genera, suggesting a loss of morphological plasticity following the invasion, even with decrease competitive interactions. The inclusion of additional specimens and taxa will allow a better assessment of taphonomic biases to further explore the validity of this pattern.