NEW EVIDENCE OF PREDATOR-PREY RELATIONSHIPS IN THE PLANKTIC ECOSYSTEM DURING THE GREAT ORDOVICIAN BIODIVERSITY EVENT: A REPORT OF A SUGGESTED PREDATOR OF GRAPTOLITES

A new discovery in the lower Martinsburg Formation (Late Ordovician) near Harrisburg, PA has uncovered evidence of Burgess-type soft-tissue preservation revealing a rare glimpse of a planktic ecosystem comprising enigmatic cone-shaped organisms and associated graptolites. The simple shape of these enigmatic fossils, along with the slight structural alteration of the Martinsburg Fm (silica-rich cleavage artifacts causing minor degradation of minute features) has made assigning taxonomic affinity challenging. However, the fossil’s biotic association, sedimentological setting, and temporal placement, suggests the organism’s affinity. Their resemblance to modern thecosome pteropods (cm scale bathy-pelagic gastropod predators) includes similar body plan, morphology, and mode of preservation. Despite these similarities, these fossils are unlikely to be related to true pteropods, which have a sparse fossil record with first occurrences in the latest Mesozoic (radiating after the K-Pg extinction). However, molecular clock studies suggest that Opisthobranchia (includes the clade Thecosomata) could have originated as early as the mid-Paleozoic. Here, we propose that these non-shelled homoplastic gastropod forms are ‘pseudopteropods’ that evolved as graptolite predators during the expansive emergence of epifaunal suspension feeders during the Great Ordovician Biodiversity Event.