NEW EVIDENCE FOR PLATYCERATID DRILLING ABILITIES FROM CRINOIDS, BRACHIOPODS, AND GASTROPODS OF THE LAKE VALLEY FORMATION (MISSISSIPPIAN, USA)

The record of drill holes in fossil invertebrates is extensive, covering the entire Phanerozoic, and possibly extending into the Precambrian. This record has proven useful for reconstructing certain aspects of behavior, and for exploring its frequency and evolutionary importance. While drill holes representing biotic interactions can generally be identified with little ambiguity, there is less certainty about whether they represent predation or some other process. Most elusive, especially for Paleozoic drill holes, has been the identity of the organism(s) that produced them. Here we review and present new evidence for drilling by platyceratid gastropods.

The most reliable evidence for identifying an organism capable of drilling involves catching it in the act, and, in fact, instances of platyceratids positioned directly over a drill hole in a crinoid have been found; these represent incontrovertible evidence. Examples of drilled crinoids and blastoids known to be infested by platyceratids have also been reported. In these, drill holes are positioned where the platyceratid is normally attached to its host and, more telling, they are often positioned within a sub-circular scar that matches platyceratids in size and shape.

Given the evidence of platyceratid drilling abilities, could they also have been responsible for drill holes in non-echinoderms? The Nunn Member of the Lake Valley Formation (Mississippian of New Mexico) contains a diverse fauna of invertebrates. We found circular drill holes on several Lake Valley brachiopods and platyceratids. Moreover, specimens of the camerate crinoid, Physetocrinus, with platyceratid-like attachment scars on the tegmen and a circular drill hole penetrating the plates of the tegmen indicate that Lake Valley platyceratids were capable of drilling. While the existence of other drillers at Lake Valley cannot be refuted, we view platyceratids as the most probable culprits for all single Lake Valley drill holes. This suggests that platyceratid drilling, either predatory or parasitic, was taxonomically wide-ranging and that platyceratids may have been the perpetrators of the majority, if not of all, single, circular drill holes in Ordovician through Permian invertebrates.