Predatory drill holes and partial mortality in Devonian colonial metazoans

Abstract

Elliptical holes ∼0.2–0.3 mm in diameter with beveled edges have been found penetrating the zooids of encrusting hederellids (colonial metazoans of uncertain affinity) from several localities in the Middle Devonian (Givetian) of North America. These are the first known predatory drill holes in pre-Cretaceous colonial animals and a key addition to the record of Paleozoic predation. Some drill holes were subsequently patched from within by new skeletal material, proving that the drilling occurred during the life of the colony. These drill holes are analogous to predatory drill holes in some modern cheilostome bryozoans, which can be similarly patched, in this case by the intramural budding of a new zooid into the empty chamber of the old zooid. A drilling predator of unknown affinity evidently consumed hederellid zooids one at a time, inflicting partial mortality on the colonies. The mode of drilling suggests that the predator specialized in this type of colonial prey, and the repaired drill holes show that the hederellids had a response to such damage. Reports of small circular holes interpreted as predatory drill holes are becoming more common in noncolonial shelled invertebrates from the Paleozoic, notably brachiopods, bivalves, and crinoids. These drill holes are far less frequent than in post-Paleozoic shells (where most are the work of gastropods), and many are of questionable origin, with some representing the traces of parasites and others postmortem domichnia.