PREDATION DAMAGE TO BRACHIOPODS FROM THE PENNSYLVANIAN OF ARIZONA AND NEW MEXICO

Predation studies on marine invertebrate faunas are important in illuminating their paleoecology as well as providing insights into the behavior of their predators. Direct evidence of predation upon brachiopods is rare in the fossil record and the majority of studies regarding such damage have been carried out on shell borings produced by predatory gastropods, as these are preserved whether the predatory attempt is successful or not and are very easily recognized due to their distinctive appearance on the preserved valves. Evidence of crushing or biting damage is much rarer as the predated organism is usually destroyed, leaving no recognizable evidence in the event of a successful attack. Unsuccessful predatory attacks upon brachiopods provide rare evidence of such predator/prey interactions in the form of repaired damage to their valves. Some brachiopods of the species Composita subtilita from the Pennsylvanian Naco Formation of Central Arizona and Madera Formation of North Central New Mexico show distinctive repaired damage that consists of paired clefts in the valves. The clefts appear to be caused by damage to the generative zone of the mantle resulting in the destruction of the cells responsible for the deposition of new shell material. This results in a very narrow zone along which no shell is produced, although shell is deposited to either side as the valves grow. This injury does not appear to be the result of environmental effects or disease as it occurs symmetrically to both valves. The most likely cause of this damage is predation attempts by a clawed or beaked predator capable of inflicting discrete damage to both valves simultaneously. Similar damage in modern brachiopods has been attributed to crustaceans, but in the Pennsylvanian faunas the most likely culprits appear to be nautiloids or conchostracan arthropods.