Trace fossils on penguin bones from the Miocene of Chubut, southern Argentina

Several traces of biological interaction were found on penguin bones from the basal levels (Aquitanian) of theMiocene Gaiman Formation in the lower Chubut valley of the Provincia del Chubut, Argentina. The fossil-bearingbeds were deposited in littoral to sublittoral environments within sediments of mostly pyroclastic origin. We interpretmany traces to have been produced by predators and/or scavengers while the penguins were still in a breeding area.Many bones show cracking marks due to aerial exposure. The material is disarticulated as is usual in recent breedingareas. Potential predators were coeval terrestrial mammals, most probably marsupial carnivores. After a marinetransgression, these bones were buried or exposed on the sea bottom where they could be colonized by algae,sponges, cnidarians, and other benthic organisms. We identified sponge borings in several bones. Other traces areinterpreted to have been produced by echinoderms feeding on sponges or algae. No evidence of other invertebratepredators such as muricid or naticid gastropods, or decapods was found. Finally, other traces appear to have beengenerated by shark and possibly teleostean vertebrates feeding on epibionts. One coracoid is interpreted to have beenmarked by a shark that is common in the Gaiman Formation, the carcharhiniform Galeocerdo aduncus. From anethological (Seilacherian) classification, traces on bones from the Gaiman Formation include Domichnia (spongeperforations), Praedichnia (terrestrial marsupials, sharks, teleosteans) and Pasichnia (echinoderms). Remarkably,remains of marine organisms with skeletons made of calcium carbonate are very poorly preserved in the GaimanFormation. Only large oysters, sparse shell fragments, skeletal moulds, and bioturbation is evident. The fossilassemblage is mainly composed of phosphatic (e.g. teeth, bones, crustacean parts) and siliceous (sponge spicules,diatoms) remains.