Just a different place to graze? An unusual occurrence of the echinoid feeding trace Gnathichnus pentax on a marine vertebrate coprolite (Miocene, Italy) and its palaeoethological implications

Faeces produced by marine vertebrates and macro-invertebrates contain sufficient organic matter to represent a usable food source for a wide array of macroscopic animals. In some extant marine environments, coprophagy even represents a crucial trophic interaction in food webs. In ancient ecosystems, coprophagy by macroscopic animals is occasionally exemplified by coprolites that exhibit biting traces or burrows. Here, we report Gnathichnus pentax on an exquisitely preserved vertebratebitten vertebrate coprolite from the marine calcareous deposits of the Pietra leccese (Miocene, southern Italy). This unusual occurrence is interpreted as evidence of the feeding activity of a regular echinoid; in particular, it may represent either exploratory coprophagy or the browsing of an algal (microbial) film that locally developed on the exterior of the faeces. Strengthening the former interpretation, the development of microbial communities on submerged faeces often leads to their destruction; furthermore, some extant Antarctic echinoderms are well known to ordinarily feed on vertebrate faeces, and coprophagy is believed to be fairly widespread among sea urchins. Supporting the algal browsing hypothesis, in turn, only a limited area of the external surface of the faeces was subject to grazing, and the resulting trace is neatly defined, which suggest that the feeding sea urchin targeted a precise location on the dung's exterior when the latter was already rather firm. To our knowledge, the G. pentax specimen studied here represents the first published record of this ichnotaxon on a coprolite.