Did the giant fossil shark Carcharocles megalodon eat small prey? Insights from bitten mammalian bones from the late Miocene of Peru.

Unlike bony fish, sharks are K-selected animals whose life history strategies generally include the use of protected nursery areas by young of the year and juveniles. Nursery areas can be primary (i.e., grounds where the sharks are born and spend the very first part of their lives) or secondary (i.e., grounds inhabited by slightly older but not yet adolescent or mature individuals). Criteria utilized to recognize and define these strategic habitats include: high concentration of young sharks, continuous or repeated use on various temporal scales, high food availability, and low predation risk. Since the fossil record of sharks is mainly composed of isolated teeth, identification of paleo-nurseries involves a series of problems due to difficult application of the above reported criteria. In fact, only four putative shark paleo-nurseries have been individuated to date: three of them regard lamniform sharks and were proposed on the basis of a few isolated small-sized teeth, whereas the fourth one comprises hybodontid and xenacanthid taxa and is testified by both selachian eggs and juvenile teeth. In the upper Miocene (Tortonian) deposits of the Pisco Formation exposed at Cerro Colorado (South Peru), a very rich shark tooth bearing level has recently been discovered. About 80% of the teeth collected from this level belong to the extant copper shark Carcharhinus brachyurus (Chondrichthyes: Carcharhinidae). These teeth are small-sized and compatible with extant individuals ranging from 80 cm to 210 cm in total length (i.e., immature and subadult copper sharks ranging in age between 4 and 16 years); adult teeth of C. brachyurus are in turn completely absent (although they are known from other Neogene deposits of Southeast Pacific, including a single site of the Pisco Formation). By analyzing the paleoenvironment of the Cerro Colorado shark tooth bearing level, we found that it meets the ecological criteria for individuating extant shark nurseries. The absence of very small-sized teeth (i.e., teeth referable to neonates and young of the year) allows us to hypothesize a secondary nursery ground inhabited by preadolescent copper sharks. The teeth size distribution of other chondrichthyan taxa (Lamniformes, Carcharhiniformes, and Myliobatiformes) that occur along with C. brachyurus put in evidence a significantly juvenile composition of the fossil shark assemblage, thus supporting the hypothesis of a communal use of the Cerro Colorado paleo-nursery by various mesopredator shark species. This study resulted in the identification of the first paleo-nursery for a carcharhiniform taxon, as well as in the first fossil shark nursery investigated with an ecology-based, actualistic approach.