Observations on heterodonty within the dentition of the Atlantic Sharpnose Shark, (Richardson, 1836), from the north-central Gulf of Mexico, USA, with implications on the fossil record.

The Atlantic Sharpnose Shark, (Richardson, 1836), is the most common small coastal requiem shark in the north-central Gulf of Mexico, USA. Despite this fact, little is known about the dental variation within this taxon. To help rectify this shortcoming, we examined 126 male and female jaws sets across all maturity stages to document the various types of heterodonty occurring in the dentition of this taxon. Quantitative data gathered from a subset of our sample allowed for us to place teeth within the dentition of into standardized upper and lower parasymphyseal/symphyseal, anterior lateral, and posterior tooth groups. As with all carcharhinid sharks, the dentition of exhibits monognathic and dignathic heterodonty. We also observed significant ontogenetic heterodonty in the species, as the teeth and dentition progress through five generalized developmental stages as the shark matures. The ontogenetic development of serrations on the teeth appears to be closely related to documented dietary changes as the shark matures. Initial diets are comprised of high percentages of invertebrate prey like shrimp, crabs, and squid, but this transitions through ontogeny to a diet that is more reliant on fishes. We also provide the first documentation of gynandric heterodonty in mature male , with development of these seasonal teeth likely enabling a male to grasp female sharks during copulation. Our analysis revealed a tremendous amount of variation in the dentition of , which has direct implications on the taxonomy of fossil . A comparison of the jaws in our sample to those of the extant species of and the morphologically similar , , and allowed us to formulate a list of generic-level characteristics to assist with the identification of isolated teeth. When applied to the fossil record, it is shown that some species previously assigned to likely belong to one of the other aforementioned genera. The earliest occurrence of unequivocal teeth in the fossil record are those of the Eocene † (Arambourg, 1952), the oldest records of which occur in early Ypresian deposits in Alabama and Mississippi, USA. The early Eocene occurrence of unequivocal fossil teeth in Alabama predates the first occurrence of , , and teeth in the state, supporting published molecular and morphological phylogenies positing a basal position for within the Carcharhinidae.