Shark teeth are the most abundant vertebrate fossil, and because tooth size generally correlates with body size, their accumulations document the size structure of populations. Understanding how ecological and environmental processes influence size structure, and how this extends to influence these dental distributions, may offer a window into the ecological and environmental dynamics of past and present shark populations. Here we examine the dental distributions of sand tigers, including extant Carcharias taurus and extinct Striatolamia macrota, to reconstruct the size structure for a contemporary locality and four Eocene localities. We compare empirical distributions against expectations from a population simulation to gain insight into potential governing ecological processes. Specifically, we investigate the influence of dispersal flexibility to and from protected nurseries. We show that changing the flexibility of initial dispersal of juveniles from the nursery and annual migration of adults to the nursery explains a large amount of dental distribution variability. Our framework predicts dispersal strategies of an extant sand tiger population, and supports nurseries as important components of sand tiger life history in both extant and Eocene populations. These results suggest nursery protection may be vital for shark conservation with increasing anthropogenic impacts and climate change.

Shark species in the fossil record are largely identified by their tooth morphology due to the poor preservation of cartilaginous skeletons. Striatolamia macrota teeth are identified by emphasized striations on the lingual side relative to the smooth labial side. Anterior teeth (A1-2 and a1-2) are recognized by their long and narrow shape, compared to the lateral and posterior teeth that have a short, blade-like appearance. The anterior teeth have an acute angle between the two roots and have two small lateral cusplets. This tooth position was chosen as a proxy for body size because its large size and distinct morphology compared to other tooth positions within the jaw. Limiting the positions measured from fossil teeth prevents potential for over representation of a single individual within the assemblage. Because the shark body length and tooth crown height relationship established by Shimada (2002) for C. taurus may not be representative and accurate for the Eocene species S. macrota, we reconstruct body length using anterior crown height. We measured anterior tooth height from the enameloid base to the blade tip with digital calipers to an accuracy of 0.1 mm. The labial and lingual sides of the tooth, and the maximum width was also measured and recorded. The labial side of the tooth is adjacent to the cheek of the shark, and the lingual is that side adjacent to the tongue. Every seventh tooth was re-measured for 0.3 mm accuracy. The modern analogue for the Eocene S. macrota is the extant sand tiger C. taurus based on the similarities in tooth shape throughout the entire dentition. We transformed total length measurements from the 2012 tagging season in Delaware Bay to anterior tooth crown height based on the allometric relationships from Shimada 2002 and previously applied to fossil S. macrota in Kim et al. 2020. For each assemblage, we tested the sample size needed to estimate the mean and standard deviation of the tooth distribution. We then calculated the means and standard deviations of each tooth distribution and performed a power test to calculate the sample size required to estimate the mean and standard deviation within a 95% confidence level and 10% margin of error. Striatolamia macrota teeth from Banks Island are curated at the Canadian Museum of Nature (Ottawa, ON Canada); Seymour Island are curated at the University of California Museum of Paleontology (UCMP; Berkeley, CA USA), Paleontological Research Institute (PRI; Ithaca, NY USA), and Swedish Natural History Museum (NRM; Stockholm, Sweden); Red Hot Truck Stop locality are curated at the Carnegie Museum of Natural History (CM; Pittsburgh, PA); and Whiskey Bridge locality are curated at the Whiteside Museum of Natural History (WMNH; Seymour, TX). Locality descriptions are included in the Supplementary Materials.

Details regarding the population simulation are detailed in the method section.

The anterior tooth length data from Eocene fossil sharks are labeled as the following columns: "Red_Hot" is the Red Hot Truck Stop locality of the Bashi/Tuscahoma Formations (Fm); Whiskey_Bridge is the Whiskey Bridge locality in the Stone City Member of the Crockett Fm.; Banks is Banks Island in the Cyclic Member of the Eureka Sound Fm. in the Arctic; Seymour is Seymour Island in the La Meseta Fm. of Antartica. The modern data from Delaware Bay takes total length data from the 2012 Delaware State and University of Delaware shark tagging program and transforms it to labial measurements of anterior tooth crown height (shown as column "modern_DE").