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The extensive chondrichthyan fossil record spans 400+ million years and has a global distribution. Paleontological studies provide a foundation of description and taxonomy to support deeper forays into ecology and evolution considering geographic, morphologic, and functional changes through time with nonanalog species and climate states. Although chondrichthyan teeth are most studied, analyses of dermal denticle metrics and soft tissue imprints are increasing. Recent methodological advances in morphology and geochemistry are elucidating fine-scale details, whereas large datasets and ecological modeling are broadening taxonomic, temporal, and geographic perspectives. The combination of ecological metrics and modeling with environmental reconstruction and climate simulations is opening new horizons to explore form and function, demographic dynamics, and food web structure in ancient marine ecosystems. Ultimately, the traits and taxa that endured or perished during the many catastrophic upheaval events in Earth's history contribute to conservation paleobiology, which is a much-needed perspective for extant chondrichthyans.▪The longevity and abundance of the chondrichthyan fossil record elucidates facets of ecological, evolutionary, and environmental histories.▪Though lacking postcranial, mineralized skeletons, dental enameloid and dermal denticles exquisitely preserve morphology and geochemistry.▪Technical advances in imaging, geochemistry, and modeling clarify the linkages between form and function with respect to physiology, diet, and environment.▪Conservation efforts can benefit from the temporal and spatial perspective of chondrichthyan persistence through past global change events. 

Isotopic analysis of phosphate oxygen from bones and teeth (18Op/16Op, δ18Op) is a common tool used to investigate modern and ancient ecosystems and their climate. However, existing methods have expanded to use pretreatments for organic removal, require large sample sizes, or require extended precipitation timing. All together, these factors could affect accuracy and precision of δ18Op measurement by promoting the formation of oxygen-bearing or nitrogen-rich contaminants. However, the nature and occurrence of contamination are not fully explored. Here we sought to develop a method of silver phosphate precipitation that tests the effect of different sample treatments and reduced sample sizes while preserving sample isotopic composition. Our protocol (SPORA) precipitates Ag3PO4 crystals from ∼1.5 mg of starting material while purifying phosphate from contaminants, like nitrogen or carbonate. Isolation and purification of phosphate are achieved with an anion exchange resin, followed by precipitation of silver phosphate using an updated silver ammine solution that targets small amounts of phosphate in solution. We used a variety of phosphate oxygen reference materials and biogenic apatite materials, such as modern and fossil specimens with varying collagen content, to test the SPORA protocol and its effects on the resultant phosphate oxygen isotopic composition. Results were then compared to those from another published silver phosphate precipitation method (i.e., Rapid University of Chicago Dilute, Rapid UC). Overall, δ18Op values of standards and biogenic apatites were similar between protocols (R2 = 0.99, p << 0.05). In addition to isotope composition comparisons, UV–Vis spectroscopy and Fourier Transform Infrared (FTIR) analyses discerned phosphate recovery and material composition of crystals precipitated via different protocols, respectively. We found that the resin i) may retain ∼10% of phosphate with no isotopic effects and ii) the SPORA protocol produces Ag3PO4 with more accurate δ18Op measurements by preventing the formation of contaminant oxygen phases, silver oxide (Ag2O) and silver carbonate (Ag2CO3), that confound the phosphate oxygen isotope composition. The SPORA Ag3PO4 precipitation procedure overcomes analytical limitations such as sample size and collagen contamination, conditions that other procedures for δ18Op analysis cannot address simultaneously. The SPORA protocol can be used on a large array of bioapatite materials for paleoecological, paleoclimatic, and archeological applications, while reducing the required sample size and ensuring pure Ag3PO4 for isotopic analysis. 

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Abstract Trophic ecology and resource use are challenging to discern in migratory marine species, including sharks. However, effective management and conservation strategies depend on understanding these life history details. Here we investigate whether dental enameloid zinc isotope (δ⁶⁶Zn_en) values can be used to infer intrapopulation differences in foraging ecology by comparing δ⁶⁶Zn_enwith same-tooth collagen carbon and nitrogen (δ¹³C_coll, δ¹⁵N_coll) values from critically endangered sand tiger sharks (Carcharias taurus) from Delaware Bay (USA). We document ontogeny and sex-related isotopic differences indicating distinct diet and habitat use at the time of tooth formation. Adult females have the most distinct isotopic niche, likely feeding on higher trophic level prey in a distinct habitat. This multi-proxy approach characterises an animal’s isotopic niche in greater detail than traditional isotope analysis alone and shows that δ⁶⁶Zn_enanalysis can highlight intrapopulation dietary variability thereby informing conservation management and, due to good δ⁶⁶Zn_enfossil tooth preservation, palaeoecological reconstructions. 

Shark teeth are one of the most abundant vertebrate fossils, 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. 

Abstract Diet is a crucial trait of an animal’s lifestyle and ecology. The trophic level of an organism indicates its functional position within an ecosystem and holds significance for its ecology and evolution. Here, we demonstrate the use of zinc isotopes (δ 66 Zn) to geochemically assess the trophic level in diverse extant and extinct sharks, including the Neogene megatooth shark ( Otodus megalodon ) and the great white shark ( Carcharodon carcharias ). We reveal that dietary δ 66 Zn signatures are preserved in fossil shark tooth enameloid over deep geologic time and are robust recorders of each species’ trophic level. We observe significant δ 66 Zn differences among the Otodus and Carcharodon populations implying dietary shifts throughout the Neogene in both genera. Notably, Early Pliocene sympatric C. carcharias and O. megalodon appear to have occupied a similar mean trophic level, a finding that may hold clues to the extinction of the gigantic Neogene megatooth shark. 

Nitrogen isotope ratios in fossil teeth place extinct megatooth sharks at the top of the marine food web.