Search for: All records

Abstract Many explanations for Eocene climate change focus on the Southern Ocean—where tectonics influenced oceanic gateways, ocean circulation reduced heat transport, and greenhouse gas declines prompted glaciation. To date, few studies focus on marine vertebrates at high latitudes to discern paleoecological and paleoenvironmental impacts of this climate transition. The Tertiary Eocene La Meseta (TELM) Formation has a rich fossil assemblage to characterize these impacts;Striatolamia macrota, an extinct (†) sand tiger shark, is abundant throughout the La Meseta Formation. Body size is often tracked to characterize and integrate across multiple ecological dimensions. †S. macrotabody size distributions indicate limited changes during TELMs 2–5 based on anterior tooth crown height (n = 450, mean = 19.6 ± 6.4 mm). Similarly, environmental conditions remained stable through this period based on δ¹⁸O_PO4values from tooth enameloid (n = 42; 21.5 ± 1.6‰), which corresponds to a mean temperature of 22.0 ± 4.0°C. Our preliminaryε_Nd(n = 4) results indicate an early Drake Passage opening with Pacific inputs during TELM 2–3 (45–43 Ma) based on single unit variation with an overall radiogenic trend. Two possible hypotheses to explain these observations are (1) †S. macrotamodified its migration behavior to ameliorate environmental changes related to the Drake Passage opening, or (2) the local climate change was small and gateway opening had little impact. While we cannot rule out an ecological explanation, a comparison with climate model results suggests that increased CO₂produces warm conditions that also parsimoniously explain the observations. 

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. 

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.