Probing the Ecology and Climate of the Eocene Southern Ocean With Sand Tiger Sharks Striatolamia macrota

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. macrota body 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 δ 18 O PO4 values from tooth enameloid (n = 42; 21.5 ± 1.6‰), which corresponds to a mean temperature of 22.0 ± 4.0°C. Our preliminary ε 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. macrota modified 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 CO 2 produces warm conditions that also parsimoniously explain the observations. Plain Language Summary The body temperature of sharks is regulated by their surrounding waters and so there are limited species in cold, polar regions. In the past, there were periods of time (e.g., a geologic time period called the Eocene, 56-33.9 million years ago) where the Earth was much warmer and sharks were abundant in ocean waters all over the globe. For instance, fossil shark teeth have been found in Eocene sediments in Antarctica, which provides an example of this distribution. We analyzed the chemistry of these fossil shark teeth, which provides environmental clues to understand how the climate changed during the Eocene. We also measured shark tooth size, which is related to the length and age of shark, to understand the life stage of sharks living in the area. Our study found that sand tiger sharks of all ages lived in this location and water temperatures recorded by their teeth stayed constant over time, despite the clear continental movement and changes in ocean circulation. Future studies of ancient climate should consider the potential of shark teeth to contribute to their analyses and geologic reconstructions.

Decoding the dynamics of dental distributions: insights from shark demography and dispersal

“…By contrast, the faunal composition and geochemistry of the Seymour Island locality (La Meseta Fm.) suggest typical marine conditions [3,16,21]. This site encompasses seven stratigraphic biostratigraphy units that span middle to late Eocene; although the shark assemblage changes across the formation [16], † S. macrota is the most abundant spanning ca 45–41 Myr and demonstrate relatively stable oceanographic conditions [3].…”

“…suggest typical marine conditions [3,16,21]. This site encompasses seven stratigraphic biostratigraphy units that span middle to late Eocene; although the shark assemblage changes across the formation [16], † S. macrota is the most abundant spanning ca 45–41 Myr and demonstrate relatively stable oceanographic conditions [3]. While our simulations narrowly support this locality serving as a juvenile site, the plateaued error surface precludes a clear interpretation, as is evident from the similarity of juvenile and adult simulated distributions (row 2, figure 5).…”

“…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 [24]. We transformed total length measurements from the 2012 tagging season in Delaware Bay [25,26] to anterior tooth crown height based on the allometric relationships from Shimada et al [7] and previously applied to fossil † S. macrota in Kim et al [3].…”

“…This error surface limits our ability to either interpret whether Seymour Island better represents a juvenile versus adult site or to confidently estimate the size of either dispersal window. However, elevated juvenile and adult dispersal windows in the Antarctic may not be surprising, as this site is known to have accumulated across a larger temporal window [43] where changing environmental conditions associated with the opening of the Drake passage [3,44] impacted shark community assembly [16], and may have influenced how sites were used by shark populations over space and time. Contemporary shark nurseries are thought to enable resource access, promote juvenile growth, and protect vulnerable pups against mortality from potential predators [13,14].…”

“…Documenting the success of sharks as marine predators has followed a trail of fossilized teeth, accumulating in ocean sediments and indirectly recording their ecological variability and the oceanic conditions in which they lived. While fossil shark teeth assemblages have been used to elucidate water temperature and salinity [ 2 ] as well as species’ age distributions [ 3 ], ontogenetic stages [ 4 ] and the presence of nurseries [ 5 , 6 ], the ecological mechanisms driving population size structure remain enigmatic even in extant populations. Given that tooth size scales allometrically with body size [ 7 ], accumulations of shark teeth within narrow temporal windows may provide insight into the functioning of shark populations and communities.…”