NITROGEN ISOTOPIC COMPOSITION OF ENAMEL-BOUND ORGANIC MATTER PRESERVES TROPHIC INFORMATION IN MESOZOIC TERRESTRIAL COMMUNITIES

The nitrogen isotopic composition of animal tissue is frequently used as a trophic level proxy. However, because this measurement relies on the preservation of primary organic material, its application to the fossil record has been limited to samples that are less than ~100 ka. To extend this tool further back in geological time, we turn to enamel as an archive of primary organics.

As teeth grow, an organic matrix guides the development of enamel and, as mineralization proceeds, this scaffolding breaks down leaving behind remnant organic material that is trapped within the crystalline mineral matrix. This remnant organic material is well protected from degradation through time compared to the organic material in other more porous tissue types like bone. However, the amount of enamel-bound organic nitrogen is low (0.002-0.025 wt%), so specialized methods (the oxidation-denitrification process) have been developed to measure extremely small amounts of nitrogen (5 nmol), allowing measurements from small quantities of fossil material (<10 mg). Studies of modern tooth enamel of rodents from a controlled feeding experiment and African mammals living in natural ecosystems confirm that δ¹⁵N of enamel-bound organic matter (δ¹⁵N_EBOM) records a diet and trophic level signal, while analyses of fossil tooth enamel from terrestrial Pleistocene fauna and Miocene age megalodon sharks demonstrate that δ¹⁵N_EBOM preserves trophic information in deep time.

Here, we extend the application of δ¹⁵N_EBOM to the Late Jurassic and reconstruct the trophic structure of four terrestrial dinosaur communities. We find no evidence for diagenetic alteration of the nitrogen isotopic composition of our enamel samples from an analysis of N content, comparison with dentine and visibly altered enamel, and elemental ratios. Carnivore-herbivore δ¹⁵N offsets of 5.8 (n=3), 5.1 (n=20), 2.7 (n=4), and 3.3‰ (n=24) from the Morrison, Dinosaur Park, Lance, and Hell Creek formations, respectively, are comparable with the range of offsets observed in the modern (~3-5‰). At a finer scale, we probe differences between the feeding strategies of sympatric carnivores, such as dietary reliance on terrestrial herbivores or fish or cannibalism. We conclude that δ¹⁵N of enamel-bound organic matter is a powerful tool for palaeoecological and trophic reconstruction in deep time.