MULTI-PROXY GEOCHEMICAL ANALYSES REVEAL NICHE-PARTITIONING OF AQUATIC VERTEBRATE COMMUNITIES IN COASTAL PLAIN DEPOSITS FROM THE UPPER CRETACEOUS OF WESTERN NORTH AMERICA

The coastal plain deposits of the Upper Cretaceous of North America preserve some of the most diverse terrestrial assemblages known in the fossil record, and have consequently received considerable attention in studies of paleoecology. Less focus has been given to the aquatic taxa and paleocommunities preserved alongside these terrestrial systems, and consequently our knowledge of their paleoecology remains somewhat poorly constrained. Here we apply a suite of geochemical proxies to reconstruct baseline paleocommunity structure among a sample of these aquatic vertebrates, and test hypotheses of niche partitioning among co-occurring aquatic reptile groups.

Samples were obtained from a spatiotemporally-constrained deposit from the upper Oldman Formation (Belly River Group; Campanian) of Alberta, Canada. They were isotopically analyzed for δ¹³C, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, alongside analyses of major and trace element concentrations (performed for diagenetic assessments and to examine ecologically-relevant element ratios), and additional preservational tests. Sampled taxa include representatives of holostean fish (‘Holostean A’, Lepisosteidae), guitarfish (Myledaphus), choristoderes (Champsosaurus), and crocodylians (Alligatoroidea).

We find that alligatoroids and choristoderes are distinct in their ranges and mean values of δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, and Sr/Ca. The first two suggest potential differences in habitat use along the gradient of freshwater to estuarine/marine in this coastal plain setting. A lack of strong overlap in the elemental ratio of Sr/Ca is indicative of differences in diet and trophic position. The sampled holostean fish and guitarfish overlap with each other in Sr/Ca and also overlap with the superficially gharial-like choristoderes, but overlap far less with the crocodylians. This suggests that dietary/trophic differences between choristoderes and crocodylians likely relate to niche partitioning, with the former having a more strongly piscivorous diet, and the latter feeding more broadly (including on terrestrial taxa). Further analysis, and the incorporation of additional lines of evidence such as morphometrics and tooth microwear, should provide further insights into the paleoecology of this system, and bolster existing parallel studies using these proxies to analyze the terrestrial components of the ecosystem.