INVESTIGATION OF CARBON AND NITROGEN STABLE ISOTOPES IN A VERTEBRATE COPROLITE ASSEMBLAGE FROM THE LATE TRIASSIC OF CONTINENTAL EQUATORIAL PANGAEA

Coprolites, fossilized faeces, provide evidence of ecological interactions in the fossil record and provide a rare window into the biology of extinct taxa. Ecological interactions, such as trophic level and carbon source, can be gleaned from carbon and nitrogen stable isotope compositions of excrement from extant and recent vertebrates. However, the utility of stable isotopes for reconstructing ancient ecological dynamics from fossil vertebrate coprolites is little-known. Here we investigated the carbon and nitrogen isotope compositions of an assemblage of coprolites collected from a single horizon (PFV 456 - representing a marginal lacustrine paleoenvironment in the humid equatorial paleotropics of continental central Pangaea) in the Upper Triassic (~ 220 Ma) Chinle Formation of Arizona. The exact taxonomic affinity of these coprolites to their makers were unknown due to their association with disarticulated skeletal elements from at least 53 vertebrate taxa. We organized the sample set of 52 coprolites into 13 discrete morphotypes based on size, shape, color, inclusions, and internal and external structures, and analyzed their organic carbon (𝛿¹³C) and nitrogen (𝛿¹⁵N) isotope compositions. 𝛿¹³C_org of the coprolites ranged from –29.9‰ to –23.6‰. This range is consistent with both the range of 𝛿¹³C from modern C3 plants and other coprolites, and living tetrapod excrement, which suggests the 𝛿¹³C_org in these coprolites represent the original isotopic compositions of the excrement from the source animals. Low nitrogen contents limited the number of nitrogen isotope compositions we were able to collect. However, two coprolites yielded 𝛿¹⁵N from 2.2‰ to 3.5‰. Reconstruction of trophic interactions based on these results is limited by the paucity of nitrogen values; however, analysis of relationships between the isotopic results and characteristics of the coprolite morphotypes such as diameter, color, and presence of inclusions allows for some insights to be drawn. For example, fish were common to the diets of the makers of several coprolite morphotypes, and the distribution of 𝛿¹³C_org values suggests the presence of a range of coprolite-producer diets with the exclusion of terrestrial apex predators given the relative lack of trophic isotope enrichment.