Assessment of trophic structure of Cretaceous communities based on stable nitrogen isotope analyses

Abstract

New δ 15 N data suggest the retention of an indigenous signal in ancient high molecular weight organic material. These data open the possibility of obtaining new paleoecological information, based on isotopic analyses, on ancient, well-preserved fossil communities. Stable carbon and nitrogen isotope analyses were performed on high molecular weight organic material isolated from 22 taxa of Late Cretaceous vertebrates (Judith River Formation, Alberta,Canada). The majority of δ 13 C and δ 15 N values (-27‰ to -23‰ and 4‰ to 12‰, for δ 13 C and δ 15 N, respectively) are similar to those reported for modern consumers. An assessment of trophic levels based on δ 15 N is consistent with previous interpretations of food web structure derived from paleoecological interpretations. Among terrestrial consumers, carnivorous theropods (tyrannosaurids and dromaeosaurids) have high δ 15 N values (6.6‰ ±0.4‰ and 7.9‰, respectively) relative to those of the dominant megaherbivore (hadrosaurids, 4.7‰ ±0.5‰). Within aquatic environments, the values of δ 15 N of the bowfin Amia (11.6‰) and plesiosaur (11.0‰),distinguish the piscivorous tendencies of these organisms from those of tower trophic level consumers such as the benthic feeding sturgeon Acipenser and the turtle Aspideretes (δ 15 N = 5.1‰ and 4.5‰, respectively). The correlation in trophic position between δ 15 N values and paleoecological evidence is unlikely to be coincidental.