ON THE PROBABLE RARITY OF SMALL DINOSAUR COPROLITES

Most known coprolites were produced by carnivores. Possible herbivorous coprolites of dinosaurs are known from the Middle and Late Jurassic and the Late Cretaceous. The majority of putative dinosaur coprolites pertain to carnivorous theropods. Large coprolites from the Late Cretaceous of Canada clearly pertain to large theropods based on size and content (proxy of digestive metabolism) that includes muscle tissue and bone fragments. The majority of coprolites in dinosaur-bearing strata (Late Triassic-Late Cretaceous) represent small carnivore coprolites. In the Jurassic-Cretaceous, these presumably represent theropods or crocodylomorphs/crocodylians based on the diversity and abundance of nonmarine predators in osseous faunas. The Late Triassic has a broader range of predators, including paracrocodylomorphs, crocodylomorphs and phytosaurs in addition to theropods.

The San Juan Basin of northwestern New Mexico yields a series of Campanian, Maastrichtian, Paleocene and Eocene nonmarine faunas that yield vertebrate coprolites. Analysis of these ichnofaunas indicates that there is limited change in coprolite morphotypes across the Cretaceous/Tertiary boundary. Thus, for example, Alococopros and Eucoprus extend across the boundary. There is also no significant change in the overall median size of coprolites (except for loss of rare large putative tyrannosaurid coprolites). This suggests that many Cretaceous coprolites probably represent crocodylomorphs, which do not demonstrate significant changes across the boundary.

Similarly, there is also no significant change in coprolite morphologies across the Cretaceous/Tertiary boundary in marine environments. This is consistent with the hypothesis that many marine coprolites represents chondrichthyans, which underwent limited extinctions across the boundary.

Clearly, some Mesozoic coprolites represent small theropods, but there are currently no clear criteria to identify them. This highlights the fact that although the coprolite record is significant and abundant it has considerable biases. Thus, caution needs to be applied when coprolites are utilized in the analysis of ecological systems or extinctions. Indeed, the general rarity of definitive dinosaur coprolites in Upper Triassic-Upper Cretaceous strata merits further investigation.