22-6 Testing dietary hypotheses and trophic paleoecology using Ca isotope analyses of dinosaur enamel bioapatite from a well-constrained microvertebrate locality in the Oldman Formation of Canada

Session: Working Up an Apatite: Teeth as Paleo -Ecological and -Climatological Archives

Presenting Author:

Authors:

Abstract:

Dietary investigations of dinosaurs have traditionally relied upon a mixture of morphological inferences, extant comparisons, biomechanical models, and rare examples of preserved stomach content. These lines of evidence have led to confident assessments of herbivory in large ornithischians and carnivory in theropods such as tyrannosaurids and dromaeosaurids.

Increasingly, geochemical proxies analyzed from fossilized hard tissues, and particularly enamel bioapatite, have been applied to test dietary hypotheses in dinosaurs. These approaches have the potential to be particularly valuable in elucidating patterns in taxa with multiple dietary hypotheses and/or where inferences from other approaches produce either inconclusive or potentially conflicting results. One notable example of this can be observed in troodontids, a small-bodied theropod dinosaur related to dromaeosaurids and birds. Troodontids have been variably hypothesized as herbivorous, carnivorous, or omnivorous. Biomechanical data suggest lower potential for active capture of larger prey, in contrast to dromaeosaurids, yet troodontid fossils are also found in association with nesting sites of large herbivorous dinosaurs and with acid-etched fossil mammal remains interpreted as regurgitated gut contents. Prior isotopic and elemental analyses of troodontid teeth are potentially consistent with omnivory, though specialized diets such as insectivory or ovivory have yet to be reliably assessed.

Here we further these tests of troodontid diet using δ^44/42Ca, recovering compositions intermediate between large ornithischian herbivores (hadrosaurids, ceratopsids, ankylosaurs) and carnivorous theropods (dromaeosaurids and tyrannosaurids). When compared to extant reptile and mammal δ^44/42Ca, troodontid diets were not consistent with primary insectivory, though we cannot exclude ovivory.

Additionally, a broad range of δ^44/42Ca exists among herbivorous ornithischian taxa. Prior studies have hypothesized this as representing feeding height stratification, a behavior supported by other evidence to have occurred in these groups, but more recent studies suggest δ^44/42Ca differences in herbivorous dinosaurs are instead related to differences in consumed plant tissues. Our results are potentially consistent with greater consumption of roots and woody tissues in ankylosaurs, a mix of woody and leafy tissues in ceratopsids, and greater consumption of leafy materials in hadrosaurids (with woody tissue feeding in some individuals, consistent with coprolite records from some hadrosaurids).

Taken together, this adds support to the growing body of research suggesting non-traditional isotopes, such as calcium, can play a critical role in our understanding of dinosaur paleoecology.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-4476

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