Clumped isotope paleothermometry of eggshells as an indicator of vertebrate endothermy

Abstract:

Isotopic analyses of the calcite or aragonite shells of aquatic organisms are often used in the study of the environmental conditions in which they grow; however, this approach is less straightforward in the terrestrial realm, where environments may be more heterogeneous. In such terrestrial localities, the bioapatite of vertebrate teeth comprises the typical archival material for isotopic analyses. The calcitic eggshells of birds and other reptiles may provide suitable material for isotopic analyses that are aimed at studying their physiology and ecology. Here we apply a novel thermometer, carbonate clumped isotopes (Δ₄₇), to test for endothermy in extinct non-avian dinosaurs in the context provided by eggs of modern reptiles and birds. These Δ_47-derived temperatures should reflect the temperature of shell formation, which in endothermic animals such as birds should represent the mother’s internal body temperature. In ectothermic animals, the same is true although their body temperatures are more affected by the external environment and thus Δ₄₇ temperatures could more accurately describe local environmental temperatures during eggshell formation.

Fossil eggshells represent appropriate material for reconstructing internal body temperatures of extinct non-avian dinosaurs since they mineralized within the mother’s body, and fragments of eggshell are commonly recovered from dinosaur-bearing fossil deposits. The dimensions of these fragments provide sufficient material for the relatively large sample required for clumped isotope analysis (~20mg). Fossil eggshell samples from several taxa of Late Cretaceous non-avian dinosaurs were analyzed using Δ₄₇ paleothermometry. Textural inspection was used as a first test for diagenetic alteration of the original calcite, and histological indicators were used for broad taxonomic identifications. Preliminary results of Δ₄₇-derived body temperature estimates from eggshells are consistent with previous body temperatures estimates from bioapatite Δ₄₇ analyses performed on non-avian dinosaur teeth.