Dual clumped isotope thermometry and fossil eggshells: a way to better understand the dinosaur-bird transition

Birds evolved from dinosaurs during the Late Jurassic and they are now one of the most widespread group of vertebrates on the planet. However, some aspects of the dinosaur-bird transition, like the timing of switch to an endothermic thermophysiology and the loss of an ovary, are still obscure. Modern reptiles, as living representatives of the reptiles-dinosaurs-birds lineage, can be used to determine the ancestral conditions for both dinosaurs and birds.

For this study, we analyzed the dual clumped isotopic composition of eggshells of modern reptiles, modern birds and Troodon, one of the last bird-like dinosaur. Dual clumped isotope thermometry (combined analysis of ∆47 and ∆48) not only allows the determination of the carbonate precipitation temperature, but consent the identification of the kinetic effects prevailing during precipitation, hence opening way to a more comprehensive understanding of the (bio)mineralization process.

Our results suggest that Troodon, despite presenting bird-like body temperature, still presented a reptile-like eggshell mineralization process that not required involvement of Amorphous Calcium Carbonate as transitional phase. These results might also imply that Troodon was still in posses of two fully functional ovaries.

Critically, this work proves that dual clumped isotope thermometry can extract biological information from mineralized fossil material, allowing investigation of physiological information of extinct animals even in absence of reliable soft tissue evidence.