Evidence for heterothermic endothermy and reptile-like eggshell mineralization in Troodon, a non-avian maniraptoran theropod
Tagliavento, Mattia; Davies, Amelia J.; Bernecker, Miguel; Staudigel, Philip T.; Dawson, Robin R.; Dietzel, Martin; Götschl, Katja; Guo, Weifu; Schulp, Anne S.; Therrien, François; Zelenitsky, Darla K.; Gerdes, Axel; Müller, Wolfgang; Fiebig, Jens
(2023) Proceedings of the National Academy of Sciences of the United States of America, volume 120, issue 15
(Article)
Abstract
The dinosaur–bird transition involved several anatomical, biomechanical, and physiological modifications of the theropod bauplan. Non-avian maniraptoran theropods, such as Troodon, are key to better understand changes in thermophysiology and reproduction occurring during this transition. Here, we applied dual clumped isotope (Δ47 and Δ48) thermometry, a technique that resolves mineralization temperature
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and other nonthermal information recorded in carbonates, to eggshells from Troodon, modern reptiles, and modern birds. Troodon eggshells show variable temperatures, namely 42 and 29 ± 2 °C, supporting the hypothesis of an endothermic thermophysiology with a heterothermic strategy for this extinct taxon. Dual clumped isotope data also reveal physiological differences in the reproductive systems between Troodon, reptiles, and birds. Troodon and modern reptiles mineralize their eggshells indistinguishable from dual clumped isotope equilibrium, while birds precipitate eggshells characterized by a positive disequilibrium offset in Δ48. Analyses of inorganic calcites suggest that the observed disequilibrium pattern in birds is linked to an amorphous calcium carbonate (ACC) precursor, a carbonate phase known to accelerate eggshell formation in birds. Lack of disequilibrium patterns in reptile and Troodon eggshells implies these vertebrates had not acquired the fast, ACC-based eggshell calcification process characteristic of birds. Observation that Troodon retained a slow reptile-like calcification suggests that it possessed two functional ovaries and was limited in the number of eggs it could produce; thus its large clutches would have been laid by several females. Dual clumped isotope analysis of eggshells of extinct vertebrates sheds light on physiological information otherwise inaccessible in the fossil record.
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Keywords: amorphous calcium carbonate, dinosaur–bird transition, dual clumped isotope thermometry, eggshell mineralization, General
ISSN: 0027-8424
Publisher: National Academy of Sciences
Note: Funding Information: R. Amos (Troodon), and S. Traver (Chicken). John Eiler and three anonymous reviewers are gratefully thanked for their constructive criticism during the revision process, which greatly improved the quality of this manuscript. Dr. Valentina Rossi, Dr. Alfio Alessandro Chiarenza, and Dr. David Varricchio are thanked for productive discussion during the preparation of the manuscript. Sven Hofmann is thanked for technical support, and M.M. Joachimski for assistance during cathodoluminescence analysis. We are especially grateful to Martin Stark for providing opportunity to collect freshly laid chicken eggs, as well as Zoo Leipzig, Zoo Hoyerswerda, and Zoo Frankfurt for kindly providing eggshells from reptiles and birds. Frankfurt Isotope and Element Research Center (FIERCE, A.G. and W.M.) is financially supported by the Wilhelm and Else Heraeus Foundation and by the Deutsche Forschungsgemeinschaft (DFG: INST 161/921-1 FUGG, INST 161/923-1 FUGG, and INST 161/1073-1 FUGG), which is gratefully acknowledged (FIERCE #121). This work became possible through DFG grants FI-948-11/1 and FI-948-13/1, granted to J.F. Publisher Copyright: Copyright © 2023 the Author(s).
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