Eggshell geochemistry reveals ancestral metabolic thermoregulation in Dinosauria

Abstract

Studying the origin of avian thermoregulation is complicated by a lack of reliable methods for measuring body temperatures in extinct dinosaurs. Evidence from bone histology and stableisotopes often relies on uncertain assumptions about the relationship between growth rate and body temperature, or the isotopic composition (δ18O) of body water. Clumped isotope (Δ47) paleothermometry, based on binding of 13C to 18O, provides a more robust tool, but has yet to be applied across a broad phylogenetic range of dinosaurs while accounting for paleoenvironmental conditions. Applying this method to well-preserved fossil eggshells demonstrates that the three major clades of dinosaurs, Ornithischia, Sauropodomorpha, and Theropoda, were characterized by warm body temperatures. Dwarf titanosaurs may have exhibited similar body temperatures to larger sauropods, although this conclusion isprovisional, given current uncertainties in taxonomic assignment of dwarf titanosaur eggshell. Our results nevertheless reveal that metabolically controlled thermoregulation was the ancestral condition for Dinosauria.

Variable thermoregulation of Late Cretaceous dinosaurs inferred by clumped isotope analysis of fossilized eggshell carbonates

“…It is clear that the average of the estimated body temperatures is close to that of the modern mammals. However, there is a large fluctuation, which probably indicates variable thermoregulation between and among the different species, a conclusion consistent with a recent study of dinosaur eggshells ( Eagle et al., 2015 ; Dawson et al., 2020 ). It is also possible that some of the dinosaur species regulated their body temperatures to some extent by altered behaviour, while some others could maintain theirs like modern endotherms ( Dawson et al., 2020 ).…”

“…A few researchers have shown that the clumped isotope thermometry can be used to estimate the body temperature of oviparous animals using the eggshell carbonates (e.g., Wacker et al., 2014 ; Eagle et al., 2015 ; Dawson et al., 2020 ). To confirm the applicability of this thermometer to infer the oviduct temperature of egg laying animals, we analysed eggshells of modern birds and reptiles including snakes and crocodiles collected from two Taiwan zoos.…”

“…In this context, the body temperature estimates of dinosaurs provide crucial information in tracing the biological evolution of metabolism and thermoregulation. Since the establishment of the taxonomic group of dinosaurs in 1842 CE, there have been many discussions about the nature of their metabolism, particularly who were warm blooded (endothermic) and who were cold blooded (ectothermic) ( Farlow, 1990 ; Amiot et al., 2006 ; Grady et al., 2014 ; Eagle et al., 2011 , 2015 ; Dawson et al., 2020 ). Endothermic mammals and birds maintain stable body temperatures, irrespective of the environmental temperature, by internal metabolic heat production but ectothermic animals fail to do so.…”

“…Two specific arguments were given in favour of endothermic metabolism: (1) aggressive physical performance such as high running speed estimated from preserved tracks and (2) a favourable predator/prey ratio determined by comparing the fossil records with the number ratio in modern ecosystems ( Bakker, 1972 ; Ostrom, 1980 ; Farlow, 1990 ). These arguments and the manner of investigations have been widely discussed and revisited based on an extensive study of dinosaur thermoregulation using biophysical modelling ( O'Conner and Dodson, 1999 ; Seebacher, 2003 ; Gillooly et al., 2006 ; Pontzer et al., 2009 ), bone histology ( Chinsamy, 1993 ), growth rate analysis ( Erickson, 2005 ; Klein and Sander, 2008 ; Woodward and Lehman, 2009 ; Sander et al., 2011 ), anatomical observations ( Bakker, 1972 ; Ruben et al., 1997 ), oxygen isotope paleothermometry ( Fricke and Rogers, 2000 ; Amiot et al., 2006 ), and clumped isotope thermometry ( Eagle et al., 2010 , 2011 , 2015 ; Löffler et al., 2017 ; Dawson et al., 2020 ).…”

“…This is consistent with the bone isotopic data, which indicated a relatively uniform body temperature in larger dinosaurs ( Barrick et al., 1996 , 1997 ). Some limited clumped isotope analysis of dinosaur eggshells, recovered mainly from high latitudes, also suggested variable thermoregulation for at least some of the species, if not all ( Eagle et al., 2015 ; Dawson et al., 2020 ).…”