Some species of dinosaurs had the ability to warm themselves by drawing heat from the Sun, according to an international team of paleontologists from the United States, Argentina, France, and Denmark.
Paleontologists used a novel technique to find out dinosaurs’ body temperatures based on their eggshells. Image credit: Gerald Grenet-Tinnes.
The team, led by Prof. Robert Eagle of the University of California, Los Angeles, believes the ancient reptiles were probably more active than modern-day alligators and crocodiles, which can be energetic, but only for brief spurts.
The evidence also shows that some dinosaurs had lower body temperatures than modern birds, their only living relatives, and were probably less active.
Dr Eagle and his colleagues examined fossilized dinosaur eggshells from Argentina and Mongolia. Analyzing the shells’ chemistry allowed them to determine the temperatures at which the eggshells formed.
“This technique tells you about the internal body temperature of the female dinosaur when she was ovulating,” said co-author Dr Aradhna Tripati, a scientist at the Natural History Museum of Denmark and the University of Brest.
“This presents the first direct measurements of theropod body temperatures.”
The results were published online October 13 in the journal Nature Communications.
The Mongolia shells, which are 71 million to 75 million years old, are from oviraptorid theropods – small dinosaurs that were related to Tyrannosaurus rex and birds. The eggshells from Argentine are from large titanosaur sauropods that lived 80 million years ago.
The isotopic composition of the eggshells showed that oviraptorid dinosaurs had body temperatures of 90 degrees Fahrenheit (32 degrees Celsius) – decidedly cooler than modern mammals and birds.
The body temperatures of the larger sauropods were 100 degrees Fahrenheit (38 degrees Celsius).
This finding – that larger dinosaurs maintained body temperatures like ours whereas smaller ones more closely resembled modern reptiles – has implications for our understanding of dinosaur physiology.
“Measuring cooler temperatures in small dinosaurs is the first evidence to suggest that at least some of them had lower basal metabolisms than most modern mammals and birds, and therefore the emergence of modern mechanisms of endothermy hadn’t occurred in these dinosaurs,” said study senior author Prof. John Eiler from California Institute of Technology in Pasadena, California.
Paleontologists have long debated whether dinosaurs were endotherms or ectotherms. The research indicates that the answer could lie somewhere in between.
“Some dinosaurs were not fully endotherms like modern birds. They may have been intermediate – somewhere between modern alligators and crocodiles, and modern birds. That’s the implication for the oviraptorid theropods,” Prof. Eagle said.
“This could mean that they produced some heat internally and elevated their body temperatures above that of the environment, but didn’t maintain as high temperatures or as controlled temperatures as modern birds.”
“If dinosaurs were endothermic to a degree, they had more capacity to run around searching for food than alligators would.”
The team also analyzed fossil soils, including minerals that formed in the upper layers of soils on which the oviraptorid theropod nests were built.
That enabled them to estimate that the environmental temperature in Mongolia shortly before the dinosaurs went extinct was 79 degrees Fahrenheit (26 degrees Celsius).
“The oviraptorid dinosaur body temperatures were higher than the environmental temperatures – suggesting they were not truly cold-blooded, but intermediate,” Dr Tripati said.
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Robert A. Eagle et al. 2015. Isotopic ordering in eggshells reflects body temperatures and suggests differing thermophysiology in two Cretaceous dinosaurs. Nature Communications 6, article number: 8296; doi: 10.1038/ncomms9296
