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Delving into Dinosaur Body Temperatures

By John Long

New research finds that the dinosaur ancestors of birds had quite high body temperatures.

In 2010 I was working in Los Angeles when I met a young PhD student from England working at Caltech named Rob Eagle. Working with Prof John Eiler, they set up a lab to try and solve one of the greatest mysteries of palaeontology: determining the body temperatures of dinosaurs.

Since the revelation that birds had evolved from feathered theropod dinosaurs, an hypothesis supported by the late 1990s discoveries of Chinese dinosaurs with feathered bodies and wing-like arms, the burning question had been about when birds first acquired their warm-blooded metabolism. Did the theropod dinosaurs have warm-blooded lifestyles well before they evolved into birds? The functional biomechanics of their skeletons indicated that they should have had a high metabolism and were likely to be warm-blooded, as birds and mammals are today.

To solve this problem, Eagle and Eiler developed an entirely new approach using clumped isotope thermometry, a technique to measure the temperature that calcitic bonds form in making organic frameworks inside bone or teeth. It works because rare isotopes of carbon-13 and oxygen-18 in apatite, the mineral that forms bone, bond with each other at certain temperatures. Measuring the degree of this “clumping” determines the exact temperature of the environment in which the bonds form inside bone or teeth –and thus the body temperature of the living animal.

The control used for this work was to measure the body temperatures of several living reptiles and see what the isotopes predicted for their body temperatures. Calibrating this data, Eagle applied the technique to a range of fossilised dinosaur teeth, as enamel is less prone to contamination by other isotopes. In 2011 they announced in Science that the gigantic long-necked sauropod dinosaurs Brachiosaurus and Camarasaurus had body temperatures of 36–38°C, a range that our human body temperature falls within (36.9°C).

Giant sauropod dinosaurs could also be warm- blooded if they lived in a very hot world, or by virtue of their great body mass, as larger body size retains heat more efficiently. This kind of regulation is termed “giganto­thermy” and explains how large marine turtles living in cold northern seas can have slightly higher body temperatures than the seawater they live in, yet strictly speaking they do not have a warm-blooded metabolism.

But the burning question about dinosaurs was really to do with small theropod dinosaurs, which were the precursors to modern birds. The team then turned their attention to getting suitable specimens and testing the materials.

They recently completed a detailed study of several specimens of small theropod dinosaur eggshells, which they then compared with many kinds of living birds and their eggs. The results, published in Nature Communications, showed that the theropod Oviraptor formed its eggs inside its body at around 32°C, well above the ambient temperature of the environment they lived in, which was found to be about 26°C based on independent tests on fossil soil samples from the same formation.

This fulfils the prediction of palaeontological studies on body functional mechanics, implying that the ancestors of the first birds had quite high body temperatures but not as high as modern flying birds. The team now aims to determine the body temperatures of much older dinosaurs to find out how and when warm-blooded metabolism first evolved in dinosaurs. The same methods could also be applied to look at when high metabolic rates first appeared in mammals and their ancestors, the mammal-like reptiles.

John Long is Strategic Professor in Palaeontology at Flinders University, and current President of the Society of Vertebrate Paleontology.