Monday, 2 November 2015

Taking The Temperatures Of Dinosaurs

The isotopic composition of dinosaur egg shell
can be used to calculate their body temperatures
Were the dinosaurs warm blooded or cold blooded? Ever since John Ostrom confirmed the evolutionary connection between birds and reptiles in the 1960s, there has been a flurry of studies which have tried to resolve this conundrum.

As time progresses these have become increasingly sophisticated. Many studies have been comparative, looking at the anatomical similarities between birds and dinosaurs in greater detail. More recently we have been able to take thin sections of bones and compare the vascular systems of the two groups.

So far evidence has been inconclusive. Now an isotopic analysis conducted by geologists from UCLA has added another powerful line of evidence to suggest that dinosaurs occupied a middle ground between a hot and cold blooded state. Isotopes are elemental atoms with a differing number of protons. As such they have identical chemical but different physical behaviours. Heavier isotopes require more energy to change state or take part in chemical reactions.

Eggshells are composed of calcium carbonate. The carbonate ion contains both oxygen and carbon, each of which possesses uncommon isotopes. During shell formation, these rare isotopes will cluster together. The degree of clustering is determined by the temperature at which the shell forms, which in turn is affected by the body temperature of the creature in which it forms. As such, the isotopic composition of the shell can act as a proxy for the body temperature of the creature which laid it. The researchers examined the degree of clustering in the shells of dinosaur eggs and then compared their data to previous studies which used similar techniques to calculate the body temperatures of giant dinosaurs.

Cryogenic preparation of gases used in the standardisation
of the egg shell sample analyses by John Eiler
Their results showed that smaller dinosaurs, like the oviraptorids, had body temperatures of around 32 degrees Celsius. This is cooler than that of mammals and birds, suggesting that their metabolism was closer to those of modern reptiles.

'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 Professor Robert Eiler from UCLA.

The results told a different story for larger dinosaurs, however. The sauropods had higher body temperatures of around 38 degrees - much closer to those of mammals and birds.

'Either they [dinosaurs] had a range of different metabolic strategies, or they all had low basal metabolisms, and the large ones were only warm due to gigantothermy,' said Eiler.

Gigantothermy is an explanation of high body temperatures, a consequence of the giant dinosaurs' size. All organisms lose heat through their skin. The rate at which this occurs is determined, in part, by their surface area to volume ratio. A mouse has a high surface area to volume ratio and so loses heat very quickly. An elephant with its low surface area to volume ratio loses heat slowly.

Giant dinosaurs may well have had reptilian metabolisms, but their giant size would have greatly reduced the loss of any heat generated by their metabolic processes, resulting in a higher body temperature. Determining whether giant dinosaurs were gigantotherms or actually did have metabolisms characteristic of hot blooded creatures requires further research. This study, however, adds a fascinating new facet to the cold-blooded verses warm-blooded debate and dinosaur physiology.