Between 200 and 65 million years ago, fearsome marine reptiles reigned over the oceans. Were they warm-blooded like today's mammals and birds or cold-blooded like nowadays fish and reptiles? For the first time, a study has settled the debate: some large marine reptiles were warm-blooded (in other words, they were endothermic), giving them a considerable advantage to swim fast over long distances and to conquer cold regions.
This work, conducted by researchers from the Laboratoire PaléoEnvironnements et PaléobioSphère (PEPS, CNRS/Université de Lyon 1) in collaboration with scientists from the Muséum National d'Histoire Naturelle and the École Normale Supérieure, has been published in the journal Science on 11 June 2010.
During the Mesozoic era (between 200 and 65 million years ago), when dinosaurs roamed the continents, fearsome predatory reptiles such as ichthyosaurs, plesiosaurs and mosasaurs reigned over the oceans. How did these large marine reptiles regulate their temperature? This question, linked to body temperature and thermoregulation processes, is essential in deciphering the feeding, ecology and evolution strategies of these now disappeared vertebrates.
To determine the body temperature of certain marine reptiles, a French team of geochemists and paleontologists(1) used, for the first time, the compositions of stable oxygen isotopes (18O/16O) in the phosphate of their skeletons. The researchers analyzed the dental remains of three major groups of large marine reptiles: ichthyosaurs, plesiosaurs and mosasaurs. They compared the 18O/16O ratio present in the tooth enamel of these reptiles to that of fish remains from the same period, found at the same latitudes. Fish, which are cold-blooded animals (ectotherms), have an isotopic composition that reflects the temperature of the sea water in which they live. In fish, the 18O/16O ratio increases as the temperature of the ocean decreases. The differences in isotopic composition between marine reptiles and fish that lived in the same body of water reflect the differences in their body temperatures.
The researchers found that the body temperature of the studied reptiles was constant, whatever the water temperature. Thus, ichthyosaurs and plesiosaurs regulated their body temperature independently of sea water temperature within a temperature range from around 12 degrees (±2°C) to around 36 degrees (±2°C). In the case of the three large groups of reptiles studied, the body temperature estimations lie between 35 and 39°C (±2°C).
Some large marine reptiles, now extinct, were thus capable of maintaining a higher body temperature than that of their living environment, suggesting a high metabolism adapted to predation and fast swimming over long distances, even in cold water. These animals consequently had an "endothermic" metabolism (i.e. capable of producing heat), similar to that of present day Cetacea. How and from what point these warm-blooded animals produced such energy remains to be determined.
(1) Researchers from the Laboratoire PEPS (CNRS/Université de Lyon 1), the Centre de Recherche sur la Paléobiodiversité et les Paléoenvironnements (CNRS/MNHN/UPMC) and the Laboratoire de Géologie de l'École Normale Supérieure (CNRS/ENS)
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