A thermometer for dinosaurs

Small heads, large bodies, and a slow metabolism -- these are the characteristics that make us think of dinosaurs as dull, lethargic and cold-blooded giants. However, this image seems to be deceiving. These giant saurians that have been extinct for 65 million years may have been high-performance models of evolution. In cooperation with colleagues from the US, researchers from the University of Bonn have just determined that the body temperature of some large herbivorous dinosaurs was between 36 and 38 degrees Celsius.

"Originally, dinosaurs were considered to have been cold-blooded animals because they are reptiles, just like salamanders or crocodiles," explained Dr. Thomas Tütken, a biochemist from the Steinmann Institut at the University of Bonn. Their body temperature depends on the ambient temperature. "This is why after a cold night, the mobility of today's reptiles is very limited, and so is their activity," he added. Warm-blooded animals such as mammals or birds, in contrast, are able to keep their body temperature constant by combusting food. Did dinosaurs also have such an active "heating system"? A homeothermic organism could be compared to a race car that, while it is a high-performance system, uses a lot of "fuel" too. In contrast, a cold-blooded animal will start up more slowly during operation in the cold, but it will also need only about one tenth of the energy a warm-blooded animal will require.

With their colleagues in the U.S., the Bonn researchers developed a method that allows determining the absolute body temperature of dinosaurs with the accuracy of a thermometer by analyzing their dental enamel. "The original chemical composition of their dental enamel has been much better preserved than that of dinosaur bones," said Tütken. Enamel contains a certain percentage of carbonate, a carbon/oxygen compound. Both elements have a heavier and a lighter variant called isotopes. "The mineral formation temperature determines how frequently the two heavy carbon isotopes (13C) and the two heavy oxygen isotopes (18O) will enter a 13C-18O bond within a dinosaur's tooth," explained the geochemist. The warmer it was while the dental enamel was formed, the more infrequently will the two heavy isotopes enter such a bond. "We used this correlation as a thermometer that allowed us to determine the body temperature accurately to within two degrees," explained Tütken.

Using this chemical thermometer, the scientists analyzed teeth from different dinosaurs - on the Camarasaurus, which reached a length of up to 20 meters and a weight of up to 15 tonnes, and the Brachiosaurus, which even topped out at 23 meters and about 40 tonnes. These two giant saurians from the sub-order of herbivorous sauropods lived during the Jurassic Era, about 150 million years ago. "We analyzed thirteen teeth from a Camarasaurus, and three from a Brachiosaurus," the Bonn researcher reported. However, only seven of them were preserved well enough for the analyses to be deemed conclusive.

The heavier carbon and oxygen isotope variants occur in enamel only in minute concentrations – on average, only in 45 ppm (parts per million.) "Consequently, we needed just a little more than a tenth of a gram of tooth enamel, which can be much thinner than a millimeter on dinosaur teeth," explained Tütken. Dinosaurs actually had bigger teeth measuring up to several centimeters. "Since they did not chew, but only cut off their food with their teeth, their teeth were replaced permanently - often as frequently as once a month, as still happens with some reptiles today," he added.

For the Camarasaurus from the U.S., dental enamel analyses resulted in a body temperature of about 36 degrees Celsius, and for the Brachiosaurus from Tanzania, in approximately 38 degrees. "Our method enabled us to determine the body temperature of giant saurians for the first time," declared the researcher from Bonn. In a previous study, the scientists had already successfully applied the chemical thermometer to fossil teeth from 30,000 year-old mammoths. Tütken added, "Our dinosaur tooth analyses have now expanded the time scale to 150 million years."

The question whether dinosaurs were warm-blooded has not been finally resolved yet. "Our data provides clear indications that their body temperature was clearly higher and more stable than ambient temperatures," said Tütken. This could, however, also be a function of the giant saurians' sheer size since a large body mass is also very good at keeping its temperature constant. The scientists now want to study the body temperatures of smaller dinosaurs because they were not able to store heat as well due to the fact that their body surface was large compared to their body volume. If they showed similarly high body temperatures as warm-blooded animals, it would be a clear indication that they were also warm-blooded.

Publication: Robert A. Eagle, Thomas Tütken, Taylor S. Martin, Aradhna K. Tripati, Henry C. Fricke, Melissa Connely, Richard L. Cifelli and John M. Eiler: "Dinosaur Body Temperatures Determined from Isotopic (13C-18O) Ordering in Fossil Biominerals," Science, 23 June 2011 (10.1126/science.1206196)