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May 28, 2015

Jurassic debate

Were dinosaurs warm or cold blooded? Scientists can't agree.

Dinosaurs, those bygone masters of the planet, were warm-blooded just like today's mammals, according to a scientist who judged their metabolism using body mass and growth rates deduced from fossils of species including Tyrannosaurus rex.

Paleontologist Michael D'Emic of Stony Brook University in New York took issue with the conclusion of other researchers last year that dinosaurs were neither cold-blooded nor warm-blooded, but had a metabolism somewhere in between.

Scientists have debated since the 19th century whether dinosaurs were slow, lumbering, cold-blooded creatures, as originally thought, or boasted a more warm-blooded physiology allowing for a vigorous lifestyle.

"The main point of my study is that the dinosaurs that have been studied so far were on average as warm-blooded as mammals living today," said D'Emic, who argued that the 2014 study under-estimated dinosaur growth rates and should have analyzed dinosaurs statistically within the same group as today's birds.

Birds, which evolved from small feathered dinosaurs roughly 150 million years ago, are warm-blooded.

The researchers in last year's study evaluated the metabolism of 21 dinosaur species using a formula based on their body mass, as revealed by the bulk of their thigh bones, and their growth rates, indicated by growth rings in fossil bones akin to those in trees. The species included predators like T. rex, long-necked and duckbilled plant-eaters and others.

They compared this information to data on living mammals, birds, fish and reptiles.

D'Emic re-analyzed the same data to reach his conclusions, published in the journal Science.

The authors of last year's study on Thursday disputed D'Emic's conclusions. "We disagree with his central criticisms and we emphasize that all of our original conclusions stand," said University of New Mexico biologist John Grady.

"Comparing dinosaur growth with the observed growth rate of living vertebrates clearly shows that non-avian dinosaurs were mesotherms," added Grady, using the term for an intermediate metabolism.

Warm-blooded land animals like mammals and birds have never achieved the size of the biggest dinosaurs. But D'Emic said merely being warm-blooded would not have prevented dinosaurs from getting large although they may have needed adaptations to prevent their huge bodies from over-heating.

"The Earth was generally warmer during the time of the dinosaurs, and so overheating could have been a problem for them. However, most large dinosaurs had some hollow, air-filled bones in their skeleton and likely had large air sacs in other parts of their bodies, just like birds today," D'Emic said.

This provided a "natural ventilation system" within their bodies that helped to keep them cool, D'Emic said.