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Were Dinosaurs Cold-Blooded or Warm-Blooded?

There has been a debate among scientists about whether dinosaurs were cold-blooded and sluggish (like reptiles today) or warm-blooded and active (like mammals today).  Professor  Roger Seymour from the University of Adelaide in Australia believes that one indicator of dinosaurs’ activity level can be found in their bones.

His theory boils down to this: In the thigh bone of animals, there are tiny holes, known as nutrient foramen, that distribute blood to the bone cells.  In highly active animals like mammals, these holes are relatively large because their bones require more blood keep them healthy.  In less active animals like reptiles, the holes are relatively small.

Centrosaurus apertus

Measuring the nutrient foramen of a ceratopsian dinosaur, Centrosaurus apertus.
Photo by Dr Donald Henderson, Curator of Dinosaurs from the Royal Tyrrell Museum in Alberta, Canada.

Sarah Smith, a student of Professor Seymour, measured holes in the bones of a range of mammals, from mice to elephants, and reptiles from lizards to crocodiles using specimens from Australian museums.  With the data in tow and comparisons made to the size of the holes, the size of the body, and the animal’s metabolic rate, Seymour determined that the “sizes of the holes were related closely to the maximum metabolic rates during peak movement in mammals and reptiles. The holes found in mammals were about 10 times larger than those in reptiles.”

Museum curators from Canada and Germany then collected data on the holes found in dinosaur fossil bones in their collections.  The specimens were from a wide range of dinosaurs from bipedal and quadrupedal carnivores and herbivores, large and small.

Seymour explained their findings:

“On a relative comparison to eliminate the differences in body size, all of the dinosaurs had holes in their thigh bones larger than those of mammals. The dinosaurs appeared to be even more active than the mammals.  We certainly didn’t expect to see that.  These results provide additional weight to theories that dinosaurs were warm-blooded and highly active creatures, rather than cold-blooded and sluggish.”

To learn more, see Seymour’s article, “Blood flow to long bones  indicates activity metabolism in mammals, reptiles and dinosaurs”.

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