Tiny holes associated with blood supply inside fossilized dinosaur long bone shafts suggest that dinosaurs had higher metabolic rates than present-day mammals, according to new research from an international team of scientists.
In living animals, these holes or nutrient foramina nourish bone cells with a rich blood supply, and were recently shown to be linked to the maximum activity rate during aerobic exercise in humans.
“Far from being lifeless, bone cells have a relatively high metabolic rate and they therefore require a large blood supply to deliver oxygen,” said study author Roger Seymour at Australia’s University of Adelaide in a press release.
“On the inside of the bone, the blood supply comes usually from a single artery and vein that pass through a hole on the shaft—the nutrient foramen.”
Seymour hypothesized that the size of these holes in fossil dinosaur bones could indicate the metabolic rate of the bone and potentially the whole body.
"One of the big controversies among paleobiologists is whether dinosaurs were cold-blooded and sluggish or warm-blooded and active,” Seymour said.
The researchers correlated metabolic rates with foramen sizes of thigh bone specimens in existing reptile and mammal species, and then compared these data with hole sizes in 10 dinosaur species from five different groups, ranging in weight from 50 kilos to 20 tonnes, and including bipedal and quadrupedal herbivores and carnivores.
"The results were unequivocal,” Seymour said. “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."
"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.”
Surprisingly, based on these figures, the dinosaurs appear to have been even more active than the mammals.
"We certainly didn’t expect to see that,” Seymour commented. “These results provide additional weight to theories that dinosaurs were warm-blooded and highly active creatures, rather than cold-blooded and sluggish."
The scientists concluded that this simple measurement could be used to assess other groups of extinct and living vertebrates in conjunction with body size, behavior, and habitat.
The findings were presented in Proceedings B, the Proceedings of the Royal Society B (Biological Sciences) on July 6.