They didn’t mark the wall in crayon or pencil with a date to monitor how they grew, the way parents do in suburban homes with their children. Millions of years ago, however, dinosaurs left clues in their bones about their annual growth.
Dinosaur bones have concentric rings, which are analogous to the ones trees have in their trunks.
Michael D’Emic, a paleontologist and Research Instructor in the Department of Anatomical Sciences at Stony Brook, studied these bones and the size of these rings and concluded that dinosaurs were warm-blooded.
In a paper published in the journal Science, D’Emic demonstrates how the growth rates of these bones indicate dinosaurs were much more like birds than reptiles in their metabolism.
“This supports the idea that dinosaurs were warm-blooded,” said Holly Woodward Ballard, an Assistant Professor of Anatomy in the Center for Health Sciences at Oklahoma State University.
D’Emic re-analyzed data that appeared in a 2014 Science article, in which other scientists had suggested dinosaurs were mesothermic, which is somewhere in between cold blooded organisms, like reptiles, and warm-blooded creatures, like birds, three-toed sloths, and humans.
D’Emic was on a dinosaur dig in Wyoming when the paper came out last June. When he returned to Stony Brook in July, he took a closer look at the results. “When I read the paper, I thought they hadn’t accounted for a couple of factors that would bias the results,” he said. “I was curious how changing some of those factors” would affect the conclusions.
D’Emic studies the smallest parts of bones. Indeed, for creatures that lived millions of years ago and weighed as much as 40 tons, he looked closely at cells that were a fraction of the width of a human hair.
In his approach to the data, D’Emic adjusted for seasonal growth patterns. Typically, dinosaurs grow only half the year. In the other half, when food is scarce or the temperature drops enough, the dinosaurs would have needed that energy to survive. When he accounted for this, he said the rate of growth doubled.
Comparing his estimated growth rate for dinosaurs with the rate for mammals and reptiles of similar size suggested the dinosaurs “fell right in line with mammals,” he said.
A dinosaur’s metabolism could affect life histories including how the dinosaurs raised their young, as well as elements to their physiology, he said. “Such a fundamental aspect of an organism has implications for the kind of animals we expect them to be,” he said.
D’Emic recognizes that some paleontologists will question his conclusions about dinosaur metabolism. When looking at a broad group of paleontologists, he “still finds a pretty big spectrum of ideas” about metabolism and the “debate is probably still open.” After this recent work, D’Emic reached out to partners from around the world to explore bone growth in other groups of dinosaurs.
Ballard, who studies the growth and development of Maiasaura (duck-billed) dinosaurs from hatchling to adults primarily in Montana, supports D’Emic’s conclusions. She said his analysis will reinforce some of the hypotheses she had about dinosaur metabolism. Ballard said D’Emic was “well thought of” and has“definitely made an impact in the histological field.”
When he was in high school, D’Emic had the opportunity to join a dinosaur dig in New York, where he found a mastodon tusk. He was living in Manhattan at the time and went to Hyde Park with a summer class. After two weeks at the site with the class, he asked if he could come back, and wound up returning regularly for months, until school started.
“I didn’t want to go back to high school when September rolled around,” D’Emic recalled.
D’Emic, who recently left a dig in Utah and was on his way to join other Stony Brook researchers in Madagascar, said he still feels inspired by the opportunity to learn about dinosaurs. When he came to the University of Michigan in 2006 to start his PhD program, he planned to focus on Titanosaurs. By the time he left, the number of species of Titanosaurs scientists had discovered and categorized had doubled.
“It’s a cool time to be a paleontologist,” he said.