Today’s mammals range in body size from pygmy shrews, with a mass of around 2g, to blue whales that weigh 180 tons. These vastly different body sizes are associated with different growth rates. Not only do large mammals grow faster than reptiles, they also grow faster than small mammals. However, this is not the case with dinosaurs.
The meat-eating theropod dinosaurs that once roamed the Earth also had wildly differing body sizes. Some were likely only a few kilograms in mass, while others, such as the giant T. rex, weighted in at around 8 tons. Recently, scientists investigating differences in growth rates between dinosaur species found, to their surprise, that there was no relationship between body size and growth rate in these dinosaurs.
“Most animals are thought to evolve to be larger by growing faster than their ancestors, but this study shows that it’s just as likely that bigger and smaller animals grew for longer or shorter periods of time during growth spurts,” said Michael D. D’Emic, a paleontologist at Adelphi University and lead author of the study.
The research, published in the journal Science, makes use of data from fossil bones to estimate the growth rates of different species of dinosaurs. The bones of dinosaurs show lines of arrested or slowed growth every year, leaving marks like tree rings that indicate the animal’s age and can be used to estimate the rate of growth.
“Rings like these are called cortical growth marks,” said D’Emic. “Widely spaced rings indicate faster growth and narrowly spaced rings tell us that an animal was growing more slowly.”
D’Emic, along with Ohio University professor Patrick O’Connor and Ph.D. student Riley Sombathy, and a team of international researchers, measured about 500 growth rings in the fossilized bones of about 80 different species of theropods.
“We found that there was no relationship between growth rate and size,” said D’Emic. “Some gigantic dinosaurs grew very slowly, slower than alligators do today. And some smaller dinosaurs grew very fast, as fast as mammals that are alive today.”
This made sense to co-author Thomas Pascucci, whose graduate thesis contributed to the project: “Extinct animals like dinosaurs inspire awe because of how different they seem from our modern world, but they were animals that grew under similar constraints and environmental factors as those that exist today,” he said.
According to O’Connor, this study opens the door to future investigations of how animals regulate their growth. “Alteration of different growth control mechanisms, at molecular or genetic levels, likely accounts for the range of developmental strategies our team observed in theropod dinosaurs. Future studies of living organisms provide an opportunity to elucidate mechanisms related to the evolution of body size in vertebrates more generally.”
Sombathy hopes to take up some of those investigations in future: “One of the things that interests me most about the results is the apparent decoupling between growth rate and body size. My Ph.D. dissertation will investigate the impacts of growth rate and body size on bone shape and function.”
“This has really important implications because changes in rate versus timing can correlate to many other things, like how many or how large your offspring are, how long you live, or how susceptible to predators you are,” D’Emic added. “Hopefully this research spurs investigations into other groups, both alive and extinct, to see what developmental mechanisms are most important in other types of animals.”
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