Researchers in North America have discovered the remains of the world’s fastest dinosaur, the ornithomimosaur. The dinosaur that lived between 145 million and 66 million years ago was said to be like “ostriches or emus” in the food web. It had large eyes, long arms with relatively large, clawed hands, long legs and tails, and either small or no teeth.
Researchers have filled in a gaping hole in North America’s fossil record—by analyzing fossilized remains from the Eutaw Formation of Mississippi, they identified large-bodied specimens of what was perhaps the world’s fastest dinosaur, the ornithomimosaur.
“An ornithomimosaur refers to a particular group of bipedal dinosaurs, most of which are generally ostrich-like in appearance,” Tom Cullen, one of the study’s authors, told Newsweek. “They generally have large eyes, long arms with relatively large clawed hands, long legs, a long tail, and either have small or no teeth…the teeth are absent in later ornithomimosaurs with them having a keratinous beak to assist in processing food.”
The bird-like dinosaurs are thought to have existed between 145 million and 66 million years ago and were distributed across the Northern Hemisphere. “They would likely be similar to ostriches or emus in the position they fill in the food web: a medium-sized, mostly-herbivorous animal that is very fast on its feet,” Cullen said.
The earliest species of ornithomimosaur were universally small-bodied, weighing approximately 26 pounds. Over time, larger species began to evolve, as Chase Brownstein, a research associate at the Stamford Museum and Nature Center who wasn’t involved in the research, told Newsweek.
“The smallest species, such as the 125-million-year-old Chinese species Hexing qingyi, were barely over a meter long, whereas the biggest one, the 72 million-year-old Deinocheirus mirificus from Mongolia, was a massive, hump-backed, giant-armed animal over 33 feet long!”
Until recently, our understanding of the evolution of this bird-like dinosaur across North America has been fairly patchy, Cullen said. “We know so much more about the dinosaurs that lived in western North America than we do about those that lived in eastern North America, and this is large because there are so many more extensive deposits of rocks in western North America that preserve the environments dinosaurs lived in.
“As a result, we have these gaps in our knowledge, and it can take much longer to figure out what species were living here [in eastern North America] when we are only looking at the comparatively rare records that do exist.”
During this period, North America was divided by a large body of water called the Western Interior Seaway, Brownstein said: “Appalachia was formed when eastern North America was isolated from western North America about 90 million years ago by flooding of the American interior, which produced the Western Interior Seaway. We really don’t know as much about Appalachian dinosaurs as western North American ones, so any bits of information are essential!”
Despite this rich fossil data from western North America, there was still a temporal gap in our understanding of the ostrich-like dinosaurs’ evolution, between 100 million and 83 million years ago.
“Prior to this time, we have a somewhat patchy record of ornithomimosaurs in North America, some of which were likely decently large,” Cullen said. “After this time gap we have a fairly rich record of ornithomimosaurs in western North America that are almost all fairly small-bodied. So we weren’t sure what this mid-point would look like.”
In a recent study, published in PLoS One, Cullen and his team used femur bone fossils from two different ornithomimosaur specimens to estimate the body mass range of the individuals.
“We were able to use the growth rings preserved in these bones in order to determine if we were dealing with two different species or just the juvenile and adult individuals of a single species,” Cullen said. “Since our small individual was over 7 years old but less than half the size of the over 10-year-old large individual, we could be reasonably sure we were dealing with different species, as we saw no evidence that their growth rates increased enough in that intervening time to make up for the difference in size observed.”
Their findings suggest that large and small ornithomimosaur species were able to co-exist during this period. “We previously knew there were large and small ornithomimosaurs at various points in time, but we did not know what was going on at this particular period of time in the Cretaceous in North America, and this helps to fill that gap,” Cullen said.
“Our finding helps fill in a previously missing piece of the puzzle of the evolutionary and biogeographic history of ornithomimosaurs.”