Enormous, birdlike dinosaurs strutted across ancient Mississippi around 85 million years ago. The precise appearance of these saurian behemoths is as yet unknown. Paleontologists have only found a smattering of leg and foot bones from these massive reptiles. Yet that small collection of fossil remains hints at a little-known realm of dinosaurs that roamed the eastern half of North America during the Cretaceous. The haunts of Tyrannosaurus and Styracosaurus to the west are familiar to experts, but paleontologists are only just beginning to piece together the world of the dinosaurs east of the Mississippi River.
The new fossils were found along Luxapallila Creek in Mississippi. The collection of bones belong to dinosaurs called ornithomimosaurs—“bird mimics” that often resembled ostriches with tails and clawed hands. What makes these bones special, North Carolina Museum of Natural Sciences paleontologist Chinzorig Tsogtbaatar and colleagues propose, is that they represent two as-yet-unknown dinosaur species, one of which attained sizes far larger than any ornithomimosaur found on the continent. The researchers have published their research today in PLOS One.
Despite only representing a few parts of the body, the bones from Mississippi still contain some telltale clues about the dinosaurs those remains belonged to. By comparing those bones to other better-known ornithomimosaurs, as well as studying the microscopic structure of the bones to estimate the growth patterns of the dinosaurs, Tsogtbaatar and co-authors were able to calculate size and age estimates for the dinosaurs in the sample.
Rather than just one species, the paleontologists propose, the collection from the banks of Luxapallila Creek represent two different ornithomimosaurs. One of them was similar in size to Gallimimus—a dinosaur that has had cameos in the Jurassic Park films and could grow to be 20 feet long and weigh nearly half a ton. At least one of the bones from this ornithomimosaur indicates that the dinosaur was about 14 years old when it died, with the age and body size being similar to other large ornithomimosaurs found elsewhere. But the new study also proposes that an even bigger ostrich-like dinosaur roamed ancient Mississippi.
Some of the foot bones found in Mississippi look comparable in size to one of the strangest dinosaurs ever found. Deinocheirus, the “terrible hands,” was an ornithomimosaur roughly the size of Tyrannosaurus that roamed Cretaceous Mongolia. The duck-beaked dinosaur had immense arms, large claws and a small sail along its back. The larger bones from Mississippi suggest that the creature like Deinocheirus would be the largest ornithomimosaur yet found in North America. “It’s a bit of a surprise to learn that Deinocheirus-sized ornithomimosaurs were also living in North America back in the Cretaceous,” says University of Calgary paleontologist Darla Zelenitsky, who was not involved in the new study. Why such giants have turned up on the eastern half of the continent, rather than in the fossil-rich rocks of the western side, is one of the new questions paleontologists will dig into.
So far, the bones from Mississippi aren’t complete enough to reliably name new species. Nevertheless, Zelenitsky says, “multiple ornithomimosaur species living side by side is interesting.” In other geological formations in western North America, she notes, multiple species of these dinosaurs have been found from the same rock layers and may indicate that ornithomimosaurs preferred different food sources or other habitat requirements to coexist. Future discoveries will help researchers compare how the dinosaur communities differed across the continent. “Hopefully more fossils will be discovered from the east,” Zelenitsky says.
Such discoveries are hard-won. “It’s hard to put into words just how much more we know about dinosaurs in the western U.S. versus the east,” says Smithsonian National Museum of Natural History paleontologist Matthew Carrano, who was not involved in the new paper. Even though some of the earliest major dinosaur discoveries in North America were made on the East Coast, not far from Philadelphia, paleontologists have been focused on the fossil-rich outcrops of western North America for more than 150 years. Scientists have an easier time finding dinosaurs in the west as a result of both natural and human factors.
The fossil record for dinosaurs across North America isn’t even. Where dinosaurs lived was shaped by a now-vanished ocean. Around 100 million years ago, a warm, shallow sea split North America in two. Known as the Western Interior Seaway, these salty waters divided the continent into a subcontinent known as Laramidia to the west and Appalachia to the east. Wet, lowland habitats along the coasts of these subcontinents generally stood a better chance of preserving dinosaurs than areas that were more inland—until the seaway receded by 66 million years ago. To find dinosaurs, paleontologists working toward the East Coast have to trace the outline of these vanished coasts and hope that vegetation or human construction have not covered up or otherwise obscured the rocks that might contain dinosaurs.
“Most of the rocks are just not very accessible,” Carrano notes. Where forests and vegetation don’t cover over the right rocks in the east, cities and towns do. Even then, he notes, during the last Ice Age immense glacial sheets extended far south and scoured off many of the rocks that formed during the heyday of the dinosaurs. Even when paleontologists in the east find rocks that could preserve dinosaurs, they often locate them on road cuts, construction projects and stream banks. And yet, now and then, eastern fossil hunters come across these rare dinosaurs. Each one is part of a prehistoric tale that is only just beginning to come into focus, surely full of new species that will hopefully be unveiled as paleontologists continue their search.