Collating only skeletons containing three or more bones, the researchers counted 23 Triceratops, five Tyrannosaurus and five Edmontosaurus within the Upper Hell Creek Formation. The youngest or “upper” formation dates from between 65 and 70 million years ago, just before the purported mass extinction of the dinosaurs that was attributed to a comet or asteroid impact.
A census of older sediments—the lower Hell Creek formation—turned up 11 Triceratops, 11 T. rex and six Edmontosaurus partial skeletons, along with fossil bones of three other dinosaurs: Thescelosaurus and Ornithomimus, two bird-like, bipedal meat-eaters reaching some 12 feet in length at maturity; and Ankylosaurus, an armored, four-legged plant-eater with a club tail.
“Small juveniles and older adults were relatively rare compared to large juveniles and subadults for all the dinosaurs,” Goodwin said. This could be explained if juveniles lived in other locations, which is not uncommon in some species. The largest adults may simply have been relatively rare.
“This adds to an emerging picture of what the dinosaur fauna looked like during the late Cretaceous,” he said.
Horner noted the greater variety of dinosaurs in the older sediments, the Lower Hell Creek Formation, compared to the younger “Upper” formation.
“Definitely there was a change in population leading up to the Cretaceous-Tertiary boundary, so something was happening to the faunas prior to the impact,” he said. “During the 10 million years after dinosaur diversity peaked 75 million years ago, the dinosaurs dwindled pretty fast, and there weren’t many left at the end.”
The work was supported by individual donations from James Kinsey, Catherine B. Reynolds and Homer Hickam, as well as Intellectual Ventures, the Windway Foundation, the Smithsonian Institution and the University of California Museum of Paleontology.
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