2 dome-headed dinosaurs the size of German shepherds discovered

An artist’s interpretation of the newly discovered pachycephalosaurus found in Utah.

(Danielle Dufault)

The discovery of a few fossil skulls of dome-headed dinosaurs is shedding light on how these bizarre creatures called pachycephalosaurs evolved, researchers say.

Both of the skulls are relatively complete. A, discovered in the Kaiparowits Formation of Utah, dating from about 76.5 million years ago. The other, found in the Kirtland Formation of New Mexico, is about 73.5 million years old, the researchers said.

The location of these skulls — in the southern Mountain states — indicates that pachycephalosaurids may have diversified in the south before they moved, and gave rise to the pachycephalosaur known as Stegoceras, said study principal investigator David Evans, a professor in the Department of Ecology and Evolutionary Biology at the University of Toronto. [Dinosaur Detective: Find Out What You Really Know]

Pachycephalosaurids (which means “thick-headed lizards”) were bipedal, herbivorous, and possibly head-butting dinosaurs that lived in the Cretaceous (145.5 million to 65.5 million years ago). At that time, a vast seaway divided the eastern part of North America (the so-called Appalachian mountains) of the western part (the so-called Laramidia). Most pachycephalosaurid fossils are found in northern Laramidia, such as modern-day Alberta and Montana, bringing the two new skulls in the south of Laramidia rather remarkable discoveries, Evans said.

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“There are a few fragmentary specimens which have been found as far south as Texas, but having a good quality material and relatively complete skulls is a true rarity,” Evans told Live Science. “The two new ones really stand out in terms of their completeness, and this allows us a better understanding of their anatomy and their relationships.”

Both of the pachycephalosaurids were small — about the size of a German shepherd, but the Utah specimen was approximately 20 percent greater than the New Mexico one, Evans said.

Bony bumps

Despite their small differences in size, both had a unique bony knobs on the back of the skull, which “is very different from what we have seen in other species,” Evans said. The different boney knobs suggested that they have two new genuses (also called genera) and species, Evans said.

The pachycephalosaurids probably used these bony knobs as decoration — as a way to distinguish between the different types and woo friends, Evans said. Maybe these ornaments, just like the pachycephalosaurids’ arched heads larger as the creature has become, he said.

Interestingly, a variety of dinosaur groups, including pachycephalosaurids, Tyrannosaurs and Ankylosaurs, moved in about 80 million years ago. It is unclear what the cause of this northward move, but one idea is that the seaway changed shape, extending into parts of the country that dinosaurs once inhabited and causing them to leave behind their southern stomping grounds, Evans said.

Perhaps one of the pachycephalosaurid populations in the south moved north, and eventually gave rise to Stegoceras, Evans said. In other words, the two new findings suggest that the “Stegoceras pedigree can actually have their origin in the southern part of North America, that is unexpected,” Evans said. “It tells an interesting story about the evolution of this group that we do not know.”

The study, which has yet to be published in a peer-reviewed journal, presented Oct. 27 at the 2016 Society of Vertebrate Paleontology meeting in Salt Lake City.

Original article on Live Science.

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