A newly discovered tyrannosaur species with a mask of flat scales and armor-like patches covering its face roamed the earth 75 million years ago, feasting on duckbilled hadrosaurs, dome-headed pachycephalosaurs and smaller carnivores, according to a team of scientists that includes Montana State University paleontologist David Varricchio.
The new species, Daspletosaurus horneri, which means “Horner’s Frightful Lizard," is named after renowned former MSU paleontologist and Museum of the Rockies curator Jack Horner, said Varricchio, an associate professor of paleontology in the College of Letters and Science. Varricchio said he suggested the name in honor of Horner’s many dinosaur fossil discoveries and the mentorship he provided to up-and-coming paleontologists that launched them in their careers, including his own.
Thomas Carr, director of the Carthage Institute of Paleontology at Carthage College in Kenosha, Wisconsin, is lead author of the discovery paper, which is the culmination of expeditions, excavations, preparation and research that spanned 25 years and involved scientists from across the globe. The paper, “A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system,” was published March 30 in the journal Scientific Reports.
The findings revealed that tyrannosaurs share some traits with crocodylians, an order of mostly large predatory, semiaquatic reptiles that appeared 83.5 million years ago in the Late Cretaceous period.
Like crocodylians, the bones in tyrannosaurs' snout and jaws are rough, except for a narrow band of smooth bone along the tooth row, said Carr, whose expertise is in the evolution and growth of tyrannosaurs.
“We did not find any evidence for lips in tyrannosaurs; the rough texture covered by scales extends nearly to the tooth row, providing no space for lips,” he said.
Also like crocodylians, horneri had numerous small nerve openings along its snout and jaws, indicating it had a pressure-sensing snout that would have been as sensitive as the human fingertip, he added.
Despite the absence of lips, there is evidence of other types of skin on the face, including areas of extremely coarse bone that supported armor-like skin on the snout and sides of the lower jaw, which would have protected tyrannosaurs from abrasions, perhaps sustained when hunting and feeding, Carr said. Horneri also had small horns in front of its eyes, and a large horn behind its eyes that was made up of keratin, the same shiny material that makes up human fingernails.
The researchers determined that horneri evolved through anagenesis, a rare type of non-branching evolution in which a species gradually changes to become a new species. It is different from the more common cladogenesis, in which an ancestral species splits into two or more branches, or descendant species.
“Daspletosaurus horneri was the youngest, and last, of its lineage that lived after its closest relative, D. torosus, which is found in Alberta, Canada,” Carr said. “The close evolutionary relationship between the species taken with their geographic proximity and their sequential occurrence suggests that together they represent a single lineage that changed over geological time, where D. torosus has morphed into D. horneri.”
Carr said that though rare, studies have revealed other examples of dinosaurs that evolved through anagenesis, including some duck-billed and horned dinosaurs.
“This animal is interesting as it adds another example of anagenesis in a dinosaur lineage,” Varricchio said. “It will be interesting to see how these patterns hold up to more discoveries and what implication this evolutionary path holds for dinosaurs.”
Along with Carr and Varricchio, other co-authors of the paper were Jayc Sedlmayr, professor at the Louisiana State University Health Sciences Center in New Orleans; Jason Moor, professor in the Honors College at the University of New Mexico in Albuquerque; and Eric Roberts, professor of geosciences at James Cook University in Australia.
The horneri specimens, which are housed at Museum of the Rockies, were discovered beginning in the early ‘90s in Montana’s Two Medicine Formation near Choteau. The fossils include a sub-adult skull and skeleton that Vicki Clouse, an assistant professor at MSU Northern, discovered on an expedition with Horner; an adult skull and skeleton Varricchio discovered with a friend and then excavated; and a partial lower jaw and isolated bones of sub-adults and juveniles that Varricchio excavated as part of his doctoral studies at a site Horner discovered.
Varricchio said it is satisfying to see the specimen of the adult skull and skeleton he discovered in 1992, now categorized as MOR 1130, described after all these years and placed in scientific context.
“In the field, we could tell there were many bones and that it was an exciting find, but we really couldn’t see any of the anatomy,” he said.
Preparation of the specimen was especially difficult, Varricchio said, because the bone was fragmented and the rock it was found in was especially hard, making it a long and painstaking process.
“One volunteer, Jamie Jette, worked very diligently for several years on the MOR 1130 specimen,” he said. “It took a long time and much effort for the animal to be finally revealed, and, actually, there are still more bones to prepare.”
Varricchio said that the well-preserved horneri specimens discovered during the various expeditions emphasize the excellent record of dinosaurs to be found in Montana.
“They highlight both the quality of the specimens, the preservation revealing the details of how these giant carnivores once looked in life, as well as the overall collection of specimens that provides insight into the evolution of the tyrannosaur group,” he said. “Montana remains a wonderful place to explore the Cretaceous.”