A group of superbly preserved dinosaur fossils discovered in Montana are giving scientists a fuller understanding of the history, evolution, and appearance of the giant theropods known as tyrannosaurs.
Right down to what their faces looked like.
In case you thought otherwise, it turns out the dinosaurs didn’t have lips.
The newly discovered fossils include the skeletons of an adult and subadult tyrannosaur, along with the lower jaw and other scattered remains of juvenile dinos that together comprise a new species.
It’s been named Daspletosaurus horneri, or “Horner’s frightful lizard,” after famed paleontologist Jack Horner.
The fossils were uncovered in the Two Medicine Formation of northwestern Montana, a region well-known for producing fossils of duck-billed dinos such as Maiasaura, as well as the first dinosaur eggs discovered in North America.
(Learn about new insights into Maiasaura: “Montana Dinosaur Species Grew Fast, Died Young, May Have Been Warm-Blooded“)
The fossils are so “excellently preserved,” paleontologists said, that they’ve made possible a new, closer-than-ever comparison between the anatomy of tyrannosaurs and that of birds and reptiles, living and extinct.
And the results show that, at least above the neck, the giant theropods bore the closest resemblance to crocodylians — the reptilian group that includes today’s crocodiles, alligators, caimans, and gharials.
“It turns out that tyrannosaurs are identical to crocodylians in that the bones of their snouts and jaws are rough, except for a narrow band of smooth bone along the tooth row,” said Dr. Thomas Carr, a paleontologist at Carthage College, in a press statement.
Different textures of bone have been found to correspond to different types of soft tissue, Carr explained, so scientists can study fossilized bones to determine where there might have been a beak, for example, or horn tissue, or scales of various shapes and sizes.
(Read about soft tissue found in fossils: “Dinosaur Blood Cells, Soft Tissue Discovered in 75-Million-Year-Old Fossils“)
And the textures of the D. horneri skulls strongly resemble those in crocodylians, giving the dinos what the researchers describe as a “mask” of large, flat scales, accompanied by patches of thick, armor-like skin on the snout and jaw.
“In crocodylians, the rough texture [in the skull bones] occurs deep to large, flat scales,” he added.
“Given the identical texture, tyrannosaurs had the same covering.
“The armor-like skin would have protected tyrannosaurs from abrasions, perhaps sustained when hunting and feeding,” Carr said.
“We did not find any evidence for lips in tyrannosaurs,” he added. “The rough texture covered by scales extends nearly to the tooth row, providing no space for lips.”
But the comparison of the fossils provided insights into much more than what the dinosaurs looked like.
It also yielded clues about how physically sensitive tyrannosaurs were, especially around the snout.
The key clues here were foramina — the grooves inside bones that hold nerves and arteries.
The network of these grooves inside the tyrannosaur skulls is, again, nearly identical to those of crocodylians, suggesting that D. horneri had sensory organs like those found in modern crocs, providing the dinosaurs with a kind of “sixth sense.”
“Our findings of a complex sensory web is especially interesting, because it is derived from the trigeminal nerve, which has an extraordinary evolutionary history of developing into wildly different ‘sixth senses’ in different vertebrates,” said Dr. Jayc Sedlmayr, an anatomist at Louisiana State University, who contributed to the research.
The trigeminal nerve is responsible for a wide variety of sensory abilities in all kinds of animals, he said, such as “sensing magnetic fields for bird migration, sensing infrared in pit vipers to identify prey, guiding mammals’ movements through the use of whiskers, sensing vibrations through the water by alligators, and turning the elephant trunk into a sensitive hand similar to what has been done to the entire face of tyrannosaurs.”
D. horneri lived in what’s now Montana from about 75.2 million to 74.4 million years ago.
But that is much more recent, and slightly farther south, than the only other known species of Daspletosaurus — D. torosus, which lived millions of years earlier.
“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 physical similarity of the two tyrannosaurs, combined with their separation over time, suggests that D. horneri evolved directly from D. torosus, he noted.
“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 explained.
This rare, non-branching kind of evolutionary development is known as anagenesis, wherein one species descends directly from another.
“Although uncommon in many evolutionary studies, anagenesis has been reported in some duck-billed dinosaurs and horned dinosaurs,” Carr said.
“Daspletosaurus, and these other dinosaurs, point the way forward in picking out the evidence for anagenesis in the fossil record.”
This unusual evolutionary discovery was made possible not only by the find of the fossils themselves, but also thanks to new dates the team obtained for parts of Montana’s Two Medicine Formation.
(Read about another recent dino find: “Huge T. Rex Fossil, a ‘Fantastic Specimen,’ Unearthed in Montana“)
Using a technique known as radiometry, the team was able to more accurately date the rocky stratum near where the fossils were found, which made clear how distant in time the two dinosaur species really were.
“The new radiometric dates we measured from the Two Medicine Formation not only help support that D. torosus and D. horneri did not live at the same time, but also help us refine the timeline of environmental and ecological changes recorded by the Two Medicine Formation,” said the University of New Mexico’s Dr. Jason Moore.
Dr. Eric Roberts, a geoscientist at James Cook University, added, “New age dates presented in this study are just the tip of the iceberg.
“Ongoing work in this field will provide unprecedented improvements in the dating of Late Cretaceous dinosaurs from western North America over the next few years.”
The team reports their discovery in the journal Scientific Reports.