It may be hard to believe, but a 27-foot long meat-eating dinosaur that died in Montana about 75 million years ago had a sensitive side.
Named Daspletosaurus horneri, the ancient animal’s snout and jaws were perforated with hundreds of small openings through which nerves passed, making the front of its face as responsive to touch as a human’s fingertips or an elephant’s trunk, according to recently published research. Such sensitivity makes sense if you consider how a tyrannosaur was built.
"Being a tyrannosaur, they had really small arms," said Jason Moore, a University of New Mexico Honors College professor and co-author of the paper. "They wouldn't be able to interact with their environment with their hands the way mammals do — find food, build nests, tend to eggs and young. In order to do these things, Daspletosaurus needed to use its feet or head.”
"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 Jayc Sedlmayr, LSU Health New Orleans anatomist and co-author of the paper.
Examples of these "sixth senses" include:
- Birds sensing magnetic fields to migrate.
- The ability of a platypus to sense electric fields (called electroreception) to locate and attack prey.
- Pit vipers' use of infrared to find food.
What’s in a name
The tyrannosaur’s name translates to Horner’s Frightful Lizard, in honor of Montana State University emeritus paleontologist and retired Museum of the Rockies curator Jack Horner. The tyrannosaur’s prey were other horned, crested duckbill, dome-headed and smaller theropod dinosaurs. It is believed to be the last species of frightful lizards to have evolved in the American West.
It was on one of Horner’s expeditions to the Cut Bank area, east of the Rocky Mountain Front, in 1989 that Vicki Clouse, now an assistant professor at MSU Northern, found the first horneri fossil, a sub-adult skull and skeleton. Then in 1998, David Varricchio, who is now a Montana State University associate professor of paleontology, and a friend discovered and excavated an adult horneri skull and skeleton along the Sun River. Varricchio also found a partial lower jaw and isolated bones of sub-adults and juveniles that he excavated as part of his doctoral studies.
“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,” Varricchio said.
It wasn’t until the fossil was cleaned, a difficult and slow task since the bone was soft and the rock was hard, that its unique features were revealed.
“One volunteer, Jamie Jette, worked very diligently for several years on the MOR 1130 specimen,” Varricchio 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.”
Once cleaned, examination of the fossil by scientists revealed the dinosaur’s other facial features, as well. The creature had no lips, many flat scales and tough patches on its face.
The findings were published in the March 30 edition of “Scientific Reports.” Thomas Carr, director of the Carthage Institute of Paleontology at Carthage College in Kenosha, Wisconsin, is lead author of the paper, which is titled: “A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system.” Carr is an expert on the evolution and growth of Tyrannosaurus rex and its closest relatives, collectively called tyrannosaurs.
The team compared the Montana specimens with those of crocodylians, birds and mammals as well as earlier research that matched bone texture with different types of skin covering.
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"Much of our research went beyond field paleontology — it was generated from lab-based comparative anatomy, the dissection of birds as living dinosaurs and crocodilians as their closest living relatives, and based on the similarities of the facial nerves and arteries we found in those same groups, which left a trace on the bone," Sedlmayr said. “We were able to then reconstruct in the new tyrannosaur species.”
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.
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 of keratin, the same shiny material in human fingernails and the hard core of pronghorn antelope horns.
"In some ways, the facial components of the trigeminal nerve of these dinosaurs mirrors that of humans," Sedlmayr said. "The human trigeminal nerve provides significant touch sensitivity to the face. It brings back sensation from our facial muscles allowing us to fine tune and coordinate the emotional and social displays so important to human communication. This nerve is so sensitive that in pathological conditions, trigeminal neuraligia, it can be responsible for some of the most severe pain our species can endure; in extreme cases, the pain is so great that many people suffering from it end up committing suicide."
The research paper also focuses on the tyrannosaur’s unique process of evolution.
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.”
The research confirms that the evolution of the dinosaur was slow — happening over a span of 2.3 million years.
"One of the difficulties in demonstrating this style of evolution is establishing that the different species don't overlap in time,” Moore said. “The new radiometric dates we measured help support this temporal separation between D. torosus and D. horneri."
Although such evolution is rare, Carr said 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.”
Eric Roberts, professor of geosciences at James Cook University in Australia, also contributed to the research.
Varricchio said that the horneri specimens, which are housed at the Museum of the Rockies in Bozeman, emphasize the excellent record of dinosaurs 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.”