Before Tyrannosaurus rex terrorized Cretaceous North America, another frightful lizard ruled Montana. Some 75 million years ago, Daspletosaurus horneri, at nearly 30 feet long and over seven feet tall, dominated the landscape.
And although D. horneri doesn't have the movie star reputation of its younger, larger cousin, T. rex, its bones could be revolutionary pieces to the puzzle of dinosaur evolution and our picture of the family of tyrannosaurids.
"With this dinosaur, we've literally changed the face of tyrannosaurs," paleontologist Thomas Carr says of D. horneri.
Dr. Carr and colleagues don't just describe and name the new dinosaur species in a paper published Thursday in the journal Scientific Reports. They also investigated what the face of these beasts may have looked like in life, blood, flesh, scales, and all.
And it turns out, these tyrannosaurs may have had patches of armor-like skin across their faces, horns covered in a hard, protective layer much like fingernails or bird beaks, and a highly sensitive snout protected by flat scales.
This would have made their snouts a lot like those of dinosaurs' close relatives: crocodilians, Carr and his coauthors say. In crocodiles, sensitive snouts help them sense tiny vibrations in murky waters or measure the temperature of nests.
"Crocodiles' faces are as sensitive as human fingertips," Carr, who is director of the Carthage Institute of Paleontology and a professor at Carthage College in Kenosha, Wis., says in a phone interview with The Christian Science Monitor. "Basically their entire face is a fingertip. And we are floating the hypothesis that tyrannosaurs were no different."
Giving evolution a facelift
It's not just the faces of dinosaurs that D. horneri might be changing. It could also help reshape biologists' ideas of how evolution works.
That dinosaur was first mentioned in scientific literature in 1992, and eventually was known by the nickname 'Two Medicine tyrannosaurine' among paleontologists for the site where it was found. But D. horneri wasn't officially named as a species until now, 25 years later. The animals' bones had been shelved.
When Carr first began to examine the bones, the first task was to find out if this was indeed a new species of dinosaur, or if it was perhaps a transitional species, perhaps between D. horneri's closest relative, Daspletosaurus torosus and T. rex.
D. horneri is in fact distinct enough in its morphology to be a new species, Carr and his colleagues say. So they gave it a species name. The name Daspletosaurus horneri, or "Horner’s Frightful Lizard", honors famed American paleontologist John R. "Jack" Horner.
But that presents a new dilemma. D. horneri is younger than D. torosus, but the two are quite closely related, likely roamed the same lands (D. torosus was unearthed in Alberta), and lived perhaps just 100,000 years apart – geologically "a blink of an eye," Carr says.
Scientists think the most common mechanism for speciation is cladogenesis, in which new species branches off from ancestral species, typically due to geographic isolation or some other force. But Carr and his colleagues think this is a case of anagenesis, when one species simply evolves into another another, without the branching.
"That's not easy to show in the fossil record," Lawrence Witmer, a paleontologist at Ohio University who was not involved in the research, says in a phone interview with the Monitor. But Dr. Witmer thinks Carr and his team made the case well.
But, cautions Hans-Dieter Sues, curator and chair of vertebrate paleontology at the National Museum of Natural History of the Smithsonian Institution who also was not part of the research team, "The fossil record for most animals, especially dinosaurs, is far too meager to confidently make inferences regarding anagenesis."
"The authors’ interpretation is plausible but difficult to test rigorously unless there are much larger samples," he writes in an email to the Monitor.
That may be changing, says Philip Currie, a paleontologist at the University of Alberta who was not involved in the research. "There wasn't a lot that could be said before about anagenesis. But now certainly there's enough tyrannosaur specimens that have been found, and especially this group of things from Montana that seem to be different from the ones in Alberta," he says in a phone interview with the Monitor. And "the radiometric dating has improved so much so you can be a lot more specific about what level these things are coming from."
And it's not just tyrannosaurs, Dr. Currie says. Some paleontologists are pointing to evidence in the fossils of anagenesis in other families of dinosaurs, like hadrosaurs and ceratopsian dinosaurs.
"Most people in recent years have tended to think that cladogenesis is the way that evolution tends to work most of the time," Currie says. But research is beginning to show that that might not be the case.
"We think it's worth investigating just how widespread anagenesis might be," Carr says. "We really don't know.
To help with that, Carr and his colleagues hope their evidence for suggesting D. horneri might be the product of anagenesis could help shape criteria for other paleontologists to look for in the fossil record.
Does D. horneri really suggest that tyrannosaurs had crocodile-like faces?
Carr's investigation of tyrannosaur facial flesh began with the bones of D. horneri.
The tyrannosaur expert and his team compared the texture on the dinosaur's skull bones with the bones of other animals and found that the roughness on D. horneri matched the roughness on crocodilian snout bones. Because the skin overlying the bones in crocodilians actually changes the texture of the bone itself, Carr says, the scientists can use that as clues to figure out what soft tissue was where on the dinosaur's face.
If the parallel between the dinosaurs and crocodilians that Carr and his colleagues draw is correct, this could provide intriguing insight into how the dinosaur's scales develop, suggests Paul Gignac, a paleontologist at Oklahoma State University who was not involved in the research.
"Crocodiles grow facial scales by actually cracking their skin, like drying paint, because the underlying bone grows faster than the skin above it," Dr. Gignac explains in an email to the Monitor. "The parallel that Carr and colleagues draw between tyrannosaurs and crocodiles suggests a similar kind of development, which we previously thought was unique to crocodiles and their kin, but that may no longer be the case."
Carr and his colleagues took the parallel a step further and, pointing to small holes in the facial bones thought to provide channels for blood vessels and nerves, suggest that D. horneri's snout was as sensitive to touch as crocodilian snouts. They argue that the arrangement of these holes, called foramina, and the texture of the bones are clues that the tyrannosaur had the same kind of specialized sense organs found in crocodilian snout skin called integumentary sensory organs (ISOs).
"The authors interpret the openings on the tips of the snout and lower jaw of tyrannosaurs as transmitting nerves that provide sensation, and this make a lot of sense," Gignac says. "The major nerve that allows people to sense touch on their faces ... is the same nerve that allows birds to sense touch and temperature on their beaks and crocodiles to sense pressure waves under water."
"This nerve ('the trigeminal nerve') also innervates the whiskers in cats and dogs," he adds. "It is a ubiquitous and important neural component of how vertebrates interact with their environments. So, it seems apropos that evolution would also equip an apex predator – one thought to engage in social, head-biting behaviors, just like crocodiles – with a touch-sensitive snout that would, for example, reinforce cues of social dominance and weakness."
But not everyone is sold on this hypothesis.
"I question the authors' inferences regarding crocodile-like facial sensitivity in tyrannosaurs," Dr. Sues says. "Lizards have numerous tiny openings in their jaw bones for the passage of nerves and blood vessels that supply the superficial tissues of the snout," as do other animals as well, he points out.
"It's certainly not just dinosaurs, certainly not tyrannosaurs and crocodiles" that have these holes in their facial bones, Currie agrees.
And, the vastly different lifestyles of the beasts adds to both Sues and Currie's skepticism. "Most crocodilians and their extinct relatives are/were semi-aquatic and have highly specialized receptors for detecting pressure changes in water. (Not surprisingly, certain land-adapted extinct crocs lack these structures.)," Sues says.
During the Cretaceous, Montana would have looked a bit different than it does today. A shallow sea called the Western Interior Seaway ran through the middle of North America at the time.
"D. horneri lived on the coastal plain, which was forested and carved by an extensive stream system," Carr writes in an email. "The climate would have been comparable to, say, modern-day Mississippi."
Carr admits that more research needs to be done. "We really need to get a handle on the distribution, density, and number of these foramina. I think this really needs to be quantified to make the comparisons a bit more rigorous," he says.
But ultimately, Carr says, "What we've proposed as a hypothesis, and what will really test it is the discovery of a tyrannosaur with its skin preserved on its face. And I think that day will come, and that will be the test of our hypothesis."
"I really hope someone finds a T. rex mummy," he says. "Wouldn't that be great?"
[Editor's note: This article was updated to add details about the Cretaceous environment.]