Were dinosaurs copycats? This ancient, domed skull suggests so.
A 230-million-year-old reptile may have looked like a dinosaur, but it wasn't one. What can it tell us about evolutionary patterns?
Dinosaurs might not have been so special after all.
"People always think that these dinosaurs that you see in the Jurassic and the Cretaceous are so unique and so different, and there's nothing ever like that before or after," says Michelle Stocker, a vertebrate paleontologist at Virginia Polytechnic Institute and State University. "But it's actually not true."
And a 230-million-year-old skull Dr. Stocker and colleagues pulled out of a drawer at the Texas Vertebrate Paleontology Collections at the University of Texas at Austin shows just that, she says.
Stocker and her colleagues came upon the skull when sorting through specimens collected across Texas by the Works Progress Administration in the late 1930s and early 1940s. "We were just going through the drawers and found this blob, essentially," she recalls in a phone interview with The Christian Science Monitor. "It was still covered in mud. It had never been washed off."
"Once we cleaned it even just a little bit, we knew it was something new," Stocker says.
New, but also familiar. When the paleontologists began to examine the skull, they quickly noticed the remarkable thickness of the roof of the skull. The dome-shaped skull struck them as particularly similar to those of animals that lived about 140 million years later: the pachycephalosaur dinosaurs.
And when the team CT-scanned the specimen, they also found three different zones of bone growth and structures of the brain case much like the pachycephalosaurs.
So did the paleontologists have on their hands a dinosaur that lived before its time? No, Stocker says. "It's missing some key features, certain features of the way the brain case is set up" to be a dinosaur, she says.
Instead, the specimen was a relative of dinosaurs and crocodilians, Stocker says. And that means this is a prime example of convergent evolution, a process by which two different animals independently evolve a similar feature.
The team dubbed the extinct animal, Triopticus primus, in a paper describing the specimen published Thursday in the journal Current Biology.
When Stephen Brusatte, a paleontologist at the University of Edinburgh who was not part of the study, saw a cast of the fossil, "my first inclination was that it was a dinosaur – a small dome-headed pachycephalosaur," he tells the Monitor in an email. And when he learned it was from the Triassic – over 100 million years earlier than those dinosaurs roamed – he was particularly confused. "Finding a small reptile with a domed head in Triassic rocks was totally unexpected. It would be like finding a monkey living in the cornfields of the Midwest."
"It tells us that dinosaurs didn't invent that body type, but rather they were just copying it from their ancient cousins," Dr. Brusatte says.
Evolution in the wake of extinction
Tropticus wasn't the only animal of its time with traits that reappeared in later animals. A Triassic reptile called a phytosaur looks just like crocodiles that live today, Stocker says. The only visible difference is the location of the nostrils. And, Stocker adds, ankylosaurus, a dinosaur that lived at the very end of the age of the dinosaur era, some 68 to 66 million years ago, also looked a lot like a Triassic animal, the Aetosaur.
And they're not alone, she says. Many other features found in fossils from the same site as Tropticus reappear in the Jurassic and the Cretaceous periods.
Stocker and her colleagues have an idea why body shapes might repeat during different time periods. Some 252 million years ago, Earth experienced the worst mass extinction in its history. The end-Permian mass extinction left countless ecological niches open for the remaining organisms, and this would have led to what's called an adaptive radiation, as the organisms evolved to fill those niches.
Then came another mass extinction, at the end of the Triassic period some 200 million years ago, starting the process all over again.
But why would so many body shapes and forms have reappeared so frequently? "Maybe there is some sort of underlying constraint into what kinds of shapes animals can evolve based on what their ancestors and close relatives were able to do," Stocker says. These constraints could be driven by something intrinsic to the animals themselves, perhaps in their genetics or developmental patterns, she says, or it could be driven by an ecological challenge.
One example of convergent evolution is flight. Birds do it, bats do it, and the extinct pterosaurs did it. But their wings were constructed quite differently, using different morphologies to accomplish the same feat.
Could a shared activity have driven the evolutions of the thick, domed skulls of Tropticus and the pachycephalosaurs?
"The massive overgrowth of the skull roof bones – 5 cm thick instead of only a few millimeters … this is not trivial in a modestly sized animal, and so must have had a function," Michael Benton, a paleontologist at the University of Bristol who was not part of the study, writes in an email to the Monitor. "The best, and probably only, function of the thickened skull would be head-to-head clashes in fighting, and the analogy is always the mountain sheep today."
Dr. Benton adds that the animals may not necessarily have used the thick skulls for the same purpose, but it's likely. "At least it suggests there is a good way that particular beasts can evolve to do particular things, and so independently different distantly related organisms follow the same lines of natural selection towards that structure."
Ongoing mystery of Tropticus
With just a partial skull, paleontologists have yet to paint a full picture of what Tropticus looked like. Without an intact snout, scientists can't even say whether the beast was carnivorous or herbivorous.
"We can say that it was in the smallish range compared with some of the other animals that are found with it there, probably between maybe 7 and 10 feet long," Stocker says. "A lot of these animals have really long tails as well, so a lot of Triopticus would be tail. But other than that, we can't say too much right now until we find some more."
Learning more about the 230-million-year-old reptile may not take traditional fieldwork. With many more fossils remaining to be studied from the Works Progress Administration excavations, perhaps more Triopticus bones are already in the drawers at the University of Texas or in another WPA collection, Stocker hopes.
Paleontologists are eager to find its jaws and pelvis, for clues into what it ate and whether it walked on two legs or four.
"Of course we would want to have a whole, articulated skeleton with baby Triopticus around it so we could see what the small ones look like in addition to the bigger one. We can cross our fingers for that one," Stocker laughs.