You might not expect to find many similarities between a mammal and a reptile, particularly if they lived millions of years apart. But scientists have found that two such extinct beasts share a rare, distinctive facial feature.
An extinct relative of the wildebeest and a duck-billed dinosaur both had bizarre crests on their heads. But it wasn't the protruding bump that has most intrigued scientists, it's what they found beneath.
The bony crest is hollow, forming a trumpet-shaped nasal passage unlike any seen outside these two species. No other animal, living or dead, has been found with such a feature.
So how did two beasts from two very different taxa come to have such a mysterious commonality? Convergent evolution, scientists say in a paper published Thursday in the journal Current Biology.
"We have an animal that its skeleton looks a lot like a wildebeest – it's actually very closely related to modern wildebeests – but its face looks a lot more like something you would see if you went way back in time to the Cretaceous and looked at hadrosaur dinosaurs," study lead author Haley O'Brien tells The Christian Science Monitor in an interview.
Rusingoryx atopocranion, the mammal, lived about 65 thousand years ago, during the late Pleistocene, while Lambeosaurine hadrosaurs, the dinosaur, lived closer to 65 million years ago, during the late Cretaceous – and yet both animals evolved the same strange nose.
And not only do their nasal passages look alike, she said, the feature also appears to develop the same way as the animals grow up from juveniles to adults, as a variety of fossils display.
"When I first saw the complete skulls, I was blown away," vertebrate paleontologist David C. Evans, who was not part of the study, writes in an email to the Monitor. "The resemblance between Rusingoryx and some hollow-crested dinosaurs in the form of their nasal structures is truly striking, and there are clear parallels in how they evolved and grew. Both groups elongated their noses to such a degree that they evolved highly domed skulls to house their nasal passages on top of their heads, above their eyes."
Different origins, same result
"It's probably one of the best examples of convergence in large animals that I've seen in a long time," Ali Nabavizadeh, a researcher in evolutionary biology and anatomy at the University of Chicago, who was not involved in the study, tells the Monitor.
One was a mammal and the other a reptile, and millions of years elapsed between their tenure on Earth, but still, these animals developed the same adaptation.
Convergent evolution occurs when two species along different lineages independently evolve the same, or similar, features for the same function. One example is how insects, birds, and bats can all fly.
Convergence typically occurs when different species face the same ecological pressures. So what did Rusingoryx and the hadrosaurs have in common?
Both animals were herbivores and lived in herds. Rusingoryx was a ruminant and hadrosaurs have been called the cows of the Cretaceous, but the similarities, besides the shared nose, stop there.
Rusingoryx lived on the savanna, a dry wide open plain, while Lambeosaurine hadrosaurs were thought to have lived in a tropical rainforest.
Understanding this mysterious convergence might hinge on the purpose that these strange nasal passages served.
Without looking inside the animals' skulls, the crest might appear to be simply for visual display or some other external use.
"We have known for decades that visual display and physical combat have strongly shaped skull evolution in many groups of animals with elaborate horns and crests," Dr. Evans says. But the long, trumpet-shaped interior suggests a more complex purpose.
The hollow cavity, part of the respiratory tract, loops up over the animal's head and seems to connect to the vocal tract.
To determine the purpose behind this strange nose, scientists focused on the mammal's living cousins, wildebeests and antelopes. While researchers can look at their soft tissue for clues, all that's left of the dinosaurs is bone.
The unusual nose could have helped the animals smell, bugle, or even regulate their temperature, Evans says. "The case for vocalization as the primary function of the nasal dome in Rusingoryx is by far the most convincing, as the authors advocate."
The Rusingoryx are very social, says Ms. O'Brien. "They live in herds and they use a lot of vocal signals to communicate. When we looked into the function of what this skull type might be doing in Rusingoryx, we really couldn't prescribe a function outside of that social vocalization."
"There are obviously a lot of things that animals do with their faces," she says. "But we don't think that this crazy nasal dome would have really changed those more normal functions for this animal. We think that it was using the nasal crest to modify the way that it's producing these vocalizations and communicating."
That makes sense, says Thomas E. Williamson, curator of paleontology at the New Mexico Museum of Natural History and Science, who was not part of the study.
"When you have any kind of a tubing, it becomes naturally resonant," he explains. "So the idea that it's being used somehow to amplify certain frequencies of sound, it will do that,"
Not your average moo
O'Brien and her colleagues suggest that Rusingoryx, and perhaps the dinosaurs by extension, used this bizarre nasal dome to communicate at frequencies other animals cannot hear. This is called infrasound, and animals like elephants and cassowaries use it to communicate under the radar.
That's possible, says Dr. Nabavizadeh. "If you have a very gregarious group of animals and they're in a big arid, open environment, as these bovids are, then you are under the selective pressure to start to create more lower bellowing sounds that are possibly outside of the hearing range of carnivores, so they can communicate without being found in big open environments."
But the environment doesn't preclude the dinosaurs from needing this ability too, says Dr. Williamson. "Infrasound ... is able to travel over great distances and open areas and in closed environments. It pretty much goes everywhere," he says. And cassowaries, the living birds thought to communicate in infrasound, live in dense tropical rainforests.