What could a dinosaur with teeth and a toothless dinosaur with a beak possibly have in common? Well, paleontologists say, they could actually be the same species. And that could tell us a lot about how that dinosaur lived throughout its life, and perhaps even yield clues into the evolution of different diets.
The 19 dinosaur skeletons in question were unearthed in the Gobi desert in China. When paleontologists pulled the 160-million-year-old bones out of the ground, they quickly realized they might have something special on their hands.
That's because some of the dinosaurs' jaws had teeth, some just shallow depressions that may have been tooth sockets, and some had no evidence of teeth but looked like they probably had a keratinous beak in life.
"It was puzzling because the juveniles had teeth, and the adults had no teeth," Josef Stiegler, a paleontology PhD candidate at George Washington University tells The Christian Science Monitor. Mr. Stiegler and his colleagues describe their study of the dinosaurs' teeth – and lack thereof – in a paper published Thursday in the journal Current Biology.
Not only did the more mature specimens lack teeth and seem to have had a beak, they also had rocks in their gullet, called gastroliths, that modern birds use for mechanical digestion. The younger ones didn't have that.
But despite having such different eating mechanisms, the animals shared a remarkable amount of other physical features. And, when Stiegler and his colleagues went to place the dinosaurs on a family tree, they realized that all 19 specimens were indeed the same species, dubbed Limusaurus inextricabilis.
Armed with that conclusion, the scientists took a closer look at the dinosaurs' jaws and figured out that the animals must have been losing their teeth by adulthood.
And this would suggest L. inextricabilis switched diets as it grew up, too.
The teeth in the juveniles would have been well-suited for a carnivorous, or at least omnivorous diet. So the babies were probably eating small insects, says James Clark, a co-author on the study and Stiegler's PhD advisor. But the toothless, beaked jaws of the adults would have been well-suited for an herbivore's diet, and the gastroliths would have helped them digest grains.
"Who would have thought that a dinosaur would start off with teeth and then replace them with a beak when it became an adult?" writes Stephen Brusatte, a paleontologist at the University of Edinburgh who was not involved in the research, in an email to the Monitor.
And, he says, "the change in anatomy seems to have enabled this dinosaur to change its diet as it grew, which probably allowed juveniles and adults to live together without competing for food."
Why the change in diet?
It might have something to do with those rocks the researchers found in the adult dinosaurs' gullets.
In order to have those gastroliths that aid in digesting grains and other things an herbivore might munch on, an animal must swallow rocks. "You're not going to be born with those," Stiegler says, "And you still have to be able to eat something."
Some animals, like birds, get around that problem by having the parents first digest food and then regurgitate it into the babies' mouths.
But "these things come from carnivorous ancestors," Stiegler says. "The group that Limusaurus is in is called Ceratosauria, and every other Ceratosaur that we know of has carnivorous-type teeth." So using this adaptation that already existed may have been the simplest way for the baby dinosaur to get its own food before it could ingest the rocks necessary to adopt a vegetarian lifestyle.
Writing a dinosaur's life story
In their study of the L. inextricabilis bones, Stiegler, Dr. Clark, and their colleagues looked at the internal structure of the bones to figure out just how old each dinosaur was when it died. When bones grow, they deposit growth rings – much like the rings of a tree, Stiegler explains.
In the assemblage, the oldest adult was 10 years old when it died, and the youngest ones were under three years old, including five infants.
The scientists discovered that the animals matured until they were 6 years old, but they lost their teeth by the time they were just a year old.
With 19 specimens, the researchers had 19 snapshots of that tooth-loss process. It began with the animal shedding its teeth. Then, although most reptiles shed and regenerate teeth continuously throughout life (a process that only happens once in humans), Stiegler explains, replacement teeth never formed in the jaws of these dinosaurs. Instead, the tooth socket closed over as if teeth had never been there.
From toothy dinosaurs to beaked birds
Limusaurus sits among theropod dinosaurs, a group that includes everything from the famous, fearsome Tyrannosaurus rex to birds (yes, those flying, feathered animals are the only living dinosaurs). So could L. inextricabilis tell researchers anything about how birds came to have toothless beaks?
There's a maxim " 'ontogeny recapitulates phylogeny,' or growth replays evolution," Thomas Carr, director of the institute of paleontology at Carthage College, who was not involved in the study, tells the Monitor in a phone interview.
The idea is that the growth an animal goes through in its lifetime could mimic the evolutionary path of a certain characteristic. For example, humans evolved from quadrupedal ancestors, and human babies still crawl around on all fours and then later walk on hind legs as adults, Dr. Carr says.
But there's a catch when comparing L. inextricabilis to birds. The animals belong to very different lineages within the vast group that is theropoda. So "this is definitely not on the way to bird beaks," Clark says in a phone interview with the Monitor.
So what is going on in birds?
"We know a lot about the mechanism of tooth-loss in birds," David Fastovsky, a paleontologist and chair of the department of geosciences at the University of Rhode Island who was not part of the research, says in a phone interview with the Monitor. "We know that the structural genes that code for teeth in birds are shut off basically before the heads of birds even form. They're shut off embryologically."
So that suggests the way that L. inextricabilis lost its teeth is an entirely different model from how birds evolved to their toothless, beaked state.
Still, Carr says, L. inextricabilis provides another model of how an animal might lose teeth.
This is a question close to Dr. Carr's own interests, as he has proposed that advanced Tyrannosaurs also lose teeth as they grow up.
But that happens in a different way, he explains. The space for teeth in a T. rex's mouth is probably a fixed length, but the animal's teeth get bigger during growth. As a result, they crowd each other out and so the dinosaur has fewer teeth in its mouth as it gets older.
Taken with the story of L. inextricabilis, Carr says, there may be a lot of ways to lose teeth.
And, as "several different groups of theropod dinosaurs evolve completely toothless beaks from fully toothed ancestors," David Evans, curator of vertebrate paleontology at the Royal Ontario Museum and a paleontologist at the University of Toronto who was also not involved in the research, writes in an email to the Monitor, "the detailed study of Limusaurus provides insight into the mechanisms of how this evolutionary transition might have occurred, and underscores just how quickly and easily these transitions could happen within theropods."
Still, the dramatic tooth loss that occurs within the lifetime of a L. inextricabilis is unique and surprising, Dr. Evans says.
"It's a very rare phenomenon," Clark agrees. "This is the first time it has been found in the fossil record," he says, but that could be because you need an extensive set of specimens that died as both juveniles and adults.
Such growth-related tooth-loss has been seen in a few living animals. The platypus, for example, is born with a few teeth, but loses them when it becomes an adult. The phenomenon is also known in a few species of fish and amphibians.
"This tells us to reexamine how we think about the evolution of tooth-loss," Stiegler says. "Limusaurus shows us is that we have to not only think about the evolutionary component, or the phylogenetic component, but also the ontogenetic [growth-related] component. How does the development of the animal figure into tooth loss [evolutionarily]?"