Some baby dinosaurs lost their chompers and never got them back

If the Tooth Fairy operated during the Jurassic, the dinosaur called Limusaurus would have kept her busy.

Scientists have discovered that the pony-sized Limusaurus lost all its teeth as it grew from hatchling to adult, the first known reptile to do so. A Limusaurus hatchling came into the world armed with at least 42 sharp little gnashers, but an adult had only a toothless beak, according to a new study. One explanation for the tooth loss: the beaked adults were vegetarians, their toothy offspring omnivores.

The find is “fascinating,” says Gregory Erickson of Florida State University, who was not involved with the study. Most dinosaurs gained new teeth as they got older, but “here we’re seeing it go the other way, where they start off with teeth and just abandon them.”

As an animal that dispensed with all its teeth at it matured, Limusaurus was definitely the weird kid in class. Dinosaurs generally had more teeth as adults than as hatchlings, Erickson says. And they didn’t just add teeth as they grew; many also constantly swapped out their existing teeth for new teeth. Champion tooth-grower Nigersaurus, for example, replaced each tooth as often as every two to four weeks, according to a previous study, and Erickson has found a number of dinosaurs that replaced their teeth every month or two.

So when the first Limusaurus skeletons were discovered in China some 15 years ago, paleontologists were confused. Researchers thought at first that they’d found two very similar species, one with teeth and one without, says study author Shuo Wang of China’s Capital Normal University. They named the one with a beak Limusaurus inextricabilis, or inextricable mud lizard, because the little animals had died in mud holes that probably formed from the footsteps of bigger animals, such as giant dinosaurs.

But a closer look at the fossils, which date to roughly 160 million years ago, showed that all the specimens belonged to just one species. “We couldn’t believe it,” Wang says.

The specimens ranged in age from hatchling to almost 10 years old and in size from 20 inches long — about as big as a cat — to well over 6 feet. The youngest animals, those less than a year, would’ve had more than 40 teeth, Wang and his co-authors report in this week’s Current Biology. Their slightly older siblings would’ve had 34 teeth. By the time they were teenagers and adults, ages 2 to 6, their teeth had all dropped out, never to return.

The youngest animals’ skeletons bear the chemical signature of a varied diet, which might have included insects and small lizards as well as plants, Wang says. The older animals’ bones, however, hint that they munched mostly on plants. The older animals also have “stomach stones,” collections of pebbles and gravel in their bodies that would’ve been useful for grinding up tough plant materials.

The study is “very compelling,” Erickson said. Scientists have found other toothless dinosaurs, as well as dinosaurs with puny remnant teeth, and the new finding “opens up the door to rethinking what some of these other dinosaurs were doing.” It’s not a bad accomplishment for a bunch of little dinosaurs that got stuck in the mud.