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These weird little dinosaurs lost all their teeth and grew beaks as they aged, new science says

It’s a nightmare-caliber metamorphosis

Artist’s rendering of limusaurus
Image courtesy of Yu Chen

In a fossil first, paleontologists have discovered that a dinosaur related to the T. rex and modern birds started out life with a full set of chompers, but grew up to lose them all. In place of pearly whites, the adults sprouted a beak instead.

It’s a strange metamorphosis, like an extra twisted, dinosaur-version of that nightmare where all your teeth fall out. And it’s the first time this phenomenon has ever been seen in living, dead, or extinct reptiles — which typically replace lost teeth with new ones. In fact, only a couple of animals, like the platypus and a few species of fish, go through this kind of developmental tooth loss. And until this discovery, no dino in the fossil record lost its teeth like this to grow a beak.

The findings, published Thursday in the journal Current Biology, could possibly help scientists understand how bird beaks evolved. But these dinos didn’t directly give rise to birds — they’re more like great aunts and uncles. So it’s also likely that they’re an evolutionary oddity with little bearing on modern bird beaks, David Evans, curator of vertebrate paleontology at the Royal Ontario Museum, told the Christian Science Monitor.

Called Limusaurus inextricabilis, which means inextricable mud lizard, these dog-sized dinosaurs once roamed northwestern China during the late Jurassic between 156 and 161 million years ago. Many died in muddy pits, which some scientists think were formed when larger dinosaurs wallowed in the mud, like pigs. The small limusaurus dinosaurs weren’t able to extricate themselves from the sticky mire and died there, emerging millions of years later as fossils.

Enough of these dinosaurs died in those mud pits that Xing Xu, a paleontologist at the Chinese Academy of Sciences, and his colleagues were able to recognize that different specimens looked like they were different ages. Of 19 different individuals, they were able to group 13 into different age groups from hatchling to adult.

Now, scientists already knew these dinosaurs were weird: they’re the only ones from the Jurassic period to have both a beak and a gizzard. (A gizzard is a pouch in modern birds’ stomachs that they use to grind up tough foods, with the help of grit they eat.)

But when the researchers compared the Limusaurus specimens across the different age groups, they discovered something even weirder. The hatchlings had a full set of teeth. But as they grew older, the teeth disappeared — leaving behind empty sockets in the jawbone. The researchers think the young dinos’ teeth could probably tear into meat. That’s also weird, because the adults most likely ate a vegetarian diet.

The development from toothy juveniles to toothless and beaked adults probably let dinos young and old live together without competing for food. And with more food options, the young animals were more likely to get enough food to survive when resources were scarce. They just needed to stay far, far away from mud puddles, if they wanted to make it to adulthood.