WIKIMEDIA, NOBU TAMURAAt the height of the Jurassic Period, 19 dog-size dinosaurs drowned in a mud pit in what is now northwestern China. But 160 million years later, these Limusaurus inextricabilis specimens—aptly named “inextricable mud lizards”—would be declared the only known reptiles to lose their teeth after birth and develop bird-like beaks as they matured. This odd phenomenon, described in a study published in Current Biology last week (December 22), may shed light on the mechanism of beak evolution.
The discovery came after a team of biologists and paleontologists re-examined the 19 specimens and noticed marked differences in the fossilized facial features. “At first we thought they were different dinosaurs—one with teeth and one without,” coauthor Wang Shuo, an evolutionary biologist at Capital Normal University in Beijing, told CNN. “But they were largely identical and we found solid evidence that teeth were lost. There were empty tooth sockets in their jaw bones.”
While the younger specimens had all of the usual trappings of carnivorous, reptilian dinosaurs, the adults seemed to have lost their teeth entirely, while the most mature specimens had developed bird-like beaks. Some of the adult specimens even had rocks in their gullet, one of the telltale signs of modern bird digestion.
"Who would have thought that a dinosaur would start off with teeth and then replace them with a beak when it became an adult?” Stephen Brusatte, a paleontologist at the University of Edinburgh, U.K., who was not involved in the study, told The Christian Science Monitor. These different diets may have allowed adult and juvenile dinosaurs to live together without competing for food, Brusatte added.
While the findings may provide some information about how beaks could theoretically evolve, the authors stressed that further investigations are needed. Limusaurus is part of the vast group of theropod dinosaurs (which includes birds) but part of a distant lineage. “This is definitely not on the way to bird beaks,” coauthor James Clark of George Washington University told The Christian Science Monitor. Still, he added, “this is the first time it has been found in the fossil record.”