Limusaurus inextricabilis was a dinosaur that lost its teeth during its growth

Limusaurus inextricabilis with the juvenile's teeth (Image courtesy George Washington University)
Limusaurus inextricabilis with the juvenile’s teeth (Image courtesy George Washington University)

An article published in the journal “Current Biology” describes a research on a dinosaur with characteristics similar to birds called Limusaurus inextricabilis. A team of researchers studied 19 specimens ranging from babies to adults discovered in today’s Xinjiang province in China to analyze how they developed teeth and then lost them over time.

Limusaurus inextricabilis was a very small dinosaur with a length that could reach 170 cm (5′ 7″) and a weight that could reach about 15 kg (33 lbs). They belonged to the ceratosaurs (Ceratosauria) group and, while they were a rather ancient and primitive species, had several characteristics similar to those of birds. Ceratosaurs were theropod dinosaurs, just like those that evolved into birds and in some cases even species that are distantly related may show similar characteristics.

The first specimens of Limusaurus inextricabilis were discovered in 2009, at the time only two subadults. The big step forward arrived with the discovery of 17 more specimens of various ages divided into three groups: six were juveniles, ten were subadults with age estimated between 2 and 6 years and one adult. They lived about 159 million years ago, during the Jurassic period.

The study of the teeth showed the most distinctive feature of Limusaurus inextricabilis because the youngest had small sharp teeth while adults and subadults were completely toothless. These differences made the paleontologists believe they had discovered two species of ceratosaurs, so much so that they started describing them separately. However, in the course of their study, they realized that the specimens showed many similarities except for the teeth and concluded that it was a single species that lost their teeth throughout its life.

This is an unprecedented discovery regarding dinosaurs and in general fossil vertebrates. Today similar changes occur in the platypus and in some species of fish and amphibians. However, these are rare cases so the study of Limusaurus inextricabilis is even more important and suggests that these dinosaurs radically changed their diet from adolescence to adulthood.

The analyzes of the fossils, also at isotopic level, indicate that Limusaurus inextricabilis juveniles were probably carnivores or omnivores while the adults were herbivores. This discovery is interesting also because it’s possible that such a thing happened to the ancestors of birds as well which may explain why they have no teeth.

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