Some 116 million years ago, a common ancestor to all modern birds had teeth.

According to LiveScience, authors of a new study published in the journal Science analyzed deteriorated tooth genes. Other than birds, edentulism, which is the absence of teeth, is most notable in vertebrates and mammals such as turtles, anteaters and baleen whales.

"One of the larger lessons of our finding is that 'dead genes,' like the remnants of dead organisms that are preserved in the fossil record, have a story to tell," study lead author Mark Springer, a professor of biology at the University of California - Riverside, said in a press release. "DNA from the crypt is a powerful tool for unlocking secrets of evolutionary history.

"Ever since the discovery of the fossil bird Archaeopteryx in 1861, it has been clear that living birds are descended from toothed ancestors," he said. "However, the history of tooth loss in the ancestry of modern birds has remained elusive for more than 150 years."

Therapod dinosaurs are the ancestor of modern birds that had teeth. Over time, they no longer relied on teeth to mash up food for digestion and instead did so with their horned beak, which is the norm for birds.

Of all the genes involved in forming teeth, the researchers found six that are directly responsible for dentin and enamel. For their study, they examined those six genes in 48 different living species for a wide representation of modern birds.

"All edentulous vertebrate genomes that we examined are characterized by inactivating mutations in DSPP, AMBN, AMELX, AMTN, ENAM, and MMP20, rendering these genes non-functional," Springer said. "The dentin-related gene DSPP is functional in vertebrates with enamelless teeth - sloth, aardvark, armadillo. All six genes are functional in the American alligator, a representative of Crocodylia, the closest living relatives of birds, and mammalian taxa with enamel capped teeth.

"The new bird genomes represent a major advance given that only a handful of bird genomes - zebra finch, turkey, chicken and duck - were previously available."