Sports mascots aside, birds have no teeth in their bills. In fact, the absence of teeth has evolved in multiple vertebrates: turtles, anteaters, baleen whales, and pangolins, for example. And then there are mammals who have teeth, but they’re not covered by protective enamel; these are your aardvarks, sloths, and armadillos. In an evolutionary twist, all toothless and enamel-less vertebrates actually descended from ancestors with enamel-capped teeth, and for birds, that was a theropod dinosaur.
Instead of teeth, modern birds use their beak and a muscular gizzard to grind up food. Now, researchers comparing the genomes of 48 birds reveal that tooth loss occurred in the common ancestor of all modern birds around 116 million years ago. The findings were published in Science this week, as part of a special issue from the Avian Phylogenomics Consortium.
"Ever since the discovery of the fossil bird Archaeopteryx in 1861, it has been clear that living birds are descended from toothed ancestors," says Mark Springer from the University of California, Riverside. "However, the history of tooth loss in the ancestry of modern birds has remained elusive for more than 150 years." Did tooth loss happen in a single common ancestor or more than once in independent lineages?
Producing teeth involves many genes, and of these, six are needed just for the proper formation of dentin (the bony tissue) and enamel. So, Springer, Robert Meredith of Montclair State University, and colleagues examined the degraded remnants of these six genes in the genomes of 48 bird species representing nearly all living orders. In particular, they looked for the presence of mutations that inactivated those half dozen genes.
"'Dead genes,' like the remnants of dead organisms that are preserved in the fossil record, have a story to tell," Springer says in a news release. "DNA from the crypt is a powerful tool for unlocking secrets of evolutionary history."
All four dozen bird species share inactivating mutations in their dentin- and enamel-related genes. That means all the genetic machinery necessary for tooth formation was lost in the common ancestor of all modern birds. And that single loss of mineralized teeth, they found, occurred about 116 million years ago.
The team thinks that tooth loss and beak development evolved together in that common avian ancestor in two stages, beginning with the front part of the upper and lower jaws, followed by the back half. Fossil evidence suggests that some birds had beaks at the front of their mouths and teeth in the back, Washington Post explains. Beaks slowly began taking over the back of the mouth as well.
Images: Aija Lehtonen (top), rickyd (middle) via Shutterstock.com