Pulling Bird's Teeth

The hummingbird's bill is perfectly adapted
to feeding on nectar within flowers

A defining feature of all birds is the beak. Two bony projections of the jaw covered by a thin, hard layer of keratin - the same protein found in hair and finger nails - it has evolved into a myriad of forms across the thousands of bird species alive today and in the fossil record.

The beaks of raptors are perfectly designed to cut into flesh whilst the delicate needles of hummingbirds are suited to reaching deep into flowers to extract sugar-rich nectar. From the challenges presented by feeding to the difficulties of grooming, from colorful displays to vicious weapons the beak has stood up to the pressures of natural selection.

Intriguingly, this coincides with the absence of teeth. In other jawed vertebrates not only has the jaw evolved depending on diet and lifestyle, but changes to the teeth increased feeding strategies. A few other vertebrate species, such as whales and pangolins have also lost their teeth, yet birds are the only group to have lost them entirely, relying on changes to the beak's shape over the past 116 million years for continued survival. 

An embryonic mouse tooth. The genes involved in dentin
and enamel synthesis are vital to its formation 

The birds' antecedents, the theropod dinosaurs, had teeth composed of dentin covered by a layer of enamel with a cavity at the centre for the pulp and nerve. This structure is much the same across all teeth in the vertebrate family.

Tooth formation itself is controlled by a complex set of genes. Of these genes, six are essential for the proper formation of dentin (DSPP) and enamel (AMTN, AMBN, ENAM, AMELX, MMP20). By examining these genes across 48 bird species representing most living groups of avians, researchers from the University of California, Riverside and Montclair State University, New Jersey, found a common set of mutations which rendered these genes inactive. The sequences were then compared to find out when these mutations arose.

The results showed that birds lost the ability to manufacture tooth enamel around 116 million years ago. Without this mineral layer which is responsible for giving teeth their durability (it is thanks to enamel that they can survive bones and last in perfect shape for hundreds of millions of years), the entire tooth structure was gradually lost, making birds reliant on the beak. On the basis of fossil and molecular evidence, the researchers proposed a two-step scenario whereby tooth loss and beak development evolved together in the common ancestor of all modern birds. In the first stage, tooth loss and partial beak development began on the anterior (front) portion of both the upper and lower jaws. The second stage involved concurrent progression of tooth loss and beak development from the anterior portion of both jaws to the back of the rostrum.

The fossil record of early birds show that they once had teeth. Archaeopteryx, the common ancestor of all birds both living and extinct, had sharp, dinosaur-like teeth. Later species in extinct groups, for example the 70 million year old Hesperornis, retained their teeth whilst modern bird groups had already lost theirs. As an addition to their study the researchers conducted genetic studies of other toothless vertebrates and found that the similar mutations in the same six genes responsible for dentin and enamel formation had resulted in their inactivation, showing a common genetic root for tooth loss (edentulism).

 'One of the larger lessons of our findings is that 'dead genes,' like the remnants of dead organisms that are preserved in the fossil record, have a story to tell,' said Mark Springer, Montclair State University. 'DNA from the crypt is a powerful tool for unlocking secrets of evolutionary history.'