Friday, December 26, 2014
Birds Lost Their Teeth 116 Million Years Ago, Scientists Say?
A group of genetic researchers led by Prof Mark Springer from Montclair State University has found that teeth were lost in the common ancestor of all living birds about 116 million years ago (the end of the Early Cretaceous).
All toothless vertebrates are descended from an ancestor with enamel-capped teeth. In the case of birds, it is theropod dinosaurs. Modern birds use a horny beak instead of teeth, and part of their digestive tract to grind up and process food.
Tooth formation in vertebrates is a complicated process that involves many different genes. Of these genes, six are essential for the proper formation of dentin (DSPP) and enamel (AMTN, AMBN, ENAM, AMELX, MMP20).
Prof Springer and his colleagues form Denmark, China, Australia and the United States examined these six genes in the genomes of 48 bird species for the presence of inactivating mutations that are shared by all these birds. The presence of such mutations in dentin and enamel-related genes would suggest a single loss of mineralized teeth in the common ancestor of all living birds.
The scientists found that the 48 bird species share inactivating mutations in both dentin- and enamel-related genes, indicating that the genetic machinery necessary for tooth formation was lost in the common ancestor of all modern birds.
“The presence of several inactivating mutations that are shared by all 48 bird species suggests that the outer enamel covering of teeth was lost around 116 million years ago,” said Prof Springer, who is the senior author of the paper published in the journal Science.
On the basis of fossil and molecular evidence, the team proposes 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 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.
“We propose that this progression ultimately resulted in a complete horny beak that effectively replaced the teeth and may have contributed to the diversification of living birds,” Prof Springer said.
The scientists also examined the genomes of additional toothless / enamelless vertebrates including three turtles and four mammals – pangolin, aardvark, sloth, and armadillo – for inactivating mutations in the dentin- and enamel-related genes. For comparison, they looked at the genomes of mammalian taxa with enamel-capped teeth.
“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,” Prof 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.”