When did the tooth loss occur in Birds?

A question that has intrigued biologists is: Were teeth lost in the common ancestor of all living birds or convergently in two or more independent lineages of birds. A research team used the degraded remnants of tooth genes in birds to determine that teeth were lost in the common ancestor of all living birds more than 100 million years ago. Birds are evolutionarily derived from theropod dinosaurs. Absence of teeth or “edentulism” has evolved on multiple occasions within vertebrates including birds, turtles, and a few groups of mammals such as anteaters, baleen whales and pangolins. Where early birds are concerned, the fossil record is fragmentary.DNA from the crypt is a powerful tool for unlocking secrets of evolutionary history.  All toothless/enamelless 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. The researchers examined these six genes in the genomes of 48 bird species, which represent nearly all living bird orders, for the presence of inactivating mutations that are shared by all 48 birds. The presence of such shared mutations in dentin and enamel-related genes would suggest a single loss of mineralized teeth in the common ancestor of all living birds. All edentulous vertebrate genomes that were examined are characterized by inactivating mutations in DSPP, AMBN, AMELX, AMTN, ENAM, and MMP20, rendering these genes non-functional.