Researchers have unearthed a fossil of a robin-sized bird (Sulcavis geeorum) from the Cretaceous Period in China that had teeth! This species belonged to a class of birds with teeth (Enantiornithines) that were plentiful in the age of the dinosaurs. However, the teeth of this well-preserved specimen were different. The teeth were sharp and had serrated ridges. The researchers think the ridges observed on the teeth were designed to crack open insects with hard shells, snails or perhaps even crabs. What I also found interesting about this study was the mention that modern birds still have the genes for teeth but, for whatever reason, those genes are still turned off. In fact, the report mentions that birds have lost their teeth at least 4 times according to fossil records. The traditional theory according to the author, Chiappe, is that teeth are heavy so birds evolved beaks which are lighter.
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O’Connor, J.K., Y. Zhang, L. M. Chiappe, Q. Meng, L. Quanguo, and L. Di. 2013. A new enantiornithine from the Yixian formation with the first recognized avian enamel specialization. Journal of Vertebrate Paleontology 33(1):1-12.
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It is not impossible that teeth genes could be switched back on again, I suppose, but the modern beak would not accomodate them well. Once the body has evolved to be toothless, it must be hard to regain them. This is surely a good example of evolutionary channeling. I was at the Holzmaden Museum in southern Germany at Christmas, where there are great fossils from the same period.
I doubt that this is also the explaination for why turtles have no teeth. ;-)