I have no friggin clue.
Can you help me think through this? Or, just let me know you read this by commenting? haha!
Think of it, it really is an interesting question to think about how and why evolution caused them to lose their teeth. Birds are one thing. It seems to be for them because it's lightweight <http://thesymbiont.blogspot.com/2010/03/why-do-all-birds-have-beaks.html> (but made turtles will make us completely rethink that!). But, why turtles? Is there something special about their diet? Was there a special need that they had being aquatic? Why don't they need to chew? Why just nip/cut? Were there other adaptations that made chewing superfluous?
Extra foder: pangolins and anteaters have lost most of their teeth in evolution, too. The earliest turtle fossils had small remnants of teeth.
Further thoughts written later:
I would suppose they'd lose them for two reason:
1) It was better to not have them. As in, birds are better off without tiny ballasts in their face...
2) They just didn't need them and evolution slowly parsed them away. So, why didn't they need them? They must have some other digestive adaptation that made mastication obsolete. Like what, though?
I love to think about this stuff. AND I love that there are websites like this that I can bounce questions off of bona fide experts.
A very helpful contrast to make between mammals (fast metabolism, fast digestion, much chewing) and reptiles (slow metab, slow diges and hardly any chewing). I can't think of a single reptile that chews a lot...anyone else?
It's cool to see different animals answer the same "question" in different ways (eg, how to survive off of what we have). A for instance: manatees and green turtles have a lot of diet cross over: sea grasses. And yet they do it in very different ways. One chews and the other doesn't.
Perhaps in the end, it should surprise me more that we, mammals, chew. Reptiles have found a much less costly way of doing it: just let the acid of your gut 'chew' it for several weeks.