The turtles and birds are the only living archosaurian animals today with a “beak,” a keratinized rhamphotheca covering the margins of the jaw [n1]. These features are associated with a lack of teeth, which has caused a question of correlation: Whether the absence of teeth in fossils indicates the presence of a beak. I talked about assessing this correlation for turtles here, but today I am expanding on the other taxon I mentioned here, Amazona ochrocephala ochrocephala.
Unlike the turtle, the parrot’s beak here is much, much thinner in cross-section. The dorsal lamina of the rhamphotheca, covering the dorsal surface of the mandibular symphysis, is parallel to and close to the ventral lamina. Similarly, the rostral margin of the bony region of the symphysis is highly porous, thus the shape of the rostral bony beak doesn’t match the rhamphotheca overlying it. This results in a larger extension of the rhamphothecal portion of the beak than the bone. It further suggests that turtle beaks are easier to “guess” the shape of the living beak than in parrots.
Caveat: Not all birds match this morphology, this holds primarily for parrots, and in some birds the beak matches the bone more closely.
[n1] As Jerry Harris in the comments below points out, of course I failed to mentioned that more than just turtles and birds have “beaks,” keratinized structures, hard or soft, formed around or within the oral tissues. Eventually I should get to covering them too, and soon.
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