It’s interesting to note that that beaks of animals come in all sorts of strange shapes, including hooks and serrations. Sometimes, none of this matches the underlying bone structure. In many birds and turtles, the rostral tip of the bony beak will be incredibly short and different in shape from the overlying rhamphotheca. This often coincides with large numbers of pits and foramina. As the above illustration shows above, this is not limited to the tomial margins or tip, but to the palatal surfaces of the beaks. Extending medially in some taxa (not all sea turtles look like Caretta caretta, above; some have much smaller extents to their rhamphotheca) the rhamphothecal plate forms a broad platform that contacts its opposite side and forms a V-shaped structure. I found that when this happens, the surface of the underlying bone attains a “porous” appearance: numerous foramina penetrate the bone, sometimes deeply through the cortical bone.
This allows me to hypothesize: 1) The density of the foramina appears to match the depth of the overlying rhamphotheca, so that the more numerous the pitting or foramina, the less likely the rhamphotheca matches the underlying surface. 2) Conversely, the rhamphotheca is more likely to be smoother and match the underlying bone’s contours and shape when there are few, if any, foramina.
In Caretta ceretta, the upper palatal rhamphotheca and the lower mandibular rhamphotheca largely match one another, forming a broad V-shaped form in transverse view, while on their posterior edges the rhamphotheca forms a high, slightly arcuate ridge that runs transversely, forming something of a secondary, rear “beak”. It is fascinating to note why this occurs, and when secondary rhamphothecal ridges occur in turtles, what dietary or behavioral habits have to do with their appearance. We can formulate further questions, such as:
What does more foramina have to do with more extensive rhamphotheca? Is it related to higher layers of keratin, or perhaps it is general to keratin itself and we can expect it in other keratinous coverings, such as nails, horns and claws?
Can we predict the shape of the rhamphotheca when all we have is the underlying bone?
Can we predict the appearance of serrations, hooks, “nails” or even the extent of the tomial height or really, the presence of a tomia from the underlying bone?
Can we predict diet and infer other functional features from these shapes?
I certainly would like to find out. In oviraptorosaurs, the upper rostral beak often have unique serrations on them in the absence of teeth, but not really on the lower jaw. Is it related to preservation, with looser, spongier bone being lost, or perhaps it is enforcing the shape the bone takes, without deviating at all? New data is forthcoming, as research from a few quarters is working to resolve this question, across a broad range of taxa (birds, turtles, and so forth).
It is interesting to note that it may be very possible that the outward appearance of the bony beak will have nothing to do with the actual shape of the living beak, or at the least that the more porous the bone the least consistent the rhamphotheca has to the shape of the jaw. It is interesting to note that in some birds where the beak most strongly deviates from the underlying bone (in the kiwi and other ratites, and ducks, for example) the rostral beak is formed into a sort of “nail” and the underlying soft tissue is preoccupied with specialized organs called Herbst corpuscles which may allow the bird to sense the movements of organisms in soft sediments, and as such are electrosensory. While these form in noticeable pits in such birds, including herons and the like, such appear to be absent in most dinosaurs. Instead, the bony features appear to relate to external superficial “tracts” on the bone surfaces, indications that the foramina are points of invagination for external nervous or nutrient tissues, which might instead suggest a close-adherence of keratinous tissue, rather than a thicker, less regular rhamphotheca.
Whatever of these options may hold true for extinct taxa for which close analogues do not exists, the investigation should certainly be interesting.
Clark, J. M., Norell, M. A. & Rowe, T. 2002. Cranial anatomy of Citipati osmolskae (Theropoda, Oviraptorosauria), and a reinterpretation of the holotype of Oviraptor philoceratops. American Museum Novitates 3364:1-24.
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