A quick post, on what I would prefer to be a “big post” day, due to focus on my oviraptorid study and other projects (but still about jaws and stuff). This one is also about jaws — surprise! — in a few groups of birds. Note that this project is generally focusing on broader variation of jaw shape and the associated keratinous coverings (rhamphotheca) that are associated with them.
As in earlier examples of avian rhamphotheca, this represents the mandible in labial (lateral), superior or oral (dorsal) and lingual (medial) views vertically, while horizontally they are the rostral half of the mandibular ramus (with fused symphysis split down the middle), rhamphothecal sheath in position over the mandible, the rhamphothecal sheath isolated, and then the mandible again but this time with an underlay of the rhamphothecal outline to describe the shape of the beak to the underlying bone.
First up, Procellariiformes — or, more accurately, Diomedea immutabilis (the Laysan albatross):
Procellariiformes have complicated rhamphotheca, with many distinct divisions indicating the various plates that comprise the rhamphotheca, here demonstrating the two mandibular plates, the rostral mandibular nail (sometimes called the “mental” plate) and the ramicorn, which forms the bulk of the rhamphotheca and are divided from one another by the mentolabial fold. In this bird, there is a small ventral projection of the nail that underlies the beak, which itself has a bony “tongue” that slides into a grove on the nail’s surface. The slot between the rhamphothecal “tongue” and the ramicorn is called the “psuedomental fold.” Terminology follows Hieronymous & Witmer (2010).
The “spout-shaped” rostral mandible accomodates the shape of the upper beak, which forms a rostral “hook” that matches the contoured aspect in lateral view, a feature that distinctly departs from the shape of the beak in parrots, in which the mandible forms a dorsally sigmoid curve that does not match the ventral concave margin of the upper jaw in lateral view. A broad lingual platform is also visible forming a narrow oval shape in aspect along the sheath’s lingual margin; this feature closely matches the shape of the bone underlying it, which is generally smooth and featureless.
Then, Acciptriformes — this one is Haliaeetus leucocephala (the bald eagle):
Like Procellariiformes, Accipitriformes comprise a large number of taxa with “spout-shaped” rostral mandibles, but this time the rami are more broadly separated, the symphysis shallower and broader, and the rhamphothecal sheath higher relative to the bone height. A ramicorn is absent, so that what is seen here is merely the nail; further caudally along the jaw, the skin is keratinized into a “cere,” but does not form a ramicorn. The length of the mandibular rhamphotheca is much, much shorter than that of the rostrum rhamphotheca, which is famously large. I’ve seen photos of rhamphotheca, but not handled them, that often extend for greater lengths beyond the shape seen above, but largely supplemented with that toughened skin. Most, if not all “birds of prey” possess short mandibular rhamphothecae, which implies less constraint on retaining expanded surfaces of the mandible for triturating food, an effect of what are effectively rostrum-based feeders: birds of prey, virtually all of them, feed by tearing prey apart with the rostral “hook”, reinforced by an extensive and durable bony core, and with ample application of pressure from the talons (raptorial prey restraint, or “RPR” [Fowler et al., 2009], which has been hypothesized to be the primary prey handling method for earlier, and larger, theropod dinosaurs [Fowler et al., 2011]). Thus, mandibular morphology need not be complex, nor the underlying bone elaborate, and indeed raptorial birds of prey typically have shorter mandibles relative to the upper jaw than most other birds (parrots, of course, are an exception, although this may be due to the functional utility of processing and locomotion employed using the rostral “hook” in these birds).
(Aside: As I was researching this, the story of Beauty — a bald eagle shot in the face but survived, despite horrible damage to the upper beak, and barely able to eat much less drink — came to my attention. The bird [currently housed in Coeur d'Alene, Idaho's Birds of Prey Northwest facilities] has now been fitted with a bionic implant, a molded plastic [nylon polymer] beak fitted which was 3D “printed” from a scan of an intact beak and mounted with titanium to the rostrum has provided the bird the ability to feed itself unaided.)
Fowler, D. W., Freedman, E. A. & Scannella, J. B. 2009. Predatory functional morphology in raptors: Interdigital variation in talon size is related to prey restraint and immobilisation technique. PLoS ONE 4(11):e7999.
Fowler, D. W., Freedman, E. A., Scannella, J. B. & Kambic, R. E. 2011. The predatory ecology of Deinonychus and the origin of flapping in birds. PLoS ONE 6(12):e28964.
Hieronymus, T. L. & Witmer, L. M. 2010. Homology and evolution of avian compound rhamphothecae. The Auk 127:590–604. [PDF link]