Rhynchops niger is a fun animal. Not only does it have this wonderfully huge lower bill, there’s lots of fun little structures of the jaw that interact in ways one doesn’t really expect in birds.(The gap in the upper and lower jaws when the jaw is shut is real. The bill of the skimmer is very, very narrow, and the mandible so very long, both so much so that the typical avian method of picking things up doesn’t work for them. Instead, the birds use the gap to hold objects after picking them up sideways in the middle of the beak and rather than with the jaw tips. The gap provides a higher surface area for gripping objects cross-wise than could be held lengthwise, and permit the jaws to stay extremely thin.)
When I set about putting this skull together, I did so knowing that this bird would be necessary to explore in my study of the relationship between jaws and their keratinous coverings. The upper rhamphotheca largely conforms to the underlying bone, but not so much the lower; what’s more, the lower jaw changes shape over time, having a nice squarish tip when these birds are younger but becoming worn down over the life of the bird until it becomes more rounded and angular, as above. The surface features of the mandibular rhamphotheca, arrayed with those rasp-like ridges, are another fun feature, but do not appear on the inner surface of the rhamphotheca and have no relation to the underlying bone (Zusi, 1962). The upper and lower beaks are so narrow that the neurovascular foramina that penetrate the bone are exposed in longitudinal groove rather than less oblique or perpendicular foramina; and there’s a lot of them, suggestive of higher degrees of tactile sensitivity in the jaws.
Bird skulls are very flexible, with various bending zones. I actually illustrated the skull is more than just skull, jaw, and rhamphotheca, and indeed I illustrated it with the cranium and rostrum as separate pieces to allow me to flex the things. Like many other birds, Rhynchops niger is capable of prokinesis, and the upper bill flexes in front of the orbit in a standard flex zone (Holliday & Witmer, 2008). And despite skimming behavior, in which the mandible is extended into the water, producing forces that might tend to rip the jaw off backwards, the jaw displays several other slender, and open synovial joints as in typical birds, although a specialized joint between the basicranium and mandible, formed from a unique process on both bones (Bock, 1960), allows the bird to “lock” the mandible in place when it is opened and the bird is skimming. (Skimming is a behavior this blog will eventually get to. There’s some interesting things to note when it comes to the relatively peculiar morphology of Rhynchops niger and that of skimming behavior.)
Bock, W. J. 1960. Secondary articulation of the avian mandible. The Auk 77: 130-132.
Holliday, C. M. & Witmer, L. M. 2008. Cranial kinesis in dinosaurs: Intracranial joints, protractor muscles, and their significance for cranial evolution and function in diapsids. Journal of Vertebrate Paleontology 28 (4): 1073-1088.
Zusi, R. L. 1962. Structural adaptations of the head and neck in the black skimmer. The Nuttall Ornithological Club. 101 pages.