Skulls of this lambeosaurine hadrosaurian ornithischian dinosaur are numerous, complete, and range in size between apparent oldest adults to animals 1/3 the size, or smaller. They all have an elongate, upright spine of a bony crest. The most unusual feature of a typical lambeosaurine is its crest; however, for Saurolophus in general, it may be the tip of the snout. Here, I’ve taken an unusual and unprecedented step of illustrating the upper snout of a hadrosaur without the characteristically common rhamphothecal beak. Why?
Since nearly the beginning, the premaxilla of ornithischian dinosaurs has been illustrated or described as bearing a beak, comprised of rhamphothecal plates that attach to the lateral and ventral margins of the toothless portions of the jaw. Recent work by numerous scientists demonstrate that there are several features of the bone of the skull which relates to the presence of a rhamphothecal sheathe, and peculiarly several of these features are missing in Saurolophus.
Rhamphothecal beaks can be attributed with strong correlation by 1) sharpened tomia lining the toothless region of the jaw — absence of teeth itself is not characteristic of a beak; 2) delicate grooves in the bone formed by blood vessels or nerves, termed neurovascular canals, which “etch” along the outer surface, often forking in the manner blood vessels do; 3) small, numerous foramina, usually at the ends of several of these canals, but elsewhere as well, rather than a few larger such foramina; 4) such foramina leading into the bone at an angle obliquely, rather than perpendicular to the bone surface; 5) and a smooth bone texture at the macro level, but tiny spiculate bone texture at the micro level.
The general appearance of bone over which lies rhamphothecal sheets is smooth, vascularizes with numerous small foramina, and with canals etching the surface. Importantly, in ornithischians with which actual rhamphotheca have been found, other hadrosaurs, the rim of the toothless area is formed into a thin tomium, a blade of bone along the rim. The sheet of the rhamphotheca lies on the surface of the bone, and an additional sheet on the other side might occur, forming a blade-like beak edge. Such a beak is found in many hadrosaurs, ceratopsians, but also living animals such as birds, turtles; the surface is comparable to the claws of birds, the horns of cattle.
There are other types of horny skin coverings, which have their own analogues on bone, such as a rough texture at the macro level, perpendicular canals entering the bone instead of oblique, and generally no other macro features aside from a somewhat “lumpy” appearance. These support features such as the toes of some large-hoofed ungulates, the nose of the rhinocerus, underlying the famous horns, the tip of the snout in many artiodactyls, lacking premaxillary teeth. These features can be found elsewhere, including the palate of some turtles where the rhamphotheca doesn’t extend, but a hard keratin does; indeed, the palate of most mammals, especially that of certain ant- and termite-eating ones, have this structure and support a hardened keratinous palate surface. If you feel the ridges on the roof of your mouth, you will be enlightened to know humans have this feature on the bony palate that underlies the soft tissue. And it is this type of keratin that seems to best fit the premaxilla of Saurolophus.
The peculiarities of the snout of Saurolophus does not end there. Typically, hadrosaurs have a large circumnarial fossa around the bony nares, anterior in which, somewhere, the fleshy nostril lies. A similar feature was described for various sauropods by a few studies, following comparison of other animals in which the bony nostril has been moved backward or is surrounded by a large bony depression. This depression is surrounded by a slight, upraised rim of bone, which lacks the characteristics of the rhamphothecal bearing rim found in birds. But unlike other hadrosaurs in which this large fossa exists, it is as if the rim of the beak had been “popped” up, such as inverting a frisbee, or turning a bowl downside up. The premaxilla of Saurolophus is long, from the tip of the snout to the end of the spike, and on its midline is a mid-line ridge, being extremely thin at the front, thickening at the middle, then thinning again towards the end. The two sides of this ridge form paired cup-like depressions around the snout, and around the tip of the spike. It is possible these regions housed extranasal vascular tissues, possible inflatable air sacs extending from the nostrils, which might be inflated as an elephant seal’s nose does. This has not been often illustrated, but is shown here. This feature is present in other hadrosaurs, but not to the extent as in Saurolophus. In this animal particularly, I think it likely the fleshy sacs that filled these depressions would have been extensive and rather visually potent. I have illustrated the skin here to be wrinkled, with a distinct and striking color contrast within the wrinkles; should the sacs be inflated, the striking color pops out, the overlying wrinkles vanish, their own color washed out in the display.