Over at Saurian, Mark Wildman discusses one of the under-cared-for groups of large terrestrial reptiles, those lovable and odd hadrosauroids. In it, he made the following comment:
Although hadrosaurines generally lacked raised cranial ornamentation, they, never the less, may have possessed a resonating sac that was located in the deep depression that surrounded the external nares. This could also have been brightly coloured and the combination of sound and inflated air sac would have made for an impressive display – something similar to the frigatebird of today.
This is interesting because, a while back, I started illustrating things like this:
It should be noted that this was based on an extreme in conjecture. Primarily, I reasoned, there had to be something that formed the huge fossae you see on the surfaces of the premaxillae and nasals. In hadrosaurs, these two bones are almost exclusively developed into the large variety of cranial crests and ridges that qualify various taxa. It is variation in these that have been predominant in developing hadrosaur taxonomy, chiefly in the lambeosaurines from the Dinosaur Park Formation.
Hadrosaur skulls generally have large external nares, the bony opening surrounded by the premaxilla and nasal (or only by the premaxilla in some lambeosaurines). This is either an elongated oval, or a narrow slit.
I initially showed this image waaaaay back in one of my earliest posts, on Glishades ericksoni (here). I later elaborated on the cranial crests here, when discussing Acristavus gagslarsoni, in which elaborations of the “crest” (or lack thereof) can be used to define taxa. As shown above, the premaxillae and nasals in hadrosaurine and saurolophine hadrosaurids (not including the lambeosaurines — or lambeosaurids, depending on your terminology preferences). But left unsaid in regards to this image, or previous discussion, was that the circumnarial fossa, the depressions on the premaxillae and nasals that surround the bony narial fenestra, varies in its extent and in fact definition.
It has long been assumed that these regions of the snout were filled with soft-tissue of some sort, although few reconstructions and authors have elaborated on exactly what. For the most part, Larry Witmer and others have been working out the analogues for various soft-tissues as applied to the skulls of various animals, in extension of one of the most ongoing and taxically expansive projects on archosaurs currently being run by one group (the Witmer Lab). This study, resulting in projects proposing anterior placement of the external fleshy naris (Witmer, 2001) or the absence of a fleshy cheek in ornithischians (Papp & Witmer, 1998, presented before the Society for Vertebrate Paleontology at Snowbird, Utah, USA), intrigue me so, as they also extend into projections of rhamphothecal extent and many other interesting things.
One potential filler for this has been assisted by Witmer’s work with various “proboscidal” (or whatever the adjective is) mammals such as saiga and moose, which preserve extensive soft-tissue around the bony narial region, and were then extended to ceratopsians (Sampson & Witmer, 1999). Extending this premise to hadrosaurs was then relatively simple, and I quickly produced the images at top. These also developed the theory that potentially various cranial air sacs extended and opening through the narial region but, unlike the narge circumnarial region in ceratopsians, they did not open fenestrae but rather expanded the size of the naris, then formed extensions of the fossae posteriorly.
This may have facilitated the presence of large bony crests in hadrosaurs such as the Brachylophosaurini in which Acristavus gagslarsoni exhibits a hadrosaurine-like condition (as in Edmontosaurus regalis, above) with a large external narial region, a definitive posterior emargination of the circumnarial fossa, and no expansion of the nasals into a form of crest. Such a condition would then progress into a condition as in Maiasaura peeblesorum, where the circumnarial fossa extended posteriorly onto the nasal and towards the orbit, or above it, and forming a “saddle-shape” depression, with the midline of the nasals dividing the fossae into left and right sides; such a division becomes more or less apparent when above the orbit these fossae develop into a vertical crest with expanded lateral flanges (see further above). In Brachylophosaurus canadensis, the posterior narial fossa extends the crest into an elongated, paddle-like structure and past the orbit, but exhibits a raised lateral margin indicating the fossae preserve a natural margin; however, the midline ridge is shallow or even absent in several specimens, and so I would speculate that the fossae merged or are not definitively separated from one another.
This is all very hypothetical, of course; practical research would require assessing the textural, morphological and histological correlates to these tissues and see if they exist in these regions and if it could be deduced the shape and extent of the cranial pneumatic diverticula on the cranial surfaces. Moreover, this hypothesis extends to a premise that these tissues, certainly responsible for the internal expansion of the cranial bones of lambeosaurine hadrosaurs into larger and more fantastic crests, are homologous with tissues that expand into the nasal and various other cranial bones of the skull, as in other dinosaurs such as theropods like Monolophosaurus jiangi. But more on that later.
I projected that hadrosaurs could, perhaps (like Mark noted at Saurian) inflate their nasal tissues (a feature known for elephant seals as well), which would likely communicate in some fashion with the diverticula so that forced air could expand into them and “balloon” outward. This would create a sort of aerostatopic condition (from the Greek αεροστάτοπες, literally “ones with balloon faces”), and perhaps this extended from the circumnarial region and onto any crests. Moreover, it implies that the function of cranial crests in hadrosaurines was to support these tissues and present them in a variety of manners and sizes, and that the aspect of the internarial bar (bony strut above the external bony naris) has nothing at all to do with visual displays — sucks to be gryposaurs, I guess. Instead, the tissues themselves are the primary display features, and while they are unpreserved (and unlikely to preserve) there is much that can be said of them.
Papp, M. J. & Witmer, L. M. 1998. Cheeks, beaks, or freaks: a critical appraisal of buccal soft-tissue anatomy in ornithischian dinosaurs. Journal of Vertebrate Paleontology 18 (Supplement to 3):69A.
Sampson, S. D. & Witmer, L. M. 1999. Novel narial anatomy in ceratopsid dinosaurs. Journal of Vertebrate Paleontology 19(Supplement to 3):72A-73A.
Witmer, L. M. 2001. Nostril position in dinosaurs and other vertebrates and its significance for nasal function. Science 293:850–853.