Last week, I asked readers to guess the intended identity for a hasty sketch (a doodle, really) of some reptile or other, with the intention of seeing if viewers could get a reasonable guess on the otherwise fossilized animal it is based on.Several got the answer correct, but not all did. I deliberately chose a strange animal, and rendered a very lacertilian-like depiction for the purpose of throwing readers for a loop. Some answers were on FaceBook, some on the blog, and some on my DA version of the illustration. Today, the answer:
Rhynchosaurs are curious archosauromorphans, with some comparability to the tuataras and rhynchocephalians in general, but otherwise are fairly distinct. Like them, however, as well as the uromastyx lizards discussed in another post, rhynchosaur dentition is fused to the jaw or fixed through dense cementum– but in rhynchosaurs they are in relatively deep sockets. They are thus ankylosed thecodont. Also, like those extant tuataras and Uromastyx, the dentition is developed in life in the posterior region of the skull while wear in the rostral end of the skull wears it down until it forms nothing but ridge and spike-like prominences on the jaw margins, maxilla and dentary both. Like tuataras, but not Uromastyx, the teeth form multiple rows on upper and lower jaws, and the jaws occlude in a form of ridges and troughs.
But rhynchosaurs have a certain degree of distinctiveness from those lizards we are broadly comparing it to: their skulls are very triangular in dorsal and ventral views, with extremely narrow, V-shaped jaws and pointed jaw tips. Rather than approaching each other broadly, the mandibular rami instead converge narrowly and resemble nothing so much as a triangle with inwardly curved sides:
Now, what I depict above is not Hyperodapedon gordoni; it shows development of the temporal arcade, with an extremely large temporal cavity in ventral view, and several concordant features as a distinction from the more gracile and basal features seen in Stenaulorhynchus and Rhynchosaurus-grades of rhynchosaur, of which the latter comprise smaller forms (and includes Fodonyx spenceri, Hone & Benton, 2008). In fact, the illustration I created does not depict a specific rhynchosaur, but generalizes the postcrania with a Scaphonyx-like body and a Rhynchosaurus-like head.
Rhynchosaur snouts, however, bear some interesting features it will be important to dicuss (at a later time), with direct concern to both mobility of the jaw (of which there is a large propalinal component) and the presence of occlusion between the toothless rostral end of the dentary with the nearly vertical and toothless premaxilla.
The rostrum is typically depicted in art as not merely exposed, but forming keratin-covered beaks, or toothlike “beaks”, depicting them as teeth, exposed as though they were mole rats (Bathyergidae, Spalacidae):
But these features are not, and they do not appear to preserve features of keratin covering, and instead may best be characterized as uromastyx and tuatara “beaks,” being wrapped in softer tissue. The one very interesting thing, though, is that the tips of these toothy elements might have been exposed through any form of epithelial-like tissues, as they show evidence of wear facets, with the processes of the dentary forming lingual wear facets and the caudolabial surface of the premaxillae forming a pit-like “facet” that seems perfectly receptive of the dentary rostral tips.
It is part of my ongoing perspective of restoring fossils that, despite these distinctions, the head should be largely encased in flesh, and the rostral tips of the snout surrounded by a tuatara-like “lip”. And, that it should bear little resemblance to the preserved lateral view of the skull. Embedded in flesh, the head obscures these bony features.
Lying Body Parts
Mike Taylor and Matt Wedel have pushed for an argument on how the fleshy shape of the neck in birds and mammals belies the shapes and arrangements of the bones within them, insisting the that the necks of birds and similarly of mammals “lies” to us. Yes, as far as assuming the shape of the bony neck doesn’t resemble the actual shape of the fleshy neck, this is true; but, depending on the ability of the researcher to extrapolate or infer data from extant organisms to reconstruct the bony neck (and their relative attitudes), we can in fact presume the fleshy necks shape, attitude, and even mobility. The same of all of these is true of the skull, influenced as it is by the position, depression, or extent of the bony hyoid system, muscles of the jaw and neck, placement of the fleshy naris, the extent of oral tissues and their likely shapes and composition, and so forth. With extensive knowledge of not only the actual anatomy of animals, and the inferences that can be drawn from them and applied to fossil taxa, including the influence in archosaurs for the presence and size of large external pneumatic diverticula, as has been suggested for sauropods (Schwart-Wings [as Schwartz], Frey and Wings, 2007).
I do not think necks lie: I think that they are telling us, among other features such as the remainder of the postcrania, much of what we might know about the living appearance of the neck. Yes, there are constraints, such as fleshy wattles, that skeletal anatomy cannot infer, or the dorsal “frill” found on some hadrosaurid mummies, but these are largely limited to “thingamabobs” and such, rather anything we might appreciate based on constraints inferred from looking at other taxa.
By similar token, I do not think skulls lie: I think that they tell us quite a bit that we simply do not have the information to be aware of what they are telling us, and one of those things they are telling us is that you cannot say practically anything you want about them, or shrug your shoulders and go “what if?” These types of statements simply go nowhere, and leave no new questions, allow little or no testing.
Benton, M. J. 1983. The Triassic reptile Hyperodapedon from Elgin: functional morphology and relationships. Philosophical Transactions of the Royal Society of London B: Biological Sciences 302:605-718.
Hone, D. W. E. & Benton, M. J. 2008. A new genus of rhynchosaur from the Middle Triassic of south-west England. Palaeontology 51(1):95–115.
Schwartz, D., Frey, E. & Meyer, C. A. 2007. Pneumaticity and soft-tissue reconstructions in the neck of diplodocid and dicraeosaurid sauropods. Acta Palaeontologica Polonica 52(1):167-188. [PDF]