Oviraptorid Head Tissues Described


In my last post, I presented an image representing an oviraptorid with the head essentially completely and distinctly fleshed out. This is partly the culmination of examining likely tissues based on skull-only analogues. Some additional suggestions were made based on comparison to birds (closest living and structurally similar animals), and this gives us a decent idea about what an oviraptorid head might look like. For simplicity’s sake, I illustrated this head without feathers, so all the integument is skin or modifications thereof.

Conch Head with Labels

Head of an oviraptorid, based on Conchoraptor gracilis (MPC-D 100/20). Labels represent major superficial structures examined here.

The first thing to note is that while I argue that the cranial fenestrae should not typically be visible, due to the presence of distinct cranial air sacs or muscles in them, this is not always the case in birds, and birds with naked heads often expose margins for the depressions around the bony naris and antorbital fossa. I don’t think the full infratemporal fenestra is visible in this manner, due to features of the dorsal and caudal margins, but the anterior margin (formed from the postorbital) might have been. This is a 3rd level inference, as similar features in birds sometimes do show up, but not always.

Also, given the medium and resolution (the illustration is naturally only about 8in wide and I used a Pigma Micron 02) the detailing is not high. It wasn’t intended to be an instructional illustration. That said, I hope I’ve managed to convey the idea of the flesh here somewhat reasonably.

There are several components to the life reconstruction shown above, first and foremost being the form of the skin. Typical skin is soft, pliant and elastic. But animals also preserved modifications of their skin with the reduction of elastin fibres, and increase in collagen fibres, which increases their stiffness and resistance to bending. This tissue arises from and is continuous with normal skin, but usually only involves the upper dermis and epidermis; the deep dermis remains the same, but the upper skin changes dramatically. There are several types, depending on the arrangement, growth and toughness of the tissue: cornified, or keratinized, skin, which is largely epidermal and composed of folds which may be bunched together — in birds, this is called “cere”; cornified pads, which develop from slightly deeper in the skin, and develop more or less perpendicular to normal skin, often involving plates overlapping one another, and which are bound to the deeper skin with Sharpey’s fibers — which leave marks on the underlying bone; and finally keratinized plates, which develop into sheaths or nails, but also beaks and the like, and which have distinct edges though the thin edge may “grade” into normal or keratinzed skin and almost blend. Hair, feathers and sauropsidan scales also arise from skin, though from normal skin into “placodes,” and diversifying from there.

As I mentioned in a previous post (or two), different types of skin will leave traces on the underlying bone, and much of this has to do with how close the tissues come to the bone, and how that bone might support them. In rhinos, the nasal boss supports a large network of dense fibers which form a pad that the nasal horn grows on, and this bone is spiky, bears numerous tiny foramina for enervation of the pad and horn, and the structure of bone is irregular and somewhat porous, but not cancellous. This type of bone structure also appears in the palates of turtles, and on the mandibles of some species (specifically, Caretta caretta appears to have a particularly dense network of foramina in irregular bone with this morphology, and the entire mandibular symphysis is covered in a pad which forms a dome-like structure in a slight crescent shape — this is matched on the palate, with a similar structure, by a peaked dome), in which a distinct dense pad is present as a continuation of the marginal beak. As far as I can tell, while this is described in general in various turtles, the exact and particular structure of this has escaped distinct analysis.

Tissue, or rather the density of connective fibers, leaves marks on bones, and the more dense the amount of fibers, the more marks are left on the bone.  Keratinized skin doesn’t really leave that much of a mark on the bone, though adhering, armor-like skin does, in the form of highly-spiky bone surfaces, as seen in crocodilians. Instead, keratinized skin may be somewhat loose and thus not be directly inferrable. There are some suggestions, drawn from extant birds, that tough skin, such as around the brows of raptors, or around the casques of birds that have them, will be present when the features of the skull suggest they may be useful. In oviraptorids, this involves the broad interorbital rim but in ornithischians it will involve the palpebral, and it is why I always illustrate my ornithischians with “angry eye” brows — they also look cool that way. Such brows form the dorsal margin of the eye-socket, and somewhat wrap around the dorsolateral margins and thus enclose the eye to a degree. In oviraptorids, then this would expand the “orbital width” even further than it already is, and give them a “heavy eye.” Below is an exemplar, from a white-headed vulture (Trigoniceps occipitalis):

© 2009, Peter van Zoest. Borrowed under fair use, although I am linking to the original posting and am unsure of the copyright allowances.

© 2009, Peter van Zoest. Borrowed under fair use, although I am linking to the original posting and am unsure of the copyright allowances.

The orbit of oviraptorids faces largely laterally, with very, very little rostral tilt, suggesting that there was a limited amount of stereoscopy involved. This, one might say, would be a sure sign that oviraptorids might not make perfect predators. I’ll get to this another day.

Oviraptorids also exhibit extensive pneumatization of the facial bones, so extensive in fact that the bone has become super-thin in places, while in others the bone appears to be absenct. This, one might suggest, meant the internal pneumatic diverticula of the frontal, nasal, premaxilla and lacrimal (when present) were exposed on the skull surface, covered only in skin (and feathers). I find myself somewhat at odds with this concept, as our only evidence for such features in birds is based on bone-resorption or pneumatic fossae, or bone around hollow chambers not actually filled with pneumatic diverticula, as in casqued hornbills.

Section through most of skull of Rhinoplax vigil, courtesy Dr. Matt Wedel via Darren Naish at Tetrapod Zoology. Although not stated, I assume the copyright would be Wedel's, @ 2011. Again, us is fair and linked to the original which is about hornbills, and should be read anyway.

Section through most of skull of Rhinoplax vigil, courtesy Dr. Matt Wedel via Darren Naish at Tetrapod Zoology. Although not stated, I assume the copyright would be Wedel’s, @ 2011. Again, us is fair and linked to the original which is about hornbills, and should be read anyway.

In this way, then, the bones of the oviraptorid face (and by extension the cranial crests) would bear a thin sheet of bone around the internal pneumatic features, over which rhamphotheca, skin, and “cere” would extend. Holes, as in the famous MPC-D 100/42, would be taphonomic, due to the extreme fragility of the <1mm thick bone present originally. There’s some evidence that, with careful preparation or even exceptional preservation, a fully intact crest is present. Regardless, where evidence for a covering rhamphothecal sheet or cornified pad (as in Rhinoplax vigil) is absent, I err on the side of caution and take the moderate path towards skin, but can elaborate by “cornifying” it a bit. Thus:

Skull of a "conchoraptorine" (PIN collection) with rhamphotheca (KH), cornified pads and "cere" (KS) demarcated in varying shades of orange. "?" referred to a valuable question about the extent and types of tissue on the hard palate and the "prongs."

Skull of a “conchoraptorine” (PIN collection) with rhamphotheca (KH), cornified pads and “cere” (KS) demarcated in varying shades of orange. “?” referred to a valuable question about the extent and types of tissue on the hard palate and the “prongs.”

As in some birds but all other sauropsids, the softer keratinized skin likely surrounded the nostril, and was present on the lower jaw. I have shown the density of keratinization here with darker orange, but do not neccessarily distinguish “beak” from cornified pads. I will project, however, that the broad nasals supported a pad-like tissue rather than “cere” because of their more irregular surface when not broken through by pneumatic diverticula. Detailed microscopic examination will either refute this, or support it.

The rest of the head was covered in simple skin, and the eye was large (although I’ve moderated the size somewhat).

Skull of a "conchoraptorine" oviraptorid (Big Beak, PIN collection) with the outline of the flesh and the position and size of the eye (E) in brown. Eye is placed caudodorsally in the fenestra and may be larger than shown.

Skull of a “conchoraptorine” oviraptorid (Big Beak, PIN collection) with the outline of the flesh and the position and size of the eye (E) in brown. Eye is placed caudodorsally in the fenestra and may be larger than shown.

What may be more surprising is that, because of the huge adductor musculature, and its rostral placement relative to the length of the skull, the sides of the jaws exhibit a pretty large muscular “cheek” region, which would be extended by a rictus only slightly. Almost immediately rostral to the coronoid process of the jaw, the mandible’s dorsal margin becomes a broad, rounded “bar,” which occludes with the ventral maxilla and jugal, themselves broadened into plates with a slight ventral sulcus. Thus, no matter what one might think of the idea of “cheeks” in the sense of mammals or whatever in dinosaurs, oviraptorids had something similar, at least on a superficial level. I’ve drawn that above with reference to other animals which exhibit a large rictus (as in Andean condors, Vultur gryphus; but amazingly similar features also present in hippos, Hippopotamus amphibius), which cause the elastic skin on the side to “wrinkle” up significantly, rathe than form a smooth, nice margin. (I will be discussing the adductor muscles in a future post.)

Lastly, there’s the placement of the fleshy nostril (naris) and the auricle (external opening for the auditory canal). These are based on general analogues, as has been described by Witmer (2001) for nostril placement (anterior within the narial fossa of the premaxilla, but also ventral and close to the margin of the jaw, though here I’ve put a beak in the way); the auricle is generally caudal to the quadrate, but cannot be too close to the condyle but was probably not tucked right up with the caudal (paroccipital) process of the squamosal. I’ve contemporized and placed it equidistant between the two. The nostril is illustrated rather than a round, always-open aperture but with a more “reptilian” fleshy flap that would allow the narial muscles to seal the nostril. This was done because I do not think that the narial anatomy necessarily favors “flow-through nostrils” as in birds, often requiring them to stay open; there is a further consideration that many oviraptorids (such as this one) lived in arid environments, and a sealable nostril would be advantageous. The large size of the narial fossa and the external bony naris itself suggests that there was a large amount of narial soft-tissue present, as part of the nasal vestibular vascular plexus (Witmer, 2001, again); such a structure is not present in birds and mammals, which contain much of the relevant tissues within the bony enclosure of the nasopharyngeal canal, but in other sauropsidans such as lizards and crocs, an extensive and sometimes erectile vascular structure suggests that, as in those animals which possess them, the nostrils may be closed. So, no open, bony nostrils for these guys.

So, for all that the illustration up top was done in about 25 minutes, free hand, there’s a lot of little bits of science behind it, making it a product of analysis rather than just pretty art (although I do like it). My next post on this topic will deal with the muscles and what their reconstruction by size and placement might mean. As I’ve said before, oviraptorids have proportionately huge adductor muscles, and moreover, the largest adductor-chamber relative to skull size of any dinosaur, living or not. What this means, while seeming a passive herbivore or whatever, is something I’ve hinted at before, but it will bear reiteration when I start laying out the groundwork arguments that support it.

Witmer, L. M. 2001. Nostril position in dinosaurs and other vertebrates and its significance for nasal function. Science 293:850-853.

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One Response to Oviraptorid Head Tissues Described

  1. Pingback: Whether Shrink-wrapping | The Bite Stuff

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