A Skull Reconstruction for Daemonosaurus

Recall my earlier post on Daemonosaurus chauliodus. At the time, I thought this was a pretty silly dinosaur to provide a reconstruction for, since for the most part I agreed with the authors’ reconstruction, it matched the skull as i saw it, and their own interpretations. There are some issues, but those issues are minor, but broadly noticeable and thus something that can be accounted for relatively easily. First, the reconstruction from Sues et al.:

CM 76821, skull and anterior cervical vertebrae with cervical ribs of Daemonosaurus chauliodus (Sues et al., 2011). Top row, left and ride sides of the slab bearing the skull; middle row, interpretive drawing of the slab; and bottom, interpretic reconstruction of skull.

CM 76821, skull and anterior cervical vertebrae with cervical ribs of Daemonosaurus chauliodus (Sues et al., 2011). Top row, left and ride sides of the slab bearing the skull; middle row, interpretive drawing of the slab; and bottom, interpretic reconstruction of skull.

One of the more noticeable features of the skull is the oddly juvenile aspect: the overly large orbital fenestra, and short snout, the short triangular aspect of the snout, and the giant, snaggly teeth. It’s those teeth I’m interested in, and in many cases, find that extraordinarily-elongated teeth in conventionally-shaped skulls are worthy of heightened scrutiny. That is to say, it seems unusual, but one shouldn’t accept it and it bears further investigation.

Elongated teeth usually arise through either natural recovery, when the teeth have exceptionally large crowns, or exceptionally long roots (when the tooth is thecodont, found within a deep socket surrounded by bone), or because the tooth as partially become dislodged within its socket, which can arise due to replacement teeth lying underneath and pushing outward, or because the soft-tissues that bind the root to its socket have been degraded. These loose teeth can look exceptional, and have occasionally resulted in being described as a natural feature of the skull in question, and even feature in fleshy reconstructions of the belonging skull. A great example is in a juvenile Ceratosaurus ?nasicornis, in which the teeth seem to overhang the mandible when the jaw is shut. This reconstruction may actually put the lie to the hypothesis for an enclosing, squamate-like fleshy “lip,” but if the teeth are slipped from their sockets, and hanging by a thread of tissue when the skull was fossilized, then it seems more likely the skull is “conventionally toothed.

Skull of the Carnegie Museum cast of a juvenile Ceratosaurus ?nasicornis specimen, courtesy and copyright of Dr. John Merck. The extraordinarily long teeth may be the result of slipped roots.

I suspect the same happened in the jaws of Daemonosaurus chauliodus, largely because the shape of the teeth themselves show a distinct caudal recurvature and basal expansion of the distal carina but in the mandibular teeth and some teeth of the upper jaw, these seems to end halfway along the exposed crown. Almost certainly, this can be settled by examining extent of enamel, but also extent of soft-tissue to enamel exposure on crowns in living reptiles and/or mammals, and consider how extrapolable this is to dinosaurs. Part of this argument stems from an observation that generally (though not always) the upper dentition of a closed reptile’s jaws (including lizards and tuataras for the most part) tend not to pass beyond the line of foramina that follows the mandibular branch of the trigeminal nerve that innervates the lateral surface of the mandible. This nerve innervates the lateral tissues of the jaw, including the oral margin and sensory organs that lie around the jaw surface, including in crocodilians and mammals, and innervate the “lips” in mammals, lizards and snakes alike, but is also found in birds, which do also possess foramina tracts that follow the trigeminal nerve; they would, by logical extrapolation from the EPB, do so in theropod dinosaurs as well. I find it generally unlikely that this tract would exhibit tissues flush to the jaw’s outer wall, show no evidence of this in the form of impressions on the jaw surface for blood vessels typically associated with close-lying hard tissues such as cornified keratin sheaths, as this is the only process by which the jaws can have a sensory surface, but the teeth overlie them. I pointed out this improbability here. It is an hypothesis, then, that teeth should not generally extend far beyond the mandibular foraminal tract, and that this tract represents a general boundary for the base of the lower extraoral tissues where they separate from the mandible, forming a “trough” lined with epithelium into which the upper teeth are contained. Rather than see snaggly-toothed theropods, the teeth should be kept “pocketed” as they are in lizards and snakes and most likely most non-eusuchian crurotarsans. I’m beginning to wonder if the same is true for sauropterygians (ichthyosaurs, plesiosaurus) and other reptiles as well (e.g., placodonts) often shown with teeth fully exposed when the jaw is shut, though those groups and various others haven’t received much scrutiny on facial reconstruction.

Nonetheless, this forms the basis of a more “conservative” cranial reconstruction for Daemonosaurus chauliodus, which also alters how Sues et al. restored the postorbital skull, which was damaged.

Skull of CM 76821, holotype of Daemonosaurus chauliodus (Sues et al., 2012).

Skull of CM 76821, holotype of Daemonosaurus chauliodus (Sues et al., 2012).

Sues, H.-D., Nesbitt, S. J., Berman, D. S. & Henrici, A. C. 2012. A late-surviving basal theropod dinosaur from the latest triassic of North America. Proceedings of the Royal Society of London, B 278:3459-3464.

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8 Responses to A Skull Reconstruction for Daemonosaurus

  1. Interesting. Your hypothesis replaces an “is” with a “should be” with regard to the teeth. This can cause problems. Also wondering why your maxilla has no antorbital fossa? (Sues et al. dismissed it too.) Seems you also shortened the retroarticular process to minimize the overbite compared to the Sues et al reconstruction. Just curious, why the changes?

    • I looked for an antorbital fossa. It doesn’t seem to be there. Basal dinosaurs also don’t seem to have a fairly extensive one, so I;m giving this one the benefit of the doubt that one did not extend onto the laterla maxillary surface or create a median pocket on the caudal surface of the ascending process of the maxilla defining the medial and lateral surfaces of the maxilla. There seems to be a trace of the fossa on the lacrimal, but this may be due to the uneven structure of the lacrimal-maxilla contact, and there doesn’t seem to be an extension of the fossa onto the lacrimal or jugal, either. This is not unexpected, and may be due to preservation, exposure, etc. The fossa may be present, but just not visible. No amount of photographic kejiggering would reveal it were it buried in matrix.

      The retroarticular process has nothing to do with the overbite, I don’t know why you’d even bring that up. The pretroarticular process starts from the caudal end of the articular depression into which the quadrate articulates, and no further anterior to that. It is also incomplete on either side of the skull, indicating a possible conservative estimate of following the curvature as was apparent instead of something ridiculous and elongated, as in oviraptorids. I didn’t do that, either. It seems to be short, stubby, and rounded, as high as long or so. Might be longer, but incomplete.

      And finally, the reason “should” be is that determining the actual position of the enamel/dentine interface or the crown/root boundary in a photograph is nigh-on impossible without exceptional preservation that colors them differently, or shows reflectivity differences. Otherwise, this MUST be examined and determined in person or by microscopic examination or sectioning. CT and/or Xray are usually good at revealing these, so this can be done non-invasively, so long as the bone material and the matrix have sufficiently distinct densities and do not show up the same under either type of of examination (since both use Xrays, this is possible). It is actually an important thing to note the degree of “slippage” for crowns relative to their neighbors and possible replacement teeth, to examine for Zähnreihen, or other possible defects; these things happen, but aren’t always looked for in “thecodont” jaws.

      • I brought up the retro process and overbite because that’s where yours differs from Sues’s. Nothing more than that. Why not ask the skull owners if they intend to do an X-ray for the tooth problem? They might have already done one for you.

        • There seem to be a fair number of archosaurs with overbites. So much so that rostral occlusion is far more particular and exception than we’ve noticed. Tyrannosauroids have distinct wear facets on their premaxillary teeth, but these appear to be related to how the tyrant uses them, not to occlusion. Virtually all ornithischians have premaxillary teeth that oppose the predentary and NOT the rostralmost dentary teeth, indicating a virtual overbite, even though the predentary extends the relative law margin rostrally and maxes for a possible occlusal surface for the preaxillary teeth. Occlusion of dentition requires more indepth analysis than a supposition that the rostral teeth from upper and lower jaws SHOULD occlude: such as in ornithocheiroid pterosaurs and eusuchian crocs where the dentition pass one another by around the jaw margin, or in many lacertilians where the lowers form a rounded arcade that matched the uppers. In many dinosaurs, the shape of the rostral dentary tooth row either does (ornithischians, some sauropodomorphans) or doesn’t (most theropods, other sauropodomorphans) precisely match the premax-maxillary tooth row, which means there is imperfect occlusion, or a gap. In the skull illustration of Velociraptor mongoliensis, baased on MPC-D 100/25, I also show a “gap” between the upper and lower dentition, or an “overbite,” and in tyrannosaurs this can increase to the entire length of the premaxillary tooth row. Additionally, if we were applying the typical mammalian condition of an “overbite” here, than virtually all reptiles, lissamphibians, and non-mammalian cynodontians would have overbites. The upper tooth row passes rostral to and lateral to the lower tooth row, containing it.

          It is an assumption that the upper and lower teeth should perfectly align somehow, and one which requires secondary information (wear facets, festooning of the tooth row, some understanding of the 3d nature of the mandibular articular joint, an accurate reconstruction of the jaw shape on both sides, and a cursory understanding of how the jaw should occlude, if it occludes.

  2. There’s also a disturbing lack of predentaries, palpebrals and large coronoid processes. Regarding tooth length, most don’t matter, but it obviously had some fangs because this is related to Heterodontosaurus after all. No doubt Big Paleo payed you well for this bit of disinformation…

    • The initial draft of this post dealt with all that. I figured it wasn’t relevant, and knocked it out. Only Dave argues FOR these, and the arguments are not substantiated in a way that the interested observer could reasonably be able to replicate the results.

  3. Alessio says:

    Teeth slipping out of their sockets is a rather common happening, as far as i know, and in many dino skulls it’s fairly evident; in BHI 3033 (or Stan) it’s quite easy to notice the difference between the root and the real tooth; same for the young Ceratosaurus in the pic and well, why it would be different for Daemonosaurus?
    I may be wrong, but i suspect many theropods hadn’t those toothy grins people usually give ’em credit for…

  4. Pingback: Something About Overbites | The Bite Stuff

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