Poposaur mandibles

There’s still the question of Effigia’s mandible hanging out there.
The question is: “Is that a predentary or a dentary at the tip?” Fig. 1). Nesbitt (2007) says dentary. I say predentaries. Let’s look at the evidence.

To answer that,
I took a comparative survey of poposaur mandibles (Fig. 1), looking for evolutionary patterns and thereby strive to provide an update to the predentary/dentary question. Surprisingly, in the case of Effigia, when you add in the splenials, which neither Nesbitt nor I did before, the mandibular fenestra becomes substantially reduced. That may be similar to what one sees in Lotosaurus, in which the elements are not jumbled. And that provides more substance to the “predentary” argument. Other than Lotosaurus, the closest sister is Shuvosaurus, which is known from an incomplete mandible (Fig.1) showing similar patterns over the remaining portions. Shuvosaurus has something similar to what I saw in Daemonosaurus, that others consider something else. In any case, at some point, something interesting developed in front of the dentaries in certain phytodinosaurs.

The other question is,
when something similar to a predentary appears in front of the dentary, as in Sacisaurus (Figure 1), should it be considered a “beak” rather than a premaxilla? This bone may be paired, as it is in Sacisaurus, rather than a single median bone, as in the predentary of Heterodontosaurus (Fig. 1).

Figure 1. Poposaur (and kin) mandibles. Here are Daemonosaurus, Poposaurus, Pisanosaurus, Heterodontosaurus, Sacisaurus, Lotosaurus, Effigia and Shuvosaurus. The mandibles of Lotosaurus and Effigia appear to share a common heritage of design.  In Effigia the splenial reduces the mandibular fenestra helping to clarify the identify of the dentary and premaxilla (or beak).

Figure 1. Poposaur (and kin) mandibles. Here are Daemonosaurus, Poposaurus, Pisanosaurus, Heterodontosaurus, Sacisaurus, Lotosaurus, Effigia and Shuvosaurus. The mandibles of Lotosaurus, Shuvosaurus and Effigia appear to share a common heritage of design. In Effigia the splenial reduces the mandibular fenestra helping to clarify the identify of the dentary and premaxilla (or beak). The extension of the angular to the predentary is unique to this clade.

If all these other mandibles had a premaxilla or beak (or the possibility of one), is there any reason to suspect that Effigia did not?

The original reconstructions of the Effigia mandible
introduced us to the largest mandibular fenestra I have ever seen relative to the size of the jaw. The new reconstruction reduces the fenestra length and, no doubt, produces a stronger jaw with the splenial (lavendar to iris blue bone) laminated to the medial side and edges.

Typically the mandibular fenestra splits the surangular from the angular,
as it does in Heterodontosaurus. However, in Lotosaurus the mandibular fenestra develops largely below the dentary with very little surangular and angular exposure. In Shuvosaurus the same pattern could play out, but unfortunately the key parts are missing (perhaps due to a very large mandibular fenestra?). This is a different pattern than in ornithischians, saurischians and theropods. And this pattern is also different from rauisuchians. Among euarchosauriforms, only in aetosaurs does the very large mandibular fenestra develop largely below the dentary. In others, the fenestra develops midway or beneath the surangular and it doesn’t get to the size seen in Effigia and Lotosaurus.

One final point
The suture between the two premaxillae in Effigia is convoluted like a puzzle piece. In this way they are locking themselves together, convergent with the central or fused premaxilla of ornithischians, but homologous with the premaxilla in Lotosaurus and Shuvosaurus.

If I’m wrong, show me some data. At this  point, at least it’s worth talking about.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Ferigolo J and Langer MC 2006. “A Late Triassic dinosauriform from south Brazil and the origin of the ornithischian predentary bone”Historical Biology 19 (1): 1–11. online pdf.
Nesbitt SJ and Norell MA 2006. Extreme convergence in the body plans of an early suchian (Archosauria) and ornithomimid dinosaurs (Theropoda). Proceedings of the Royal Society B 273:1045–1048. online
Nesbitt S 2007. The anatomy of Effigia okeeffeae (Archosauria, Suchia), theropod-like convergence, and the distribution of related taxa. Bulletin of the American Museum of Natural History, 302: 84 pp. online pdf

AMNH Effigia webpage

wiki/Effigia

7 thoughts on “Poposaur mandibles

  1. Certainly some oviraptosaurs would be in contention for the “the largest mandibular fenestra I have ever seen relative to the size of the jaw” title?

  2. Lotosaurus mandible is heavily reconstructed. Most of it that you see on the mount — I am told — isn’t really there, and that seems to inlude about half of the mandible’s “material,” i.e., plaster. You also ignore the evidence in silesaurid jaws that the edentulous region isn’t a predentary (lack of sutures) and pretend that pixelation can somehow recover this. This fantasizing needs to stop, Dave.

  3. Jaime, I’m going with what I see. That, by definition, is not fantasizing. If that’s wrong, we’ll fix it. If you have different data, send it and be helpful. Your ad hominem comments are inappropriate for a scientist. Pull it together, Jaime, and rise to your better self.

  4. Like your palate post, what you’re showing here is largely from your imagination. Just look at the dentary closeups of Shuvosaurus and Lotosaurus in Holliday and Nesbitt (2013). No predentaries. You think they would miss that in a paper dedicated to symphyseal anatomy? The way you put Pisanosaurus’ predentary against the broken dentary margin, and Poposaurus’ predentary against its broken dentary margin just shows you don’t pay attention to the papers and view elements as shapes you can rearrange with impunity, just like your Gracilisuchus pelvis with its added outlines over natural edges. The same is probably true of the posteroventral dentary margin in Sacisaurus. In Poposaurus your articular is actually mostly the surangular which fused to it (this is the way it is in most archosaurs, so e.g. your widely exposed Silesaurus and Heterodontosaurus articulars are also wrong). The surangular has a broken anterior margin that you use as a suture for … the surangular. And you have a dentary tooth going through the predentary in Poposaurus. How’s that even supposed to work? Your sutures for Lotosaurus are basically all made up. Might I point out your dentary-surangular suture is wrong, and the real one seems to be about in the middle of the fenestra (Nesbitt et al., 2013)? As far as I know, the splenial is never part of the lateral surface inside the external mandibular fenestra, and the dentary is always exposed ventrally. The left splenial of Effigia is _articulated_ to the dentary and anteriorly positioned. You can’t just pretend it’s not and move it back to show through the external mandibular fenestra. I don’t even need to mention again that your Silesaurus and Sacisaurus predentaries are just part of the dentary, your whole idea of “paired predentaries” has no basis in reality (the only real predentary in your lineup is Heterodontosaurus’), or your Daemonosaurus mandible used palatal elements to create a coronoid process.

    Again, it’s clear you are unfamiliar with archosaur anatomy, don’t read papers comprehensively, and trace elements incorrectly.

  5. Just read the Effigia paper in a bit of detail. I have to agree with Mickey – I don’t see any evidence of a predentary AND it sure looks like the preserved elements would exclude the possibility of a predentary.

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