I just wanted to pick up on an image I included in an earlier post. This one demonstrates what we know of an unusual theropod palate.
Oviraptorid palates, as I’ve noted before, are particularly odd, both due their eversion below the jaw margin and the formation of small bony “prongs” on the maxillary palatal processes, but due to a variety of other features:
1. The vomer is the longest bone in the palate. This is the triangular-headed looking bone in the front, with all the hatching. This bone is split roughly 70% of its length into two long, thin strips which articulate with the pterygoids in the back and the palatines in the middle.
2. In typical archosaurs, the palatine is a plate-like rectangular bone with a set of struts sticking out of it on the sides; this is usually expanded into a “tetraradiate” plate, is flat and broad, and the “spokes” contact the pteryoid, maxilla, and vomer. In oviraptorids, and potentially some of their oviraptorosaurian ancestors, this bone has virtually lost the struts, or at least has reduced them to thin expansions of the ends, where the pterygoid and vomer come close together. This form has been termed “biramal,” but I (as always) would prefer a more consistent set of terminology with variations in numbers of radial spokes. The loss of these “spokes” also results in the loss of the suborbital fenestra.
3. Usually, the pterygoid is a whopping huge, hourglass-shaped bone, broad at the posterior end where it expands over the anterior edge of the quadrate, and expanded vertically at the anterior end. In other archosaurs, this anterior end produces a large “pterygoid flange” (processus pterygoideus) that anchors the pterygoideus muscles. In other archosaurs, this muscle site is below the jaw line, and inserts on the medial surfaces of the angular and prearticular, and the lateral surface of the surangular (or angular, in some animals), and forms the anterior portion of the jaw adductor muscle super-complex. In oviraptorids, this origin site in the palate is flat horizontally, rather than vertically, and is hardly hourglass-shaped; it is rendered into a fairly L-shaped, ventrally concave plate where it contacts a conjunction of the palatine, epipterygoid, and vomer. The insertion on the mandible will be covered in another post.
4. Ectopterygoids are also often “triradiate” but like the palate it’s “biramal” here as well. This is a long, C-shaped bone, resembling nothing so much as farfatte pasta with a twist in the middle. The most exceptional thing here is that in typical archosaurs, the ectopterygoid contacts the maxilla (and sometimes the jugal) with a slight hook-shaped lateral spoke, but this is merged here with the anterior pterygoid “spoke” into a long contact along the maxilla and probably the anterior jugal. This shape narrows the space between palatine and ectopterygoid, such that the intervening lacuna, the postpalatine vacuity (or fenestra) is reduced to a tiny slot and only apparently in lateral view, but not necessarily in ventral view.
5. Seldom preserved (or mentioned) is a large, flattened epipterygoid. In typical lizards and birds, this is a thin, rod-like bone, while in some archosaurs, and many theropods, it is expanded into a broader structure. The bone in tyrannosaurids and oviraptorids, as shown here, is a large element. The upper end is in contact with the sphenoid elements of the braincase (latero and orbito), while the lower end is in contact (and may be fused with) the ptergoid and even quadrate. It’s broad form enforces the concept of cranial akinesis, whereas this bone is lose, smaller, and mobile relative to the palate and braincase in cranially-kinetic animals.
In extension of this discussion, the palate of other oviraptorosaurs may be interesting to compare, as it provides us with something of a solution to how the amazingly unique palate arose in oviraptorids. However, little has been described, or preserved, of this system of bones in other oviraptorosaurs. A bone was noted as a palatine in ROM 12450, the Horseshoe Canyon caenagnathid (Sues, 1997, as Chirostenotes sp.), and it is “tetraradiate.” The palates of some specimens of Caudipteryx zoui are also apparent, although poorly described; Greg Paul reconstructed the palate as everted, based largely on discussion noted with Nick Longrich (Paul, 2002). This is based on crushed specimens, however, and their likely morphology should await better description and discussion. Their similarity to Incisivosaurus gauthieri (Xu et al., 2000; = Protarchaeopteryx gauthieri of Senter, 2004) implies they may not be everted, in which form only the pterygoid flange with its ectopterygoid component (as in other archosaurs) are the only ventrally extended elements.
Note in the image above that the palate is extended only partially below the jaw margin, unlike in oviraptorids. This may imply an original “oviraptorid” may not have had an everted palate.
So what would be the point of an everted palate? That question has never been asked, nor has it featured in the various bite-force/mechanic studies that have been done on oviraptorids. I have an idea, but I will wait until I can talk about the lower jaw first.
Paul, G. S. 2002. Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. (Johns Hopkins University Press, Baltimore.)
Senter, P. 2007. A new look at the phylogeny of Coelurosauria (Dinosauria, Theropoda). Journal of Systematic Palaeontology 5(4):429-463.
Sues, H.-D. 1997. On Chirostenotes, a Late Cretaceous oviraptorosaur (Dinosauria: Theropoda) from western North America. Journal of Vertebrate Paleontology 17(4):698-716.
Xu X., Cheng Y.-n. Wang X.-l. & Chang C.-h. 2002. An unusual oviraptorosaurian dinosaur from China. Nature 419:291-293.
Zhou Z., Wang X.-l., Zhang F.-c. & Xu X. 2000. Important features of Caudipteryx – Evidence from two nearly complete new specimens. Vertebrata Palasiatica 38(4):241–254.