Diet in Oviraptorosaurs (Oviraptorosaurs, 101)
Continuing from the last post, we proceed with describing the oviraptorosaurs and how these animals may have eaten.
The basal, toothed taxa are fairly distinct from one another, although in some cases they are quite similar this is largely considered to be plesiomorphic (a term meaning “broadly similar,” and referring to animals that are similar by reason of common features, without direct relationship). All known have fairly long legs, short arms, a short tail; they have a small head that is fairly boxy with huge orbits and a roughly triangular snout; their teeth are all located in the anterior portions of their jaws, although some taxa have longer tooth rows than others. They all seem to have masses of gastroliths preserved in their stomachs. This will be touched on after I describe their skulls and potential jaw mechanics.
Protarchaeopteryx was named for an animal that the authors thought not that different from Archaeopteryx , albeit with shorter arms and less teeth; they thought it was a bird, along with Archaeopteryx, and grouped the two of them together in the Archaeopterygidae. Phylogenetic analysis performed shortly thereafter  affirmed its position as a non-bird, and certainly not a close relative of Archaeopteryx, but at the time it was assumed to be an ambiguous maniraptoran theropod, especially as virtually all of its skull was missing, and the postcrania hard to define. This has since been resolved with both the discovery of Incisivosaurus  and with further analyses positioning Protarchaeopteryx at the base (or close to it) of Oviraptorosauria.
Details of the dentition are limited; the skull material is mostly damaged, with the bone split between the two slabs, thus rendering surface material of the bone missing (as it is not exposed). Teeth are exposed from crown to their roots, but in the premaxillary teeth, they are only exposed as roots. These are nonetheless very large, while those of the maxilla and dentary are very small. The tip of the mandible appears edentulous naturally (the other teeth are exposed in the rest of the jaw). The shapes of the teeth are unknown, and very little is exposed of the rest of the skull. What there is appears very similar to that of Incisivosaurus (see below).
The skull [3,4] is boxy, narrow, and complete with a short series of small, semiconical teeth on the maxilla and dentary, and two pairs of teeth on each premaxilla. These latter teeth are larger than all of the others; while all teeth are labiolingually compressed, the first pair is rotated almost 90 degrees to the others so that it appears to be transversely broad, chisel-like in appearance, and form an incisor-like aspect. As in other theropods, the dentigerous (tooth-bearing) portion of the jaw is about half of the total jaw length. The form of the maxillary and dentary teeth are low-crowned (with a basoapical height less than the mesiodistal length), none are recurved or even appear to have a remotely pointed apex (all are rounded). All teeth, including the large first pair, have constricted bases above the roots. Each crown has a carina that is continuous mesially to distally, around the apex.
The mandible is deep, and the halves are not fused to one another, unlike in caenagnathids, and there is no medial curvature of the mesial end. The dentigerous portion of the mandible is much shorter than the dentary’s dorsal margin, as the symphyseal region is edentulous.
Unlike in typical oviraptorosaurs, the distal quadrate condyles are narrower than they are craniocaudally long, and are segregated into distinct condyles. This is typical for a mandibular articulation that fit neatly and securely into the mandibular cotylus (or socket). Because the only known skull of Incisivosaurus (IVPP V13326) is taphonomically distorted (compressed mediolaterally)  and the posterior region of the skull attenuated, the quadrates are slightly rotated around their long axes from their natural condition.
One of the best known basal oviraptorosaurs, largely due to its completeness and the number of specimens known, as well as the condition that some of the fossils are in. Of the specimens [2,5,6], only the type and paratype of C. zoui (the type species ) are difficult to assess, and this is due to their scattered and partially shattered condition; skulls of other specimens include one referred to C. zoui  but differs quite substantially from it. However, in general, they are similar enough that describing the jaw apparatus and dentition is equivalent among all specimens.
The skull is far more triangular than in Incisivosaurus , with a shallower snout and larger orbit. The snout, the region just anterior to the orbit and all the way to the front tip of the skull, is very short and rounded at the end. It terminates in a small maxilla and a relatively large premaxilla. The ventral edges of the premaxilla and maxilla seem to bear scalloping on their edges, and this may indicate they bore an extensive rhamphotheca that would have acted much like a bird’s beak. The anterior half of the premaxilla has four small, but long, needle-like teeth; these are essentially conical and slightly recurved [2,6]. C. zoui and C. dongi are not comparable, as no skull material has been described for the latter , and it may be likely that the two are synonymous as they differ only in a few proportions of the limbs and sternum; however, specimens referred to C. zoui  include skulls that are seemingly more robust, deeper in the snout, and slightly larger relative to the body length, than in C. zoui; these may belong to a distinct species, but based on skeletal anatomy, they are not Similicaudipteryx . Nonetheless, the teeth and mandible do not differ that much between these specimens.
The lower jaw is entirely edentulous, and also appears to lack a coronoid eminence. Like caenagnathids, the mandible is opened by a very large external mandibular fenestra both internally and externally; in Caudipteryx, this fenestra is 1/3 the mandibular length, including the retroarticular process (which is short and deep), and the articular surface of the articular is cupped, concavely cotylar rather than rounded and condylar [2,6].
Similicaudipteryx preserves no cranial material. However, it does preserve gastroliths in the stomach region (along with virtually all specimens of Caudipteryx described to date) . This will be discussed later.
Kurzanov’s original description of this unusual taxon  described only a partial cranium. However, subsequent publications and a monograph [9,10] describe the skull is more detail from a referred specimen. This specimen, although not detailed in any of Kurzanov’s or other publications treating Avimimus, is actually smaller than the holotype cranium, which is seldom mentioned, although this is generally due to the relative lack of discernible sutures in the referred skull, while those in the holotype are clearly visible [8,10]. It is interesting to note that in even the most recent publications to regard Avimimus in detail, Vickers-Rich et al.  and Norman  do not mention the holotype skull.
Very little of the oral skull is known, including a tiny flat, rounded fragment probably pertaining to the mesial mandible, the poster half of one mandibular ramus, and a paired triangular bone that has been interpreted as the conjoined premaxillae. Some of the identifications may be off, especially as the premaxillae bear a distinct fossa bordering what may be the anterior and ventral margin of the external narial fenestra, but the ventral margin is very shallow unlike what is observed in any other oviraptorosaur. The morphology does correspond to anterior maxillae, where the fossa would pertain to the antorbital fossa, but the elements as conjoined seem to be naturally attached to one another, forming a perfectly pyramidal structure. Regardless, the ventral, tomial margin is crenellated, with what appear to be 4-5 denticulate processes resembling the ventral maxillae and premaxillae in Caudipteryx [2,6]. The remainder of the cranium is complete from the occipital condyle to the anterior edge of the orbit, although the lachrymal doesn’t appear to be preserved; this leaves a large gap missing in the skull.
The partial mandible is better placed to be analyzed. Like Caudipteryx, the external mandibular fenestra is very large, over 50% the height of the mandible itself; unlike Caudipteryx or Incisivosaurus, however, and more like other oviraptorosaurs, the surangular, articular and angular are all fused into a single unit and there is a prominent coronoid eminence which is inflected medially; the coronoid is also probably fused to the surangular, and the prearticular also seems to be so. The dorsal margin of the surangular is bowed, although this may not be as significant as the inflected coronoid eminence. The articular surface of the articular is cotylar and narrow, corresponding to a narrow condyle of the quadrate, as in Caudipteryx and Incisivosaurus.
A fragment of the mesial dentary seems to be preserved, but it has so far only been illustrated in a line drawing, and a recent atlas  of skeletal elements failed to illustrate it; it may not actually be a dentary [9,10]. As shown, however, it is edentulous and mesially bowed in a manner similar to oviraptorids and caenagnathids, although lacking a substantial symphyseal shelf. In this way the fragment, if correctly identified, represents a condition unknown in any other oviraptorosaur (Caudipteryx [2,6], Incisivosaurus  and Microraptor  all possess a straight, unbowed dentary, but possess a large symphyseal shelf posterior to the anterior margin. Lingual details of the bone are obscured by its size, and Kurzanov is light on the details of its surfaces [9,10].
Very few bits of the skeleton afford a potential look at the skull of Microraptor, at first  being surmised that none of it could be reasonably be identified; subsequent examination  proposed that a fragment once considered a portion of the posterior mandible by its discoverer , then a potential pelvic element  was more likely to be a nearly complete right dentary . Some ambiguities of this fragment make this identification peculiar, and until more substantive material comes to light, it seems more or less the best idea yet. I will consider this fragment a nearly complete dentary for the time being.
It is very short, and resembles that of referred skulls of Caudipteryx  in general, as well as that of oviraptorids. Narrowly triangular and very thin, it shows no transverse bowing and no curvature mesially. A small region of the symphysis appears to be inflected mesially. The dorsal margin of the dentary is tomiate. Otherwise, features of the lingual surface of the bone correspond to a Meckelian groove and a symphysis, supporting the identification, but one should be nonetheless cautious. The symphysis raised from the lingual surface and thus extended mesially, producing a shallow “shelf.”
Until it is further described, I am not regarding Gigantoraptor and its large jaw. Needless to say, it indicates a superficially oviraptorid-like morphology with some specifically caenagnathid-like aspects. This material should be interesting to analyze.
Some specimens of oviraptorosaur have been found with masses of gastroliths. To date, the only known specimens of Protarchaeopteryx  and Similicaudipteryx  and all specimens of Caudipteryx [2,5,6] have been found with such masses. They are normally found adjacent to the pubis and the ventral end of the thoracic region of the trunk, near to the gastralia. Generally, this corresponds to the intestinal tract in birds, whereas in said birds such a mass is located in the gizzard at the anterior end of the thoracic cavity near the sternum, while in crocodilians it is located within the stomach at the dorsal end of the thoracic cavity. Thus, the positions of the masses in these specimens, while consistent, appear to be displaced.
Gastroliths appear to serve two different functions, depending on the group they are found in. Birds appear to use them for aid in reducing food to pulp in the gizzard, while some predatory bones may use it to process bone, while crocodilians seem to use them for ballast. The latter hypothesis is unlikely to hold for the oviraptorosaurians, so we are given to the other two assumptions, grinding food.
They are generally found in herbivores, but as noted above, predatory birds such as ospreys also prefer them.
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