Oviraptorosauria represents a very small portion of overall dinosaur-dom – if you include birds in the discussion of Dinosauria, they represent an even smaller portion, but then their relative similarities become more interesting.
Unlike virtually all other dinosaurs (excluding birds and ornithomimosaurs, another group we’ll eventually get to), most members of Oviraptorosauria lack teeth. This is interesting in that when first discovered, they were interpreted as having very different diets from conventional theropod dinosaurs (and some were not considered dinosaurs as a result). When Sternberg discovered, then described, Caenagnathus  (now considered to belong to Chirostenotes ) , he inferred it was in fact a large bird’s jaw. However, when the fossil Oviraptor philoceratops was discovered , its proximity to a nest of eggs, it was secure in its nature as a theropod dinosaur despite completely lacking teeth; this was recognized because of the rest of the skeleton preserved with Oviraptor … but not with Caenagnathus.
Not all oviraptorosaurs lack teeth – just most of them. The earliest members of the group apparently had teeth, some of which were particularly large. Protarchaeopteryx  (variously synonymized with Incisivosaurus) is noted for having large teeth in the anterior tips of its jaws; P. robusta, the type species, had needle-shaped teeth, while P. gauthieri (=Incisivosaurus) have larger, blade-like teeth (especially the first pair of premaxillary teeth). Finally, several species of Caudipteryx  have been named, but all of them have just 3-4 small, needle-shaped teeth in the premaxilla; at least both species of Protarchaeopteryx had teeth in the premaxilla, maxilla, and dentary as well.
Diet is normally easily inferred from the morphology and arrangement of the teeth, and secondarily from the structure of the jaw and cranial muscular apparatus. With oviraptorosaurs, and with the above-mentioned exceptions, dentition is absent, so this leaves us with less direct means of determining diet. A few fossils have permitted an example of an actual consumed or likely consumed food, but this is particularly rare. In this series, I will explore these dietary features of oviraptorosaurs, presenting published theories on their diet, and arguments on why these bird-like dinosaurs lost their teeth (and what else that may mean).
There are generally three predefined groups of oviraptorosaurs, separated into three distinct gradients. These groups are the Oviraptoridae (the monophyletic one, although how much of Oviraptorosauria it might contain is still up in the air), the basal, toothed oviraptorosaurs (which may form a smaller monophyletic group, but also appear to produce a grade of ancestrally toothed to progressively less-toothed taxa), and the non-toothed oviraptorids which lay outside Oviraptoridae (some of which may belong to a monophyletic Caenagnathidae). These are then three gradients of toothless to semi-toothless taxa, but the toothless taxa represent the derived clades, while the basalmost forms are toothed, indicating that tooth loss was progressive in the Oviraptorosauria. It is possible that this loss of teeth occured only once, and certainly it seems to be that way (as all of the toothed taxa fail to group with the untoothed ones) but there is limited information in this regard: Some of the toothless taxa may derive from within the toothed taxa separate from the Oviraptoridae-Caenagnathidae clade. Because this issue is not cut and dry, the sections following this post will deal with the grades rather than the clades of discreet taxa, so we will begin with Toothed Oviraptorosaurs, followed by the Caenagnathids (some of them former birds!), and then finally by the Oviraptorids (many of which sported unique crests).
Any study or examination of the jaws of any animal requires its own particular lexicon of terms and phrases, often built on specific systems (e.g., mammalian teeth are qualified by their cusps, which help define points of note when describing such teeth). For most reptiles, the general complexity of the teeth is less likely to require a unique set of terms (there are groups of archosaurs with complex dentitions, but Oviraptorosauria is not one of them). But a typical reptile jaw is oriented from the jaw-joint to the jaw symphysis, and is descriptive in three planes of observation; in most reptile jaws, this jaw is relatively unremarkable in one of these planes (the horizontal), but again, this is not the case in Oviraptorosauria. The peculiar condition of the teeth when it exists, and the unusually complex anatomy of the symphysis in Caenagnathidae require elaboration, however, and this index of terminology helps to clarify this.
Apex = “The tip of the dental crown. There may be more than one apex, in the case of multiple cusps on a crown.”
Apical = “Towards the tip of the dental crown, or on a plane equivalent to a tip of the dental crown (in cases where there is more than one). Its opposite is basal.” Adv. Apically.
Basal = “Towards the root of the dental crown. Its opposite is apex.” Adv. Basally.
Base = “The area directly above the root of the dental crown, where the tooth emerges from the substrate, or the portion of the tooth that is in contact with, but not subsumed into, the substrate.”
Basoapical = “Along a line parallel to the dead-center of the base in mesiodistal and Labiolingual axes to the highest apex of the dental crown (or on a plane equivalent to the highest apex).” Adv. Basoapically.
Carina = “A ridge produced on margin of the crown. The placement of a carina is either typically mesial, distal, or lingual, but rarely labial.”
Carinate = “Of or possessing a carina. In non-dental anatomy, of or possessing a keel on the sternum.”
Condylar = “The portion of a joint having a convex, or externally bulbous shape. Its opposite is cotylar.”
Condyle/Condylus = “A projection of bone having a semi-spherical shape, usually as an articulation or joint with another bone.”
Cotylar = “The portion of a joint having a concave, or internally cupped shape. Its opposite is condylar.”
Cotyle/Cotylus = “A region of bone recessed to articulate with a condyle.”
Dentigerous = “Bearing or capable of bearing teeth, usually as far as having tooth sockets, slots or channels for teeth, or with teeth in situ.”
Dental margin = “The portion of a jaw that corresponds to the tooth row, in matters where the tooth row itself is not being accounted for.”
Distal = When referring to the skull or jaw, “Towards the rear of the skull or jaw. Its opposite is mesial.” When referring to a specific non-oral bone, “Away from the origin point, as proximal is its opposite.” Adv. Distally.
Edentulous = “Lacking any evidence for teeth.” This is used without the additional meaning of simply not preserving teeth, as it is used in Mammology.
Labial = “Away from the midline of the skull or jaw. Its opposite is lingual.” Adv. Labially.
Labiolingual = “Along a line perpendicular to the sagittal plane, but also perpendicular to the mesiodistal axis.” Adv. Labiolingually.
Lingual = “Towards the midline of the skull or jaw. Its opposite is labial.” Adv. Lingually.
Mesial = “Towards the front of the skull or jaw. Its opposite is distal.” Adv. Mesially.
Mesiodistal = “Along line formed from the front to the back of the skull or jaw, which may curve.” Adv. Mesiodistally.
Oral Bone = “Any of a bone that forms the margin of the oral orifice, especially a tooth-bearing bone; any bone that contributes to the palate or lingual apparatus.”
Ramus = “1. Generally, any bar of bone forming a radial strut. 2. Specifically, one of the two halves of the mandible, forming a V-shape when conjoined at the symphysis at the mesial end, hinged and attached to the cranium at the proximal end (when the mandible is isolated, at the distal end).”
Symphysis = “The region of the mandibular ramus that contacts the other mandibular ramus; this is generally confined to the dentary, although in some archosaurs it includes the splenial, a condition that is almost the case in some oviraptorosaurs.”
Tomial = “Pertaining to a tomus.” Adv. Tomially.
Tomiate = “Having or being a tomus.”
Tomus/Tomia = “A ridge or region of bone along a margin forming a sharpened ridge, especially as the rim of a strap-like region of bone.”
Proceed to Part One, Toothed Oviraptorosaurs
 Sternberg, R. M. 1940. A toothless bird from the Cretaceous of Alberta. Journal of Paleontology 14(1):81-85.
 Gilmore, C. W. 1924. A new coelurid dinosaur from the Belly River Cretaceous of Alberta. Canada Department of Mines Geological Survey Bulletin (Geological Series) 38(43):1-12.
 Sues, H.-D. 1997. On Chirostenotes, a Late Cretaceous oviraptorosaur (Dinosauria: Theropoda) from western North America. Journal of Vertebrate Paleontology, 17(4): 698-716.
 Osborn, H. F. 1924. Three new Theropoda, Protoceratops zone, central Mongolia. American Museum Novitates 144: 1-12 pp.
 Ji Q. & Ji S-a.. 1997. A Chinese archaeopterygian, Protarchaeopteryx gen. nov. Geological Science and Technology (Di Zhi Ke Ji), 238:38-41.
 Xu X., Cheng Y.-n. Wang X.-l. & Chang C.-h. 2002. An unusual oviraptorosaurian dinosaur from China. Nature, 419:291-293.
 Ji Q., Currie, P. J., Norell, M. A. & Ji S.-a. 1998. Two feathered dinosaurs from northeastern China. Nature, 393(6687):753–761.