Oviraptorosauria

Oviraptorosaurs Temporal range: Ma PreꞒ Ꞓ O S D C P T J K Pg N
Oviraptorosauria diversity. Clockwise from top left: GIN 100/42 (which may represent Citipati or a different taxon), "Ronaldoraptor" (undescribed), Avimimus, Anzu and Gigantoraptor
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Clade:	Pennaraptora
Clade:	†Oviraptorosauria Barsbold, 1976
Subgroups
†Incisivosaurus †Ningyuansaurus?[1] †Protarchaeopteryx †Shanyangosaurus? †Scansoriopterygidae? †Thecocoelurus? †Caudipteridae[2] †Edentoraptora[3] †Avimimus †Caenagnathoidea
Synonyms
Caenagnathiformes Sternberg, 1940 Avimimiformes Chatterjee, 1991

Oviraptorosaurs ("egg thief lizards") are a group of feathered

Oviraptorosaurians have shortened rostrums, massive, beaklike mandibles, and long parietal bones. With the exception of the 8-meter long Gigantoraptor, they are generally medium-sized and rarely exceeded 2 meters in length. The most primitive members have four pairs of teeth in the premaxillae, such as in Caudipteryx^[9] and in Incisivosaurus they are enlarged and form bizarrely prominent bucktoothed incisors. The more advanced members have no teeth in the jaws.

Oviraptorosaurians are different from most other maniraptorans in the form of their skulls. They have shortened snouts, beak-like jaws with few or no teeth, and a large opening in the lower jaw bone. Some have bony crests atop the skull. The most primitive members have a few teeth in the front of the mouth; in Incisivosaurus, they are enlarged and form bizarrely prominent "bucktoothed" incisors. The arms and hands are generally long (though very reduced in some advanced species) and the shoulder girdle is large and massive, with flexed coracoid bones and prominent attachments for strong arm muscles.

Their tails are very short compared to other maniraptorans. In Nomingia and Similicaudipteryx, the tail ends in four fused vertebrae which Osmólska, He, and others have referred to as a "pygostyle", but which Witmer found was anatomically different and evolved separately from the pygostyle of birds (a bone which serves as the attachment point for a fan of tail feathers).^[2][10]

Feathers

Evidence for feathered oviraptorosaurs exists in several forms. Most directly, four species of primitive oviraptorosaurs (in the genera

Notably, a study on flight feathers has concluded that Caudipteryx was secondarily flightless, implying an ancestral volant ancestor for oviraptorosaurs.[14]

Paleobiology

Diet

The eating habits of these animals are not fully known: they have been suggested to have been either carnivorous, herbivorous, mollusk-eating or egg-eating (the evidence that originally supported the latter is no longer considered valid); these options are not necessarily incompatible.

Some ate small

Originally these animals were thought to be egg raiders, based on a Mongolian find showing Oviraptor on top of a nest. Recent studies have shown that the animal was actually on top of its own nest.[15]

A 2022 study of the bite forces of oviraptorosaurs such as Incisivosaurus, Khaan, Citipati, and Conchoraptor suggests that most if not all oviraptorosaurs had a very strong bite force. The moderate jaw gape seen in oviraptorosaurs is indicative of herbivory in the majority of the group, but it is clear they were likely feeding on much tougher or more types of vegetation than other herbivorous theropods in their environment, such as ornithomimosaurs and therizinosaurs were able to. The examinations suggest oviraptorosaurs may have been powerful-biting generalists or specialists that partook of niche partitioning both in body size and cranial function. One particular group, the Caenagnathidae, may have also been more omnivorous or even carnivorous than other oviraptorosaurs.^[16]

Reproduction

Several oviraptorosaurian nests are known, with several oviraptorid specimens preserved in a brooding position over large clutches of up to a dozen or more eggs. The eggs are usually arranged in pairs, and forming a circular pattern within the nest. One oviraptorosaurian specimen from China has been found with two unlaid eggs within the pelvic canal. This suggests that, unlike modern crocodilians, oviraptorosaurs did not produce and lay many eggs at the same time. Rather, the eggs were produced within the reproductive organs in pairs, and laid two at a time, with the mother positioned in the center of the nest and rotating in a circle as each pair was laid. This behavior is supported by the fact that the eggs were shaped like highly elongated ovals — the greatest egg elongation among diapsids — with the more pointed end pointing backward from the cloaca and oriented toward the center of the nest.^[17][18] Geochemical analysis also revealed that oviraptorosaurs incubated their eggs in the 35–40 °C (95–104 °F) range, as many modern bird species do today, based on the oxygen isotope ratios in the bones of the fossil embryos of various species during development.^[19]

The presence of two shelled eggs within the birth canal shows that oviraptorosaurs were intermediate between the reproductive biology of crocodilians and modern birds. Like crocodilians, they had two oviducts. However, crocodilians produce multiple shelled eggs per oviduct at a time, whereas oviraptorosaurs, like birds, produced only one egg per oviduct at a time.^[17]

Relationship to birds

Oviraptorosaurs, like

The internal classification of the oviraptorosaurs has also been controversial. Most studies divide them into two primary sub-groups, the Caenagnathidae and the Oviraptoridae. The Oviraptoridae is further divided into the small, short-armed, and crestless subfamily Ingeniinae, and the larger, crested, long-armed Oviraptorinae. However, some phylogenetic studies have suggested that many traditional members of the Caenagnathidae were more closely related to the crested oviraptorids.

Phylogeny

The 2007 cladistic analysis of Turner and colleagues recovered the Oviraptorosauria as a maniraptoran clade (natural grouping) of maniraptorans more primitive than true birds. They found that the oviraptorosaurs are the sister group to the Therizinosauria and that the two, together, are more basal than any member of Paraves.^[23] However, a more recent study by Zanno and colleagues challenged that finding, showing therizinosaurs to be more primitive and not closely related to oviraptorosaurs.^[24]

The following cladogram was found by an analysis published with the description of the caenagnathid Anzu.^[25]

See also

• Timeline of oviraptorosaur research

References