Oviraptoridae

Skull of Nemegtomaia, featuring deep lower jaws and a large palatal region

The diet of oviraptorids is not fully understood. Though some appear to have been at least partially carnivorous, they were probably primarily herbivorous or omnivorous.[5]

Originally, oviraptorids were thought to be specialized egg raiders, based on a Mongolian find showing Oviraptor on top of a nest erroneously attributed to the ceratopsian dinosaur Protoceratops. However, discoveries in the 1990s, including Citipati specimens clearly brooding (rather than preying on) the same types of nests, and a Citipati embryo inside the same type of egg preserved in these nests, showed that the "specialized egg thief" idea was incorrect.[6] Still, some scientists have suggested that oviraptorids may have fed on shelled food items like eggs or shellfish. However, animals specialized for eating shelled food typically have broad, crushing beaks or teeth. In contrast, the jaws of oviraptorids had thin, sharp edges probably supporting shearing beaks, ill-suited for cracking shells. Among other known animals, the beaks of oviraptorids most closely resemble those of herbivorous dicynodont synapsids, which are usually considered herbivorous.(This leads to the possibility that these animals are omnivorous).[5]

Their beaks also share similarities with the beaks of herbivorous parrots and tortoises.[7]

Evidence of partial carnivory among some oviraptorines comes from a lizard skeleton preserved in the body cavity of the type specimen of Oviraptor[8] and two hatchling Byronosaurus skulls found in a Citipati nest.[9] Some scientists have also suggested that some oviraptorids (especially the small-handed, weak-clawed "ingeniines") fed mainly on plant material.[5]

Collection of several embryo-bearing eggs of oviraptorids

Although fossilized dinosaur eggs are generally rare, oviraptorid eggs are relatively well known. Several oviraptorid nests, eggs, and embryos are known, mostly uncovered in the Gobi Desert. Some specimens of Oviraptor philoceratops, Citipati osmolskae, Nemegtomaia and cf. Machairasaurus have been found in brooding positions in association with nests.[8][10][11] All of the nesting specimens are situated on top of egg clutches, with their limbs spread symmetrically on each side of the nest, front limbs covering the nest perimeter. This brooding posture is found today only in birds and supports a behavioral link between birds and theropod dinosaurs.[10]

Oviraptorid eggs are shaped like elongated ovals (elongatoolithid) and resemble the eggs of ratite birds (such as ostriches) in texture and shell structure. In the nest, eggs are typically found in pairs and arranged in concentric circles of up to three layers, with complete clutches consisting of as many of 22 eggs in some species.[12] The eggs of Citipati are the largest known definitive oviraptorid eggs, at 18 cm. In contrast, eggs associated with Oviraptor are only up to 14 cm long.[10]

The first oviraptorid eggs (of the genus Oviraptor, which mean "Egg thief") were found in close proximity to the remains of the ceratopsian dinosaur Protoceratops and it was assumed that the oviraptorids were preying upon the eggs of the ceratopsians.[13] It was not until 1993, when a Citipati embryo was discovered inside an egg of the type assigned to Protoceratops, that the error was corrected.[6] Norell et al., who recognized the embryo as oviraptorid, assigned it to the genus Citipati. The egg containing the embryo was smaller than most known Citipati eggs at only 12 cm, though it was partially eroded and broken into three pieces, making an accurate estimate of its original size difficult.[10] The embryo-bearing egg was otherwise identical to other oviraptorid eggs in shell structure and was found in an isolated nest, again arranged in a circular pattern.[6]

Nesting Nemegtomaia specimen MPC-D 107/15

An oviraptorosaurian specimen from China described in 2005 was found to have 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 oval shape, with the more narrow end pointing backward from the birth canal, matching their orientation toward the center of the nest after being laid.[14]

Oviraptorid nest/egg clutch (specimen PFMM 0010403018)

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.[14]

In 2017, paleontologists discovered colored pigments in some fossilized oviraptorid embryos of the egg-shell genus Macroolithus (which may represent eggs of Heyuannia). Examinations of eggs attributed to Heyuannia by Jasmina Wiemann and Tzu-Ruei Yang et al revealed the eggs preserved the blue-green pigment biliverdin and the reddish-brown pigment protoporphyrin, the same pigments found in many modern birds' eggshells. The eggs are thought to have been a blue-green color, because biliverdin is preserved in much greater abundance the photoporphyrin. In modern bird eggs, coloration can camouflage the eggs or help parents identify eggs, and it is correlated with more intensive parental care.[15]

A study by Robert Eagle et al. of the University of California-Los Angeles indicates that from specimens of eggs found in Mongolia and examination of the isotopes carbon-13 and oxygen 18 found within, Oviraptorids had body temperatures that could be elevated higher than that of the surrounding environment but lower than that of birds. This is very different from the isotope ratios of sauropod dinosaurs like Brachiosaurus, which had body temperatures of up to 100 °F (38 °C) and were fully endothermic.[16]

Restoration of a nesting Nemegtomaia, featuring feathers used to incubate eggs

Oviraptorids were probably feathered, since some close relatives were found with feathers preserved (including species of Caudipteryx, Protarchaeopteryx and Similicaudipteryx).[17][18] Another finding pointing to this is the discovery in Nomingia of a pygostyle, a bone that results from the fusion of the last tail vertebrae and is responsible in birds to hold a fan of feathers in the tail.[19] Finally, the arm position of the brooding Citipati would have been far more effective if feathers were present to cover the eggs.[20]

The brooding oviraptorid specimen IGM 100/979 showed a callus and possible longitudinal groove left over from a healed fracture of the right ulna. Other oviraptorids have had pathological features reported in their phalanges but these have not been described in detail in the scientific literature.[21]