Oviraptorosauria

Oviraptorosaurs ("egg thief lizards") are a group of feathered maniraptoran dinosaurs from the Cretaceous Period of what are now Asia and North America. They are distinct for their characteristically short, beaked, parrot - like skulls, with or without bony crests atop the head. The group includes the Oviraptoridae, the Caenagnathidae and several species which do not belong to either of these families, including Avimimus and Caudipteryx, and Incisivosaurus. They ranged in size from Caudipteryx, which was the size of a turkey, to the 8 meter long, 1.4 ton Gigantoraptor. The group (along with all maniraptoran dinosaurs) is close to the ancestry of birds. Analyses like those of Osmolska et al. (2004) suggest that they may in fact represent primitive flightless birds.

Paul Sereno, 2005 named the group Oviraptoriformes for all dinosaurs closer to Oviraptor than to modern birds. Therizinosaurs are usually considered members of this larger group, alongside the oviraptorosaurs.

Anatomy
Oviraptorosaurians are most different from other maniraptorans in the form of their skulls. They have shortened snouts, massive, beaklike jaws, and a parrot -like head overall. 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 tail is very short and thick at the base. The arms and hands are long and the shoulder girdle is large and massive, with flexed coracoids and prominent attachments for strong arm muscles.

In Nomingia the tail ends in four fused vertebrae which Osmolska et al. call a pygostyle, but which Witmer (2002), found was anatomically different and non - homologous to the pygostyle of birds.

Feathers
Evidence for feathered oviraptorosaurs exists in several forms. Most directly, two species of primitive oviraptorosaurs (Caudipteryx) have been found with impressions of well developed feathers, most notably on the wings and tail, suggesting that they functioned at least partially for display. Secondly, at least one oviraptorosaur (Nomingia) was preserved with a tail ending in something like a pygostyle, a bony structure at the end of the tail that, in modern birds, is used to support a fan of feathers. Similarly, quill knobs (anchor points for wing feathers on the ulna) have been reported in the oviraptorosaurian species, Avimimus portentosus. Additionally, a number of oviraptorid specimens have famously been discovered in a nesting position similar to that of modern birds. The arms of these specimens are positioned in such a way that they could perfectly cover their eggs if they had small wings and a substantial covering of feathers.

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 latter is no longer considered valid); these options are not necessarily incompatible.

Some ate small vertebrates. Evidence for this comes from a lizard skeleton preserved in the body cavity of Oviraptor and two baby Troodontid skulls found in a Citipati nest. Evidence in favor of a herbivorous diet includes the presence of gastroliths preserved with Caudipteryx. There are also arguments for the inclusion of mollusks in their diet.

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 in fact the animal was on top of its own nest.

Relationship to birds
The cladistic analysis of Turner et al. (2007) recovered the Oviraptorosauria as a maniraptoran clade that branches off from the main lineage of maniraptorans far before birds. They find that the Oviraptorosauria are the sister group to the Therizinosauria and that the two, together, are more basal than any member of Paraves.

Oviraptorosaurs, like dromaeosaurs, are so bird-like that several scientists consider them to be true birds, more advanced than Archaeopteryx. Gregory S. Paul (1998, 2002) has written extensively on this possibility and Maryanska et al. published a technical paper, detailing this idea in 2002. Michael Benton (2004), in his widely-respected text Vertebrate Paleontology, also includes oviraptorosaurs as an order within the class Aves. However, a number of researchers disagree with this classification, retaining oviraptorosaurs as non-avian maniraptorans slightly more primitive than the dromaeosaurs. For a detailed technical discussion of this debate, see Discussion of Maryanska et al. (2002).

Taxonomy

 * Infraorder Oviraptorosauria
 * Incisivosaurus
 * Protarchaeopteryx
 * Family Avimimidae
 * ''Avimimus
 * Family Caudipteridae
 * ''Caudipteryx
 * ''Similicaudipteryx
 * Superfamily Caenagnathoidea
 * Hagryphus
 * Family Caenagnathidae
 * Caenagnathasia
 * Chirostenotes
 * Elmisaurus
 * ?Nomingia
 * Family Oviraptoridae
 * Gigantoraptor
 * Microvenator
 * Shixinggia
 * Subfamily Ingeniinae
 * Conchoraptor
 * Heyuannia
 * "Ingenia"
 * Khaan
 * Subfamily Oviraptorinae
 * Citipati
 * Nemegtomaia
 * Oviraptor
 * Rinchenia

Phylogeny
The following cladogram follows an analysis by Phil Senter, 2007.

Osteology
Oviraptorosaurians have shortened rostrums, massive, beaklike mandibles, and long parietals. The most primitive members have four pairs of teeth in the premaxillae, in Incisivosaurus they are enlarged and form bizarrely prominent bucktoothed incisors. The more advanced members have no teeth in the jaws. Pneumatization is extensive in the skulls and vertebrae of the more advanced members. Oviraptorosauria have thick, U - shaped furculae and a large sternal plates that are wider (together) than they are long, unlike in birds and dromaeosaurs. The arms are around half the length of the legs and over half the length of the presacral vertebral column. The Hands are long, and tridactyl, with a reduced third finger in Caudipteryx and Ingenia. There are between 5 and 8 sacral vertebrae. The pubis is vertical or subvertical. The tibia is 15%-25% longer than the femur. The tail is short, with the number of vertebrae reduced to 24 or so, and proximally very thick, with broad transverse processes. The ischium retains the primitive character of a prominent, triangular obturator process and lack the proximodorsal process that is found in birds. The pectoral girdle is also primitive; the scapula is a broad blade that is distally expanded, it lies on the lateral aspect of the thorax at an angle to the vertebral column, and the coracoid has the primitive coelurosaur shape with a proximal supracoracoidal nerve foramen and a moderate biceps tubercle.