Caenagnathidae is a family of bird-like maniraptoran theropod dinosaurs from the Late Cretaceous of North America and Asia. They are a member of the Oviraptorosauria, and close relatives of the Oviraptoridae. Like other oviraptorosaurs, caenagnathids had specialized beaks, long necks, and short tails, and would have been covered in feathers. The relationships of caenagnathids were long a puzzle. The family was originally named by Charles Hazelius Sternberg in 1940  as a family of flightless birds. The discovery of skeletons of the related oviraptorids revealed that they were in fact non-avian theropods, and the discovery of more complete caenagnathid remains  revealed that Chirostenotes pergracilis, originally named on the basis of a pair of hands, and "Ornithomimus" elegans, named from a foot, were caenagnathids as well.
|Reconstructed skull of Anzu wyliei|
Overall, the anatomy of the caenagnathids is similar to that of the closely related Oviraptoridae, but there are a number of differences. In particular, caenagnathid jaws exhibited a distinct suite of specializations not seen in other oviraptorosaurs. Compared to the oviraptorids, the jaws tended to be relatively long and shallow, suggesting that the bite was not as powerful. The inside of the lower jaws also bore a complex series of ridges and toothlike processes, as well as a pair of horizontal, shelf-like structures. Furthermore, the jaws were unusual in being hollow and air filled, apparently being connected to the air sac system. Caenagnathids also tended to be more lightly built than the oviraptorids. They had slender arms and long, gracile legs, although they lacked the extreme cursorial specializations seen in avimimids and Caudipteryx.
The name Caenagnathus (and hence Caenagnathidae) means "recent jaws"—when first discovered, it was thought that caenagnathids were close relatives of paleognath birds (such as the ostrich) based on features of the lower jaw. Since it would be unusual to find a recent group of birds in the Cretaceous, the name "recent jaws" was applied. Most paleontologists, however, now think that the birdlike features of the jaw were acquired convergently with modern birds.
The earliest known caenagnathid is Caenagnathasia martinsoni, from the Turonian Bissekty Formation of Uzbekistan. The jaw of Caenagnathasia already has the specialized ridges and crushing surfaces seen in later forms. This suggests that caenagnathids originated well before the Turonian, but currently, there are no Early Cretaceous caenagnathid fossils. Caenagnathids later appeared in western North America, during the Campanian, suggesting that they may have originated in Asia, then migrated into western North America. Caenagnathids showed considerable variation in form. The tiny jaws of Caenagnathasia suggest a small animal, perhaps the size of a turkey. An unnamed species from the Hell Creek Formation  indicates a much larger animal, considerably larger than a human. If Gigantoraptor erlianensis is a caenagnathid, then it would represent far and away the largest member of the group, measuring up to 8 meters in length and weighing up to 1.4 tons. Their beaks also show considerable variation; that of Caenagnathasia is relatively short and deep, while that of Caenagnathus is long and shovel-shaped. This variation in size and beak shape suggests that caenagnathids evolved to exploit a range of ecological niches. Caenagnathids persisted up until the end of the Cretaceous period, as shown by the presence of at least two species in the late Maastrichtian Hell Creek Formation, before vanishing at the end of the Cretaceous along with all other non-avian dinosaurs.
The family Caenagnathidae, together with its sister group the Oviraptoridae, comprises the superfamily Caenagnathoidea. In phylogenetic taxonomy, the clade Caenagnathidae is defined as the most inclusive group containing Chirostenotes pergracilis but not Oviraptor philoceratops. While before 2010s only about two to six species were commonly recognized as belonging to the Caenagnathidae, currently that number may be much greater, with new discoveries and theories about older species that may inflate this number to up to ten. Much of this historical difference centers on the first caenagnathid to be described, Chirostenotes pergracilis. Due to the poor preservation of most caenagnathid remains and resulting misidentifications, different bones and different specimens of Chirostenotes have historically been assigned to a number of different species. For example, the feet of one species, named Macrophalangia canadensis, were known from the same region from which Chirostenotes pergracilis was recovered, but the discovery of a new specimen with both hands and feet preserved provided the support to combine them, while the later discovery of a partial skull with hands and feet  suggested that Chirostenotes and Caenagnathus were the same animal, and current studies of caenagnathid relationships continue to find them as closely related genera.
Longrich and colleagues (2013) defined a subgroup of Caenagnathidae, the Caenagnathinae, as all caenagnathids more closely related to Caenagnathus collinsi than to Caenagnathasia martinsoni or Elmisaurus rarus. In 2015, the group Elmisaurinae was defined, including all species more closely related to Elmisaurus rarus than to Caenagnathus collinsi.
Roughly a dozen caenagnathid species have been named, but it remains unclear how many are valid. Many species are known from fragmentary remains, such as jaws, hands, or feet, making comparisons between them difficult. Caenagnathus sternbergi, for example, was described on the basis of a jaw bone. It has been interpreted as either the jaws of Chirostenotes pergracilis (described on the basis of a pair of hands) or Chirostenotes elegans (described on the basis of a foot), but because no complete skeleton is known, it is difficult to be certain which animal it belongs to. The relationships of other species remain in doubt. Gigantoraptor was originally interpreted as an oviraptorid, but may in fact represent a primitive caenagnathid.
Caenagnathids are only known from the Late Cretaceous of North America and Asia. The earliest and most primitive known caenagnathid is Caenagnathasia martinsoni, from the Bissekty Formation of Uzbekistan. The largest is the enormous Gigantoraptor erlianensis.
Anomalipes zhaoi ("meaning Zhao Xijin's unusual foot") is an extinct caenagnathid dinosaur discovered in the Campanian-aged Wangshi Group in China. It is the only species in the genus Anomalipes.Anzu wyliei
Anzu (named for Anzû, a feathered demon in ancient Mesopotamian mythology) is a genus of large oviraptorosaurian dinosaurs from the late Cretaceous (66 million years ago) of North Dakota and South Dakota, US. The type species is Anzu wyliei.
In 2015, the International Institute for Species Exploration names it as one of the "Top 10 New Species" for new species discovered in 2014.Apatoraptor
Apatoraptor ("Apatè robber") is a genus of caenagnathid dinosaur which contains a single species, A. pennatus. The only known specimen was discovered in the Campanian-age Horseshoe Canyon Formation of Alberta.Beibeilong
Beibeilong is an extinct genus of oviraptorosaurian dinosaur. The genus contains a single species B. sinensis, named in 2017 by Pu and colleagues based on a nest with a possible embryo (nicknamed "Baby Louie") and eggs. It was similar to but more basal than Gigantoraptor, and may have been of similar size as an adult. This result was consistently recovered by phylogenetic analyses incorporating ontogenetically variable characteristics, which led Pu et al. to conclude that they were distinct taxa.Caenagnathasia
Caenagnathasia ('recent jaw from Asia') is a small caenagnathid oviraptorosaurian theropod from the Late Cretaceous of Uzbekistan.Caenagnathoidea
Caenagnathoidea ("recent jaw forms") is a group of advanced oviraptorosaurian dinosaurs from the Cretaceous Period of what are now Asia and North America. They are distinct for their characteristically short, beaked, parrot-like skulls, often with bony crests atop the head. 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. The most complete specimens have been found in Asia, representing members of the sub-group Oviraptorinae. Notable but fragmentary remains are also known from North America, almost all of which belong to the subgroup Elmisaurinae.The earliest definitive caenangnathoid is Microvenator celer, which dates to the late Aptian age of the early Cretaceous period, though the slightly earlier Caudipteryx from the lower Yixian Formation of China, may also be a member of this group.Caenagnathus
Caenagnathus ('recent jaw') is a genus of caenagnathid oviraptorosaurian dinosaur from the late Cretaceous (Campanian; ~75 million years ago). It is known from partial remains including lower jaws, a tail vertebra, hand bones, and hind limbs, all found in the Dinosaur Park Formation of Alberta, Canada. Caenagnathus weighted about a maximum of 96 kg (212 lb).Chirostenotes
Chirostenotes ( KY-ro-sti-NOH-teez; named from Greek 'narrow-handed') is a genus of oviraptorosaurian dinosaur from the late Cretaceous (about 76.5 million years ago) of Alberta, Canada. The type species is Chirostenotes pergracilis.Elmisaurus
Elmisaurus is an extinct genus of dinosaur from the Late Cretaceous. It was a theropod belonging to the Oviraptorosauria. Its fossils have been found in Mongolia. It is known from foot and hand bones.Epichirostenotes
Epichirostenotes is a genus of oviraptorosaurian dinosaur from the late Cretaceous. Epichirostenotes is known from an incomplete skeleton found in 1923 at the Horseshoe Canyon Formation, in strata dated to about 72 million years ago. It was first named by Robert M. Sullivan, Steven E. Jasinski and Mark P.A. van Tomme in 2011 and the type species is Epichirostenotes curriei. Its holotype, ROM 43250, had been assigned to Chirostenotes pergracilis by Hans-Dieter Sues in 1997.Gigantoraptor
Gigantoraptor is a genus of giant oviraptorosaurian theropod dinosaur.Hagryphus
Hagryphus ("Ha's griffin"), is an oviraptorosaurian theropod dinosaur from the Upper Cretaceous Period of what is now Utah.Leptorhynchos (dinosaur)
Leptorhynchos is an extinct genus of caenagnathid dinosaurs known from the Late Cretaceous Dinosaur Park, Hell Creek and Aguja Formations of west Texas and Montana, United States and southern Alberta, Canada. It lived about 75 to 66 million years ago.Nomingia
Nomingia is a genus of oviraptorid theropod dinosaur hailing from the Late Cretaceous Bugin Tsav Beds of Mongolia.Ojoraptorsaurus
Ojoraptorsaurus is a genus of oviraptorosaurian dinosaur from the late Cretaceous. Ojoraptorsaurus is only known from pubic bones found at the Naashoibito Member of the Ojo Alamo Formation dating to the early Maastrichtian, about 69 million years ago. It was first named by Robert M. Sullivan, Steven E. Jasinski and Mark P.A. van Tomme in 2011 and the type species is Ojoraptorsaurus boerei. The generic name combines a reference to the formation with a Latin raptor, "plunderer", and a Latinised Greek saurus, "lizard". The specific name honours oceanographer Arjan Boeré who found the specimen.Oviraptoridae
Oviraptoridae is a group of bird-like, herbivorous and omnivorous maniraptoran dinosaurs. Oviraptorids are characterized by their toothless, parrot-like beaks and, in some cases, elaborate crests. They were generally small, measuring between one and two metres long in most cases, though some possible oviraptorids were enormous. Oviraptorids are currently known only from the Late Cretaceous of Asia, with the most well-known species and complete specimens found only in the Gobi Desert of Mongolia and northwestern China.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. They ranged in size from Caudipteryx, which was the size of a turkey, to the 8 metre long, 1.4 ton Gigantoraptor. The group (along with all maniraptoran dinosaurs) is close to the ancestry of birds. Analyses like those of Maryanska et al (2002) and Osmólska et al. (2004) suggest that they may represent primitive flightless birds. The most complete oviraptorosaur specimens have been found in Asia. The North American oviraptorosaur record is sparse.The earliest and most basal ("primitive") known oviraptorosaurs are Ningyuansaurus wangi, Protarchaeopteryx robusta and Incisivosaurus gauthieri, both from the lower Yixian Formation of China, dating to about 125 million years ago during the Aptian age of the early Cretaceous period. A tiny neck vertebra reported from the Wadhurst Clay Formation of England shares some features in common with oviraptorosaurs, and may represent an earlier occurrence of this group (at about 140 million years ago).Timeline of oviraptorosaur research
This timeline of oviraptorosaur research is a chronological listing of events in the history of paleontology focused on the oviraptorosaurs, a group of beaked, bird-like theropod dinosaurs. The early history of oviraptorosaur paleontology is characterized by taxonomic confusion due to the unusual characteristics of these dinosaurs. When initially described in 1924 Oviraptor itself was thought to be a member of the Ornithomimidae, popularly known as the "ostrich" dinosaurs, because both taxa share toothless beaks. Early caenagnathid oviraptorosaur discoveries like Caenagnathus itself were also incorrectly classified at the time, having been misidentified as birds.The hypothesis that caenagnathids were birds was questioned as early as 1956 by Romer, but not corrected until Osmolska formally reclassified them as dinosaurs in 1976. Meanwhile, the classification of Oviraptor as an ornithomimid persisted unquestioned by researchers like Romer and Steel until the early 1970s when Dale Russell argued against the idea in 1972. In 1976 when Osmolska recognized Oviraptor's relationship with the Caenagnathids, she also recognized that it was not an ornithomimid and reclassified it as a member of the former family. However, that same year Rinchen Barsbold argued that Oviraptor belonged to a distinct family he named the Oviraptoridae and he also formally named the Oviraptorosauria later in the same year.Like their classification, the paleobiology of oviraptorosaurs has been subject to controversy and reinterpretation. The first scientifically documented Oviraptor skeleton was found lying on a nest of eggs. Because its powerful parrot-like beak appeared well-adapted to crushing hard food items and the eggs were thought to belonged to the neoceratopsian Protoceratops, oviraptorosaurs were thought to be nest-raiders that preyed on the eggs of other dinosaurs. In the 1980s, Barsbold proposed that oviraptorosaurs used their beaks to crack mollusk shells as well. In 1993, Currie and colleagues hypothesized that small vertebrate prey may have also been part of the oviraptorosaur diet. Not long after, fossil embryonic remains cast doubt on the popular reconstruction of oviraptorosaurs as egg thieves when it was discovered that the "Protoceratops" eggs that Oviraptor was thought to be "stealing" actually belonged to Oviraptor itself. The discovery of additional Oviraptor preserved on top of nests in lifelike brooding posture firmly established that oviraptorosaurs had been "framed" as egg thieves and were actually caring parents incubating their own nests.Wulatelong
Wulatelong is an extinct genus of basal oviraptorid dinosaur known from the Late Cretaceous Wulansuhai Formation (Campanian stage) of Bayan Mandahu, Linhe District of Inner Mongolia, northern China. It contains a single species, Wulatelong gobiensis.