Caenagnathidae

Anzu wyliei skeleton cast in the Rocky Mountain Dinosaur Resource Center in Woodland Park, Colorado, USA.

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.[2] Caenagnathids also tended to be more lightly built than the oviraptorids. They had slender arms and long, gracile legs,[7] 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.[8][9]

The earliest known caenagnathid is Microvenator celer, from the Early Cretaceous Cloverly Formation. Caenagnathids likely dispersed to Asia from North America with some caenagnathids later reappearing in western North America, during the Campanian. Caenagnathids showed considerable variation in form. The tiny jaws of Caenagnathasia suggest a small animal, perhaps the size of a turkey. Anzu wyliei, from the Hell Creek Formation is 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 Anzu and another, unnamed species similar to Citipes and Elmisaurus 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,[10] 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[7] provided the support to combine them, while the later discovery of a partial skull with hands and feet [3] suggested that Chirostenotes and Caenagnathus were the same animal, and current studies of caenagnathid relationships continue to find them as closely related genera.[11]

Caenagnathid skeletons to scale

Hendrickx and colleagues (2015) defined a subgroup of Caenagnathidae, the Caenagnathinae, as all caenagnathids more closely related to Caenagnathus collinsi than to Elmisaurus rarus.[12] The group Elmisaurinae is defined as including all species more closely related to Elmisaurus rarus than to Caenagnathus collinsi.[12][13]

The cladogram below follows an analysis by Gregory Funston in 2020.[14]

• Timeline of oviraptorosaur research

• Overview of Caenagnathidae by Jaime Headden.