Caenagnathidae

Caenagnathidae were a family of advanced caenagnathoid dinosaurs that lived during the Cretaceous in North America and Asia. They belonged to the group Oviraptorosauria and were closely related to the Oviraptoridae. Like their relatives, they had specialized beaks, long necks, short tails, and were probably covered in feathers.

For a long time, scientists were confused about how caenagnathids fit into the dinosaur family tree. In 1940, Raymond Martin Sternberg originally named the family, thinking it belonged to a group of flightless birds. Later, when skeletons of related oviraptorids were found, it became clear that caenagnathids were actually non-avian theropods. More complete caenagnathid fossils also showed that two other dinosaurs—*Chirostenotes pergracilis* (first known only from a pair of hands) and *Citipes elegans* (first thought to be an ornithomimid, based on a foot)—were actually caenagnathids.

The name *Caenagnathus* (and therefore Caenagnathidae) means "recent jaws." When first discovered, researchers believed caenagnathids were close relatives of paleognath birds like ostriches, based on features of the lower jaw. Because finding a recent bird group in the Cretaceous would have been unusual, the name "recent jaws" was chosen. Most paleontologists now think the birdlike jaw features evolved separately, through convergent evolution with modern birds.

Caenagnathids included some of the largest oviraptorosaurs ever. The biggest members were *Beibeilong* and *Gigantoraptor*, both enormous in size. Others were smaller, like *Hagryphus* and *Anzu*. Their overall anatomy was similar to oviraptorids, but with key differences. Their jaws were longer and shallower than those of oviraptorids, suggesting a weaker bite. The inside of the lower jaw had complex ridges, toothlike processes, and a pair of horizontal shelves. Unusually, the jaws were hollow and connected to the air sac system.

Studies of how oviraptorosaurs moved and fed show that caenagnathids were less suited to eating plants than more primitive oviraptorosaurs or their close relatives the oviraptorids. Their lower jaws had less mechanical advantage at the front but not at the back, similar to dromaeosaurids, and were less robust. This made them better at quickly capturing prey, with pointed beaks that could slash.

Caenagnathids were also more lightly built than oviraptorids. They had slender arms and long, graceful legs, though they did not have the extreme running adaptations seen in avimimids or *Caudipteryx*.

Caenagnathidae, along with its sister group Oviraptoridae, forms the superfamily Caenagnathoidea. In phylogenetic terms, the clade Caenagnathidae includes all species more closely related to *Caenagnathus collinsi* than to *Oviraptor philoceratops*. Before the 2010s, only about two to six species were commonly recognized as caenagnathids, but now that number may reach up to ten, thanks to new discoveries and revised ideas about older species. Much of this confusion centers on the first described caenagnathid, *Chirostenotes pergracilis*. Because caenagnathid fossils are often poorly preserved and misidentified, different bones and specimens of *Chirostenotes* were once assigned to many different species. For example, the feet of a species called *Macrophalangia canadensis* came from the same area as *Chirostenotes*, and a later specimen with both hands and feet showed they were the same animal. Another partial skull with hands and feet suggested *Chirostenotes* and *Caenagnathus* might be identical, though current studies still treat them as closely related but separate genera.

In 2015, Hendrickx and colleagues defined a subgroup called Caenagnathinae, which includes all caenagnathids more closely related to *Caenagnathus collinsi* than to *Elmisaurus rarus*. The group Elmisaurinae includes species more closely related to *Elmisaurus rarus* than to *Caenagnathus collinsi*. A cladogram from Gregory Funston’s 2020 analysis shows these relationships.

The earliest known caenagnathid is *Microvenator celer*, from the Early Cretaceous Cloverly Formation. Caenagnathids likely spread from North America to Asia, with some later reappearing in western North America during the Campanian. They showed considerable variation in size and shape. The tiny jaws of *Caenagnathasia* suggest an animal about the size of a turkey. *Anzu wyliei*, from the Hell Creek Formation, was much larger than a human. If *Gigantoraptor erlianensis* is a caenagnathid, it would be by far the largest member, reaching great length and weight.

Their beaks also varied widely: *Caenagnathasia* had a short, deep beak, while *Caenagnathus* had a long, shovel-shaped one. This variation in size and beak shape suggests caenagnathids evolved to fill a range of ecological niches. They survived until the end of the Cretaceous, as shown by the presence of *Anzu* and an unnamed elmisaurine species in the late Maastrichtian Hell Creek Formation.