Caenagnathidae

Caenagnathidae is a family of advanced caenagnathoid dinosaurs that lived during the Cretaceous in North America and Asia. They belong to the group Oviraptorosauria and are closely related to the Oviraptoridae. Like other oviraptorosaurs, they had specialized beaks, long necks, short tails, and were likely covered in feathers.

For a long time, the relationships of caenagnathids were unclear. The family was first named in 1940 by Raymond Martin Sternberg, who thought they were flightless birds. Later, skeletons of related oviraptorids showed they were actually non-avian theropods. More complete caenagnathid remains also revealed that *Chirostenotes pergracilis* (originally known only from a pair of hands) and *Citipes elegans* (originally thought to be an ornithomimid, based on a foot) were caenagnathids.

The name *Caenagnathus*—and thus Caenagnathidae—means "recent jaws." When first discovered, the lower jaw features suggested they were close relatives of paleognath birds like ostriches. Because finding a recent bird group in the Cretaceous was unusual, the name "recent jaws" was chosen. Most paleontologists now believe the birdlike jaw features evolved independently, through convergent evolution with modern birds.

Caenagnathids included some of the largest oviraptorosaurs. The biggest members, such as *Beibeilong* and *Gigantoraptor*, were enormous. Others were smaller, like *Hagryphus* and *Anzu*. Their overall anatomy was similar to oviraptorids, but with key differences. Their jaws were longer and shallower, suggesting a weaker bite. The inside of the lower jaw had complex ridges, toothlike processes, and horizontal shelf-like structures. The jaws were also hollow and air-filled, connected to the air sac system.

Studies of oviraptorosaur function show caenagnathids were less suited to herbivory than more primitive oviraptorosaurs or oviraptorids. Their mandibles had lower mechanical advantage in the front but not the back, similar to dromaeosaurids, and were less robust. This made them better at quick prey capture, with pointed beaks that could slash. They were also more lightly built than oviraptorids, with slender arms and long, graceful legs, though they lacked the extreme running adaptations of avimimids and *Caudipteryx*.

Caenagnathidae, along with its sister group Oviraptoridae, makes up the superfamily Caenagnathoidea. In phylogenetic terms, Caenagnathidae is defined as the most inclusive group containing *Caenagnathus collinsi* but not *Oviraptor philoceratops*. Before the 2010s, only about two to six species were commonly recognized, but that number may now reach up to ten, thanks to new discoveries and reinterpretations. Much of this change involves *Chirostenotes pergracilis*, the first caenagnathid described. Poor preservation led to misidentifications, with different bones assigned to different species. For example, the feet of *Macrophalangia canadensis* were later linked to *Chirostenotes* when a specimen with both hands and feet was found. A partial skull with hands and feet later suggested *Chirostenotes* and *Caenagnathus* were the same animal, though current studies still treat them as closely related genera.

In 2015, Hendrickx and colleagues defined Caenagnathinae as all caenagnathids closer to *Caenagnathus collinsi* than to *Elmisaurus rarus*, and Elmisaurinae as those closer to *Elmisaurus rarus* than to *Caenagnathus collinsi*. A cladogram from Gregory Funston in 2020 outlines 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 form. The tiny jaws of *Caenagnathasia* suggest an animal 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 the largest member, reaching great length and weight. Their beaks also varied: *Caenagnathasia* had a short, deep beak, while *Caenagnathus* had a long, shovel-shaped one. This variation in size and beak shape suggests they evolved to fill different ecological niches. Caenagnathids survived until the end of the Cretaceous, as shown by *Anzu* and an unnamed elmisaurine species from the late Maastrichtian Hell Creek Formation.