The skull of Anzu is deep and narrow, as previously noted for Epichirostenotes [40] , and is crowned by a tall, cassowary-like crest. Surprisingly, the jugal resembles those of non-oviraptorosaurian theropods in being dorsoventrally deep and posteriorly bifid. The jaws are edentulous, and, as in other derived caenagnathids, the occlusal surface of the coossified dentaries is characterized by a complex array of ridges and grooves. With 12 cervical vertebrae, the neck is long, but it is also transversely wide across the cervical ribs, especially toward its posterior end. The tail is extensively pneumatized and terminates in a short sequence of highly modified vertebrae that collectively comprise a pygostyle-like structure. The sternal plates closely resemble those of oviraptorids in having a pair of well-developed lateral processes [18] . The distal end of the radius is peculiar in being divided into a pair of tuberosities. The manus is proportionally large, and, like those of other derived caenagnathids, its unguals exhibit proximodorsal ‘lips’ that are more prominent than in most other oviraptorosaurs. Although incompletely known in Anzu, manual digits II and III were probably elongate, as in other derived caenagnathids in which the manus is better known (e.g., Chirostenotes pergracilis, Elmisaurus rarus, Hagryphus) [34] , [39] , [63] , [64] . The pubic shaft is straight and the ischium is short and distinctly curved. The hind limb elements are gracile, with the tibia being substantially longer than the femur. The pes is incompletely preserved in Anzu, but what is known suggests that its digits were elongate, as in other derived caenagnathids (e.g., Macrophalangia [35] ).

Anzu wyliei provides, for the first time, a nearly complete view of the skeletal morphology of a caenagnathid oviraptorosaur. With an approximate body length of 3.5 m, a height at the hip of roughly 1.5 m ( Figures 4A , 5A ), and a body mass of some 200–300 kg, Anzu is among the largest known oviraptorosaurs, second in size only to its probable close relative Gigantoraptor (the mass of which has been estimated at between 1,400 and 3,246 kg [4] , [79] ). Moreover, when considered in light of very small-bodied taxa such as Caenagnathasia (with an estimated mass of 5 kg [46] ) and Elmisaurus elegans [73] , it appears highly likely that caenagnathids encompassed a much greater range of body sizes than did other oviraptorosaurs, and indeed, many other non-avian theropod groups. Further studies of caenagnathid growth and the ontogenetic status of individual specimens belonging to this clade are needed to evaluate this hypothesis.

Paleoecology of caenagnathidae

The mode of life of caenagnathids and other oviraptorosaurs has been the subject of much speculation. The fact that the jaws of most oviraptorosaurs lack teeth and were probably covered by a keratinous rhamphotheca has frustrated attempts to infer the diets of these theropods. The earliest suggestion was that the oviraptorid Oviraptor philoceratops fed on the eggs of other dinosaurs, based on the association of the holotypic skeleton with a clutch of eggs that was, at the time, assigned to the ceratopsian Protoceratops [15]. Eggs of this morphotype were subsequently identified as those of oviraptorids [80], and additional oviraptorid skeletons were found, often in bird-like brooding positions, atop nests of such eggs [16]–[18], [20]. Consequently, the prevailing view is that, rather than preying on the nests of other dinosaurs, these oviraptorids were guarding their own nests at the time of death. Nevertheless, the likelihood that oviraptorosaurs brooded their nests does not preclude the possibility that these theropods may also have eaten the eggs of other vertebrates. Indeed, Currie et al. [46] likened the tooth-like ventral processes of the palatal shelves of oviraptorid maxillae to the egg-cracking vertebral processes of the extant egg-eating snake Dasypeltis scabra; based in part on this similarity, these authors argued that oviraptorosaurs may have subsisted on eggs and small-bodied vertebrates. The discovery of two embryonic or perinatal skulls of the troodontid theropod Byronosaurus in a nest with oviraptorid eggs [80], [81] may support this view, in that it suggests that these young troodontids may have been captured and killed by the adult oviraptorid that was presumably tending this nest. Nevertheless, this conclusion should be regarded as tentative, given that, according to J. Clark (pers. comm.), there are no signs of predation on the Byronosaurus specimens. Furthermore, this nest comes from one of the most densely fossiliferous localities within the Upper Cretaceous Djadokhta Formation, with dozens of lizard, dinosaur, and mammal specimens found in close proximity [80], [81]. As such, it is possible that the association of these troodontid skulls with this oviraptorid nest is due to taphonomic or preservational factors rather than representing an authentic paleoecological interaction.

Other previous works have offered alternative dietary hypotheses for Oviraptorosauria. Barsbold [82], [83] has suggested that the edentulous but robust jaws of oviraptorids were employed to crush mollusks; nevertheless, this proposal has been strongly challenged on anatomical, biomechanical, and paleoecological grounds [46], [49]. Cracraft [38] and Funston and Currie [49] noted the close structural similarities between caenagnathid mandibles and those of dicynodontian synapsids, the latter of which are considered largely or exclusively herbivorous. In both caenagnathids and dicynodonts, the morphology of the mandibular joint indicates extensive fore-and-aft motion of the mandible, which could have facilitated the slicing of plant fodder between the opposing, beak-covered jaws. Based on inferences concerning jaw mechanics, Smith [25] also proposed that Oviraptor and its relatives may have been herbivores, whereas Longrich et al. [42] recently came to the same conclusion on the basis of observed anatomical convergences between caenagnathid mandibles and those of extant herbivorous turtles and birds.

In their recent description of the well-preserved Campanian caenagnathid mandible TMP 2001.012.0012, Funston and Currie [49] conducted some of the most detailed comparisons to date of the jaws of these oviraptorosaurs with those of dicynodonts. They observed that TMP 2001.012.0012 possesses four of the five mandibular features that King et al. [84] regarded as specializations for more efficient shearing of vegetation in these synapsids. Specifically, these are a jaw joint that permits anteroposterior movement of the mandible, an intramandibular fenestra, loss of the coronoid eminence coupled with posterodorsal expansion of the dentary, and a fused dentary symphysis. Interestingly, in addition to these four characters, Anzu and Gigantoraptor possess the fifth feature as well: a lateral flange on the dentary. King et al. [84] interpreted this structure as the area of insertion for anteriorly positioned adductor musculature in dicynodonts, and it may have served the same purpose in these two caenagnathids. Regardless of its functional significance, the presence of a lateral flange on the dentaries of Anzu and Gigantoraptor indicates that the degree of convergence in mandibular anatomy between Caenagnathidae and Dicynodontia was even greater than previously appreciated.

Intriguingly, what is perhaps the most compelling evidence for oviraptorosaurian herbivory has thus far been documented only in archaic, tooth-bearing members of the clade. Several specimens of Caudipteryx have been preserved with clusters of gastroliths [2], [8], [26], which strongly suggests the presence of a gastric mill in this taxon [85], and the anteriormost premaxillary teeth of Incisivosaurus resemble rodent incisors [5]. Furthermore, the only known specimen of the recently described basal oviraptorosaur Ningyuansaurus preserves numerous ovate structures within the body cavity that may be seeds [7]. Perhaps, as suggested by Zanno and Makovicky [85], basal oviraptorosaurs were wholly or predominantly herbivorous, but the evolution of a keratinous beak in a derived subset of the clade (Avimimus plus Caenagnathoidea according to our phylogenetic results) led to a broader array of diets among members of this group, including predation on small animals and/or eggs in addition to herbivory.

The behavioral implications of the distinctive appendicular skeletal anatomy of Caenagnathidae have also been a topic of frequent discussion in the literature. Currie and Russell [39] commented on the proportionally long hind limbs and large, broad pedes of these oviraptorosaurs, and proposed that caenagnathids may have been specialized waders that hunted freshwater invertebrates. These authors further suggested that the slender third manual digit of Chirostenotes pergracilis may have been used for extricating small prey from crevices in stream bottoms or trees [39], [45]. More recently, Senter and Parrish [48] found the structure and range of inferred motion of the manus of C. pergracilis to be compatible with a hooking function. They also proposed that the manus was useful for crevice probing, but that, rather than manual digit III, it was the long second manual digit, with its straighter ungual, that served this purpose.

Various authors have provided still other interpretations of the functional morphology of caenagnathid limbs. Based on the length and proportions of the hind limb, Currie [45] suggested that these oviraptorosaurs may have been quick, agile cursors. Varricchio [47], by contrast, compared the phalangeal proportions of a small caenagnathid pes from the Hell Creek Formation of Montana (MOR 752) to those of several modern bird species, and concluded that the pedal digits of these oviraptorosaurs were better adapted for grasping than for running. On this basis he hypothesized that caenagnathids may have used their feet for climbing or prey capture. Longrich et al. [42] also proposed that caenagnathids might have been semi-arboreal.

The depositional environments in which caenagnathid fossils have been found may provide additional clues to the habitat preferences of these theropods. An apparent paleoenvironmental distinction between caenagnathids and oviraptorids has long been noted [1], [42], [46]: namely, whereas the vast majority of oviraptorid fossils have been recovered from rocks interpreted to represent arid to semi-arid settings [20], [86]–[89], most caenagnathids have been discovered in fluvial sediments that are thought to have been deposited under more mesic conditions [90]–[92]. As such, caenagnathids are thought to have been adapted to wetter, more humid surroundings than were their oviraptorid relatives.

The sedimentology of the localities in the Hell Creek Formation that have yielded associated caenagnathid specimens may provide more specific insights into the types of environments that these animals frequented [93]. The more complete CM specimens of Anzu were recovered from silty mudstones that are herein interpreted as overbank sediments, whereas the fragmentary MRF 319 was found in a channel lag deposit. A fourth associated caenagnathid skeleton from the Hell Creek Formation, discovered in 2013 by the Burpee Museum of Natural History, was also preserved in an organic-rich mudstone (S. Williams, pers. comm.; T.R.L., pers. obs.). The lithology of the site that yielded the associated pes MOR 752 was not described [47]. Consequently, the three most complete caenagnathid skeletons recovered from the Hell Creek Formation to date have all come from mudstones that are interpreted as low-energy overbank deposits. Given the limited sample of Hell Creek oviraptorosaurian fossils at present, this pattern could be the result of taphonomic filtering (as has been documented in other North American Late Cretaceous continental units, e.g., [94]), with the comparatively small and delicate (relative to those of most other Hell Creek non-avian dinosaurs) bones of oviraptorosaurs being scattered or destroyed in higher-energy fluvial systems. Nevertheless, the possible association of caenagnathid skeletons with overbank deposits might instead reflect an authentic paleoecological preference comparable to that hypothesized for other late Maastrichtian North American dinosaurs [93]. Namely, in the paleoenvironment represented by the Hell Creek Formation, these oviraptorosaurs may have favored floodplain habitats over those closer to river margins.

To our knowledge, ichnological evidence has not yet been brought to bear on the issue of caenagnathid paleoecology; nevertheless, fossil trackways may offer additional insights into the habits of these dinosaurs. The distinctive theropod ichnotaxon Saurexallopus spp. from the Maastrichtian of the Rocky Mountain region of the United States [95], [96] was recently identified as having been made by one or more, emu- to ostrich-sized, Chirostenotes-like oviraptorosaurian taxa with estimated hip heights of 1.1–1.75 m [97]. The pedal morphology, stratigraphic and geographic provenance, and inferred size range of the Saurexallopus trackmaker(s) thus corresponds closely with known body fossils of Anzu, and as such, it is conceivable that footprints of this type may have been made by A. wyliei and/or other, closely related caenagnathids. Consequently, the depositional settings of Saurexallopus trackways may shed light on the habitat preferences of latest Cretaceous caenagnathids in western North America. The only known specimens of the type ichnospecies S. lovei, from the middle Maastrichtian Harebell Formation of northwestern Wyoming, were made by at least two individual theropods that walked over bioturbated tidal flats and into shallow waters along the coast of a nearshore marine or brackish embayment [95]. Tracks of the second named ichnospecies, S. zerbsti, were discovered in a silty, fine-grained sandstone of the upper Maastrichtian Lance Formation of northeastern Wyoming that has been interpreted to represent an ephemeral pond [96]. Interestingly, in both cases, the depositional environments of the Saurexallopus tracks indicate that these putative caenagnathids entered shallow bodies of water, a circumstance that is consistent with the wading hypothesis offered by Currie and Russell [39] exclusively on the basis of body fossils.

To conclude, a considerable range of possible lifestyles has been proposed for Caenagnathidae, some of which may be mutually exclusive (e.g., strict herbivory versus predation on small vertebrates; wading versus climbing). This is in spite of the fact that, prior to the discovery of Anzu wyliei, our knowledge of the anatomy of these theropods was mostly limited to bones of the mandible, manus, and pes. Although the morphology of the new Hell Creek caenagnathid does not provide definitive evidence for choosing between any of these various paleoecological proposals, some hypotheses appear more plausible than others. For instance, if, as in caudipterygids (the only oviraptorosaurs for which forelimb integument has been extensively preserved [2], [9], [26]), manual digit II of caenagnathids supported quill feathers, the crevice-probing function proposed for this digit by Senter and Parrish [48] is difficult to envision. Furthermore, it seems unlikely that large-bodied caenagnathids such as Anzu and Hagryphus would have spent much time in trees, at least as adults (contra [47]). On the other hand, the jaw morphology of caenagnathids does suggest that these theropods were capable of processing a variety of potential food items. Furthermore, evidence from the depositional settings of caenagnathid body and possible trace fossils suggests that these theropods may have favored well-watered floodplain habitats over drier environs. In sum, in our view, Anzu and other derived caenagnathids may well have been ecological generalists that fed upon vegetation, small animals, and perhaps even eggs on the humid coastal plains of western North America at the end of the Age of Dinosaurs.