AMERICANt MUSEUM Norntates PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024 Number 3265, 36 pp., 15 figures May 4, 1999 An Oviraptorid Skeleton from the Late Cretaceous of Ukhaa Tolgod, Mongolia, Preserved in an Avianlike Brooding Position Over an Oviraptorid Nest JAMES M. CLARK,I MARK A. NORELL,2 AND LUIS M. CHIAPPE3 ABSTRACT The articulated postcranial skeleton of an ovi- presence of a single, ossified ventral segment in raptorid dinosaur (Theropoda, Coelurosauria) each rib as well as ossified uncinate processes from the Late Cretaceous Djadokhta Formation associated with the thoracic ribs. Remnants of of Ukhaa Tolgod, Mongolia, is preserved over- keratinous sheaths are preserved with four of the lying a nest. The eggs are similar in size, shape, manal claws, and the bony and keratinous claws and ornamentation to another egg from this lo- were as strongly curved as the manal claws of cality in which an oviraptorid embryo is pre- Archaeopteryx and the pedal claws of modern served, suggesting that the nest is of the same climbing birds. The skeleton is positioned over species as the adult skeleton overlying it and was the center of the nest, with its limbs arranged parented by the adult. The lack of a skull pre- symmetrically on either side and its arms spread cludes specific identification, but in several fea- out around the nest perimeter. This is one of four tures the specimen is more similar to Oviraptor known oviraptorid skeletons preserved on nests than to other oviraptorids. The ventral part of the of this type of egg, comprising 23.5% of the 17 thorax is exceptionally well preserved and pro- oviraptorid skeletons collected from the Dja- vides evidence for other avian features that were dokhta Formation before 1996. The lack of dis- previously unreported in oviraptorids, including turbance to the nest and skeleton indicate that the the articulation of the first three thoracic ribs specimen is preserved in the position in which with the costal margin of the sternum and the the adult died. Its posture is the same as that ' Research Associate, Department of Vertebrate Paleontology, American Museum of Natural History; Assistant Professor, Department of Biological Sciences, George Washington University, Washington, D.C. 20052. 2 Chairman, Department of Vertebrate Paleontology, American Museum of Natural History. 3Chapman Fellow and Research Associate, Department of Ornithology, American Museum of Natural History. Copyright ( American Museum of Natural History 1999 ISSN 0003-0082 / Price $3.90 2 AMERICAN MUSEUM NOVITATES NO. 3265 commonly taken only by birds among tetrapods indicating that the behavior of sitting on open that brood their nest, and its close proximity to nests in this posture evolved before the most re- the eggs indicates that the nest was not covered, cent common ancestor of modern birds. INTRODUCTION Among the most surprising and revealing but only circumstantial evidence for reiden- specimens collected from Upper Cretaceous tifying the eggs was forwarded. The discov- Djadokhta-like beds in Mongolia by the joint ery of Ukhaa Tolgod in 1993 provided the American Museum of Natural History-Mon- evidence crucial to deducing a definitive an- golian Academy of Sciences expeditions are swer-an oviraptorid embryo within the those of the peculiar dinosaurs of the Ovi- same type of egg as those beneath the Ovi- raptoridae (Norell and Clark, 1997). Unlike raptor philoceratops holotype (Norell et al., the skulls of most other nonavian theropods, 1994). With this reidentification of the eggs the unusually short, often highly pneuma- the association of the skeleton with eggs of tized skull of oviraptorids lacks teeth and its own kind became explicable as evidence may bear a crest, resembling superficially the of parental care, rather than predation. skull of the living cassowary. The abundance The specimen described here, IGM 100/ of oviraptorid specimens at the extraordi- 979 (mistakenly labeled 100/972 in fig. 1 of narily rich locality of Ukhaa Tolgod (Dash- Norell et al., 1995), provides further evi- zeveg et al., 1995) is most unexpected in dence that this association is indeed a result light of their rarity in other deposits (Bars- of parental behavior. The specimen (fig. 1) bold et al., 1990). This wealth of new ma- was discovered in 1993, and after its prepa- terial already has answered a 75 year old ration a preliminary note was published (No- enigma, and the specimen described here rell et al., 1995). A second specimen on a played a crucial role in this story (Norell et nest was collected from Ukhaa Tolgod in al., 1994). 1995 (see Webster, 1996), and another from The bizarre and poorly preserved holotype correlative beds at Bayan Mandahu in Inner specimen of Oviraptor philoceratops was Mongolia, China, was reported by Dong and discovered at Bayn Dzak (the "Flaming Currie (1996). These three specimens and the Cliffs") by the American Museum's Central 0. philoceratops holotype provide compel- Asiatic Expedition in 1923, fortuitously en- ling evidence that the close association be- countered while excavating the first dinosaur tween adults and nests in Oviraptoridae has nest found at this famous locality (Andrews, a biological explanation. 1932). Because eggs similar to those in this The specimen is remarkably intact, and in nest are abundant at Bayn Dzak, and because addition to its importance in preserving this of the preponderance of skeletal material of individual's relationship to the nest it pro- Protoceratops andrewsi at this locality, the vides important new data on the thoracic eggs were identified as belonging to Proto- skeleton in oviraptorids. Many of these fea- ceratops (Osborn, 1924). The skeleton over- tures were revealed by preparation following lying the nest therefore presented an enigma. publication of the preliminary note, and thus To explain this association, Henry Fairfield were not reported or illustrated there. Ovi- Osborn (Osborn, 1924) speculated that the raptorids are among the closest relatives of adult animal had been preserved while rob- Avialae, the group comprising Archaeopter- bing the nest (although there was no evi- yx, extant birds, and related taxa (Aves of dence to indicate predation), hence he en- some other authors: see Gauthier, 1986), and dowed it with a name meaning "egg seizer these new features help determine the precise fond of ceratopsians." relationships of oviraptorids among Thero- For many years the identity of the eggs as poda and the homology of these features. those of Protoceratops was accepted with little Institutional abbreviations: AMNH- doubt. In recent years this identification was American Museum of Natural History, New questioned (Sabath, 1991; Mikhailov, 1991), York; IGM-Mongolian Institute of Geolo- 19991999~CLARK ET AL.: NESTING OVIRAPTORID FROM MONGOLIA33 gy, Ulaan Baatar; IVPP-Institute of Verte- Ukhaa Tolgod, traces of burrowing inverte- brate Paleontology and Paleoanthropology, brates are preserved with the skeleton. They Beijing; MIAE-field numbers of the collec- are recognizable mainly because they are tions made by the Mongolian Academy of more strongly cemented than the surrounding Sciences-American Museum of Natural His- matrix and are lighter in color. Five burrows tory expeditions to the Gobi Desert. were exposed during preparation, including a long burrow on the dorsal surface of the ster- GEOLOGIC SETTING OF THE FIND nal plates (fig. 2). These burrows are filled with white particles which may be pieces of The specimen is from a thick-bedded digested bone, and two in the shoulder region sandstone in the lower part of the local sec- are nearly white. Four of the five are slightly tion at the Ankylosaur Flats sublocality, curved, but the smallest one is straight. One Ukhaa Tolgod. These sediments and the fau- is preserved above the right coracoid, one to na they entomb closely resemble sediments the left of the fourth digit of the left pes, and and assemblages at localities in the Djadokh- another between the right ischium and right ta Formation, such as Bayn Dzak and Bayan pes, and one was removed from above a right Mandahu, but some similarities with the fau- ventral rib just behind the right sternal plate. na of the Barun Goyot Formation have also Most are similar in shape, ranging from 3-4 been noted (Dashzeveg et al., 1995; Gao and cm in length and 4-8 mm in diameter, but Norell, 1996). Currently the stratigraphic re- the one above the sternum is over 6 cm long lationships of this locality relative to other (fig. 2). Although other evidence of contem- localities, especially the type section of the poraneous fossil arthropods at Ukhaa Tolgod Barun Goyot Formation, are unclear and un- (Loope et al., 1998) and other Djadokhta lo- der study (L. Dingus et al., in prep.). calities (see Johnston et al., 1996) may be At Ukhaa Tolgod, the specimen was col- from the taxon responsible for the burrows, lected from a unit with a lithology that has it is also possible that burrows associated been described elsewhere as a "structureless with IGM 100/979 were formed significantly sandstone" and interpreted as eolian in origin after deposition of the bed in which it is bur- (e.g., Eberth, 1993). In the Djadokhta For- ied. mation at Ukhaa Tolgod this facies initially The age of the Djadokhta Formation at was interpreted as eolian in origin (Dashzev- Ukhaa Tolgod and elsewhere in Mongolia eg et al., 1995). Recently, Loope et al. (1998) and China currently is not constrained by ra- reinterpreted these deposits as representing diometric dates, marine invertebrates, or pa- low-energy debris flows from eolian deposits leomagnetic polarity patterns. Comparisons that formed thick alluvial fans. The source of of the vertebrates suggest a correlation with these deposits was an extensive field of sta- the late Campanian to early Maastrichtian bilized sand dunes that destabilized when in- marine invertebrate stages (Lillegraven and undated with water during large rainstorms.