Browsing by Author "Clark, James Matthew, 1956-"
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Item Convergent evolution of a eusuchian-type secondary palate within Shartegosuchidae. (American Museum novitates, no. 3901)(American Museum of Natural History., 2018-06-18) Dollman, Kathleen N.; Clark, James Matthew, 1956-; Norell, Mark.; Xu, Xing, 1969-; Choiniere, Jonah N.; Mongolian-American Museum Paleontological Project.; Mongolyn Shinzhlėkh Ukhaany Akademi.Shartegosuchids are a poorly known, early-branching group of Asian and North American crocodylomorphs. Shartegosuchids have been hypothesized to have eusuchian-type secondary palates, but a paucity of described material makes assessing this difficult. Our fieldwork in western Mongolia recovered cranial material of a new Shartegosuchus specimen from the Ulan Malgait Formation, which we CT-scanned and digitally reconstructed to investigate its palatal morphology. We then incorporated this new anatomical information into a revised phylogenetic dataset to assess its affinities. Our study confirms that Shartegosuchus has a posteriorly placed choana that is fully enclosed by the pterygoids, but differs from Eusuchia in possessing a secondary palatal fenestra and reduced palatine bones. Shartegosuchus, together with Adzhosuchus, Fruitachampsa, and Nominosuchus, forms the monophyletic group Shartegosuchidae. Shartegosuchidae is nested within a larger clade Shartegosuchoidea, and this clade is an earlier-diverging lineage than Eusuchia, showing that a eusuchian-type secondary palate evolved multiple times in crocodylomorphs, including very early in the group's evolutionary history. The co-occurrence of Nominosuchus in the Ulan Malgait Formation and the Shishigou Formation allows us to assign an early Oxfordian age to Shartegosuchus. The independent evolution of a eusuchian-type secondary palate in an oreinorostral group suggests that the link between platyrostry and a closed secondary palate has been overstated.Item Cranial anatomy of Citipati osmolskae (Theropoda, Oviraptorosauria), and a reinterpretation of the holotype of Oviraptor philoceratops. American Museum novitates ; no. 3364(New York, NY : American Museum of Natural History, 2002) Clark, James Matthew, 1956-; Norell, Mark.; Rowe, Timothy, 1953-; Mongolian-American Museum Paleontological Project.; Mongolyn Shinzhlėkh Ukhaany Akademi.; Central Asiatic Expeditions (1921-1930)We describe the skull of the holotype of Citipati osmolskae, one of the best preserved oviraptorid skulls known. The skull preserves stapes and epipterygoids, and the mandible preserves a slender coronoid bone, none of which has been reported before in oviraptorids. The braincase is similar to that of other basal coelurosaurs but possesses extensive recesses presumably occupied by pneumatic diverticulae; the circumnarial region is highly pneumatized, and a large recess continues posteriorly from the narial region to invade the frontals and parietals dorsal to the braincase. Circum-otic pneumatic recesses include two dorsal recesses above the otic recess, a posterior recess on the anterior surface of the paroccipital process, and extensive cavities in the basisphenoid beneath the braincase. The more dorsal of the two dorsal tympanic recesses is very deep, and CT scans suggest that it connected medially across the midline dorsal to the otic region and anteriorly with the frontoparietal space. The otic recess is unusually shallow. Comparison of the new skull with the poorly preserved skull of the holotype of Oviraptor philoceratops demonstrates that the braincase and palate of the latter are similar to those of other oviraptorids. Its rostrum and dentary are more elongate than in other oviraptorids, however, a more plesiomorphic condition suggesting it may be the most basal oviraptorid. A well-preserved skeleton previously referred to O. philoceratops, IGM 100/42, does not belong to this genus or species, and its narial region is very similar to that of Citipati osmolskae.Item Cranial osteology of Haplocheirus sollers Choiniere et al., 2010 (Theropoda, Alvarezsauroidea). (American Museum novitates, no. 3816)(American Museum of Natural History., 2014-10-22) Choiniere, Jonah N.; Clark, James Matthew, 1956-; Norell, Mark.; Xu, Xing, 1969-The basalmost alvarezsauroid Haplocheirus sollers is known from a single specimen collected in Upper Jurassic (Oxfordian) beds of the Shishugou Formation in northwestern China. Haplocheirus provides important data about the plesiomorphic morphology of the theropod group Alvarezsauroidea, whose derived members possess numerous skeletal autapomorphies. We present here a detailed description of the cranial anatomy of Haplocheirus. These data are important for understanding cranial evolution in Alvarezsauroidea because other basal members of the clade lack cranial material entirely and because derived parvicursorine alvarezsauroids have cranial features shared exclusively with members of Avialae that have been interpreted as synapomorphies in some analyses. We discuss the implications of this anatomy for cranial evolution within Alvarezsauroidea and at the base of Maniraptora.Item The Early Cretaceous crocodylomorph Hylaeochampsa vectiana from the wealden of the Isle of Wight. American Museum novitates ; no. 3032(New York, N.Y. : American Museum of Natural History, 1992) Clark, James Matthew, 1956-; Norell, Mark.Item An embryonic oviraptorid (Dinosauria, Theropoda) from the Upper Cretaceous of Mongolia. American Museum novitates ; no. 3315(New York, NY : American Museum of Natural History, 2001) Norell, Mark.; Clark, James Matthew, 1956-; Chiappe, Luis M.; Mongolian-American Museum Paleontological Project.; Mongolyn Shinzhlėkh Ukhaany Akademi.An embryonic oviraptorid skeleton is described within an egg from the late Cretaceous Djadokh[t]a Formation of Ukhaa Tolgod, Mongolia. The specimen comprises the ventral part of the skull and most of the mandible, a poorly preserved axial skeleton missing most of the tail, and portions of the forelimbs, shoulder girdles, pelvis, and hindlimbs. The skull is readily referable to the theropod dinosaur clade Oviraptoridae on the basis of several skull specializations (edentulous, vertically oriented premaxilla, a sinusoidally shaped lower jaw, and an unusual articulation of the vomer and premaxilla), and the postcranial skeleton is consistent with this identification. The egg is equivalent in overall shape and microstructure to those found beneath several oviraptorid skeletons recovered from the same formation. The skeleton is well ossified and, in comparison with ossification patterns in living Aves, the evidence suggests that this species was closer to the precocial end of the precocial-altricial spectrum of developmental patterns.Item The Late Cretaceous alligatoroid Brachychampsa montana (Crocodylia) : new material and putative relationships. American Museum novitates ; no. 3116(New York, N.Y. : American Museum of Natural History, 1994) Norell, Mark.; Clark, James Matthew, 1956-; Hutchison, J. Howard (John Howard), 1939-Item A new dromaeosaurid theropod from Ukhaa Tolgod (Ömnögov, Mongolia) ; American Museum novitates, no. 3545(New York, NY : American Museum of Natural History, 2006) Norell, Mark.; Clark, James Matthew, 1956-; Turner, Alan H. (Alan Hamilton); Makovicky, Peter J.; Barsbold, Rinchin.; Rowe, Timothy, 1953-We describe a new dromaeosaurid theropod from the Upper Cretaceous Djadokhta Formation of Ukhaa Tolgod, Mongolia. The new taxon, Tsaagan mangas, consists of a well-preserved skull and cervical series. This specimen marks only the second dromaeosaurid taxon from a formation that has otherwise yielded numerous specimens of Velociraptor mongoliensis, and Tsaagan mangas is the only dromaeosaurid known from Ukhaa Tolgod beyond sporadic occurrences of isolated teeth. Tsaagan mangas differs from other dromaeosaurids in the possession of a straight, untwisted, and pendulous paroccipital process, a large and anteriorly located maxillary fenestra, and a jugal-squamosal contact that excludes the postorbital from the margin of the infratemporal fenestra. The phylogenetic affinities of Tsaagan mangas are determined through a comprehensive phylogenetic analysis of Coelurosauria, confirming its position within Dromaeosauridae. This new specimen, coupled with CT imaging, provides new information on the skull and braincase anatomy of dromaeosaurids.Item A new skull of Gobipteryx minuta (Aves, Enantiornithes) from the Cretaceous of the Gobi Desert. American Museum novitates ; no. 3346(New York, NY : American Museum of Natural History, 2001) Chiappe, Luis M.; Norell, Mark.; Clark, James Matthew, 1956-; Mongolian-American Museum Paleontological Project.; Mongolyn Shinzhlėkh Ukhaany Akademi.We describe an exquisitely preserved new skull of a bird from the late Cretaceous sandstones of Ukhaa Tolgod, southern Mongolia. Derived similarities shared between this skull and the holotype of Gobipteryx minuta, also from the late Cretaceous of the Gobi Desert, support the assignment of the new cranial material to this avian taxon. The new skull also proves indistinguishable from that of the enantiornithine Nanantius valifanovi from the late Cretaceous of Mongolia. The identification of the new skull as that of Gobipteryx minuta and its correspondence to that of Nanantius valifanovi indicate that the latter taxon is a junior synonym of Gobipteryx minuta. This taxonomic conclusion is crucial for understanding the phylogenetic relationships of Gobipteryx minuta because the undoubtedly enantiornithine postcranial morphology of Nanantius valifanovi provides the first uncontroversial evidence of the enantiornithine relationship of Gobipteryx minuta. The new skull from Ukhaa Tolgod and our reinterpretation of cranial aspects of the previously published material of Gobipteryx minuta and Nanantius valifanovi permit an accurate reconstruction of the palate of this enantiornithine bird, thus adding significant data for understanding the poorly known palatal structure of Mesozoic birds.Item Osteology and relationships of Byronosaurus jaffei (Theropoda, Troodontidae). American Museum novitates ; no. 3402(New York, NY : American Museum of Natural History, 2003) Makovicky, Peter J.; Norell, Mark.; Clark, James Matthew, 1956-; Rowe, Timothy, 1953-; Mongolian-American Museum Paleontological Project.; Mongolyn Shinzhlėkh Ukhaany Akademi.The troodontid Byronosaurus jaffei is known from two specimens from adjacent localities in the Nemegt basin, Omnögov Aimag, Mongolia. These specimens are composed of well-preserved cranial material and fragmentary postcrania. All of these elements are described here. Byronosaurus jaffei is included in a comprehensive phylogenetic analysis of Coelurosauria to ascertain its relationships. Several interesting characters of Byronosaurus jaffei have implications both for theropod relationships and for understanding patterns of variation within coelurosaurian theropods. These include the position of a foramen that marks the exit of the supra-alveolar canal (which we suggest is homologous with the subnarial foramen), the flattened internarial bar, the unusual interfenestral bar, and the unserrated teeth. Additionally, the well-preserved braincase allows detailed comparison with other troodontid taxa.Item An oviraptorid skeleton from the late Cretaceous of Ukhaa Tolgod, Mongolia, preserved in an avianlike brooding position over an oviraptorid nest. American Museum novitates ; no. 3265(New York, NY : American Museum of Natural History, 1999) Clark, James Matthew, 1956-; Norell, Mark.; Chiappe, Luis M.; Mongolian-American Museum Paleontological Project.; Mongolyn Shinzhlėkh Ukhaany Akademi."The articulated postcranial skeleton of an oviraptorid dinosaur (Theropoda, Coelurosauria) from the late Cretaceous Djadokhta Formation of Ukhaa Tolgod, Mongolia, is preserved overlying a nest. The eggs are similar in size, shape, and ornamentation to another egg from this locality in which an oviraptorid embryo is preserved, suggesting that the nest is of the same species as the adult skeleton overlying it and was parented by the adult. The lack of a skull precludes specific identification, but in several features the specimen is more similar to Oviraptor than to other oviraptorids. The ventral part of the thorax is exceptionally well preserved and provides evidence for other avian features that were previously unreported in oviraptorids, including the articulation of the first three thoracic ribs with the costal margin of the sternum and the presence of a single, ossified ventral segment in each rib as well as ossified uncinate processes associated with the thoracic ribs. Remnants of keratinous sheaths are preserved with four of the manal claws, and the bony and keratinous claws were as strongly curved as the manal claws of Archaeopteryx and the pedal claws of modern climbing birds. The skeleton is positioned over the center of the nest, with its limbs arranged symmetrically on either side and its arms spread out around the nest perimeter. This is one of four known oviraptorid skeletons preserved on nests of this type of egg, comprising 23.5% of the 17 oviraptorid skeletons collected from the Djadokhta Formation before 1996. The lack of disturbance to the nest and skeleton indicate that the specimen is preserved in the position in which the adult died. Its posture is the same as that commonly taken only by birds among tetrapods that brood their nest, and its close proximity to the eggs indicates that the nest was not covered, indicating that the behavior of sitting on open nests in this posture evolved before the most recent common ancestor of modern birds"--P. [1]-2.Item A review of the Mongolian Cretaceous dinosaur Saurornithoides (Troodontidae, Theropoda). (American Museum novitates, no. 3654)(2009) Norell, Mark.; Makovicky, Peter J.; Bever, Gabe S.; Balanoff, Amy M.; Clark, James Matthew, 1956-; Barsbold, Rinchin.; Rowe, Timothy, 1953-We review the morphology, taxonomy, and phylogenetic relationships of the Upper Cretaceous Mongolian troodontid Saurornithoides. Saurornithoides mongoliensis is known only by the holotype from Bayan Zag, Djadokhta Formation. This specimen includes a nearly complete, but weathered, skull and mandibles, a series of dorsal, sacral, and caudal vertebrae, and a partial pelvic girdle and hind limb. Saurornithoides junior, here referred to Zanabazar, also is known only by the holotype from Bugiin Tsav, Nemegt Formation. This specimen consists of a skull and partial mandible, a series of sacral and caudal vertebrae, a partial pelvic girdle, and the distal part of the right hind limb. Saurornithoides + Zanabazar is one of the few Mongolian taxa known from both the Djadokhta and Nemegt formations. The monophyly of Saurornithoides + Zanabazar has not been seriously questioned historically, yet empirical support for this clade is currently tenuous. A privileged phylogenetic relationship between Saurornithoides, Zanabazar, and the North American troodontid Troodon formosus is supported by numerous characters including the presence of a subotic recess, lateroventrally projecting and hollow basipterygoid processes, a lacrimal whose anterior process is significantly longer than its posterior process, a highly pneumatized parasphenoid rostrum, a constricted neck of the occipital condyle, a symphyseal region of the dentary that is slightly recurved medially, and an obturator process located near the middle of the ischiadic shaft. CT data for the skulls of both species facilitated a description of the endocranial anatomy of Saurornithoides mongoliensis and Zanabazar junior, including a reconstruction of the endocranial space of Zanabazar junior. Despite being the largest of the known troodontid species, the endocranial volume of Zanabazar junior is considerably smaller than that estimated for Troodon formosus, suggesting that the extremely high encephalization quotient of Troodon formosus may be autapomorphic among troodontids.Item Skeletal morphology of Mononykus olecranus (Theropoda, Avialae) from the late Cretaceous of Mongolia. American Museum novitates ; no.3105(New York, N.Y. : American Museum of Natural History, 1994) Altangerel, Perle.; Chiappe, Luis M.; Rinchen, Barsbold.; Clark, James Matthew, 1956-; Norell, Mark.; Mongolyn Shinzhlėkh Ukhaany Akademi.Item The skull of Erlicosaurus andrewsi, a late Cretace[o]us "Segnosaur" (Theropoda, Therizinosauridae) from Mongolia. American Museum novitates ; no.3115(New York, N.Y. : American Museum of Natural History, 1994) Clark, James Matthew, 1956-; Altangerel, Perle.; Norell, Mark.; Mongolyn Shinzhlėkh Ukhaany Akademi.