Browsing by Author "Bever, Gabe S."
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Item The braincase anatomy of the late Cretaceous dinosaur Alioramus (Theropoda, Tyrannosauroidea). (Bulletin of the American Museum of Natural History, no. 376)(American Museum of Natural History., 2013-03-15) Bever, Gabe S.; Brusatte, Stephen.; Carr, Thomas D. (Paleontologist); Xu, Xing, 1969-; Balanoff, Amy M.; Norell, Mark.; Mongolian-American Museum Paleontological Project.; Mongolyn Shinzhlėkh Ukhaany Akademi.The late Cretaceous tyrannosaurid Alioramus altai is known from a single specimen whose articulated braincase exhibits a nearly unique combination of preservational quality, subadult stage of growth, and morphological complexity. We use a detailed physical preparation combined with high-resolution computed tomography to provide an expanded description of this braincase that includes details of the neurocranium and its dermal roof, pneumatic recesses and sinuses, cranial endocast, and inner ear cavities. A few notable features include a highly developed rostral tympanic recess marked by three pneumatic fenestrae, a highly pneumatic paroccipital process with both rostral and caudal pneumatic foramina, a prootic fossa housing external foramina for the trigeminal and facial nerves, a well-developed superficial lamina of the prootic, an expanded vestibular cavity, and an osseous labyrinth that is plesiomorphic in appearance. These observations, set within the currently available comparative context, elucidate numerous neuroanatomical transformations within Tyrannosauroidea and clarify where more data and work are needed. We expand the discussion for the 21 characters from the neurocranium utilized in a recent revision of tyrannosauroid phylogeny, including a listing of which tyrannosauroid taxa can be scored for the primitive and derived states of each character.Item The braincase of Apatosaurus (Dinosauria, Sauropoda) based on computed tomography of a new specimen, with comments on variation and evolution in sauropod neuroanatomy. (American Museum novitates, no. 3677)(New York, NY : American Museum of Natural History., 2010) Balanoff, Amy M.; Bever, Gabe S.; Ikejiri, Takehito.We describe a previously unreported braincase of the sauropod dinosaur Apatosaurus from the Cactus Park Quarry, Morrison Formation of western Colorado using high-resolution X-ray computed tomography. The digital nature of these data allowed us to prepare and describe the first three-dimensional rendering of the endocranial space in this historically important dinosaur species. Results are compared with a range of taxa drawn from across the sauropod tree revealing previously underappreciated variation in the sauropod neurocranium. Examples of variable characters include the degree of cerebral and pontine flexure, the morphology of the parietal body and superior sagittal sinus and their relationship with the overlying dermal roof, and the conformation of several cranial nerve foramina. We provide preliminary evolutionary hypotheses and discussion for many of these features. The recognition that considerable variation is present in the sauropod neurocranium hopefully will encourage more detailed descriptions of this anatomically complex region, as well as facilitating a synthetic review of the sauropodomorph braincase as these descriptions become available.Item Osteology of a new late Cretaceous troodontid specimen from Ukhaa Tolgod, Ömnögovi Aimag, Mongolia. (American Museum novitates, no. 3889)(American Museum of Natural History., 2017-12-15) Pei, Rui.; Norell, Mark.; Barta, Daniel E.; Bever, Gabe S.; Pittman, Michael, 1985-; Xu, Xing, 1969-A new troodontid dinosaur, Almas ukhaa, from the Late Cretaceous deposits of the Djadokhta Formation at Ukhaa Tolgod, Mongolia, is described here. The holotype specimen (IGM 100/1323) comprises an almost complete and articulated cranium and partial articulated postcranial skeleton. This specimen has a small body size and a short snout as in basal paravians, but it exhibits a number of derived troodontid features that differentiate Almas ukhaa from the Early Cretaceous troodontids reported from China and unite this new taxon with other Late Cretaceous troodontids. Relative to other troodontids, Almas ukhaa is autapomorphic in the presence of a posteriorly curved pterygoid flange, absence of a lateral groove on the anterior part of the dentary, presence of a distinct spikelike process on the ischium, and elongate chevrons. The eggshell associated with IGM 100/1323 can be assigned to Prismatoolithidae indet. based on the smooth surface, eggshell thickness, and microstructural characteristics, and also preserves attributes similar to Protoceratopsidovum minimum. A unique relationship between Byronosaurus and the perinate troodontids IGM 100/972 and IGM 100/974 is no longer supported based on the new observations of Almas ukhaa and Gobivenator.Item The perinate skull of Byronosaurus (Troodontidae) with observations on the cranial ontogeny of paravian theropods. (American Museum novitates, no. 3657)(2009) Bever, Gabe S.; Norell, Mark.The skulls of two perinate paravians from Ukhaa Tolgod, Djadoktha Formation, Mongolia, are described here. The skulls are nearly unique in their combination of ontogenetic age and preservational quality and provide us with the first look at the morphology of such important anatomical regions as the rostrum, palate, and braincase at or near the onset of postnatal development in a nonavian paravian coelurosaur. Based on a number of derived characters, the skulls are allocated to a derived position within Troodontidae that is outside the clade consisting of Saurornithoides mongoliensis, Saurornithoides junior, Troodon formosus, and probably Sinornithoides youngi. A single synapomorphy, presence of a lateral maxillary groove, supports the Ukhaa perinates as Byronosaurus. The comparative morphology of the Ukhaa perinates with adult troodontids indicates a number of significant postnatal transformations (e.g., elongation and flattening of the rostrum, increase in the number of maxillary and dentary teeth, restructuring of the occipital plate and paroccipital process). These comparisons demonstrate that many characters historically considered important for phylogenetic and taxonomic assessments of adult maniraptorans are present at a relatively early stage of ontogeny. Differences in the developmental timing of various cranial characters have important implications for interpreting the fossil record as well as for understanding the role heterochrony has played in the evolution of derived coleurosaurs, including birds. The ontogenetic information provided by the Ukhaa perinates also allow us to comment on the enigmatic paravian Archaeornithoides deinosauriscus, which has been considered both the sister taxon to Avialae and a juvenile specimen of the troodontids Saurornithoides mongoliensis and Byronosaurus jaffei. We found no unique characters that support a priviledged relationship of Archaeornithoides deinosauriscus with avialans and only weak character support for this taxon as a basal troodontid--there is no known character evidence supporting it as a juvenile of either Saurornithoides or Byronosaurus.Item Postnatal ontogeny of the skull in the extant North American turtle Sternotherus odoratus (Cryptodira, Kinosternidae). (Bulletin of the American Museum of Natural History, no. 330)(New York : American Museum of Natural History., 2009) Bever, Gabe S.There are few published data addressing the types, levels, and patterns of intraspecific variation in gross anatomical structures for nonavian reptiles, especially considering the widespread interest in their morphology and evolutionary history. This study examines variation in both discrete and continuous characters of the postnatal skull for a single population of the kinosternid turtle Sternotherus odoratus. The primary purpose of the study was to elucidate those features that exhibit transformations (and by default those that do not) in the latter two-thirds of postnatal ontogeny--that portion heavily relied upon in phylogenetic analyses of macroanatomical features and most often preserved in the vertebrate fossil record. Data are presented for cranial characters historically used to elucidate phylogenetic relationships in turtles and to assess fossil specimens taxonomically. Results indicate that the number of characters exhibiting postnatal variation, and the levels at which they vary, are surprisingly high considering the conservative nature of both the taxonomic sample and the pool of examined characters. Features associated with the feeding apparatus are among the most variable cranial structures--for both continuous and discrete characters. A relatively large number of discrete transformations occur in structures derived from those cranial elements preformed in cartilage. This concentration demonstrates that developmental trajectories apomorphic for deep nodes in vertebrate phylogenetic history result in relatively late-stage postnatal transformations and high levels of variation in characters informative at much more restricted taxonomic levels. This study provides a baseline of data that future studies examining the ontogenetic and evolutionary history of variation can build upon and it represents a necessary step in understanding the complex system by which variability in developmental modules becomes integrated in the reptile skull.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.