Browsing by Author "Balanoff, Amy M."
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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 Cranial osteology of the theropod dinosaur Incisivosaurus gauthieri (Theropoda, Oviraptorosauria). (American Museum novitates, no. 3651)(2009) Balanoff, Amy M.; Xu, Xing, 1969-; Kobayashi, Yoshimura.; Matsufune, Yusuke.; Norell, Mark.We provide a description of the holotype skull of the unusual oviraptorosaur Incisivosaurus gauthieri. Previous phylogenetic analyses have placed this taxon firmly within Oviraptorosauria near the base of the clade; however, until now only a cursory description of this important specimen was available. The presence of many primitive characteristics (e.g., maxillary and dentary teeth as well as an extended palate and rostrum) indicates that the observed similarities between avians and derived oviraptorids are convergences rather than shared derived characters. In addition, we clarify previous descriptions of several ambiguous anatomical features, most notably of the palate. We also employ computed tomographic (CT) analysis, which allows for a more complete description of the braincase and the reconstruction of an endocranial endocast. CT imagery reveals features that were before unobtainable, such as the presence of a replacement tooth behind the large rodentiform incisor in the premaxilla. This arrangement indicates that although the incisiform teeth of I. gauthieri are morphologically distinct they are replaced in typical archosaurian fashion.Item Osteology of Khaan mckennai (Oviraptorosauria, Theropoda). (Bulletin of the American Museum of Natural History, no. 372)(American Museum of Natural History., 2012-12-28) Balanoff, Amy M.; Norell, Mark.The monophyly of Oviraptoridae, a group of theropod dinosaurs, which share a uniquely bizarre morphology, has never been called into question due in large part to their unusual complex of characters. Despite a vivid recent history of discovery and broad public appeal the nature of their morphological diversity has not been explored extensively. Many previous descriptions of oviraptorid taxa are lost in the obscurity of hard-to-find journals, and many lack illustrations of what are now recognized as phylogenetically important characters. The primary goal of this paper is to provide a relatively comprehensive descriptive morphology and illustrations for one member of Oviraptoridae, namely Khaan mckennai, with an emphasis on characters that can be used to establish a phylogenetic hypothesis for the taxon and group as a whole. K. mckennai is a small-bodied, crestless oviraptorid that is known from pristine material that has been collected from the late Cretaceous sediments of Mongolia. Similar to other oviraptorids, it shares a wide number of features in common with extant birds. However, when these characters are put in the context of Oviraptorosauria, including relatively new, more basal forms like Incisivosaurus gauthieri and Caudipteryx zoui, character states such as extreme pneumatization of the skull or the reduction in the number of caudal vertebrae are found to be either homoplastic for the two groups or plesiomorphic for a more inclusive clade.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 A second specimen of Citipati osmolskae associated with a nest of eggs from Ukhaa Tolgod, Omnogov Aimag, Mongolia. (American Museum novitates, no. 3899)(American Museum of Natural History., 2018-04-26) Norell, Mark.; Balanoff, Amy M.; Barta, Daniel E.; Erickson, Gregory M.;Adult dinosaurs preserved attending their nests in brooding positions are among the rarest vertebrate fossils. By far the most common occurrences are members of the dinosaur group Oviraptorosauria. The first finds of these were specimens recovered from the Djadokhta Formation at the Mongolian locality of Ukhaa Tolgod and the Chinese locality of Bayan Mandahu. Since the initial discovery of these specimens, a few more occurrences of nesting oviraptors have been found at other Asian localities. Here we report on a second nesting oviraptorid specimen (IGM 100/1004) sitting in a brooding position atop a nest of eggs from Ukhaa Tolgod, Omnogov, Mongolia. This is a large specimen of the ubiquitous Ukhaa Tolgod taxon Citipati osmolskae. It is approximately 11% larger based on humeral length than the original Ukhaa Tolgod nesting Citipati osmolskae specimen (IGM 100/979), yet eggshell structure and egg arrangement are identical. No evidence for colonial breeding of these animals has been recovered. Reexamination of another "nesting" oviraptorosaur, the holotype of Oviraptor philoceratops (AMNH FARB 6517) indicates that in addition to the numerous partial eggs associated with the original skeleton that originally led to its referral as a protoceratopsian predator, there are the remains of a tiny theropod. This hind limb can be provisionally assigned to Oviraptoridae. It is thus at least possible that some of the eggs associated with the holotype had hatched and the perinates had not left the nest.Item Supplemental Material for 'A second specimen of Citipati osmolskae associated with a nest of eggs from Ukhaa Tolgod, Omnogov Aimag, Mongolia. (American Museum novitates, no. 3899)'(2018-04-26) Norell, Mark; Balanoff, Amy M.; Barta, Daniel E.; Erickson, Gregory M.; Mongolian-American Museum Paleontological Project.; Mongolyn Shinzhlėkh Ukhaany Akademi.Supplemental Material for 'A second specimen of Citipati osmolskae associated with a nest of eggs from Ukhaa Tolgod, Omnogov Aimag, Mongolia. (American Museum novitates, no. 3899)'