Amy M. Balanoff

Tyrannosaur Paleobiology: New Research on Ancient Exemplar Organisms

Tyrannosaurs Revisited Tyrannosaurs represent some of the most successful and largest carnivores in Earths history. An expanding fossil record has allowed studies of their evolution and behavior that now allow broader comparisons with other groups, not just dinosaurs. Brusatte et al. (p. 1481) review the biology and evolutionary history of tyrannosaurs and update their phylogenetic relations to include several new fossils. The analysis suggests that tyrannosaurs remained relatively small (less than about 5 meters long) until the Late Cretaceous (about 80 million years ago). Tyrannosaurs, the group of dinosaurian carnivores that includes Tyrannosaurus rex and its closest relatives, are icons of prehistory. They are also the most intensively studied extinct dinosaurs, and thanks to large sample sizes and an influx of new discoveries, have become ancient exemplar organisms used to study many themes in vertebrate paleontology. A phylogeny that includes recently described species shows that tyrannosaurs originated by the Middle Jurassic but remained mostly small and ecologically marginal until the latest Cretaceous. Anatomical, biomechanical, and histological studies of T. rex and other derived tyrannosaurs show that large tyrannosaurs could not run rapidly, were capable of crushing bite forces, had accelerated growth rates and keen senses, and underwent pronounced changes during ontogeny. The biology and evolutionary history of tyrannosaurs provide a foundation for comparison with other dinosaurs and living organisms.

A Review of the Mongolian Cretaceous Dinosaur Saurornithoides (Troodontidae: Theropoda)

Abstract 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 Saurornithoid...

Evolutionary origins of the avian brain

Features that were once considered exclusive to modern birds, such as feathers and a furcula, are now known to have first appeared in non-avian dinosaurs. However, relatively little is known of the early evolutionary history of the hyperinflated brain that distinguishes birds from other living reptiles and provides the important neurological capablities required by flight. Here we use high-resolution computed tomography to estimate and compare cranial volumes of extant birds, the early avialan Archaeopteryx lithographica, and a number of non-avian maniraptoran dinosaurs that are phylogenetically close to the origins of both Avialae and avian flight. Previous work established that avian cerebral expansion began early in theropod history and that the cranial cavity of Archaeopteryx was volumetrically intermediate between these early forms and modern birds. Our new data indicate that the relative size of the cranial cavity of Archaeopteryx is reflective of a more generalized maniraptoran volumetric signature and in several instances is actually smaller than that of other non-avian dinosaurs. Thus, bird-like encephalization indices evolved multiple times, supporting the conclusion that if Archaeopteryx had the neurological capabilities required of flight, so did at least some other non-avian maniraptorans. This is congruent with recent findings that avialans were not unique among maniraptorans in their ability to fly in some form.

Osteology of Khaan mckennai (Oviraptorosauria: Theropoda)

ABSTRACT 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.

The Braincase of Apatosaurus (Dinosauria: Sauropoda) Based on Computed Tomography of a New Specimen with Comments on Variation and Evolution in Sauropod Neuroanatomy

Abstract 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.

Cranial Osteology of the Theropod Dinosaur Incisivosaurus gauthieri (Theropoda: Oviraptorosauria)

Abstract 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.

Variation, Variability, and the Origin of the Avian Endocranium: Insights from the Anatomy of Alioramus altai (Theropoda: Tyrannosauroidea)

The internal braincase anatomy of the holotype of Alioramus altai, a relatively small-bodied tyrannosauroid from the Late Cretaceous of Mongolia, was studied using high-resolution computed tomography. A number of derived characters strengthen the diagnosis of this taxon as both a tyrannosauroid and a unique, new species (e.g., endocranial position of the gasserian ganglion, internal ramification of the facial nerve). Also present are features intermediate between the basal theropod and avialan conditions that optimize as the ancestral condition for Coelurosauria—a diverse group of derived theropods that includes modern birds. The expression of several primitive theropod features as derived character states within Tyrannosauroidea establishes previously unrecognized evolutionary complexity and morphological plasticity at the base of Coelurosauria. It also demonstrates the critical role heterochrony may have played in driving patterns of endocranial variability within the group and potentially reveals stages in the evolution of neuroanatomical development that could not be inferred based solely on developmental observations of the major archosaurian crown clades. We discuss the integration of paleontology with variability studies, especially as applied to the nature of morphological transformations along the phylogenetically long branches that tend to separate the crown clades of major vertebrate groups.

The Braincase Anatomy of the Late Cretaceous Dinosaur Alioramus (Theropoda: Tyrannosauroidea)

ABSTRACT 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, e...

The Endocranial Cavity of Oviraptorosaur Dinosaurs and the Increasingly Complex, Deep History of the Avian Brain

Unraveling the origins of the character complexes diagnosing major crown clades is one of the greatest challenges in evolutionary biology. These origination events tend to optimize along extraordinarily long stem lineages where the comparative biology of extant lineages is relatively weak in its heuristic power. Here we add to a growing paleontological literature on the evolutionary origins of the modern avi an brain by describing the endocranial casts of two oviraptorosaur dinosaurs, Citipati osmolskae and Khaan mckennai. These fossil data confirm the antiquity of several avian features, including the expanded cerebrum. They also extend our appreciation of both the inherent variability in the brain-skull relationship along the avian stem and the dynamic nature of these crown characters in the earliest history of their expression.