Osteology of Khaan mckennai (Oviraptorosauria: Theropoda)

Cranial functional specialisation for strength precedes morphological evolution in Oviraptorosauria

Oviraptorosaurians were a theropod dinosaur group that reached high diversity in the Late Cretaceous. Within oviraptorosaurians, the later diverging oviraptorids evolved distinctive crania which were extensively pneumatised, short and tall, and had a robust toothless beak, interpreted as providing a powerful bite for their herbivorous to omnivorous diet. The present study explores the ability of oviraptorid crania to resist large mechanical stresses compared with other theropods and where this adaptation originated within oviraptorosaurians. Digital 3D cranial models were constructed for the earliest diverging oviraptorosaurian, Incisivosaurus gauthieri, and three oviraptorids, Citipati osmolskae, Conchoraptor gracilis, and Khaan mckennai. Finite element analyses indicate oviraptorosaurian crania were stronger than those of other herbivorous theropods (Erlikosaurus and Ornithomimus) and were more comparable to the large, carnivorous Allosaurus. The cranial biomechanics of Incisivosaurus align with oviraptorids, indicating an early establishment of distinctive strengthened cranial biomechanics in Oviraptorosauria, even before the highly modified oviraptorid cranial morphology. Bite modelling, using estimated muscle forces, suggests oviraptorid crania may have functioned closer to structural safety limits. Low mechanical stresses around the beaks of oviraptorids suggest a convergently evolved, functionally distinct rhamphotheca, serving as a cropping/feeding tool rather than for stress reduction, when compared with other herbivorous theropods.

Transformation of the pectoral girdle in pennaraptorans: critical steps in the formation of the modern avian shoulder joint

Important transformations of the pectoral girdle are related to the appearance of flight capabilities in the Dinosauria. Previous studies on this topic focused mainly on paravians yet recent data suggests flight evolved in dinosaurs several times, including at least once among non-avialan paravians. Thus, to fully explore the evolution of flight-related avian shoulder girdle characteristics, it is necessary to compare morphology more broadly. Here, we present information from pennaraptoran specimens preserving pectoral girdle elements, including all purportedly volant taxa, and extensively compare aspects of the shoulder joint. The results show that many pectoral girdle modifications appear during the evolution from basal pennaraptorans to paravians, including changes in the orientation of the coracoid body and the location of the articulation between the furcula and scapula. These modifications suggest a change in forelimb range of motion preceded the origin of flight in paravians. During the evolution of early avialans, additional flight adaptive transformations occur, such as the separation of the scapula and coracoid and reduction of the articular surface between these two bones, reduction in the angle between these two elements, and elongation of the coracoid. The diversity of coracoid morphologies and types of articulations joining the scapula-coracoid suggest that each early avialan lineage evolved these features in parallel as they independently evolved more refined flight capabilities. In early ornithothoracines, the orientation of the glenoid fossa and location of the acrocoracoid approaches the condition in extant birds, suggesting a greater range of motion in the flight stroke, which may represent the acquisition of improved powered flight capabilities, such as ground take-off. The formation of a new articulation between the coracoid and furcula in the Ornithuromorpha is the last step in the formation of an osseous triosseal canal, which may indicate the complete acquisition of the modern flight apparatus. These morphological transitions equipped birds with a greater range of motion, increased and more efficient muscular output and while at the same time transmitting the increased pressure being generated by ever more powerful flapping movements in such a way as to protect the organs. The driving factors and functional adaptations of many of these transitional morphologies are as yet unclear although ontogenetic transitions in forelimb function observed in extant birds provide an excellent framework through which we can explore the behavior of Mesozoic pennaraptorans.

Osteology of the axial skeleton of Aucasaurus garridoi: phylogenetic and paleobiological inferences

Aucasaurus garridoi is an abelisaurid theropod from the Anacleto Formation (lower Campanian, Upper Cretaceous) of Patagonia, Argentina. The holotype of Aucasaurus garridoi includes cranial material, axial elements, and almost complete fore- and hind limbs. Here we present a detailed description of the axial skeleton of this taxon, along with some paleobiological and phylogenetic inferences. The presacral elements are somewhat fragmentary, although these show features shared with other abelisaurids. The caudal series, to date the most complete among brachyrostran abelisaurids, shows several autapomorphic features including the presence of pneumatic recesses on the dorsal surface of the anterior caudal neural arches, a tubercle lateral to the prezygapophysis of mid caudal vertebrae, a marked protuberance on the lateral rim of the transverse process of the caudal vertebrae, and the presence of a small ligamentous scar near the anterior edge of the dorsal surface in the anteriormost caudal transverse process. The detailed study of the axial skeleton of Aucasaurus garridoi has also allowed us to identify characters that could be useful for future studies attempting to resolve the internal phylogenetic relationships of Abelisauridae. Computed tomography scans of some caudal vertebrae show pneumatic traits in neural arches and centra, and thus the first reported case for an abelisaurid taxon. Moreover, some osteological correlates of soft tissues present in Aucasaurus and other abelisaurids, especially derived brachyrostrans, underscore a previously proposed increase in axial rigidity within Abelisauridae.

Coracoid strength as an indicator of wing-beat propulsion in birds

Birds generate a propulsive force by flapping their wings. They use this propulsive force for various locomotion styles, such as aerodynamic flight, wing-paddle swimming and wing-assisted incline running. It is therefore important to reveal the origin of flapping ability in the evolution from theropod dinosaurs to birds. However, there are no quantitative indices to reconstruct the flapping abilities of extinct forms based on their skeletal morphology. This study compares the section modulus of the coracoid relative to body mass among various extant birds to test whether the index is correlated with flapping ability. According to a survey of 220 historical bird specimens representing 209 species, 180 genera, 83 families and 30 orders, the section modulus of the coracoid relative to body mass in non-flapping birds was significantly smaller than that of flapping birds. This indicates that coracoid strength in non-flapping birds is deemphasised, whereas in flapping birds the strength is emphasised to withstand the contractile force produced by powerful flapping muscles, such as the m. pectoralis and m. supracoracoideus. Therefore, the section modulus of the coracoid is expected to be a powerful tool to reveal the origin of powered flight in birds.

Exceptional preservation and foot structure reveal ecological transitions and lifestyles of early theropod flyers

Morphology of keratinised toe pads and foot scales, hinging of foot joints and claw shape and size all inform the grasping ability, cursoriality and feeding mode of living birds. Presented here is morphological evidence from the fossil feet of early theropod flyers. Foot soft tissues and joint articulations are qualitatively assessed using laser-stimulated fluorescence. Pedal claw shape and size are quantitatively analysed using traditional morphometrics. We interpret these foot data among existing evidence to better understand the evolutionary ecology of early theropod flyers. Jurassic flyers like Anchiornis and Archaeopteryx show adaptations suggestive of relatively ground-dwelling lifestyles. Early Cretaceous flyers then diversify into more aerial lifestyles, including generalists like Confuciusornis and specialists like the climbing Fortunguavis. Some early birds, like the Late Jurassic Berlin Archaeopteryx and Early Cretaceous Sapeornis, show complex ecologies seemingly unique among sampled modern birds. As a non-bird flyer, finding affinities of Microraptor to a more specialised raptorial lifestyle is unexpected. Its hawk-like characteristics are rare among known theropod flyers of the time suggesting that some non-bird flyers perform specialised roles filled by birds today. We demonstrate diverse ecological profiles among early theropod flyers, changing as flight developed, and some non-bird flyers have more complex ecological roles.

Cranial muscle reconstructions quantify adaptation for high bite forces in Oviraptorosauria

Oviraptorosaurians are an unusual and probably herbivorous group of theropod dinosaurs that evolved pneumatised crania with robust, toothless jaws, apparently adapted for producing a strong bite. Using 3D retrodeformed skull models of oviraptorid oviraptorosaurians Citipati, Khaan, and Conchoraptor, along with the earliest diverging oviraptorosaurian, Incisivosaurus, we digitally reconstruct jaw adductor musculature and estimate bite force to investigate cranial function in each species. We model muscle length change during jaw opening to constrain optimal and maximum gape angles. Results demonstrate oviraptorids were capable of much stronger bite forces than herbivorous theropods among Ornithomimosauria and Therizinosauria, relative to body mass and absolutely. Increased bite forces in oviraptorid oviraptorosaurians compared to the earliest diverging oviraptorosaurian result from expanded muscular space and different cranial geometry, not changes in muscular arrangement. Estimated optimal and maximum possible gapes are much smaller than published estimates for carnivorous theropods, being more similar to the herbivorous therizinosaurian theropod Erlikosaurus and modern birds. Restrictive gape and high bite force may represent adaptation towards exploiting tough vegetation, suggesting cranial function and dietary habits differed between oviraptorids and other herbivorous theropods. Differences in the relative strength of jaw adductor muscles between co-occurring oviraptorids may be a factor in niche partitioning, alongside body size.

An exquisitely preserved in-ovo theropod dinosaur embryo sheds light on avian-like prehatching postures

Despite the discovery of many dinosaur eggs and nests over the past 100 years, articulated in-ovo embryos are remarkably rare. Here we report an exceptionally preserved, articulated oviraptorid embryo inside an elongatoolithid egg, from the Late Cretaceous Hekou Formation of southern China. The head lies ventral to the body, with the feet on either side, and the back curled along the blunt pole of the egg, in a posture previously unrecognized in a non-avian dinosaur, but reminiscent of a late-stage modern bird embryo. Comparison to other late-stage oviraptorid embryos suggests that prehatch oviraptorids developed avian-like postures late in incubation, which in modern birds are related to coordinated embryonic movements associated with tucking - a behavior controlled by the central nervous system, critical for hatching success. We propose that such pre-hatching behavior, previously considered unique to birds, may have originated among non-avian theropods, which can be further investigated with additional discoveries of embryo fossils.

The phylogenetic affinities and morphological peculiarities of the bird-like dinosaur Borogovia gracilicrus from the Upper Cretaceous of Mongolia

Borogovia gracilicrus is a small-bodied theropod dinosaur from the Maastrichtian (Upper Cretaceous) Nemegt Formation of southern Mongolia. The taxon is based on a single fragmentary specimen preserving only the distal part of the hindlimbs. The morphology of Borogovia shows a peculiar combination of features, some of which are traditionally considered troodontid synapomorphies and others which are unusual for Troodontidae but are shared with other maniraptoran clades. In particular, the second toe of B. gracilicrus differs from other troodontids in lacking some of the features which contribute to the specialized 'sickle-clawed' second toe, here termed the 'falciphoran condition', shared with dromaeosaurids and some other paravians, such as the strongly compressed and falciform ungual. Phylogeny reconstructions intended to explore the affinities of Borogovia consistently support its referral within a subclade of troodontids including all Late Cretaceous taxa. The placement of Borogovia is not significantly affected by its unusual combinations of hindlimb features or by the homoplasy of the elements forming the falciphoran condition. Borogovia is supported as a valid taxon and is distinct from the other Nemegt troodontids, Tochisaurus and Zanabazar. The lack of a falciform ungual, and the distinctive morphology of the second toe in B. gracilicrus are interpreted as a derived specialization among Troodontidae and not as retention of the plesiomorphic condition of non-paravian theropods.

The evolutionary and functional implications of the unusual quadrate of Longipteryx chaoyangensis (Avialae: Enantiornithes) from the Cretaceous Jehol Biota of China

While the morphology and evolution of the quadrate among early birds and through the evolutionary origin of birds is not well known, we add to knowledge about that past diversity through description of the morphology of the quadrate in the unusually elongate skull of the Cretaceous enantiornithine bird Longipteryx chaoyangensis. The lateral and caudal surfaces of the quadrate are well exposed in two specimens revealing morphologies typical of early birds and their dinosaurian close relatives like a small otic head and two mandibular condyles. However, both skeletons exhibit quadrates with a unique, enlarged lateral crest that has not been previously described among Mesozoic birds. It is possible that the rostral surface of this lateral expansion served as the origination site for enlarged jaw musculature in a manner similar to the enlarged subcapitular tubercle in extant galloanserine birds. The caudally concave surface of the quadrate likely reflects some aspect of cranial pneumaticity, with its shape and position reminiscent of quadrates found in close non-avialan maniraptoran relatives. It is possible that this lateral crest has a wider distribution among enantiornithines and other early birds and that the crest has been misidentified as the orbital process in some more damaged specimens. In addition, the enlarged lateral mandibular condyle (relative to the medial condyle) differs from the condition typically reported among enantiornithines and could indicate a difference in jaw function or mechanics in this bird with an elongated rostrum, or simply misinterpretations of morphology. Further examination of the quadrate in temporally early and phylogenetically stemward birds, along with their close outgroups, could greatly impact the study of several different aspects of bird biology including assessment of phylogenetic relationships, interpretation of the function and kinematics of the skull, reconstruction of foraging paleoecology, and evolution of skull morphological diversity among Mesozoic birds.

A partial oviraptorosaur skeleton suggests low caenagnathid diversity in the Late Cretaceous Nemegt Formation of Mongolia

The Nemegt Formation of the Gobi Desert of Mongolia has produced one of the most abundant and diverse oviraptorosaur records globally. However, the caenagnathid component of this fauna remains poorly known. Two caenagnathid taxa are currently recognized from the Nemegt Formation: Elmisaurus rarus and Nomingia gobiensis. Because these taxa are known from mostly non-overlapping material, there are concerns that they could represent the same animal. A partial, weathered caenagnathid skeleton discovered adjacent to the holotype quarry of Nomingia gobiensis is referable to Elmisaurus rarus, revealing more of the morphology of the cranium, mandible, pectoral girdle, and pubis. Despite metatarsals clearly exhibiting autapomorphies of Elmisaurus rarus, overlapping elements are identical to those of Nomingia gobiensis, and add to a growing body of evidence that these taxa represent a single morphotype. In the absence of any positive evidence for two caenagnathid taxa in the Nemegt Formation, Nomingia gobiensis is best regarded as a junior synonym of Elmisaurus rarus. Low caenagnathid diversity in the Nemegt Formation may reflect broader coexistence patterns with other oviraptorosaur families, particularly oviraptorids. In contrast to North America, competition with the exceptionally diverse oviraptorids may have restricted caenagnathids to marginal roles in Late Cretaceous Asian ecosystems.

Cretaceous bird with dinosaur skull sheds light on avian cranial evolution

The transformation of the bird skull from an ancestral akinetic, heavy, and toothed dinosaurian morphology to a highly derived, lightweight, edentulous, and kinetic skull is an innovation as significant as powered flight and feathers. Our understanding of evolutionary assembly of the modern form and function of avian cranium has been impeded by the rarity of early bird fossils with well-preserved skulls. Here, we describe a new enantiornithine bird from the Early Cretaceous of China that preserves a nearly complete skull including the palatal elements, exposing the components of cranial kinesis. Our three-dimensional reconstruction of the entire enantiornithine skull demonstrates that this bird has an akinetic skull indicated by the unexpected retention of the plesiomorphic dinosaurian palate and diapsid temporal configurations, capped with a derived avialan rostrum and cranial roof, highlighting the highly modular and mosaic evolution of the avialan skull.

Pelecanimimus (Theropoda: Ornithomimosauria) postcranial anatomy and the evolution of the specialized manus in Ornithomimosaurs and sternum in maniraptoriforms

Pelecanimimus polyodon was discovered in 1993 in the Spanish Barremian fossil site of Las Hoyas, being the first ornithomimosaur described from Europe. So far, there has been no detailed description of the holotype of Pelecanimimus, which is composed of the anterior-half of an articulated skeleton. Here we report a new, detailed, revised and more accurate osteological description of its postcranial skeleton, comparing this new data to information about Ornithomimosauria from the last three decades. This osteological and phylogenetic analysis of Pelecanimimus shows several ornithomimosaur synapomorphies and a unique combination of characters that emend its original diagnosis. Pelecanimimus diverged early in Ornithomimosauria and reveals an enlargement trend of the manus, shared with derived ornithomimosaurians, due to a long metacarpal I and elongated distal phalanges. This evolutionary novelty, and other synapomorphies, has led to the definition of a new clade, Macrocheiriformes, including Pelecanimimus and more derived ornithomimosaurs. Pelecanimimus has the only ossified sternal plates among ornithomimosaurs and the first evidence of uncinate processes in a non-maniraptoran theropod, indicating a convergent appearance of these structures in Coelurosauria. The character combination in an early-diverging ornithomimosaur like Pelecanimimus found in this analysis provides a key step in the evolution of the manus and pectoral girdle in Ornithomimosauria.

Osteology of Unenlagia comahuensis (Theropoda, Paraves, Unenlagiidae) from the Late Cretaceous of Patagonia

Unenlagia comahuensis was originally described as a phylogenetic link between nonavian dinosaurs and birds. Later it was interpreted by some authors as belonging to the deinonychosaurian clade Dromaeosauridae, and more recently as phylogenetically closer to birds than to dromaeosaurids. The only known specimen is represented by an incomplete skeleton, including vertebrae, incomplete scapular girdle, pelvis, and limbs, coming from Upper Cretaceous beds of Neuquén province, Patagonia, Argentina. The aim of the present paper is to include a detailed anatomical description of Unenlagia (currently only known by preliminary descriptions). Detailed analysis of Unenlagia anatomy resulted in the recognition of one possible additional Unenlagiidae synapomorphy (i.e., the presence of cup-like iliac articulation on ischium). We recognize derived anatomical traits that Unenlagia and kin share with birds, lending support to the interpretation that unenlagiids are stem-Avialae. Particularly, some appendicular features (e.g., scapula with subtriangular and relatively reduced acromion, poor outward projection of the glenoid and glenoidal lips on the scapula, lateral orientation of scapular glenoid, craniolaterally oriented deltopectoral crest of humerus) may be related to the acquisition of anatomical novelties that in birds are associated with flight. The present contribution on Unenlagia provides new data regarding the early evolution of avian features.

Histology and pneumaticity of Aoniraptor libertatem (Dinosauria, Theropoda), an enigmatic mid-sized megaraptoran from Patagonia

Aoniraptor libertatem is a mid-sized megaraptoran that comes from the Late Cretaceous (Turonian) Huincul Formation at Río Negro province, Patagonia, Argentina. In this study, we conducted a detailed analysis of pneumaticity of the sacrum and tail of Aoniraptor. This shows a complex structure within these vertebrae, being composed by small diverticulae surrounding large pneumatic canals and a central chamber that opens outside through pleurocoels or pneumatic canals. Further, we carried out a histologic analysis which confirms the pneumatic nature of these anatomical features. Both analyses found that chevrons in Aoniraptor were invaded by pneumaticity, a feature that appears to be unique to this taxon. In addition, a comparative analysis between Aoniraptor and other theropods (e.g. Gualicho and other megaraptorans) was carried out. This resulted in the modification of previous schemes about the evolution of pneumaticity through Theropoda, the finding of some evolutionary pneumatic traits through Megaraptora, and the usefulness of pneumatic traits as a taxonomic tool.

A new two-fingered dinosaur sheds light on the radiation of Oviraptorosauria

Late Cretaceous trends in Asian dinosaur diversity are poorly understood, but recent discoveries have documented a radiation of oviraptorosaur theropods in China and Mongolia. However, little work has addressed the factors that facilitated this diversification. A new oviraptorid from the Late Cretaceous of Mongolia sheds light on the evolution of the forelimb, which appears to have played a role in the radiation of oviraptorosaurs. Surprisingly, the reduced arm has only two functional digits, highlighting a previously unrecognized occurrence of digit loss in theropods. Phylogenetic analysis shows that the onset of this reduction coincides with the radiation of heyuannine oviraptorids, following dispersal from southern China into the Gobi region. This suggests expansion into a new niche in the Gobi region, which relied less on the elongate, grasping forelimbs inherited by oviraptorosaurs. Variation in forelimb length and manus morphology provides another example of niche partitioning in oviraptorosaurs, which may have made possible their incredible diversity in the latest Cretaceous of Asia.

Dromaeosaurid crania demonstrate the progressive loss of facial pneumaticity in coelurosaurian dinosaurs

Dinosaurs are notable for their extensive skeletal pneumaticity, a feature that may have helped facilitate the development of various ‘extreme’ body plans in this group. Despite its relevance to understanding the evolution of the avian body plan, this feature has only been described in detail for a few non-avian dinosaurs, and cranial pneumaticity outside the braincase remains poorly documented. I describe facial pneumatic features in members of the Dromaeosauridae, a clade of hypercarnivorous dinosaurs closely allied to birds. Variation in the pneumaticity of the nasals and jugals, the position and shape of the pneumatic fenestrae of the maxilla and the border of the antorbital fossa shows that facial pneumaticity differed substantially among closely related dromaeosaurids and other bird-like dinosaurs. Ancestral state reconstructions of facial pneumaticity in coelurosaurs suggest a complex evolutionary history for these features. Surprisingly, the general trend along the path towards birds was the loss or reduction of superficial pneumatic features on the snout and cheek. Some facial pneumatic features seem to have evolved secondarily in some derived bird-like forms. The results show superficial facial pneumaticity did not increase in coelurosaurs and emphasize the complexity of the evolution of pneumatization in the lineage leading to birds.

More than one way to be a giant: Convergence and disparity in the hip joints of saurischian dinosaurs

Saurischian dinosaurs evolved seven orders of magnitude in body mass, as well as a wide diversity of hip joint morphology and locomotor postures. The very largest saurischians possess incongruent bony hip joints, suggesting that large volumes of soft tissues mediated hip articulation. To understand the evolutionary trends and functional relationships between body size and hip anatomy of saurischians, we tested the relationships among discrete and continuous morphological characters using phylogenetically corrected regression. Giant theropods and sauropods convergently evolved highly cartilaginous hip joints by reducing supraacetabular ossifications, a condition unlike that in early dinosauromorphs. However, transitions in femoral and acetabular soft tissues indicate that large sauropods and theropods built their hip joints in fundamentally different ways. In sauropods, the femoral head possesses irregularly rugose subchondral surfaces for thick hyaline cartilage. Hip articulation was achieved primarily using the highly cartilaginous femoral head and the supraacetabular labrum on the acetabular ceiling. In contrast, theropods covered their femoral head and neck with thinner hyaline cartilage and maintained extensive articulation between the fibrocartilaginous femoral neck and the antitrochanter. These findings suggest that the hip joints of giant sauropods were built to sustain large compressive loads, whereas those of giant theropods experienced compression and shear forces.

The body plan of Halszkaraptor escuilliei (Dinosauria, Theropoda) is not a transitional form along the evolution of dromaeosaurid hypercarnivory

The dromaeosaurid theropod Halszkaraptor escuilliei is characterized by several unusual features absent in other paravians, part of which has been interpreted as diagnostic of a novel lineage adapted to a semiaquatic ecology. Recently, these evolutionary and ecological interpretations have been challenged, and Halszkaraptor has been claimed to be a transitional form between non-dromaeosaurid maniraptoriforms and other dromaeosaurids: following that reevaluation, its peculiar body plan would represent the retention of several maniraptoran plesiomorphies, lost among other dromaeosaurids, and not an adaptation to a novel ecology. This alternative scenario is here carefully investigated and tested. It is shown that most statements supporting this scenario are based on misinterpretation of anatomical traits and bibliography. Once these statements have been corrected, character state transition optimization over a well-supported phylogenetic framework indicates that the large majority of the peculiar features of the Halszkaraptor lineage are derived novelties acquired by the latter after its divergence from the last ancestor shared with eudromaeosaurs, and thus are not maniraptoriform plesiomorphies. At least seven novelties of the Halszkaraptor lineage are convergently acquired with spinosaurids, and are integrated in semiaquatic adaptations: one of these is reported here for the first time. The amount of morphological divergence of Halszkaraptorinae from the ancestral dromaeosaurid condition is comparable to those of Microraptorinae and Velociraptorinae. Among extant taxa, the sawbills (Mergini, Anseriformes) show the closest ecomorphological similarity with the peculiar body plan inferred for Halszkaraptor. The halszkaraptorine bauplan is thus confirmed as a derived amphibious specialization, and does not represent a "transitional" stage along the evolution of dromaeosaurids.

Notes on the cheek region of the Late Jurassic theropod dinosaur Allosaurus

Allosaurus, from the Late Jurassic of North America and Europe, is a model taxon for Jurassic basal tetanuran theropod dinosaurs. It has achieved an almost iconic status due to its early discovery in the late, 19th century, and due to the abundance of material from the Morrison Formation of the western U.S.A., making Allosaurus one of the best-known theropod taxa. Despite this, various aspects of the cranial anatomy of Allosaurus are surprisingly poorly understood. Here, we discuss the osteology of the cheek region, comprised by the jugal, maxilla, and lacrimal. This region of the skull is of importance for Allosaurus taxonomy and phylogeny, particularly because Allosaurus has traditionally been reconstructed with an unusual cheek configuration, and because the European species Allosaurus europaeus has been said to be different from North American material in the configuration of these bones. Based on re-examination of articulated and disarticulated material from a number of repositories, we show that the jugal participates in the antorbital fenestra, contradicting the common interpretation. The jugal laterally overlies the lacrimal, and forms an extended antorbital fossa with this bone. Furthermore, we document previously unrecorded pneumatic features of the jugal of Allosaurus.

Halszkaraptor escuilliei and the evolution of the paravian bauplan

The evolution of birds from dinosaurs is a subject that has received great attention among vertebrate paleontologists. Nevertheless, the early evolution of the paravians, the group that contains birds and their closest non-avian dinosaur relatives, remains very poorly known. Even the most basal members of one paravian lineage, the Dromaeosauridae, already show a body plan that differs substantially from their closest non-paravian relatives. Recently, the dromaeosaurid Halszkaraptor escuilliei was described from the Cretaceous of Mongolia. Halszkaraptor possesses numerous unserrated premaxillary teeth, a platyrostral rostrum with a developed neurovascular system, an elongate neck, bizarrely-proportioned forearms, and a foreword-shifted center of mass, differing markedly from other paravians. A reevaluation of the anatomy, taphonomy, environmental setting, and phylogenetic position of H. escuilliei based on additional comparisons with other maniraptorans suggests that, rather than indicating it was a semiaquatic piscivore, the body plan of this dinosaur bears features widely distributed among maniraptorans and in some cases intermediate between the conditions in dromaeosaurids and related clades. I find no evidence for a semiaquatic lifestyle in Halszkaraptor. A phylogenetic reevaluation of Halszkaraptorinae places it as the sister clade to Unenlagiinae, indicating the bizarre features of unenlagiines previously interpreted as evidence of piscivory may also represent a mosaic of plesiomorphic, derived, and intermediate features. The anatomy of Halszkaraptor reveals that dromaeosaurids still possessed many features found in more basal maniraptoran and coelurosaur clades, including some that may have been tied to herbivory. Rather than being a semiaquatic piscavore, Halszkaraptor was a basal dromaeosaurid showing transitional features.

The skull evolution of oviraptorosaurian dinosaurs: the role of niche partitioning in diversification

Oviraptorosaurs are bird-like theropod dinosaurs that thrived in the final pre-extinction ecosystems during the latest Cretaceous, and the beaked, toothless skulls of derived species are regarded as some of the most peculiar among dinosaurs. Their aberrant morphologies are hypothesized to have been caused by rapid evolution triggered by an ecological/biological driver, but little is known about how their skull shapes and functional abilities diversified. Here, we use quantitative techniques to study oviraptorosaur skull form and mandibular function. We demonstrate that the snout is particularly variable, that mandibular form and upper/lower beak form are significantly correlated with phylogeny, and that there is a strong and significant correlation between mandibular function and mandible/lower beak shape, suggesting a form-function association. The form-function relationship and phylogenetic signals, along with a moderate allometric signal in lower beak form, indicate that similar mechanisms governed beak shape in oviraptorosaurs and extant birds. The two derived oviraptorosaur clades, oviraptorids and caenagnathids, are significantly separated in morphospace and functional space, indicating that they partitioned niches. Oviraptorids coexisting in the same ecosystem are also widely spread in morphological and functional space, suggesting that they finely partitioned feeding niches, whereas caenagnathids exhibit extreme disparity in beak size. The diversity of skull form and function was likely key to the diversification and evolutionary success of oviraptorosaurs in the latest Cretaceous.

A non-archaeopterygid avialan theropod from the Late Jurassic of southern Germany

The Late Jurassic 'Solnhofen Limestones' are famous for their exceptionally preserved fossils, including the urvogel Archaeopteryx, which has played a pivotal role in the discussion of bird origins. Here we describe a new, non-archaeopterygid avialan from the Lower Tithonian Mörnsheim Formation of the Solnhofen Archipelago, Alcmonavis poeschli gen. et sp. nov. Represented by a right wing, Alcmonavis shows several derived characters, including a pronounced attachment for the pectoralis muscle, a pronounced tuberculum bicipitale radii, and a robust second manual digit, indicating that it is a more derived avialan than Archaeopteryx. Several modifications, especially in muscle attachments of muscles that in modern birds are related to the downstroke of the wing, indicate an increased adaptation of the forelimb for active flapping flight in the early evolution of birds. This discovery indicates higher avialan diversity in the Late Jurassic than previously recognized.

A new caudipterid from the Lower Cretaceous of China with information on the evolution of the manus of Oviraptorosauria

Caudipteridae is a basal clade of Oviraptorosauria, all known species from the Early Cretaceous Jehol Biota of northeastern China. They were one of the first feathered dinosaur groups discovered, and possessed avian-like pennaceous remiges and rectrices. Their discovery provided significant information on early oviraptorosaurian evolution and the origins of birds and feathers. Here we describe a new caudipterid species Xingtianosaurus ganqi gen. et sp. nov. from the Lower Cretaceous Yixian Formation of Liaoning Province, China. This new taxon differs from other caudipterids by a small pleurocoel close to the dorsal edge of the lateral surface of the dorsal vertebrate centrum, a humerus longer than the scapula, a proportionally long ulna, a relatively small radiale angle, and a relatively short metacarpal I. The phylogenetic results shows X. ganqi is an early diverging caudipterid. It exhibits a mosaic morphology, providing new morphological information on early manual evolution of Oviraptorosauria, and giving new light on the evolution of radiale angle among Coelurosauria.

The sandwich structure of keratinous layers controls the form and growth orientation of chicken rhinotheca

The upper beak bone of birds is known to be overlain by the rhinotheca, which is composed of the horny sheath of keratinous layers. However, the details of the structure and growth pattern of the rhinotheca are yet to be understood. In this study, the microstructure of the rhinotheca from chicken specimens of different growth stages (ranging from 1 to ~ 80 days old) was analyzed using a combination of thin section and scanning electron microscopy observations, and small-angle X-ray scattering analysis. We found that the rhinotheca comprises three different layers - outer, intermediate, and inner layers - throughout its growth. The outer layer arises from the proximal portion of the beak bone and covers the dorsal surface of the rhinotheca, whereas the intermediate and inner layers originate in the distal portion of the beak bone and underlie the outer layer. This tri-layered structure of the rhinotheca was also observed in wild bird specimens (grey wagtail, king quail, and brown dipper). On the median plane, micro-layers making up the outer and inner layers are bedded nearly parallel to the rostral bone at the base. However, more distally positioned micro-layers of the outer layer are more anteverted distally. The micro-layers of the intermediate layer are bedded nearly perpendicular to those of the outer and inner layers on the median plane. The growth of micro-layers in the intermediate layer adds thickness to the rhinotheca, which causes the difference in profile between the beak bone and the rhinotheca in the distal portion of the beak. Moreover, the entire intermediate layer grows distally as new proximal micro-layers form. The outer layer is dragged distally by the intermediate layer as a result of its distal growth, for the three layers are closely packed to each other at their boundaries. Furthermore, the occurrence of the intermediate and inner layers in the distal portion of the rostral bone may be because the distal end of the beak is frequently used and worn, and the rhinotheca therefore needs to be replaced more frequently at the distal end. The rhinotheca structure described here will be an important and useful factor in the reconstruction of the beaks of birds in extinct taxa.

A new baby oviraptorid dinosaur (Dinosauria: Theropoda) from the Upper Cretaceous Nemegt Formation of Mongolia

Recent discoveries of new oviraptorosaurs revealed their high diversity from the Cretaceous Period in Asia and North America. Particularly, at the family level, oviraptorids are among the most diverse theropod dinosaurs in the Late Cretaceous of Mongolia and China. A new oviraptorid dinosaur Gobiraptor minutus gen. et sp. nov. from the Upper Cretaceous Nemegt Formation is described here based on a single holotype specimen that includes incomplete cranial and postcranial elements. The most prominent characters of Gobiraptor are its thickened rostrodorsal end of the mandibular symphysis and a rudimentary lingual shelf on each side of the dentary. Each lingual shelf is lined with small occlusal foramina and demarcated by a weakly developed lingual ridge. This mandibular morphology of Gobiraptor is unique among oviraptorids and likely to be linked to a specialized diet that probably included hard materials, such as seeds or bivalves. The osteohistology of the femur of the holotype specimen indicates that the individual was fairly young at the time of its death. Phylogenetic analysis recovers Gobiraptor as a derived oviraptorid close to three taxa from the Ganzhou region in southern China, but rather distantly related to other Nemegt oviraptorids which, as the results of recent studies, are also not closely related to each other. Gobiraptor increases diversity of oviraptorids in the Nemegt Formation and its presence confirms the successful adaptation of oviraptorids to a mesic environment.

New theropod (Tetanurae: Avetheropoda) material from the 'mid'-Cretaceous Griman Creek Formation at Lightning Ridge, New South Wales, Australia [corrected]

The limited fossil record of Australian Cretaceous theropods is dominated by megaraptorids, reported from associated and isolated material from the Early Cretaceous of Victoria and the 'Mid'-Cretaceous of central-north New South Wales and central Queensland. Here, we report on new postcranial theropod material from the early Late Cretaceous Griman Creek Formation at Lightning Ridge. Among this new material is an associated set consisting of two anterior caudal vertebrae and a pubic peduncle of the ilium, to which a morphologically similar partial vertebral centra from a separate locality is tentatively referred. These elements display a combination of characteristics that are present in megaraptorid and carcharodontosaurid theropods, including camellate internal organization of the vertebral centra, ventrally keeled anterior caudal centra and a pubic peduncle of the ilium with a ventral surface approximately twice as long anteroposteriorly as mediolaterally wide. Unfortunately, a lack of unambiguous synapomorphies precludes accurate taxonomic placement; however, avetheropodan affinities are inferred. This new material represents the second instance of a medium-sized theropod from this interval, and only the third known example of associated preservation in an Australian theropod. Additional isolated theropod material is also described, including an avetheropodan femoral head that shows similarities to Allosaurus and Australovenator, and a mid-caudal vertebral centrum bearing pneumatic foraminae and extensive camellae that is referrable to Megaraptora and represents the first axial skeletal element of a megaraptorid described from Lightning Ridge.

An unusual association of hadrosaur and therizinosaur tracks within Late Cretaceous rocks of Denali National Park, Alaska

We report details of a unique association of hadrosaur and therizinosaur tracks found in the Late Cretaceous lower Cantwell Formation, Denali National Park, central Alaska Range, Alaska. This rock unit is now well-documented as a source of thousands of fossil footprints of vertebrates such as fishes, pterosaurs, and avialan and non-avialan dinosaurs. The lower Cantwell Formation in this area consists of numerous fining-upward successions of conglomerates and pebbly sandstones, cross-stratified and massive sandstones, interbedded sandstones and siltstones, organic-rich siltstones and shales, and rare, thin, bentonites, typically bounded by thin coal seams, and it contains a diverse fossil flora. We report the first North American co-occurrence of tracks attributable to hadrosaurs and therizinosaurs in the lower Cantwell Formation. Although previously un-reported in North America, this association of hadrosaur and therizinosaur tracks is more characteristic of the correlative Nemegt Formation in central Asia, perhaps suggesting that parameters defining the continental ecosystem of central Asia were also present in this part of Alaska during the Latest Cretaceous.

Redescription and affinities of Hulsanpes perlei (Dinosauria, Theropoda) from the Upper Cretaceous of Mongolia

Hulsanpes perlei is an enigmatic theropod dinosaur from the Baruungoyot Formation (?mid- to upper Campanian, Upper Cretaceous) of Mongolia. It was discovered in 1970, during the third Polish-Mongolian paleontological expedition to the Nemegt Basin. The taxon is known based on a partial braincase and an incomplete right hindlimb. However, the braincase fragment has never been described nor illustrated. We redescribe all elements that form the holotype of Hulsanpes and discuss the affinities of this taxon. The braincase fragment is interpreted as belonging to the inner ear region, and includes the floccular recess and part of the labyrinth. Hulsanpes perlei is confirmed as a valid taxon, diagnosed by a unique combination of metatarsal characters, including two autapomorphies. Historically, it represents the oldest record of the recently-established clade Halszkaraptorinae. Our findings identify subcursorial adaptations for Hulsanpes, shared with Mahakala, and differentiating them from Halszkaraptor. As such, appendicular disparity in the potentially sympatric halszkaraptorines suggest a reduced ecological overlap among these taxa, which may explain the co-occurrence of multiple species of this clade during the latest Cretaceous in what is now the Nemegt Basin.

Postcranial skeletal anatomy of the holotype and referred specimens of Buitreraptor gonzalezorum Makovicky, Apesteguía and Agnolín 2005 (Theropoda, Dromaeosauridae), from the Late Cretaceous of Patagonia

Here we provide a detailed description of the postcranial skeleton of the holotype and referred specimens of Buitreraptor gonzalezorum. This taxon was recovered as an unenlagiine dromaeosaurid in several recent phylogenetic studies and is the best represented Gondwanan dromaeosaurid discovered to date. It was preliminarily described in a brief article, but a detailed account of its osteology is emerging in recent works. The holotype is the most complete specimen yet found, so an exhaustive description of it provides much valuable anatomical information. The holotype and referred specimens preserve the axial skeleton, pectoral and pelvic girdles, and both fore- and hindlimbs. Diagnostic postcranial characters of this taxon include: anterior cervical centra exceeding the posterior limit of neural arch; eighth and ninth cervical vertebral centra with lateroventral tubercles; pneumatic foramina only in anteriormost dorsals; middle and posterior caudal centra with a complex of shallow ridges on lateral surfaces; pneumatic furcula with two pneumatic foramina on the ventral surface; scapular blade transversely expanded at mid-length; well-projected flexor process on distal end of the humerus; dorsal rim of the ilium laterally everted; and concave dorsal rim of the postacetabular iliac blade. A paleohistological study of limb bones shows that the holotype represents an earlier ontogenetic stage than one of the referred specimens (MPCA 238), which correlates with the fusion of the last sacral vertebra to the rest of the sacrum in MPCA 238. A revised phylogenetic analysis recovered Buitreraptor as an unenlagiine dromaeosaurid, in agreement with previous works. The phylogenetic implications of the unenlagiine synapomorphies and other characters, such as the specialized pedal digit II and the distal ginglymus on metatarsal II, are discussed within the evolutionary framework of Paraves.

A new caenagnathid dinosaur from the Upper Cretaceous Wangshi Group of Shandong, China, with comments on size variation among oviraptorosaurs

The bone-beds of the Upper Cretaceous Wangshi Group in Zhucheng, Shandong, China are rich in fossil remains of the gigantic hadrosaurid Shantungosaurus. Here we report a new oviraptorosaur, Anomalipes zhaoi gen. et sp. nov., based on a recently collected specimen comprising a partial left hindlimb from the Kugou Locality in Zhucheng. This specimen's systematic position was assessed by three numerical cladistic analyses based on recently published theropod phylogenetic datasets, with the inclusion of several new characters. Anomalipes zhaoi differs from other known caenagnathids in having a unique combination of features: femoral head anteroposteriorly narrow and with significant posterior orientation; accessory trochanter low and confluent with lesser trochanter; lateral ridge present on femoral lateral surface; weak fourth trochanter present; metatarsal III with triangular proximal articular surface, prominent anterior flange near proximal end, highly asymmetrical hemicondyles, and longitudinal groove on distal articular surface; and ungual of pedal digit II with lateral collateral groove deeper and more dorsally located than medial groove. The holotype of Anomalipes zhaoi is smaller than is typical for Caenagnathidae but larger than is typical for the other major oviraptorosaurian subclade, Oviraptoridae. Size comparisons among oviraptorisaurians show that the Caenagnathidae vary much more widely in size than the Oviraptoridae.

The oldest Archaeopteryx (Theropoda: Avialiae): a new specimen from the Kimmeridgian/Tithonian boundary of Schamhaupten, Bavaria

The iconic primeval bird Archaeopteryx was so far mainly known from the Altmühltal Formation (early Tithonian) of Bavaria, southern Germany, with one specimen having been found in the overlying Mörnsheim Formation. A new specimen (the 12th skeletal specimen) from the earliest Tithonian Painten Formation of Schamhaupten (Bavaria) represents the so far oldest representative of the genus. The new specimen shows several interesting anatomical details, including the presence of a postorbital in contact with the jugal, the presence of a separate prefrontal and coronoid, and opisthocoelous mid-cervical vertebrae. Based on observations on the new specimen, we discuss several problematic issues concerning Archaeopteryx, including the monophyly and diagnosis of the genus, the absence/presence of the sternum, the position of the gastralia, and variation in morphometrics and dental morphology in that genus. Based on a new diagnosis for the genus Archaeopteryx, the Berlin, Eichstätt, Solnhofen, Munich, Daiting, Thermopolis, 11th, and 12th specimens can be referred to this genus with high certainty. The Maxberg specimen is very probably also an Archaeopteryx, based on overall similarity, although none of the diagnostic characters can be evaluated with certainty. The ninth specimen (‘chicken wing’) might be Archaeopteryx, but cannot be referred to the genus with any certainty. In comparison with other paravians, the presence of distally thickened anterior pectoral ribs indicates that a rather large cartilagenous sternum was present in this taxon. In contrast to non-opisthopubic theropods, opisthopubic taxa, such as Archaeopteryx and many other paravians, have the posterior end of the gastral basket preserved at about half-length of the pubis, which might reflect the post-mortem collapse of enlarged abdominal air sacs in these taxa. Specimens that can be referred to Archaeopteryx show a high amount of variation, both in the morphometrics of the limb bones as well as in the dentition. In respect to the latter aspect, variation is found in tooth number, spacing, orientation, and morphology, with no two specimens showing the exact same pattern. The significance of this variation is unclear, and possible explanations reach from high intraspecific (and possibly ontogenetic and/or sexual dimorphic) variation to the possibility that the known specimens represent a ‘species flock’ of Archaeopteryx, possibly due to island speciation after the initial dispersal of the genus into the Solnhofen Archipelago.

Synchrotron scanning reveals amphibious ecomorphology in a new clade of bird-like dinosaurs

Maniraptora includes birds and their closest relatives among theropod dinosaurs. During the Cretaceous period, several maniraptoran lineages diverged from the ancestral coelurosaurian bauplan and evolved novel ecomorphologies, including active flight, gigantism, cursoriality and herbivory. Propagation X-ray phase-contrast synchrotron microtomography of a well-preserved maniraptoran from Mongolia, still partially embedded in the rock matrix, revealed a mosaic of features, most of them absent among non-avian maniraptorans but shared by reptilian and avian groups with aquatic or semiaquatic ecologies. This new theropod, Halszkaraptor escuilliei gen. et sp. nov., is related to other enigmatic Late Cretaceous maniraptorans from Mongolia in a novel clade at the root of Dromaeosauridae. This lineage adds an amphibious ecomorphology to those evolved by maniraptorans: it acquired a predatory mode that relied mainly on neck hyperelongation for food procurement, it coupled the obligatory bipedalism of theropods with forelimb proportions that may support a swimming function, and it developed postural adaptations convergent with short-tailed birds.

Functional anatomy of a giant toothless mandible from a bird-like dinosaur: Gigantoraptor and the evolution of the oviraptorosaurian jaw

The Oviraptorosauria are a group of theropod dinosaurs that diverged from the typical carnivorous theropod diet. It includes two main lineages – Caenagnathidae and Oviraptoridae – that display a number of differences in mandibular morphology, but little is known about their functional consequences, hampering our understanding of oviraptorosaurian dietary evolution. This study presents the first in-depth description of the giant toothless mandible of Gigantoraptor, the only well-preserved stemward caenagnathid mandible. This mandible shows the greatest relative beak depth among caenagnathids, which is an adaptation seen in some modern birds for processing harder seeds. The presence of a lingual triturating shelf in caenagnathids more crownward than Gigantoraptor suggests a possible increased specialization towards shearing along this lineage. Like other oviraptorosaurs, the possession of a dorsally convex articular glenoid in Gigantoraptor indicates that propalinal jaw movement was probably an important mechanism for food processing, as in Sphenodon and dicynodonts. Oviraptorid mandibles were more suited for producing powerful bites (e.g. crushing-related) compared to caenagnathids: oviraptorids generally possess a deeper, more downturned beak, a taller coronoid process prominence and a larger medial mandibular fossa. This disparity in caenagnathid and oviraptorid mandible morphology potentially suggests specialization towards two different feeding styles – shearing and crushing-related mechanisms respectively.

A bird-like skull in a Triassic diapsid reptile increases heterogeneity of the morphological and phylogenetic radiation of Diapsida

The Triassic Period saw the first appearance of numerous amniote lineages (e.g. Lepidosauria, Archosauria, Mammalia) that defined Mesozoic ecosystems following the end Permian Mass Extinction, as well as the first major morphological diversification of crown-group reptiles. Unfortunately, much of our understanding of this event comes from the record of large-bodied reptiles (total body length > 1 m). Here we present a new species of drepanosaurid (small-bodied, chameleon-like diapsids) from the Upper Triassic Chinle Formation of New Mexico. Using reconstructions of micro-computed tomography data, we reveal the three-dimensional skull osteology of this clade for the first time. The skull presents many archaic anatomical traits unknown in Triassic crown-group reptiles (e.g. absence of bony support for the external ear), whereas other traits (e.g. toothless rostrum, anteriorly directed orbits, inflated endocranium) resemble derived avian theropods. A phylogenetic analysis of Permo-Triassic diapsids supports the hypothesis that drepanosaurs are an archaic lineage that originated in the Permian, far removed from crown-group Reptilia. The phylogenetic position of drepanosaurids indicates the presence of archaic Permian clades among Triassic small reptile assemblages and that morphological convergence produced a remarkably bird-like skull nearly 100 Myr before one is known to have emerged in Theropoda.

Perinate and eggs of a giant caenagnathid dinosaur from the Late Cretaceous of central China

The abundance of dinosaur eggs in Upper Cretaceous strata of Henan Province, China led to the collection and export of countless such fossils. One of these specimens, recently repatriated to China, is a partial clutch of large dinosaur eggs (Macroelongatoolithus) with a closely associated small theropod skeleton. Here we identify the specimen as an embryo and eggs of a new, large caenagnathid oviraptorosaur, Beibeilong sinensis. This specimen is the first known association between skeletal remains and eggs of caenagnathids. Caenagnathids and oviraptorids share similarities in their eggs and clutches, although the eggs of Beibeilong are significantly larger than those of oviraptorids and indicate an adult body size comparable to a gigantic caenagnathid. An abundance of Macroelongatoolithus eggs reported from Asia and North America contrasts with the dearth of giant caenagnathid skeletal remains. Regardless, the large caenagnathid-Macroelongatoolithus association revealed here suggests these dinosaurs were relatively common during the early Late Cretaceous.

A Late Cretaceous diversification of Asian oviraptorid dinosaurs: evidence from a new species preserved in an unusual posture

Oviraptorosaurs are a bizarre group of bird-like theropod dinosaurs, the derived forms of which have shortened, toothless skulls, and which diverged from close relatives by developing peculiar feeding adaptations. Although once among the most mysterious of dinosaurs, oviraptorosaurs are becoming better understood with the discovery of many new fossils in Asia and North America. The Ganzhou area of southern China is emerging as a hotspot of oviraptorosaur discoveries, as over the past half decade five new monotypic genera have been found in the latest Cretaceous (Maastrichtian) deposits of this region. We here report a sixth diagnostic oviraptorosaur from Ganzhou, Tongtianlong limosus gen. et sp. nov., represented by a remarkably well-preserved specimen in an unusual splayed-limb and raised-head posture. Tongtianlong is a derived oviraptorid oviraptorosaur, differentiated from other species by its unique dome-like skull roof, highly convex premaxilla, and other features of the skull. The large number of oviraptorosaurs from Ganzhou, which often differ in cranial morphologies related to feeding, document an evolutionary radiation of these dinosaurs during the very latest Cretaceous of Asia, which helped establish one of the last diverse dinosaur faunas before the end-Cretaceous extinction.

The first oviraptorosaur (Dinosauria: Theropoda) bonebed: evidence of gregarious behaviour in a maniraptoran theropod

A monodominant bonebed of Avimimus from the Nemegt Formation of Mongolia is the first oviraptorosaur bonebed described and the only recorded maniraptoran bonebed from the Late Cretaceous. Cranial elements recovered from the bonebed provide insights on the anatomy of the facial region, which was formerly unknown in Avimimus. Both adult and subadult material was recovered from the bonebed, but small juveniles are underrepresented. The taphonomic and sedimentological evidence suggests that the Avimimus bonebed represents a perimortem gregarious assemblage. The near absence of juveniles in the bonebed may be evidence of a transient age-segregated herd or 'flock', but the behaviour responsible for this assemblage is unclear. Regardless, the Avimimus bonebed is the first evidence of gregarious behaviour in oviraptorosaurs, and highlights a potential trend of increasing gregariousness in dinosaurs towards the end of the Mesozoic.

Vertebral Adaptations to Large Body Size in Theropod Dinosaurs

Rugose projections on the anterior and posterior aspects of vertebral neural spines appear throughout Amniota and result from the mineralization of the supraspinous and interspinous ligaments via metaplasia, the process of permanent tissue-type transformation. In mammals, this metaplasia is generally pathological or stress induced, but is a normal part of development in some clades of birds. Such structures, though phylogenetically sporadic, appear throughout the fossil record of non-avian theropod dinosaurs, yet their physiological and adaptive significance has remained unexamined. Here we show novel histologic and phylogenetic evidence that neural spine projections were a physiological response to biomechanical stress in large-bodied theropod species. Metaplastic projections also appear to vary between immature and mature individuals of the same species, with immature animals either lacking them or exhibiting smaller projections, supporting the hypothesis that these structures develop through ontogeny as a result of increasing bending stress subjected to the spinal column. Metaplastic mineralization of spinal ligaments would likely affect the flexibility of the spinal column, increasing passive support for body weight. A stiff spinal column would also provide biomechanical support for the primary hip flexors and, therefore, may have played a role in locomotor efficiency and mobility in large-bodied species. This new association of interspinal ligament metaplasia in Theropoda with large body size contributes additional insight to our understanding of the diverse biomechanical coping mechanisms developed throughout Dinosauria, and stresses the significance of phylogenetic methods when testing for biological trends, evolutionary or not.

Elongatoolithid eggs containing oviraptorid (Theropoda, Oviraptorosauria) embryos from the Upper Cretaceous of Southern China

BACKGROUND

Oviraptorids, like many other dinosaurs, clearly had a complex pattern of skeletal growth involving numerous morphological changes. However, many ontogenetic skeletal changes in oviraptorids were previously unclear due to the lack of well preserved specimens that represent very young developmental stages.

RESULTS

Here we report three elongatoolithid dinosaur eggs from the Upper Cretaceous Nanxiong Formation of Nankang District, Ganzhou City, Jiangxi Province, China that contain in ovo embryonic skeletons. The eggs themselves show diagnostic features of the oofamily Elongatoolithidae, whereas the embryos are identified as taxonomically indeterminate oviraptorids. The three new specimens display pathological eggshell features, including double-layered and multilayered cones in the columnar layer, which probably result from high levels of pathogenic trace elements in the environment. Nevertheless, the skeletons of the preserved embryos exhibit no structural or histological abnormalities. Comparisons between the new embryos and other oviraptorid specimens reveal 20 osteological features that appear to change substantially during ontogeny in oviraptorids. For example, the dorsoventral height of the skull increases more rapidly than the anteroposterior length during oviraptorid ontogeny, and the initially paired nasals fuse at an early stage, presumably facilitating growth of a crest.

CONCLUSIONS

The new specimens represent the first known oviraptorid embryos associated with pathological eggshells. The absence of structural and histological abnormalities indicates the environmental factor that led to the eggshell pathologies did not affect the skeletal development of the oviraptorids themselves. As in tyrannosaurids, but in contrast to the situation in other maniraptorans, the oviraptorid skull becomes proportionally dorsoventrally deeper during ontogeny. Although oviraptorids and therizinosauroids occupy broadly the same grade of maniraptoran evolution, the embryonic ossification patterns of the vertebral column and furcular hypocleidium appear to differ significantly between the two clades. The limb proportions of juvenile oviraptorids indicate that they were bipedal, like adults. Oviraptorids may have differed greatly from therizinosauroids in their growth trajectories and locomotor modes during early post-hatching ontogeny, essentially occupying a different ecological niche.

Brain modularity across the theropod-bird transition: testing the influence of flight on neuroanatomical variation

Living birds constitute the only vertebrate group whose brain volume relative to body size approaches the uniquely expanded values expressed by mammals. The broad suite of complex behaviors exhibited by crown-group birds, including sociality, vocal learning, parental care, and flying, suggests the origins of their encephalization was likely driven by a mosaic of selective pressures. If true, the historical pattern of brain expansion may be more complex than either a gradual expansion, as proposed by early studies of the avian brain, or a sudden expansion correlating with the appearance of flight. The origins of modern avian neuroanatomy are obscured by the more than 100 million years of evolution along their phylogenetic stem (from the origin of the modern radiation in the Middle Jurassic to the split from crocodile-line archosaurs). Here we use phylogenetic comparative approaches to explore which evolutionary scenarios best explain variation in measured volumes of digitally partitioned endocasts of modern birds and their non-avian ancestors. Our analyses suggest that variation in the relative volumes of the endocranium and cerebrum explain most of the structural variation in this lineage. Generalized multi-regime Ornstein-Uhlenbeck (OU) models suggest that powered flight does not appear to be a driver of observed variation, reinforcing the hypothesis that the deep history of the avian brain is complex, with nuances still to be discovered.