A gigantic bird-like dinosaur from the Late Cretaceous of China

An evolutionary trend of decreasing size is present along the line to birds in coelurosaurian theropod evolution, but size increases are seen in many coelurosaurian subgroups, in which large forms are less bird-like. Here we report on a new non-avian dinosaur, Gigantoraptor erlianensis, gen. et sp. nov., from the Late Cretaceous Iren Dabasu Formation of Nei Mongol, China. Although it has a body mass of about 1,400 kg, a phylogenetic analysis positions this new taxon within the Oviraptorosauria, a group of small, feathered theropods rarely exceeding 40 kg in body mass. A histological analysis suggests that Gigantoraptor gained this size by a growth rate considerably faster than large North American tyrannosaurs such as Albertosaurus and Gorgosaurus. Gigantoraptor possesses several salient features previously unknown in any other dinosaur and its hind limb bone scaling and proportions are significantly different from those of other coelurosaurs, thus increasing the morphological diversity among dinosaurs. Most significantly, the gigantic Gigantoraptor shows many bird-like features absent in its smaller oviraptorosaurian relatives, unlike the evolutionary trend seen in many other coelurosaurian subgroups.     

Branched integumental structures in Sinornithosaurus and the origin of feathers

The evolutionary origin of feathers has long been obscured because no morphological antecedents were known to the earliest, structurally modern feathers of Archaeopteryx. It has been proposed that the filamentous integumental appendages on several theropod dinosaurs are primitive feathers; but the homology between these filamentous structures and feathers has been disputed, and two taxa with true feathers (Caudipteryx and Protarchaeopteryx) have been proposed to be flightless birds. Confirmation of the theropod origin of feathers requires documentation of unambiguously feather-like structures in a clearly non-avian theropod. Here we describe our observations of the filamentous integumental appendages of the basal dromaeosaurid dinosaur Sinornithosaurus millenii, which indicate that they are compound structures composed of multiple filaments. Furthermore, these appendages exhibit two types of branching structure that are unique to avian feathers: filaments joined in a basal tuft, and filaments joined at their bases in series along a central filament. Combined with the independent phylogenetic evidence supporting the theropod ancestry of birds, these observations strongly corroborate the hypothesis that the integumental appendages of Sinornithosaurus are homologous with avian feathers. The plesiomorphic feathers of Sinornithosaurus also conform to the predictions of an independent, developmental model of the evolutionary origin of feathers.     

Forensic palaeontology: The Archaeoraptor forgery

The Archaeoraptor fossil was announced as a 'missing link' and purported to be possibly the best evidence since Archaeopteryx that birds did, in fact, evolve from certain types of carnivorous dinosaur. It reportedly came from Early Cretaceous beds of China that have produced other spectacular fossils transitional between birds and extinct non-avian dinosaurs. But Archaeoraptor was revealed to be a forgery in which bones of a primitive bird and a non-flying dromaeosaurid dinosaur had been combined. Here we use high-resolution X-ray computed tomography (CT) to determine the nature and extent of the forgery, as well as how it was built, by imaging the fracture pattern and distribution of materials through the entire specimen.     

A new carnivorous dinosaur from the Late Jurassic Solnhofen archipelago

Small Late Jurassic theropod dinosaurs are rare worldwide. In Europe these carnivorous dinosaurs are represented primarily by only two skeletons of Compsognathus, neither of which is well preserved. Here we describe a small new theropod dinosaur from the Late Jurassic period of Schamhaupten in southern Germany. Being exquisitely preserved and complete from the snout to the distal third of the tail, the new fossil is the best-preserved predatory, non-avian dinosaur in Europe. It possesses a suite of characters that support its identification as a basal coelurosaur. A cladistic analysis indicates that the new taxon is closer to maniraptorans than to tyrannosauroids, grouping it with taxa often considered to be compsognathids. Large portions of integument are preserved along its tail. The absence of feathers or feather-like structures in a fossil phylogenetically nested within feathered theropods indicates that the evolution of these integumentary structures might be more complex than previously thought.     

Dinosaurian growth rates and bird origins

Dinosaurs, like other tetrapods, grew more quickly just after hatching than later in life. However, they did not grow like most other non-avian reptiles, which grow slowly and gradually through life. Rather, microscopic analyses of the long-bone tissues show that dinosaurs grew to their adult size relatively quickly, much as large birds and mammals do today. The first birds reduced their adult body size by shortening the phase of rapid growth common to their larger theropod dinosaur relatives. These changes in timing were primarily related not to physiological differences but to differences in growth strategy.     

Basic avian pulmonary design and flow-through ventilation in non-avian theropod dinosaurs

Birds are unique among living vertebrates in possessing pneumaticity of the postcranial skeleton, with invasion of bone by the pulmonary air-sac system. The avian respiratory system includes high-compliance air sacs that ventilate a dorsally fixed, non-expanding parabronchial lung. Caudally positioned abdominal and thoracic air sacs are critical components of the avian aspiration pump, facilitating flow-through ventilation of the lung and near-constant airflow during both inspiration and expiration, highlighting a design optimized for efficient gas exchange. Postcranial skeletal pneumaticity has also been reported in numerous extinct archosaurs including non-avian theropod dinosaurs and Archaeopteryx. However, the relationship between osseous pneumaticity and the evolution of the avian respiratory apparatus has long remained ambiguous. Here we report, on the basis of a comparative analysis of region-specific pneumaticity with extant birds, evidence for cervical and abdominal air-sac systems in non-avian theropods, along with thoracic skeletal prerequisites of an avian-style aspiration pump. The early acquisition of this system among theropods is demonstrated by examination of an exceptional new specimen of Majungatholus atopus, documenting these features in a taxon only distantly related to birds. Taken together, these specializations imply the existence of the basic avian pulmonary Bauplan in basal neotheropods, indicating that flow-through ventilation of the lung is not restricted to birds but is probably a general theropod characteristic.     

A new feathered maniraptoran dinosaur fossil that fills a morphological gap in avian origin

Recent fossil discoveries have substantially reduced the morphological gap between non-avian and avian dinosaurs, yet avians including Archaeopteryx differ from non-avian theropods in their limb proportions. In particular, avians have proportionally longer and more robust forelimbs that are capable of supporting a large aerodynamic surface. Here we report on a new maniraptoran dinosaur, Anchiornis huxleyi gen. et sp. nov., based on a specimen collected from lacustrine deposits of uncertain age in western Liaoning, China. With an estimated mass of 110 grams, Anchiornis is the smallest known non-avian theropod dinosaur. It exhibits some wrist features indicative of high mobility, presaging the wing-folding mechanisms seen in more derived birds and suggesting rapid evolution of the carpus. Otherwise, Anchiornis is intermediate in general morphology between non-avian and avian dinosaurs, particularly with regard to relative forelimb length and thickness, and represents a transitional step toward the avian condition. In contrast with some recent comprehensive phylogenetic analyses, our phylogenetic analysis incorporates subtle morphological variations and recovers a conventional result supporting the monophyly of Avialae. Supported by Hundred Talents Project of the Chinese Academy of Sciences, National Natural Science Foundation of China (Grant Nos. 40125006, 40472018), and National Basic Research Program of China (Grant No. 2006CB806400)     

Bird-like fossil footprints from the Late Triassic

The study of fossilized footprints and tracks of dinosaurs and other vertebrates has provided insight into the origin, evolution and extinction of several major groups and their behaviour; it has also been an important complement to their body fossil record. The known history of birds starts in the Late Jurassic epoch (around 150 Myr ago) with the record of Archaeopteryx, whereas the coelurosaurian ancestors of the birds date back to the Early Jurassic. The hind limbs of Late Triassic epoch theropods lack osteological evidence for an avian reversed hallux and also display other functional differences from birds. Previous references to suggested Late Triassic to Early Jurassic bird-like footprints have been reinterpreted as produced by non-avian dinosaurs having a high angle between digits II and IV and in all cases their avian affinities have been challenged. Here we describe well-preserved and abundant footprints with clearly avian characters from a Late Triassic redbed sequence of Argentina, at least 55 Myr before the first known skeletal record of birds. These footprints document the activities, in an environment interpreted as small ponds associated with ephemeral rivers, of an unknown group of Late Triassic theropods having some avian characters.     

Palaeontology: 'modern' feathers on a non-avian dinosaur

Discoveries of integumentary coverings on non-avian theropod dinosaurs are becoming commonplace. But the only definitive evidence so far that any of these animals had feathers as we know them today has come from the oviraptorosaur Caudipteryx and the enigmatic coleurosaur Protarchaeopteryx, both of which are considered by some to be secondarily flightless birds. Here we describe the occurrence of pinnate feathers, which clearly feature a rachis and barbs, on a small, non-avian dromaeosaur from northern China. This finding indicates that feathers of modern aspect evolved in dinosaurs before the emergence of birds and flight.     

Pre-<Emphasis Type="Italic">Archaeopteryx</Emphasis> coelurosaurian dinosaurs and their implications for understanding avian origins

The last two decades have witnessed great advances in reconstructing the transition from non-avian theropods to avians, but views in opposition to the theropod hypothesis still exist. Here we highlight one issue that is often considered to raise problems for the theropod hypothesis of avian origins, i.e. the “temporal paradox” in the stratigraphic distribution of theropod fossils — the idea that the earliest known avian is from the Late Jurassic but most other coelurosaurian groups are poorly known in the Jurassic, implying that avians arose before their supposed ancestors. However, a number of Jurassic non-avian coelurosaurian theropods have recently been discovered, thus documenting the presence of most of the major coelurosaurian groups in the Jurassic alongside, or prior to, avians. These discoveries have greatly improved the congruence between stratigraphy and phylogeny for derived theropods and, effectively, they reject the “temporal paradox” concept. Most importantly, these discoveries provide significant new information that supports the relatively basal positions of the Tyrannosauroidea and Alvarezsauroidea among the Coelurosauria. Indeed, they imply a new phylogenetic hypothesis for the interrelationships of Paraves, in which Archaeopteryx, the Dromaeosauridae, and the Troodontidae form a monophyletic group while the Scansoriopterygidae, other basal birds, and probably also the Oviraptorosauria, form another clade. Mapping some of the salient features onto a temporally-calibrated theropod phylogeny indicates that characteristics related to flight and arboreality evolved at the base of the Paraves, earlier than the Late Jurassic.     

The avian nature of the brain and inner ear of Archaeopteryx

Archaeopteryx, the earliest known flying bird (avialan) from the Late Jurassic period, exhibits many shared primitive characters with more basal coelurosaurian dinosaurs (the clade including all theropods more bird-like than Allosaurus), such as teeth, a long bony tail and pinnate feathers. However, Archaeopteryx possessed asymmetrical flight feathers on its wings and tail, together with a wing feather arrangement shared with modern birds. This suggests some degree of powered flight capability but, until now, little was understood about the extent to which its brain and special senses were adapted for flight. We investigated this problem by computed tomography scanning and three-dimensional reconstruction of the braincase of the London specimen of Archaeopteryx. Here we show the reconstruction of the braincase from which we derived endocasts of the brain and inner ear. These suggest that Archaeopteryx closely resembled modern birds in the dominance of the sense of vision and in the possession of expanded auditory and spatial sensory perception in the ear. We conclude that Archaeopteryx had acquired the derived neurological and structural adaptations necessary for flight. An enlarged forebrain suggests that it had also developed enhanced somatosensory integration with these special senses demanded by a lifestyle involving flying ability.     

Cursoriality in bipedal archosaurs

Modern birds have markedly foreshortened tails and their body mass is centred anteriorly, near the wings. To provide stability during powered flight, the avian centre of mass is far from the pelvis, which poses potential balance problems for cursorial birds. To compensate, avians adapted to running maintain the femur subhorizontally, with its distal end situated anteriorly, close to the animal's centre of mass; stride generation stems largely from parasagittal rotation of the lower leg about the knee joint. In contrast, bipedal dinosaurs had a centre of mass near the hip joint and rotated the entire hindlimb during stride generation. Here we show that these contrasting styles of cursoriality are tightly linked to longer relative total hindlimb length in cursorial birds than in bipedal dinosaurs. Surprisingly, Caudipteryx, described as a theropod dinosaur, possessed an anterior centre of mass and hindlimb proportions resembling those of cursorial birds. Accordingly, Caudipteryx probably used a running mechanism more similar to that of modern cursorial birds than to that of all other bipedal dinosaurs. These observations provide valuable clues about cursoriality in Caudipteryx, but may also have implications for interpreting the locomotory status of its ancestors.     

Baryonyx, a remarkable new theropod dinosaur

An extremely large claw bone, some 30 cm long, was found in Wealden (Lower Cretaceous) deposits in a Surrey claypit in January 1983. This led to the discovery the following month of the well-preserved skeleton of a new large theropod dinosaur. Only one other theropod specimen comprising more than a few bones had ever been found in Britain, and that discovery was more than a century ago. Indeed, no large theropod, reasonably complete, had previously been discovered in Lower Cretaceous rocks anywhere in the world. Our study so far suggests that the Surrey dinosaur was a typical large theropod in certain respects, resembling, for example Allosaurus. In several other respects, however, it differs sufficiently from all known dinosaurs to merit designation as the representative of a new species, genus and family.     

A bizarre Jurassic maniraptoran from China with elongate ribbon-like feathers

Recent coelurosaurian discoveries have greatly enriched our knowledge of the transition from dinosaurs to birds, but all reported taxa close to this transition are from relatively well known coelurosaurian groups. Here we report a new basal avialan, Epidexipteryx hui gen. et sp. nov., from the Middle to Late Jurassic of Inner Mongolia, China. This new species is characterized by an unexpected combination of characters seen in several different theropod groups, particularly the Oviraptorosauria. Phylogenetic analysis shows it to be the sister taxon to Epidendrosaurus, forming a new clade at the base of Avialae. Epidexipteryx also possesses two pairs of elongate ribbon-like tail feathers, and its limbs lack contour feathers for flight. This finding shows that a member of the avialan lineage experimented with integumentary ornamentation as early as the Middle to Late Jurassic, and provides further evidence relating to this aspect of the transition from non-avian theropods to birds.     

Origin of avian genome size and structure in non-avian dinosaurs

Avian genomes are small and streamlined compared with those of other amniotes by virtue of having fewer repetitive elements and less non-coding DNA. This condition has been suggested to represent a key adaptation for flight in birds, by reducing the metabolic costs associated with having large genome and cell sizes. However, the evolution of genome architecture in birds, or any other lineage, is difficult to study because genomic information is often absent for long-extinct relatives. Here we use a novel bayesian comparative method to show that bone-cell size correlates well with genome size in extant vertebrates, and hence use this relationship to estimate the genome sizes of 31 species of extinct dinosaur, including several species of extinct birds. Our results indicate that the small genomes typically associated with avian flight evolved in the saurischian dinosaur lineage between 230 and 250 million years ago, long before this lineage gave rise to the first birds. By comparison, ornithischian dinosaurs are inferred to have had much larger genomes, which were probably typical for ancestral Dinosauria. Using comparative genomic data, we estimate that genome-wide interspersed mobile elements, a class of repetitive DNA, comprised 5-12% of the total genome size in the saurischian dinosaur lineage, but was 7-19% of total genome size in ornithischian dinosaurs, suggesting that repetitive elements became less active in the saurischian lineage. These genomic characteristics should be added to the list of attributes previously considered avian but now thought to have arisen in non-avian dinosaurs, such as feathers, pulmonary innovations, and parental care and nesting.     

The distribution of integumentary structures in a feathered dinosaur

Non-avian theropod dinosaurs with preserved integumentary coverings are becoming more common; but apart from the multiple specimens of Caudipteryx, which have true feathers, animals that are reasonably complete and entirely articulated that show these structures in relation to the body have not been reported. Here we report on an enigmatic small theropod dinosaur that is covered with filamentous feather-like structures over its entire body.     

Birds have paedomorphic dinosaur skulls

The interplay of evolution and development has been at the heart of evolutionary theory for more than a century. Heterochrony—change in the timing or rate of developmental events—has been implicated in the evolution of major vertebrate lineages such as mammals, including humans. Birds are the most speciose land vertebrates, with more than 10,000 living species representing a bewildering array of ecologies. Their anatomy is radically different from that of other vertebrates. The unique bird skull houses two highly specialized systems: the sophisticated visual and neuromuscular coordination system allows flight coordination and exploitation of diverse visual landscapes, and the astonishing variations of the beak enable a wide range of avian lifestyles. Here we use a geometric morphometric approach integrating developmental, neontological and palaeontological data to show that the heterochronic process of paedomorphosis, by which descendants resemble the juveniles of their ancestors, is responsible for several major evolutionary transitions in the origin of birds. We analysed the variability of a series of landmarks on all known theropod dinosaur skull ontogenies as well as outgroups and birds. The first dimension of variability captured ontogeny, indicating a conserved ontogenetic trajectory. The second dimension accounted for phylogenetic change towards more bird-like dinosaurs. Basally branching eumaniraptorans and avialans clustered with embryos of other archosaurs, indicating paedomorphosis. Our results reveal at least four paedomorphic episodes in the history of birds combined with localized peramorphosis (development beyond the adult state of ancestors) in the beak. Paedomorphic enlargement of the eyes and associated brain regions parallels the enlargement of the nasal cavity and olfactory brain in mammals. This study can be a model for investigations of heterochrony in evolutionary transitions, illuminating the origin of adaptive features and inspiring studies of developmental mechanisms.     

Palaeontology: leg feathers in an Early Cretaceous bird

Here we describe a fossil of an enantiornithine bird from the Early Cretaceous period in China that has substantial plumage feathers attached to its upper leg (tibiotarsus). The discovery could be important in view of the relative length and aerodynamic features of these leg feathers compared with those of the small 'four-winged' gliding dinosaur Microraptor and of the earliest known bird, Archaeopteryx. They may be remnants of earlier long, aerodynamic leg feathers, in keeping with the hypothesis that birds went through a four-winged stage during the evolution of flight.     

An Archaeopteryx-like theropod from China and the origin of Avialae

Archaeopteryx is widely accepted as being the most basal bird, and accordingly it is regarded as central to understanding avialan origins; however, recent discoveries of derived maniraptorans have weakened the avialan status of Archaeopteryx. Here we report a new Archaeopteryx-like theropod from China. This find further demonstrates that many features formerly regarded as being diagnostic of Avialae, including long and robust forelimbs, actually characterize the more inclusive group Paraves (composed of the avialans and the deinonychosaurs). Notably, adding the new taxon into a comprehensive phylogenetic analysis shifts Archaeopteryx to the Deinonychosauria. Despite only tentative statistical support, this result challenges the centrality of Archaeopteryx in the transition to birds. If this new phylogenetic hypothesis can be confirmed by further investigation, current assumptions regarding the avialan ancestral condition will need to be re-evaluated.