Xiaoting Zheng

An Early Cretaceous heterodontosaurid dinosaur with filamentous integumentary structures.

Ornithischia is one of the two major groups of dinosaurs, with heterodontosauridae as one of its major clades. Heterodontosauridae is characterized by small, gracile bodies and a problematic phylogenetic position. Recent phylogenetic work indicates that it represents the most basal group of all well-known ornithischians. Previous heterodontosaurid records are mainly from the Early Jurassic period (205–190 million years ago) of Africa. Here we report a new heterodontosaurid, Tianyulong confuciusi gen. et sp. nov., from the Early Cretaceous period (144–99 million years ago) of western Liaoning Province, China. Tianyulong extends the geographical distribution of heterodontosaurids to Asia and confirms the clade’s previously questionable temporal range extension into the Early Cretaceous period. More surprisingly, Tianyulong bears long, singular and unbranched filamentous integumentary (outer skin) structures. This represents the first confirmed report, to our knowledge, of filamentous integumentary structures in an ornithischian dinosaur.

Exceptional dinosaur fossils show ontogenetic development of early feathers

Recent discoveries of feathered dinosaur specimens have greatly improved our understanding of the origin and early evolution of feathers, but little information is available on the ontogenetic development of early feathers. Here we describe an early-juvenile specimen and a late-juvenile specimen, both referable to the oviraptorosaur Similicaudipteryx, recovered from the Lower Cretaceous Yixian Formation of western Liaoning, China. The two specimens have strikingly different remiges and rectrices, suggesting that a radical morphological change occurred during feather development, as is the case for modern feathers. However, both the remiges and the rectrices are proximally ribbon-like in the younger specimen but fully pennaceous in the older specimen, a pattern not known in any modern bird. In combination with the wide distribution of proximally ribbon-like pennaceous feathers and elongate broad filamentous feathers among extinct theropods, this find suggests that early feathers were developmentally more diverse than modern ones and that some developmental features, and the resultant morphotypes, have been lost in feather evolution.

Hind Wings in Basal Birds and the Evolution of Leg Feathers

Four-Winged Birds? Recently, nonavialan dinosaurs with feathers on their fore- and hindlimbs have been described. Zheng et al. (p. 1309) describe eleven basal avialan fossils with clear evidence of feathered hindlimbs. Together these fossils show that early avialans possessed four wings, rather than two. A gradual reduction in hindlimb feathering eventually yielded the two-wing condition in todays birds. Such a transition may have accompanied a locomotory decoupling of the fore- and hindlimbs, which facilitated the development of the forelimbs into flight-capable wings. Fossils of basal birds have feathers on all four limbs, suggesting that the present two-winged condition is a derived state. Recent discoveries of large leg feathers in some theropods have implications for our understanding of the evolution of integumentary features on the avialan leg, and particularly of their relevance for the origin of avialan flight. Here we report 11 basal avialan specimens that will greatly improve our knowledge of leg integumentary features among early birds. In particular, they provide solid evidence for the existence of enlarged leg feathers on a variety of basal birds, suggest that extensively scaled feet might have appeared secondarily at an early stage in ornithuromorph evolution, and demonstrate a distal-to-proximal reduction pattern for leg feathers in avialan evolution.

A new feather type in a nonavian theropod and the early evolution of feathers

All described feathers in nonavian theropods are composite structures formed by multiple filaments. They closely resemble relatively advanced stages predicted by developmental models of the origin of feathers, but not the earliest stage. Here, we report a feather type in two specimens of the basal therizinosaur Beipiaosaurus, in which each individual feather is represented by a single broad filament. This morphotype is congruent with the stage I morphology predicted by developmental models, and all major predicted morphotypes have now been documented in the fossil record. This congruence between the full range of paleontological and developmental data strongly supports the hypothesis that feathers evolved and initially diversified in nonavian theropods before the origin of birds and the evolution of flight.

Fossil evidence of avian crops from the Early Cretaceous of China

The crop is characteristic of seed-eating birds today, yet little is known about its early history despite remarkable discoveries of many Mesozoic seed-eating birds in the past decade. Here we report the discovery of some early fossil evidence for the presence of a crop in birds. Two Early Cretaceous birds, the basal ornithurine Hongshanornis and a basal avian Sapeornis, demonstrate that an essentially modern avian digestive system formed early in avian evolution. The discovery of a crop in two phylogenetically remote lineages of Early Cretaceous birds and its absence in most intervening forms indicates that it was independently acquired as a specialized seed-eating adaptation. Finally, the reduction or loss of teeth in the forms showing seed-filled crops suggests that granivory was possibly one of the factors that resulted in the reduction of teeth in early birds.

Preservation of ovarian follicles reveals early evolution of avian reproductive behaviour

The two groups of archosaurs, crocodilians and birds, form an extant phylogenetic bracket for understanding the reproductive behaviour of dinosaurs. This behaviour is inferred from preserved nests and eggs, and even gravid individuals. Data indicate that many ‘avian’ traits were already present in Paraves—the clade that includes birds and their close relatives—and that the early evolution of the modern avian form of reproduction was already well on its way. Like living neornithine birds, non-avian maniraptorans had daily oviposition and asymmetrical eggs with complex shell microstructure, and were known to protect their clutches. However, like crocodilians, non-avian maniraptorans had two active oviducts (one present in living birds), relatively smaller eggs, and may not have turned their eggs in the way that living birds do. Here we report on the first discovery of fossilized mature or nearly mature ovarian follicles, revealing a previously undocumented stage in dinosaur reproduction: reproductively active females near ovulation. Preserved in a specimen of the long bony-tailed Jeholornis and two enantiornithine birds from the Early Cretaceous period lacustrine Jehol Biota in northeastern China, these discoveries indicate that basal birds only had one functional ovary, but retained primitive morphologies as a result of their lower metabolic rate relative to living birds. They also indicate that basal birds reached sexual maturity before skeletal maturity, as in crocodiles and paravian dinosaurs. Differences in follicular morphology between Jeholornis and the enantiornithines are interpreted as forming an evolutionary gradient from the reproductive condition in paravian dinosaurs towards neornithine birds. Furthermore, differences between the two enantiornithines indicate that this lineage might also have evolved advanced reproductive traits in parallel to the neornithine lineage.

A bizarre Jurassic maniraptoran theropod with preserved evidence of membranous wings

The wings of birds and their closest theropod relatives share a uniform fundamental architecture, with pinnate flight feathers as the key component. Here we report a new scansoriopterygid theropod, Yi qi gen. et sp. nov., based on a new specimen from the Middle–Upper Jurassic period Tiaojishan Formation of Hebei Province, China. Yi is nested phylogenetically among winged theropods but has large stiff filamentous feathers of an unusual type on both the forelimb and hindlimb. However, the filamentous feathers of Yi resemble pinnate feathers in bearing morphologically diverse melanosomes. Most surprisingly, Yi has a long rod-like bone extending from each wrist, and patches of membranous tissue preserved between the rod-like bones and the manual digits. Analogous features are unknown in any dinosaur but occur in various flying and gliding tetrapods, suggesting the intriguing possibility that Yi had membranous aerodynamic surfaces totally different from the archetypal feathered wings of birds and their closest relatives. Documentation of the unique forelimbs of Yi greatly increases the morphological disparity known to exist among dinosaurs, and highlights the extraordinary breadth and richness of the evolutionary experimentation that took place close to the origin of birds.

Insight into the early evolution of the avian sternum from juvenile enantiornithines

The sternum is one of the most important and characteristic skeletal elements in living birds, highly adapted for flight and showing a diverse range of morphologies. New exceptional material of young juvenile specimens from the Early Cretaceous Jehol Group in northeastern China reveals the unique sequence of development in the sternum of Enantiornithes, the dominant clade of Cretaceous birds. We recognize six ossifications that together form the sternum, three of which were previously unknown. Here we show that although basal living birds apparently have retained the dinosaurian condition in which the sternum develops from a bilateral pair of ossifications (present in paravian dinosaurs and basal birds), the enantiornithine sternal body primarily develops from two unilateral proximo-distally arranged ossifications. This indicates that although superficially similar, the sternum formed very differently in enantiornithines and ornithuromorphs, suggesting that several ornithothoracine sternal features may represent parallelism. This highlights the importance of ontogenetic studies for understanding homology and the evolution of skeletal features in palaeontology.

The oldest record of ornithuromorpha from the early cretaceous of China

Ornithuromorpha is the most inclusive clade containing extant birds but not the Mesozoic Enantiornithes. The early evolutionary history of this avian clade has been advanced with recent discoveries from Cretaceous deposits, indicating that Ornithuromorpha and Enantiornithes are the two major avian groups in Mesozoic. Here we report on a new ornithuromorph bird, Archaeornithura meemannae gen. et sp. nov., from the second oldest avian-bearing deposits (130.7 Ma) in the world. The new taxon is referable to the Hongshanornithidae and constitutes the oldest record of the Ornithuromorpha. However, A. meemannae shows few primitive features relative to younger hongshanornithids and is deeply nested within the Hongshanornithidae, suggesting that this clade is already well established. The new discovery extends the record of Ornithuromorpha by five to six million years, which in turn pushes back the divergence times of early avian lingeages into the Early Cretaceous.

On the absence of sternal elements in Anchiornis (Paraves) and Sapeornis (Aves) and the complex early evolution of the avian sternum

Significance We have observed more than 200 specimens of Anchiornis, the earliest known feathered dinosaur, and nearly 100 specimens of Sapeornis, one of the basalmost birds, and recognize no sternal ossifications. We propose that the sternum may have been completely lost in these two taxa (and Archaeopteryx as well) based on histological analysis and the excellent preservation of soft-tissue structures, thus suggesting the absence of a sternum could represent the plesiomorphic avian condition. Our discovery reveals an unexpected level of complexity and high degree of inherent developmental plasticity in the early evolution of the avian sternum. Anchiornis (Deinonychosauria: Troodontidae), the earliest known feathered dinosaur, and Sapeornis (Aves: Pygostylia), one of the basalmost Cretaceous birds, are both known from hundreds of specimens, although remarkably not one specimen preserves any sternal ossifications. We use histological analysis to confirm the absence of this element in adult specimens. Furthermore, the excellent preservation of soft-tissue structures in some specimens suggests that no chondrified sternum was present. Archaeopteryx, the oldest and most basal known bird, is known from only 10 specimens and the presence of a sternum is controversial; a chondrified sternum is widely considered to have been present. However, data from Anchiornis and Sapeornis suggest that a sternum may also have been completely absent in this important taxon, suggesting that the absence of a sternum could represent the plesiomorphic avian condition. Our discovery reveals an unexpected level of complexity in the early evolution of the avian sternum; the large amount of observable homoplasy is probably a direct result of the high degree of inherent developmental plasticity of the sternum compared with observations in other skeletal elements.