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We present the first detailed description of the osteology and mineralogy of an assemblage of Psittacosaurus juveniles (DMNH D2156) associated with a larger specimen from the Lujiatun beds of the Yixian Formation in Liaoning, China. We fully describe all of the material as well as discuss intraspecific variation in the sample set of twenty-four juveniles with associated postcranial material and compare with previously described ceratopsian juvenile material. Based on the development of the ends of long bones, it is suggested that the juveniles are not embryonic, but are post-hatchlings. In comparison with previous histologic analyses on P. lujiatunensis , it is shown that the large specimen associated with the juveniles is likely not yet an adult and that this assemblage is not an exemplar of parental care, but may be an example of post-hatching cooperation. The large specimen is however, shown to be positively associated with the assemblage and was not added on after excavation. Additionally, an allometric analysis of Psittacosaurus bone lengths is performed in order to determine ontogenetic trajectories and allometry using femur length as a proxy. This demonstrates that the vast majority of long bones and girdle elements in Psittacosaurus are isometric with body size, but supports previous analyses that the forelimb grew slower than the hindlimb. Finally, a mineralogical analysis using X-ray diffraction and petrographic thin sections of the block where DMNH D2156 is preserved shows that the animals were preserved in a volcanic-lithic arenite. The statistically significant alignment of specimens and preservation in a volcanic rock suggests that the taphonomic setting and reasoning for the exceptional preservation of the specimen is due to burial by a volcaniclastic debris flow. The vast majority of specimens in the Yixian Formation are found in lacustrine strata, recently suggested to be carried into lakes by pyroclastic debris flows. The preservation of DMNH D2156 in a clastic flow further supports the volcaniclastic flow preservation model for the fauna of the Yixian Formation, but we prefer a lahar flow interpretation for DMNH D2156 since there is no evidence of the bone microstructure being affected by intense heat associated with pyroclastic debris flows.

Confuciusornis sanctus stands out among the remarkable diversity of birds of the Jehol biota (Lower Cretaceous, Liaoning Province, China). Its basal position in the phylogenetic tree of birds, combined with the exceptional number of well-preserved, largely complete and articulated specimens, makes it a perfect model system for studying the variation, development and life history of early Mesozoic birds. A comprehensive morphometric study (measurements of humerus, ulna, radius, femur and tibia) previously identified two distinct size classes of C. sanctus, while demonstrating the lack of statistical support of the association between this size dimorphism and the characteristic pair of long tail feathers present in some specimens. Four plausible explanations were discussed to account for the resultant size classes: the existence of more than one species in the sample; sexual size dimorphism; two size classes corresponding to attritional death assemblages; and/or a particular growth pattern similar to that i...

The lacustrine deposits of the Jiufotang Formation, Liaoning Province, China have yielded a diverse avifauna representing a wide spectrum of basal lineages. Here we add to this diversity by describing the longirostrine enantiornithean taxon Rapaxavis pani gen. et sp. nov. The specimen possesses the following autapomorphies: (1) caudoateral processes of the sternum with two prominent branches and a smaller third branch, (2) paired triangular thoracic elements of unknown homology, and (3) the combination of six sacral vertebrae and six free caudal vertebrae. Otherwise, this genus is closely allied with the Yixian Formation species Longirostravis hani, sharing elongate, curving rostrum with teeth restricted to the premaxilla and rostral-most dentary, and a derived manus lacking unguals. The holotype of Rapaxavis is exquisitely preserved, with such details as distal tarsals, keratinous sheaths of the pedal unguals and some three-dimensional features of the bones intact. The holotype appears to lack a...

An incomplete specimen of Sinosauropteryx prima collected from the Early Cretaceous Yixian Formation of the Dawangzhangzi area in Lingyuan, western Liaoning is depicted. It represents the first systematically described material of this feathered compsognathid outside the Sihetun area in Beipiao. This specimen shows some diagnostic features of Sinosauropteryx prima, such as the very short forelimb in relation to hindlimb with the length ratio of humerus plus radius to femur plus tibia just around 30%, and the long and massive first manual ungual subequal in length to radius. The presence of Sinosauropteryx prima only at Sihetun and Dawangzhangzi supports the suggestion that the fossil-bearing beds in the Sihetun and Dawangzhangzi areas are equivalent to each other within the Yixian Formation.

The discovery of Hongshanornis longicresta, a small ornithuromorph bird with unusually long hindlimb proportions, was followed by the discovery of two closely related species, Longicrusavis houi and Parahongshanornis chaoyangensis. Together forming the Hongshanornithidae, these species reveal important information about the early diversity and morphological specialization of ornithuromorphs, the clade that contains all living birds. Here we report on a new specimen (DNHM D2945/6) referable to Hongshanornis longicresta that contributes significant information to better understand the morphology, trophic ecology, and aerodynamics of this species, as well as the taxonomy of the Hongshanornithidae. Most notable are the well-preserved wings and feathered tail of DNHM D2945/6, which afford an accurate reconstruction of aerodynamic parameters indicating that as early as 125 million years ago, basal ornithuromorphs had evolved aerodynamic surfaces comparable in size and design to those of many modern birds, and flight modes alike to those of some small living birds.