A Psittacosaurus Bonebed Found From Aptian Cretaceous China

The osteology and taphonomy of a Psittacosaurus bonebed assemblage of the Yixian Formation (Lower Cretaceous), Liaoning, China

Authors:

Hedrick et al

Abstract:

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.