A new sauropod tracksite from the Lower Cretaceous of Ningxia, Northwestern China, with implications for overtrack preservation.

Dinosaur fossils in the Early Cretaceous Madongshan Formation (Ningxia, northwestern China) are scarce, but fossil tracks have recently been reported. We describe a new sauropod tracksite, the Beilianchi tracksite, which is the largest tracksite known from the formation. The site consists of a steeply inclined surface approximately 650 m2 in area that, although mostly inaccessible, is documented and studied using photogrammetry. Nine medium to large-sized sauropod trackways (pes lengths ranging from 58.3 to 143.5 cm), as well as numerous isolated tracks, were identified at the site. The tracks are preserved in laminated silty mudstones; layers deposited after track formation are filling many of the tracks as overtracks but are mostly eroded outside of the tracks. The site preserves evidence of probably a single tracking surface and at least two distinct track-forming intervals, suggesting a time-averaged assemblage. Characteristic features of these trackways include (1) narrow/medium to wide gauge, (2) outward rotation of both manus and pes impressions, (3) oval to subtriangular pes morphologies, and (4) semicircular manus impressions. The Beilianchi tracks are intermediate in morphology between typical Brontopodus and Parabrontopodus tracks. Our understanding of the distribution and size spectrum of Early Cretaceous sauropods in northwestern China is advanced by this discovery.