The first theropod tracks from the Middle Jurassic of Gansu, Northwest China: new and rare evidence of quadrupedal progression in theropod dinosaurs

Abstract

A new Middle Jurassic tracksite dominated by non-avian theropod footprints from the Wangjiashan Formation in Pingchuan District, Baojishan Basin, Gansu Province has yielded a unique trackway with four consecutive manus-pes sets. Only three previous examples, all Early Jurassic in age, of theropod trackways are known with convincing examples of manus tracks and in each case, only two tracks were recorded in association with pes tracks with metatarsal impressions and pelvic traces indicating crouching behavior. Thus, this is the first example of manus tracks registered while a theropod trackmaker was walking. This unique configuration is here designated as Grallator pingchuanensis ichnosp. nov. which shows the trackmaker forelimbs registering in a wide straddle gait, much wider than the pes trackway width. G. pingchuanensis confirms previous reports that theropods could occasionally register tridactyl, ectaxonic manus traces. In the case of the Pingchuan trackway, the short step indicates an animal moving at a slow speed, probably due to a soft substrate.