Large theropod and small sauropod trackmakers from the Lower Cretaceous Jingchuan Formation, Inner Mongolia, China

Abstract

At least 16 tetrapod tracksites have been reported from the Cretaceous Jingchuan Formation of the Ordos Basin, in the Chabu area, of Otog Qi, Inner Mongolia. Only a few of these sites, designated as Chabu sites 1–16, have been documented in detail. Most known sites occur in an area of ∼100 km2 and are saurischian dominated, yielding the tracks of sauropods, avian and non-avian theropods. Sites include the type localities of three previously-named theropod ichnotaxa: the non-avian theropod ichnogenus Chapus, (site 8) the avian theropod ichnogenus Tatarornipes (site 4) and the non-avian theropod ichnospecies Asianopodus robustus (site 6). We herein revise the description of ichnogenus Chapus based on the tridactyl holotype, erect the new ichnotaxon Ordexallopus zhanglifui ichnogen. et ichnosp. nov, for a distinctive tetradactyl morphotype, also from site 8, and compare both with A. robustus, and other well known large theropod track ichnotaxa from the Lower Cretaceous. Ordexallopus is shown to represent a large robust tetradactyl trackmaker with foot morphology reminiscent of that registered by the smaller, more gracile tetradactyl maker of Saurexallopus tracks. Both likely represent arctometatarsalians.Previous studies, have shown that many of the of the 16 known sites warrant detailed study, due to the abundance and ichnotaxonomic significance of tracks. We herein show that site 8, which has been developed as an interpretative destination, comprises four different sites (8A–8D), only one of which has previously been described in detail, in Chinese. Site 8D is of special significance as the type locality of Ordexallopus zhanglifui ichnogen et isp. nov. It also reveals evidence of gregarious behavior among juvenile sauropods traveling in the same direction as a large adult. The site also yields the most complete turtle trackway known from the Cretaceous of China. Ongoing studies of the Chabu area tracksites which form part of Otog Geopark indicate that the ichnological potential of the region is as yet only partially realized.