A new Minisauripus site from the Lower Cretaceous of China: Tracks of small adults or juveniles?

Abstract

Because skeletal remains of very small theropods are delicate and rare, diminutive tracks provide valuable, additional evidence of body size. The distinctive Asian ichnogenus Minisauripus has assumed importance in this debate and has played a role in the challenging question about whether it represents a small trackmaker species or juveniles of a larger species. New discoveries of Minisauripus footprints from the Lower Cretaceous Feitianshan Formation at Yangmozu in Sichuan Province, China, support the conclusion that all known examples of this ichnotaxon are small. The main Yangmozu site reveals 65 theropod tracks (~20 trackways) of different-sized trackmakers. Three trackways, comprising 10 pes imprints of 2.5–2.6cm length, are assigned to Minisauripus. The remaining 17 trackways represent small–medium-sized theropods (track lengths 9.9–19.6cm), including one assigned to cf. Jialingpus. All unequivocally identified Minisauripus tracks from Korea (five sites) and China (three sites) fall in the size range of ~1.0–6.1cm. Assuming a small adult trackmaker, and based on standard foot length, leg length and body length ratios, all Minisauripus tracks indicate trackmakers with hip heights of <~5.0 and ~28.0cm and body lengths in the range of ~12.0–72.0cm. Based on lack of large tracks (longer than 6.1cm) at any known sites, and various precedents in the ichnological literature we infer that Minisauripus represents a small theropod species. However, we cannot completely exclude the possibility that such tracks represent juveniles of larger trackmaking species.