Chirotherium: Fossil Footprints of Primitive Reptiles in the Middle Triassic Guanling Formation, Zhenfeng, Guizhou Province, China

Chirotherium, fossil footprints of primitive reptiles, is found in the Middle Triassic Guanling Formation in two sites in Zhenfeng County, Guizhou Province, China. The footprints were first reported in China 15 years ago, but they have not been studied very well so far. The tracks were made by four-foot reptiles although the animals could walk or run with the support of only hind legs on land as only a few toe prints of fore limbs have been found in the tracks. A single footprint has 5 digits, like a human hand with four fingers spreading forward and a “thumb” curling back to one side. However, the “thumb” does not point to the inner side of the hand but to the outer side. Thus, the “thumb” is, in fact, the fifth digit, the little toe. The largest footprints in a clear trackway found in the Longchang site are 16 cm in length and 14 cm in width with a pace length of 70.2 cm and a stride length of 140 cm; while the largest footprints in the Niuchang site are 25 cm in length and 17 cm in width with a pace length of 52 cm and a stride length of 98.5 cm. The hip height and the locomotion speed for the track-makers of the footprints in the two sites are calculated according to the popular formulas provided by Alexander and Thulborn. The hip heights of the track-makers are 0.64 m in the Longchang site and 1.0 m in the Niuchang site, while the locomotion speeds were about 10 km/h and 4 km/h accordingly. It is estimated that the running speed for the track-makers might exceed 20 km/h according to the calculation. Most tracks of Chirotherium in the world were found in sandstones or mudstones with mud crack structures, while the tracks in Zhenfeng are found in muddy dolomite with mud cracks. Similar to other footprints of Chirotherium, the sedimentary rocks in which the tracks are found were all formed in dry and hot climates in Triassic. The footprints in the two sites are the only Chirotherium found in China, so they provide valuable information for the study of paleo-climate of the so-called Pangea in the Triassic. Besides, because the track sites are situated on the southwestern edge of the Yangtze Plate and nearby the northwestern edge of the Triassic Nanpanjiang Flysch Basin, it is very important to study further the ecology and sedimentary environment of the footprints so as to discuss the facies change and the tectonic relationship between the basin and the platform in the Triassic.