贵州贞丰中三叠统关岭组中Chirotherium——原始爬行类足迹研究

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

在贵州省贞丰县龙场镇和牛场镇两地中三叠统关岭组下段的泥质白云岩中保存有古老的爬行类——“手兽”的足迹化石—— Chirotherium。该足迹为四足动物留下的行走轨迹 ,后足保存较完整 ,前足可见零星和不完整的趾痕。单个足迹由五个脚趾和后掌组成 ,由于其中的四个脚趾伸向前方而第五趾由后掌外侧横向伸出且向后弯曲 ,整个足迹看上去犹如人的手掌印痕。由前足脚印保存不完全可以判断 ,该造迹动物既可以四足行走又可以仅靠后足奔跑。龙场足迹点保存完整的后足脚印最大为长 16 cm,宽 14 cm,平均单步长 70 .2 cm,复步长为 14 0 cm;而牛场足迹点所见最大后足脚印长达 2 5 cm,宽 17cm,平均单步长为 5 2 cm,复步长为 98.5 cm。笔者等参照国际上流行的计算方法 ,根据脚印长度和步长大小计算了两地造迹动物的臀高和行走速度 :臀高大约 0 .6 4～ 1.0 m,估计身长可达2～ 3m ,行走速度 4～ 10 km / h。考虑到造迹动物当时仅仅处于漫步和小跑状态 ,推测其奔跑速度可以达到 2 0 km / h以上。发现于欧洲、美洲等地的 Chirotherium多数保存于三叠纪的红色砂泥岩层表面 ,其中多数岩层发育泥裂构造 ;还有些则保存于薄层碳酸盐岩层的表面 ,也多数发育泥裂构造。贞丰的两处足迹都保存于中三叠统关岭组下段发育泥裂构造的泥?

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.