新疆水龙兽新材料兼论陆相二叠-三叠系界线

THE LYSTROSAURUS FOSSILS FROM XINJIANG AND THEIR BEARING ON THE TERRESTRIAL PERMIAN-TRIASSIC BOUNDARY

记述了新疆锅底坑组的两件水龙兽标本 ,其中产自吉木萨尔县的一件层位最低 ,被鉴定为杨氏水龙兽相似种 (Lystrosauruscf.L .youngi)。在支序分析的基础上对比了中国与南非的水龙兽 ,认为两地的水龙兽大多数分别构成单系类群。总结了中国新疆水龙兽在锅底坑组和韭菜园组下部出现的层位 ,并讨论了以水龙兽来确定陆相二叠 -三叠系界线。

Two new specimens of Lystrosaurus from Xinjiang are described. They are from the Guodikeng Formation. One specimen (CAGS V 381, Fig.1) was found in the section measured across the southern limb of the Dalongkou Anticline, Jimusar. The horizon is layer 54 in the section measured by the Institute of Geology, Chinese Academy of Geological Sciences (Cheng et al., 1996). This is the lowest record of Lystrosaurus in this section up to now, it is 67m below the boundary between the Guodikeng Formation and the Jiucaiyuan Formation. This specimen is described as Lystrosaurus cf. L. youngi. Another specimen (IVPP V 13462, Fig.2) was found in the uppermost of the Guodikeng Formation in the Taoshuyuangou Section, Turfan. It is described as Lystrosaurus hedini. Lystrosaurus weidenreichi was named based on postcranial skeleton, its main diagnostic features are large lystrosaurs, and curved scapula (Young, 1939). Sun (1973) suggested that curved sca- pula is a common character in large lystrosaurs, and the difference among species of Lystrosaurus is trivial in postcranial skeleton. Colbert (1974) also suggested that' L. weidenreichi may very well be synonymous with L. hedini '. So the name of L. weidenreichi is nomen invalid. A preliminary cladistic analysis is done based on 11 characters (Appendix I),using Kwazulusaurus shakai as the outgroup (Maisch, 2002). This analysis includes all known species from South Africa and China. The data (Table 1) were analyzed using PAUP 4.0b10, 26 most parsimonious trees are resulted. The consensus tree of these trees is showed (Fig.3).We can conclude that most species of Lystrosaurus belong to two monophyletic groups: one includes L. oviceps, L. murrayi, L. declivus and L. maccaigi from South Africa (synapomorphy: character 8 (1)); the other includes L. broomi, L. hedini, L. robustus and L. shichanggouensis from China (synapomorphies: characters 9(1),10(1))?This study analyzes the relationships of the Lystrosaurus from South Africa and China. The validity of some species used in this analysis is still in question, e. g., L. oviceps from South Africa (Cosgriff et al., 1982). The holotype of L. broomi from China is deformed, and there is only one incomplete skull referred to this species after many year's collecting of Lystrosaurus in Xinjiang. Its validity is also doubtable.The general morphology of the skulls are well comparable between lystrosaurs from China and South Africa: L. youngi with L. curvatus, L. hedini with L. platyceps, L. broomi with L. oviceps (not L. murrayi that L. broomi had been merged in), L. robustus with L. declivus, L. shichanggouensis with L. maccaigi and L. murrayi. Based on the work of Zhao (1980), Cheng (1986) and our fieldwork, a stratigraphic sequence of Lystrosaurus in Xinjiang(Table 2) is presented.Cluver (1971) suggested that L. curvatus is the most primitive species in Lystrosaurus, this is coincident with its occurrence in stratum: a fossil identified as L. cf. L. curvatus was found in Madumabisa Mudstine of Zambia, in association with several Upper Permian genera (King and Jenkins, 1997). There is a similar case in China: L. youngi , which had been regarded as junior synonym as L. curvatus by some authors (Colbert, 1974; Cosgriff et al., 1982; King, 1988), is the most primitive known species of Lystrosaurus in China, L. cf. L. youngi is the oldest known Lystrosaurus in China too.The character series of transverse ridge in the middle of the frontal could be observed from L. youngi (absent) through L. hedini (present but not pronounced) to L. robustus and L. shichanggouensis (pronounced), it is consistent with their sequence occurrence in strata.This is the preliminary study about the sequence of Lystrosaurus , and more biostratigraphic work should be done all over the world about this genus befor we can use it as a good marker to correlate strata.