Читать книгу Hadrosaurs - David A. Eberth - Страница 83

Four basal hadrosauroids have been recovered from two basins in the Mazongshan area of northwestern Gansu Province (Fig. 4.1). The northern Gongpoquan Basin yielded Gongpoquansaurus mazongshanensis (Lü, 1997) comb. nov. and Equijubus normani You et al., 2003, whereas the southern Yujingzi Basin produced Jintasaurus meniscus You and Li, 2009 and Xuwulong yueluni You et al., 2011. Although all four are basal hadrosauroids, the Gongpoquan and Yujingzi basins each yield a relatively derived hadrosauroid (Gongpoquansaurus and Jintasaurus, respectively) and a relatively more basal form (Equijubus and Xuwulong, respectively) (Fig. 4.3).

Both Gongpoquansaurus and Equijubus are from the middle gray unit of the Xinminpu (= Xinminbao) Group in the Gongpoquan Basin, and the age of the Xinminpu Group in the Mazongshan area is considered as Aptian–Albian in age based on analysis of spores and pollens (Tang et al., 2001; You et al., 2003). Both Jintasaurus and Xuwulong are from the middle gray unit in the Yujingzi Basin, and the dinosaur-bearing rocks here are considered as belonging to the Xinminpu Group as well, and can be generally correlated to those in the Gongpoquan Basin with an age assignment of Aptian–Albian (You and Li, 2009; Li et al., 2010; Makovicky et al., 2010; You et al., 2010; You et al., 2011).

The Xinminpu Group includes two formations: the lower Xiagou and the upper Zhonggou formations. Our 2010 and 2011 field work confirms that the dinosaur-bearing Xinminpu Group in the Mazongshan area belongs to Zhonggou Formation (Liu, Y.-Q., pers. comm.). In the Hanxia Section about 200 km south of the Gongpoquan Basin, the Zhonggou and Xiagou formations are separated by andesites (Niu, 1987) dated at 113.7 Ma (Liu, Y.-Q., pers. comm.). This volcanic activity, which occurred at about the Aptian-Albian boundary (112 Ma), is expressed regionally across several localities and extended northeastward to Tebch in Inner Mongolia (110 Ma; Eberth et al., 1993). In this context the Zhonggou Formation is most likely of Albian age. An Albian age interpretation for the Zhonggou Formation is further supported by the palynological analysis of the lower red unit in the Gongpoquan Basin (Tang et al., 2001).