Amended Diagnosis Gongpoquansaurus mazongshanensis is a basal hadrosauroid distinguishable from other taxa by the following unique combination of features: transversely elongated supratemporal fenestra, broad and flat dorsal surface of the parietals with a deep median groove, and large and straight nuchal crest (Fig. 4.2).
4.2. Holotype (IVPP V. 11333) of Gongpoquansaurus mazongshanensis comb. nov. Caudal part of the skull in dorsal view, showing its unique combination of features: (1) transversely elongated supratemporal fenestra; (2) a deep median groove on the broad and flat dorsal surface of the parietals; and (3) large and straight nuchal crest.
Comments and Comparisons All the specimens were collected from the southeastern part of the Gongpoquan Basin in the summer of 1992 during the Sino-Japanese Silk Road Dinosaur Expedition (Dong, 1997). The locality of the holotype is shown in figure 1 (Dong, 1997:7). All the referred specimens were probably recovered from the same quarry as the holotype (Dong, Z.-M., pers. comm.). Because it is common for more than one iguanodontian taxon to occur in the same horizon and locality, and another iguanodontian (Equijubus normani) has already been recovered from this horizon in the same (southeastern) part of the Gongpoquan Basin, there is a possibility that not all the referred specimens belong to Gongpoquansaurus mazongshanensis. Therefore, we prefer to restrict the diagnosis of this taxon, basing it on the holotype only.
Based on dentary crowns, Norman (2002) considered “Probactrosaurus” mazongshanensis to be more closely related to Altirhinus than to Probactrosaurus gobiensis. Buffetaut and Suteethorn (2011) found that the maxillary teeth of Siamodon nimngami are very similar to those of “Probactrosaurus” mazongshanensis, with a prominent median primary ridge, and strongly denticulate margins. As suggested above, the association between the referred teeth and the holotype needs to be confirmed.
4.3. Cladogram showing the phylogenetic relationships of Gongpoquansaurus mazongshanensis comb. nov. This is based on the results of cladistic analyses of McDonald et al. (2010) and McDonald (2011), with the addition of Xuwulong yueluni based on You et al. (2011).
In McDonald et al. (2010)’s cladistic analysis, which included “Probactrosaurus” mazongshanensis, Equijubus normani and Jintasaurus meniscus, “Probactrosaurus” mazongshanensis is more derived than Equijubus and less derived than Jintasaurus, with several intermediate taxa in between (Fig. 4.3). This supports the elevation of “Probactrosaurus” mazongshanensis as a new genus.
Carpenter and Ishida (2010) noticed that “Probactrosaurus” mazongshanensis is an immature individual as evidenced by the incomplete fusion of the neural arch with the centrum, and found substantial differences between it and Probactrosaurus gobiensis. In addition to the diagnostic cranial and cervical features identified here, they also noted the following differences: teeth with large marginal crenulations (small, less developed in P. gobiensis); dentary teeth significantly larger than maxillary teeth (nearly same size in P. gobiensis); sacrum rounded ventrally (keeled in P. gobiensis); and scapula expanded distally (nearly parallel-sided in P. gobiensis).
Jintasaurus, a slightly more derived hadrosauroid from the nearby Yujingzi Basin, shows clear differences with Gongpoquansaurus: the supratemporal fenestra is rostrolateral-caudomedially directed in Jintasaurus but transversely elongated in Gongpoquansaurus, and the parietals of Jintasaurus do not possess a median deep groove.
DISCUSSION
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).
ACKNOWLEDGMENTS
We are grateful to the crew of the former Fossil Research and Development Center of the Third Geology and Mineral Resources Exploration Academy of the Gansu Provincial Bureau of Geo-Exploration and Mineral Development for discovering, excavating, and preparing most of the Gansu dinosaur specimens, especially Lanzhousaurus magnidens. Funding was provided by the National Natural Science Foundation of China (41072019 and 40672007) and the IVPP Key Projects to HLY, the National Science Foundation of the United States (1024671) to PD, and Gansu Department of Land and Resources to QLD.
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