Fang et al.—Ordovician actinocerid nautiloids from Tibet


Table 1. Occurrences (presence or absence) of actinocerid nautiloids in the Middle Ordovician. MYS=Malaysia (Kobayashi, 1958, 1959; Stait and Burrett, 1982); MMR=Myanmar (Niko and Sone, 2014); THA=Thailand (Brown et al., 1951; Kobayashi, 1958, 1959; Kobayashi and Hamada, 1964; Burton, 1974; Ingavat et al., 1975; Stait and Burrett, 1984); HIM=Himalaya (Chen, 1975; Chen, 1983, 1984); LHA=Lhasa (this paper); TAS=Tasmania (Teichert and Glenister, 1953; Stait, 1984); N AUS=North Australia (Teichert and Glenister, 1952; Wade, 1977); NC=North China (Chao et al., 1965; Chen and Zou, 1984; Zhu and Li, 1996); SC=South China (Yang, 1978, 1980).


Genus


Actinoceras 10 Adamsoceras 00 Armenoceras 11 Cyclonybyoceras 00 Discoactinoceras 00 Georgina 00 Hoeloceras 00 Hunjiangoceras 00 Meitanoceras 00 Mesaktoceras 00 Mesowutinoceras 00 Nybyoceras 00 Ordosoceras 01 Ormoceras 10 Orthonybyoceras 00 Pararmenoceras 00 Paratunkuskoceras 01 Parormoceras 00 Polydesmia 00 Pomphoceras 00 Sactoceras 00 Selkirkoceras 00 Tunkuskoceras 00 Wutinoceras 11


MYS MMR THA Himalaya 1


0 1 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 1


W. foerstei (Endo, 1930), Mesowutinoceras cf. M. giganteum Chen in Chen and Zou, 1984, Armenoceras tani (Grabau, 1922), A. teicherti Endo, 1932, A. xizangense new species, Deiroceras globosom Zou and Shen in Chen and Zou, 1984, and Discoactinoceras cf. D. multiplexum Kobayashi, 1927. They all belong to the order Actinocerida. A majority of the species from Xainza are diagnostic of the Darriwilian (Middle Ordovician) according to a comparison with those from North China. For example, Pomphoceras is found in the Zhuozishan Forma- tion of the Ordos Region (Chen and Zou, 1984), the Majiagou Formation of southern Jilin, North China (Zhu and Li, 1996), and the Chiatsun Formation of Nyalam, Himalaya Terrane (Chen, 1975), which are all restricted to the Darriwilian. Wutinoceras was originally described from the Majiagou Formation (Wuting Formation; Darriwilian) in Benxi (Pen- his-hu) and Nanpiao, Liaoning, North China (Endo, 1930, 1932; Kobayashi and Matumoto, 1942), and has also been recorded in the Kakit Bukit Formation (partly Darriwilian) in the Langkawi Islands, Malaysia (Stait and Burrett, 1982; Stait et al., 1987), and in the Whiterock (Darrwilian) of North America (Flower, 1968). Armenoceras is very common in the Middle Ordovician of North China (Endo, 1932; Chao et al., 1965; Zhu and Li, 1996) and was discovered in the Thung Song Group and the Tha Manao Formation of Thailand (Kobayashi, 1958, 1959; Stait and Burrett, 1984), the Whiterock part of the Kakit Bukit Formation of Malaysia (Kobayashi, 1958, 1959; Stait and Burrett, 1984; Stait et al., 1987), the Wunbye Formation of Myanmar (Niko and Sone, 2014), the Middle Ordovician in Queensland and Tasmania, Australia (Whitehouse, 1936; Teichert and Glenister, 1952), and in North America, Greenland, and Siberia, Russia (Balashov, 1962). Therefore, the age of the actinocerid nautiloids from the Lhasai Formation of Xainza is Middle Ordovician, and most probably Darriwilian.


0 0 1 1 0 1 0 0 1 0 0 0 1 1 0 0 0 1 0 1 0 0 0 1


Lhasa TAS N AUS NC SC 0


0 1 0 1 0 0 0 0 0 1 0 0 1 0 0 0 1 0 0 0 0 0 1


0 1 0 0 0 0 0 0 0 0 0 1 0 1 1 0 0 0 0 0 0 0 0 1


1 0 1 0 0 1 0 0 0 1 0 0 0 1 0 0 0 0 0 0 0 0 0 0


Paleobiogeographic implications


The Xainza region is tectonically located close to the northern margin of the Lhasa Terrane, which was part of the northeastern peri-Gondwana region during the Early–Middle Ordovician (Metcalfe, 2001; Burrett et al., 2014). The Middle Ordovician nautiloid faunas of the Lhasa Terrane and some other Chinese terranes (e.g., the Himalaya and North China terranes) are characteristically dominated by actinocerids (e.g., Chen, 1975; Chen and Zou, 1984) and indicate that the nautiloids of these regions have a strong affinity. Several global Darriwilian cephalopod biogeographic regions were identified, including Laurentian, Rheic, East Asian (i.e., southern Tibet, Tarim, South and North China), and Siberia regions (Kröger, 2013). Drawing on specimens from the Xainza region, the detailed paleogeo- graphic reconstruction of northeastern peri-Gondwana during the Middle Ordovician may now be firmly established. Pomphoceras is a key element of the Zhuozishan Formation


in InnerMongolia (North China) and in the Chiatsun Formation in Nyalam (Himalaya Terrane). In addition, Pomphoceras nyalamense is present in both the Lhasa and Himalaya terranes (Chen, 1975), and Pomphoceras yaliense is foundinthe North China, Lhasa, andHimalaya terranes (Chen, 1975; Chen and Zou, 1984). Pomphoceras resembles Ordosoceras in external form, inner construction, and systematic phylogeny; the latter is the Middle Ordovician index fossil of North China (Chang, 1959; Chen and Zou, 1984). Ordosoceras is also recorded fromMyan- mar (Sibumasu Terrane; Niko and Sone, 2014), which indicates that the actinocerids from Sibumasu have close affinities with those of North China. Discoactinoceras is recorded in Inner Mongolia (Zou, 1981) and Liaoning (Kobayashi, 1978), both from North China, so its occurrence in the Xainza Region of Tibet also supports a close relationship of the two terranes. Wutinoceras and Armenoceras are widespread genera and are


1 0 1 0 1 0 1 1 1 0 1 1 1 1 0 1 1 1 1 1 1 1 1 0


0 0 0 0 0 0 0 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0


401


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