Cephalopod Palaeobiogeography in Northeastern Peri-Gondwana During the Middle-Late Ordovician
1 CAS Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing Institute of Geology and Palaeontology, Nanjing 210008, China; xfang@nigpas.ac.cn, chenten40@163.com, ybzhang@nigpas.ac.cn, ydzhang@nigpas.ac.cn, wjli@nigpas.ac.cn;
2 University of Chinese Academy of Sciences, Beijing 100049, China.
The Middle to Late Ordovician was a critical period in the evolutionary history of cephalopods, when nautiloids diversified rapidly and reached their diversify peak by producing many new species in northeastern peri-Gondwana. Recently, several actinocerid nautiloids from Xainza (Lhasa Terrane) have been described in detail, providing some new information to reconstruct the palaeobiogeography of nautiloids in northeastern peri-Gondwana.
A dataset of the distribution of actinocerids in several northeastern peri-Gondwanan terranes during the Middle Ordovician has been assembled (Table 1, Fig. 1).
Pomphoceras is a key element of the Zhuozishan Formation in Inner Mongolia (North China) and in the Chiatsun Formation in Nyalam (Himalaya Terrane). In addition, Pomphoceras nyalamense has been discovered in both the Lhasa and Himalaya terranes(Chen, 1975)and Pomphoceras yaliense is found in the North China,Lhasa and Himalaya terranes (Chen,1975;Chen&Zou,1984).Pomphoceras resembles Ordosoceras in external form, inner construction and systematic phylogeny; the latter is a Middle Ordovician index fossil of North China(Chen&Zou,1984).Ordosoceras is also recorded from Myanmar(Sibumasu Terrane; Niko & Sone, 2014), which indicates that the actinocerid fauna from Sibumasu has close affinities with that of North China.Discoactinoceras has been reported from two areas of North China — Inner Mongolia (Zou, 1981) and Liaoning (Kobayashi, 1978) — so its occurrence in the Xainza Region of Tibet also supports a close relationship of the two terranes. Wutinoceras and Armenoceras are cosmopolitan genera and are common in the Himalaya Terrane (Chen, 1975), Sibumasu (Stait & Burrett, 1982; Niko & Sone, 2014), Australia (Stait, 1984) and North America (Flower, 1968). Some common elements of the actinocerid fauna from Xainza are shared with North Australia (Wade, 1977), but few are shared with Tasmania (Stait, 1984). There are few forms in common between the Lhasa Terrane and South China during this period (Yang, 1978, 1980). In conclusion, the Middle Ordovician actinocerid fauna of the Lhasa Terrane shows close affinities with those of the Himalaya Terrane, North China and the Sibumasu Terrane.
Table 1 Occurrences of actinocerid nautiloids during the Middle Ordovician. MYS=Malaysia, MMR =Myanmar, THA=Thailand, TAS=Tasmania, NAUS=North Australia, NC=North China, SC=South China.
In the Late Ordovician, elements of Lituitidae, Sinoceratidae, Michelinoceratinae and Oncoceratidae replace the actinocerids as the dominant forms as recorded in the Keerduo Formation and the lower part of the Gangmusang Formation (Sandbian), and Trocholitidae dominate in the upper part of the Gangmusang Formation (lower-middle Katian). These faunas correlate respectively to those of the Datianba, Pagoda and Linhsiang formations of South China (Li & Cheng, 1988; Chen, 1981, 1987). In addition, elements of these nautiloid faunas have been discovered in Himalaya, western Yunnan, Thailand (Sibumasu) and the Tarim terrane (Burton, 1974; Wang, 1981; Chen, 1984; Zhang et al., 2014), but very few are known from North China, which is dominated by pseudorthocerids. In conclusion, the nautiloid faunas of Himalaya, Lhasa, Sibumasu, South China and Tarim have strong affinities that are not shared with those of North China.
Figure 1 Global paleogeographic distribution of actinocerid faunas in the Middle Ordovician. 1. Xainza region, Tibet, China; 2. Nyalam, Tibet, China; 3. Inner Mongolia, North China; 4. Liaoning, North China; 5. Guizhou, South China; 6. Shan Plateau, Myanmar; 7. Phibum, Southern Thailand;8. Langkawi Islands, Malaysia; 9. Queensland, North Australia; 10. Tasmania (Tas.), Australia. Base map of paleogeographic reconstruction is modified from Burrett et al. (2014).
Acknowledgements Financial support from the Chinese Academy of Sciences (Nos. XDPB05, XDB10010100), the National Natural Science Foundation of China (Nos. 41290260, 41521061) and the Ministry of Science and Technology of China (No. 2013FY111000) is acknowledged. This is a contribution to IGCP Project 653.
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