Fang et al.—Ordovician actinocerid nautiloids from Tibet
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Figure 1.
(1) Location of the studied area in North Tibet, western China, with the tectonic sutures bounding the Lhasa Terrane (base map after Qu et al., 2002;
Xu et al., 2011). (2) Lithostratigraphic and biostratigraphic subdivisions of the Ordovician in the Xainza Region in Tibet, western China; lithostratigraphic and biostratigraphic data from Xia (1983) and National Commission of Stratigraphy of China (2014). (3) Locality of the Lhasai section in Xainza County.
yellow thin-bedded calcareous siltstone; and (3) the upper part, mainly consisting of bioclastic limestone, micritic limestone, and conglomeratic limestone.
Materials and methods
Most of the actinocerid specimens described herein were col- lected by some of the authors from the Lhasai section, which is located near Lhasai village, about 26km east of Xiongmei town (31.2605°N, 89.1722°E to 31.2748°N, 89.1645°E), during the 2002 and 2012 investigations, which focused on the Middle Ordovician succession of the Xainza Region (Fig. 1.3). All nautiloid shells are recrystallized and shell structures
are poorly preserved. Due to poor preservation, only one spe- cimen was found for each species. Most shells are slightly fragmented. All specimens were cut in median section, polished, and photographed with a Nikon D600 digital camera, Leica M125 microscope, and Leica DSC450C camera enhanced by Leica’s LAS software. Our identifications are based mainly on the position and shape of the siphuncle, the types of the septa and septal neck, the details of the connecting rings, the shape of the segment, and the endosiphuncular canal system, following Teichert et al. (1964), Chao et al. (1965), Flower (1968), Frey (1995), Zhu and Li (1996), and Niko and Sone (2014). The siphuncle segment compression ratio (SCR) is an important term introduced by Frey (1995) and is defined as the ratio of the maximum diameter of the connecting rings to the length of camerae. To investigate quantitatively the paleogeographic affinity
of the actinocerid nautiloids, the occurrence (presence or absence) data of 24 nautiloid genera from nine terranes or
regions (Fig. 2; Table 1) are compiled into a binary data set. Cluster analysis (CA) methods in the PAST version 3.15 soft- ware (Hammer et al., 2001) were adopted, using the Jaccard similarity coefficient (Jaccard, 1901a, b), which is commonly used in paleobiogeographic studies (e.g., Shi, 1993; Rong et al., 1995; Shen and Shi, 2000, 2004).
Repository and institutional abbreviation.—All specimens are deposited in the Nanjing Institute of Geology and Palaeontol- ogy, Chinese Academy of Sciences (NIGP).
Age of the actinocerid nautiloids of Lhasai Formation
The actinocerid nautiloids of Lhasai Formation are comparable to those from other Asian terranes (e.g., Qiangtang Terrane, Sibumasu Terrane, South and North China). According to the latest detailed restudy of the Ordovician Huaiyuan Epeirogeny in North China and adjacent regions, two distinct phases of the tectonic event are distinguished. The early phase was initiated from the Floian to early Darriwilian and is manifested by an extensive diachronous hiatus in the platform facies across North China (Zhen et al., 2016). According to a recent restudy of the conodonts from the Ordovician of North China, several strati- graphic units that were regarded as mainly of early Ordovician and Dapingian age, including the Zhuozishan, the Sandaokan, and the Majiagou formations, are revised as Darriwilian (late Middle Ordovician) (Wang et al., 2014a, b, 2016; Zhen et al., 2016).
Actinocerid nautiloids of the Lhasai Formation comprise
nine species in six genera, including Pomphoceras nyalamense (Chen, 1975), P. yaliense (Chen, 1975), Wutinoceras cf.
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