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Journal of Paleontology 92(3):398–411 Order Actinocerida Teichert, 1933 Family Meitanoceratidae Kobayashi, 1977 Genus Pomphoceras Zou and Shen in Chen and Zou, 1984
Type species.—Pomphoceras wuhaiense Zou and Shen in Chen and Zou, 1984
Pomphoceras nyalamense (Chen, 1975) Figures 4.3, 5.4–5.6
1975 Ordosoceras nyalamense Chen, p. 272, text fig. 1, pl. 1, figs. 14, 15.
Occurrence.—Lhasai Formation, Darriwilian (Middle Ordovi- cian), near Lhasai village, about 26km east of Xiongmei town, Xainza County, North Tibet.
Figure 3. Cluster analysis of the Middle Ordovician actinocerid nautiloids of peri-Gondwana using PAST (Hammer et al., 2001) based on the Jaccard similarity coefficient.
common in the Himalaya Terrane (Chen, 1975), Sibumasu (Stait and Burrett, 1982; Niko and Sone, 2014), Australia (Stait, 1984), and North America (Flower, 1968). Some common elements of the actinocerids from Xainza are shared with North Australia (Teichert and Glenister, 1952; Wade, 1977), but few with Tasmania (Stait, 1984). Nevertheless, there are few forms shared with Lhasa and South China during the Middle Ordovician (Chen and Liu, 1976; Yang, 1978, 1980). Among them, the dominant formMeitanoceras is an endemic genus,which is only reported in the Middle Ordovician Meitan Formation from Chongqing, Guizhou, and Yunnan, South China. Ormoceras is a widespread genus, distributed in most parts of the world during this period. However, in general, there are few resemblances between the nautiloids of South and North China (Yang, 1980). The Middle Ordovician actinocerids of the Lhasa Terrane
show close affinity with those of the Himalaya Terrane, North China, and the Sibumasu Terrane (Fig. 3). The similarities of the Middle Ordovician actinocerid nautiloids can be interpreted as reflecting similar paleogeographic situations in low paleolati- tude; all these terranes are located in a northern tropical region. This also supports the Middle Ordovician reconstruc- tions that place North China close to Australia and place South China close to the Middle East (e.g., Burrett and Stait, 1987; Burrett et al., 1990, 2014; Metcalfe, 2011) rather than the reconstructions that have these positions reversed (e.g., PaleoGIS 4.0 for ArcGIS, 2011). The affinities of nautiloids from these regions have a similar paleobiogeographic relation- ship to those of other marine organisms. For example, brachio- pod affinities between Tibet and Sibumasu have been recognized from 230 genera of rhynchonelliform brachiopods from 65 localities during the Dapingian and Darriwilian (Harper et al., 2013).
Systematic paleontology
Class Cephalopoda Cuvier, 1797 Subclass Nautiloidea Agassiz, 1847
Description.—The orthocone is composed of nine camerae. It is about 47mm in length and 38mm in width, with a large expansion angle of 28.6°; conch cross section is dorsoventrally slightly depressed; external shell wall is not preserved. Septal curvature and cameral length are moderate; there are seven to eight camerae within the length of the corresponding lateral conch diameter; siphuncle is large, probably central in position. Siphuncular wall consists of cyrtochoanitic septal necks and thin connecting rings. Diameter of septal foramina is probably 4.2mm. Septal necks are long and straight and brims are recumbent. Lengths of necks and brims are 1.8mm and 0.8mm, respectively. Maximum diameter of connecting rings is 11.6mm, while the corresponding conch diameter is 37.8mm. The anteriors of the connecting rings are curved and one-third part of the pos- terior is inflated and then progresses straight down obliquely; adnation area is narrow. SCR is 2.2. Cameral deposits well developed as episeptal-mural and hyposeptal deposits. The endo- siphuncular canal system consists of a large central canal, radial canals, and perispatium. Central canal is probably 3.4mm in dia- meter and distal ends of the radial canals are nearly horizontal.
Material.—An incomplete orthocone cut and polished in med- ian section. NIGP166262.
Remarks.—According to Chen and Zou (1984), the conch of the genus Pomphoceras is slightly curved adapically, circular in cross section; siphuncle is small, central or subcentral. The segment shape of Pomphoceras is similar to that of Meitanoceras and Ordosoceras, but differs from Meitanoceras in having small septal foramina and from Ordosoceras in that its radial canals seem to be straight and directly transverse. Pomphoceras nyalamense is similar to the type species of
Pomphoceras, P. wuhaiense Zou and Shen in Chen and Zou (1984, p. 73, text fig. 23, pl. 15, figs. 3, 4) fromInnerMongolia, in its general siphuncular shape and central position of the siphuncle, but differs in that the former has a larger expansion rate.
Pomphoceras yaliense (Chen, 1975) Figures 4.1, 5.7–5.9
1975 Ordosoceras yaliense Chen, p. 274, pl. 1, figs. 5, 6.
1984 Pomphoceras yaliense; Chen and Zou, p. 74, pl. 15, fig. 5, pl. 19, fig. 8.
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