Jing et al.—Ordovician conodonts from Wuhai area


between them. According to Zhen et al. (2011b), the conodont zones of Tarim in the studied interval were, in ascending order: the Yangtzeplacognathus crassus, Histiodella holodentata, H. kristinae, Pygodus serra, and P. anserinus zones (Fig. 4). Although it is a pity that no species of Histiodella was dis- covered in our sections, some other short-ranging conodonts, such as Dzikodus tablepointensis and Eoplacognathus suecicus, of the same age may serve as proxies for the biostratigraphic correlations. In Tarim, the appearance of H. holodentata Ethington and Clark, 1982 is above the upper limit of Yangtze- placognathus crassus, and Dzikodus tablepointensis ranges from the middle H. holodentata Zone to the appearance of H. kristinae (Zhen et al., 2011b), thus the lower Dzikodus tablepointensis Zone of Wuhai should be correlated with the H. holodentata Zone of Tarim. H. kristinae, the index species of the H. kristinae Zone in Tarim, extends stratigraphically from the upper part of Eoplacognathus pseudoplanus Zone (Löfgren, 2004) to the middle part of the E. suecicus Zone in Baltoscandia (Rasmussen, 2001). Although no age-diagnostic species had been obtained between the latest appearance of H. kristinae and the first appearance of Pygodus serra in Tarim (Zhen et al., 2011b), the H. kristinae Zone may be roughly correlative to the upper part of Dzikodus tablepointensis Zone and the whole Eoplacognathus suecicus Zone of Wuhai. The dominant species of the P. serra and P. anserinus zones in Wuhai, such as Costiconus ethingtoni, Dapsilodus viruensis, Protopanderodus cooperi, Pygodus serra, and P. anserinus, are also characteristic of the contemporaneous conodont faunas in Tarim (Zhen et al., 2011b). Consequently, the P. serra and P. anserinus zones of the present study are comparable with the eponymous zones of Tarim.


North China platform.—The Ordovician conodonts from Hebei, Shandong, and Henan Provinces of North China have been investigated by An et al. (1983), Pei and Cai (1987), An and Zheng (1990), and Wang et al. (2014). The conodonts of this report are significantly different from the counterparts of the North China Platform, although these regions are geographically close together. Based on the zonations proposed by the four above-mentioned publications, the following middle Darriwilian to early Sandbian zones of the North China platform may be recognized: the Histiodella holodentata–Tangshanodus tangshanensis, Eoplacognathus suecicus, Aurilobodus serratus, Scandodus handanensis, and Tasmanognathus sishuiensis zones, in ascending order (Fig. 4). Among these, the E. suecicus Zone was subdivided into the Plectodina onychodenta and Acontiodus? linxiensis subzones (Wang et al., 2014). Evidently, the zonation of platform facies is mainly based on endemic taxa, and this forms a biostratigraphic barrier to correlate with the contemporaneous successions elsewhere. Conodonts recovered in this study share several long-ranging species with coeval faunas of platform facies, such as Cornuodus longibasis, Ansella jemtlandica (=Belodella rigida of An et al. 1983), Oslodus semisymmetricus (=Dapsilodus compressus sensu An et al. 1983), and Venoistodus balticus (= Oistodus venustus of An et al. 1983). These have such long stratigraphic ranges that they are not useful for correlating the studied sections with zones of platform facies. The significant Darriwilian species, Eoplacognathus suecicus and Plectodina onychodonta,of


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Wuhai are also characteristic in the Darriwilian conodont faunas of the North China platform, and both of them are index species of the Darriwilian zones in North China platform (An et al., 1983; An and Zheng, 1990; Wang et al., 2014). Unfortunately, two stratigraphically important conodonts, Histiodella holodentata and Eoplacognathus pseudoplanus, which were discovered in the platform facies (An et al., 1983; Wang et al., 2014), were not found from our sections. Both E. suecicus and P. onychodonta occur first in sample W-6-1 and may suggest that the base of E. suecicus Zone in the Wolonggang section coincides with the lower boundary of P. onychodonta Subzone of North China platform, and that is a little higher than the base of E. suecicus Zone of the platform facies. Therefore, the E. suecicus Zone of Wuhai is comparable to the P. onychodonta Subzone of E. suecicus Zone in North China platform.


Western Newfoundland.—Conodonts of the Table Head, Green Point, and Lower Head formations in western Newfoundland were described by Stouge (1984, 2012), who established two Periodon-based phylozones and five Histiodella-based sub- phylozones in the Darriwilian. In successive order, the zones are Periodon macrodentatus and P. zgierensis, the subzones are Histiodella sinuosa, H. holodentata, H. cf. holodentata, H. kristinae, and H. bellburnensis (Fig. 4). The two sections of the present study bear abundant specimens of Periodon, including P. flabellum, P. hankensis, P. macrodentatus, P. zgierensis,and P. aculeatus (Table 1). Unfortunately, no species of Histiodella was discovered. Periodon macrodentatus, the nominate species of the P. macrodentatus zone of Western Newfoundland, ranges from the lower part of D. tablepointensis Zone to the base of P. anitae Subzone at the Wolonggang section. The range of Periodon zgierzensis, the descendant of P. macrodentatus, is restricted to E. suecicus and P. serra zones at our sections. In view of the fact that the lowest subzone of the P. zgierensis Zone is the H. kristinae Subzone (Stouge, 2012), the base of the P. zgierensis Zone is quite likely lower than that of the Eoplacognathus suecicus Zone. Other common taxa between Wuhai and Western Newfoundland, such as Cornuodus longibasis, Protopanderodus cooperi and P. calceatus, are wide-ranging and only a little or no use for correlation. The faunal evidence described above indicate that the D. tablepointensis and E. suecicus zones in the Wuhai area are correlative with the upper part of the P. macrodentatus Zone to the lower part of the P. zgierensis Zone and the upper part of the P. zgierensis Zone in western Newfoundland, respectively. Because both sections of the present study lack species of Histiodella, the comparison at the subzone level between the Wuhai area and western Newfoundland is impossible.


Conodont paleoecology


Two hypothetical ecologic models, the depth-stratification model (Seddon and Sweet, 1971) and the lateral segregation model (Barnes and Fåhraeus, 1975), of conodonts had been proposed to explain the distribution of conodonts. Sweet (1988) discussed strengths and weakness to these two hypotheses in detail, and tested them through three practical applications—the Ordovician of Cincinnati Region, the Mississippian of western United States, and the Pennsylvanian of the US Midcontinent.


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