Dzikodus tablepointensis Zone

The lowest of the recognized zones ranges from 26 m to 104 m above the base of the Wolongang Section. The zone index species is associated with 17 other species, including Ansella crassa Bauer, 1994, A. jemtlandica (Löfgren, Reference Löfgren1978), Baltoniodus sp., Coelocerodontus trigonius Ethington, 1959, Cornuodus longibasis (Lindström, 1955), Costiconus ethingtoni (Fåhræus, 1966), Dapsilodus viruensis (Fåhræus, 1966), Drepanoistodus basiovalis (Sergeeva, 1963), Periodon hankensis Stouge, Reference Stouge2012, P. macrodentatus (Graves and Ellison, 1941), P. flabellum (Lindström, 1955), Phragmodus polonicus Dzik, Reference Dzik1978, P. sp. Polonodus sp., Protopanderodus cooperi (Sweet and Bergström, 1962), P. varicostatus (Sweet and Bergström, 1962) and Spinodus spinatus (Hadding, 1913). The name-bearing species, Dzikodus tablepointensis (Stouge, Reference Stouge1984), is a characteristic species of the Darriwilian in Europe (Dzik, Reference Dzik1976; Löfgren, Reference Löfgren1978, Reference Löfgren1990, Reference Löfgren2000b, Reference Löfgren2004; Rasmussen, Reference Rasmussen1991, Reference Rasmussen2001), North America (Stouge, Reference Stouge1984; McCracken, Reference Mccracken1991) , South China (An, Reference An1987; Ding et al., Reference Ding, Chen, Zhang, Cao and Bao1993; Zhang, Reference Zhang1998a), Argentina (Ottone et al., Reference Ottone, Albanesi, Ortega and Holfeltz1999; Ortega et al., Reference Ortega, Albanesi and Frigerio2007; Feltes et al., Reference Feltes, Albanesi and Bergström2013; Mestre and Heredia, Reference Mestre and Heredia2013a), Tarim (Zhao et al., Reference Zhao, Zhang and Xiao2000; Zhen et al., Reference Zhen, Wang, Zhang, Bergström, Percival and Cheng2011b) and New Zealand (Zhen et al., Reference Zhen, Cooper, Simes and Percival2011a). An (Reference An1987) originally established the “Polonodus” tablepointensis Subzone with two subzones (the underlying “Amorphognathus” antivariabilis Subzone and the overlying Eoplacognathus suecicus Subzone) as subdivisions of the “A.” variabilis–“A.” antivariabilis Zone. Subsequently, the “Polonodus” tablepointensis Subzone was excluded from the “A.” variabilis–“A.” antivariabilis Zone (An, Reference An1987) or “A.” variabilis Zone (Ding et al., Reference Ding, Chen, Zhang, Cao and Bao1993) with Dzikodus tablepointensis becoming the index species for the conodont zone underlying the Eoplacognathus suecicus Zone (Zhang, Reference Zhang1998a). Zhang (Reference Zhang1998a) defined the lower boundary of the D. tablepointensis Zone at the last occurrence of Yangtzeplacognathus crassus (Chen and Zhang, Reference Ding, Chen, Zhang, Cao and Bao1993), and the upper boundary at the First Occurrence (FO) of E. suecicus, but the base of this zone is a little higher than the FO of the zonal species. Unlike Zhang’s definition, we place the base of the D. tablepointensis Zone at the FO of the nominate species because of the absence of Y. crassus at the Wolonggang Section. In view of the low conodont abundance under the sample W-2-2 (the FO sample of D. tablepointensis), the lower limits of the D. tablepointensis Zone of the Wolonggang section and in south-central China are probably very close. Zhang (Reference Zhang1998a) correlated the D. tablepoinetnsis Zone with the upper part of Acrograptus ellesae graptolite zone and the Nicholsonograptus fasiculatus graptolite zone, which coincide with the middle and upper parts of time slice 4b of Webby, Cooper, Bergström, and Paris (Reference Webby, Cooper, Bergström and Paris2004). A late Kundan age for the zone was suggested (Zhang, Reference Zhang1998a; Löfgren, Reference Löfgren2004), and this corresponds to the middle part of Stage slice Dw 2 (middle Darriwilian) of Bergström et al., (Reference Bergström, Chen, Gutiérrez-Marco and Dronov2009).

Eoplacognathus suecicus Zone

This zone occupies the interval between about 104m to 185m above the base of the Wolonggang Section. The zonal species, first described by Bergström (Reference Bergström1971), is widely distributed in Baltoscandia (Bergström, Reference Bergström1971, Reference Bergström1983; Lindström, Reference Lindström1971; Dzik, Reference Dzik1976, Reference Dzik1994; Löfgren, Reference Löfgren1978, Reference Löfgren1994, Reference Löfgren2000a; Bagnoli and Stouge, Reference Bagnoli and Stouge1997; Zhang, Reference Zhang1998a, Reference Zhang1999; Rasmussen, Reference Rasmussen2001; Viira et al., Reference Viira, Löfgren, Mägi and Wickström2001), North America (Harris et al., Reference Harris, Bergström, Ethington and Ross1979; Pyle and Barnes, Reference Pyle and Barnes2003), North China (An et al., Reference An, Zhang, Xiang, Zhang, Xu, Zhang, Jiang, Yang, Lin, Cui and Yang1983; An and Zheng, Reference An and Zheng1990; Wang et al., Reference Wang, Bergström, Zhen, Zhang and Wu2014), South China (An, Reference An1987; Ding et al., Reference Ding, Chen, Zhang, Cao and Bao1993; Zhang, Reference Zhang1998a; Wu et al., Reference Wu, Stouge, Li and Wang2010) and New Zealand (Zhen et al., Reference Zhen, Cooper, Simes and Percival2011a). Bergström (Reference Bergström1971) proposed an E. suecicus Subzone that occupies the lowermost part of the Middle Ordovician Pygodus serra zone in Europe. Later, Löfgren (Reference Löfgren1978) established the separate E. suecicus Zone underlying the P. serra Zone in Jämtland, Sweden. Zhang (Reference Zhang1998a) redefined this zone as the interval between the FO of E. suecicus Bergström, Reference Bergström1971 and the FO of Yangtzeplacognathus foliaceus (Fåhræus, 1966), and confined it to the Aserian. Bergström et al. (Reference Bergström, Chen, Gutiérrez-Marco and Dronov2009) dated this zone as the upper part of Dw 2. Zhang (Reference Zhang1998d) subdivided the E. suecicus Zone into two subzones, the lower P. lunnensis Subzone and the upper P. anitae Subzone. The base and top of the P. lunnensis Subzone were defined by the FOs of P. lunnensis Zhang, 1998 and P. anitae Bergström, Reference Bergström1983. The lower and upper boundaries of the P. anitae Subzone were taken as the FOs of P. anitae and Yangtzeplacognathus foliaceus. Such subdivisions have been generally accepted (e.g., Löfgren and Zhang, Reference Löfgren and Zhang2003; Bergström, Reference Bergström2007b; Agematsu et al., Reference Agematsu, Sashida and Sardsud2013; Wang et al., Reference Wang, Bergström, Zhen, Chen and Zhang2013b). The E. suecicus Zone of the Wolonggang Section can also be subdivided into the same two subzones as those were proposed by Zhang (Reference Zhang1998d).

Pygodus lunnensis Subzone

This level is represented by a diverse assemblage recovered from sample W-6-1. Being the most diverse one in the Wolonggang section, this sample includes 26 species, Ansella jemtlandica, Bergstroemognathus sp., Besselodus semisymmetricus (Hamar, 1966), Cornuodus longibasis, Costiconus ethingtoni, Drepnodus arcuatus Pander, 1856, D. reclinatus (Lindström, 1955), Drepanoistodus basiovalis, Dzikodus tablepointensis, Eoplacognathus suecicus, Erraticodon hexianensis An and Ding, 1985, Gen. et sp. indet., Onyxodus acuoliratus Watson, 1988, Panderodus sulcatus (Fåhræus, 1966), Periodon hankensis, P. macrodentatus, P. zgierzensis Dzik, Reference Dzik1976, Phragmodus polonicus, Plectodina onychodenta An and Xu, 1983, Polonodus sp., Protopanderodus caleatus Bagnoli and Stouge, Reference Bagnoli and Stouge1997, P. cooperi, P. graeai (Hamar, 1966), P. varicostatus, Pygodus lunnensis, and Spinodus spinatus. The subzone index is the most distinctive species of this assemblage, along with several other biostratigraphically important species, including Plectodina onychodenta, Periodon zgierzensis, Phragmodus polonicus and Panderodus sulcatus. Although P. lunnensis has previously been reported only from Sweden (Zhang, Reference Zhang1998d; Zhang and Sturkell, Reference Zhang and Sturkell1998; Bergström, Reference Bergström2007b) and Thailand (Agematsu et al., Reference Agematsu, Sashida and Sardsud2013), and was also recorded as Polonodus magnum Albanesi 1998 from Argentina (Albanesi et al., Reference Albanesi, Hünicken and Barnes1998; Ortega, 2007), it is a potentially important age-diagnostic taxon in deep-water Darriwilian bio-realms.

Pygodus anitae Subzone

This subzone is ranging from 128m to 185m above the base of the Wolonggang Section, and is characterized by the occurrence of P. anitae along with 23 other species, including Ansella crassa, A. jemtlandica, A. nevadensis (Ethington and Schumacher, 1969), Besselodus semisymmetricus, Cornuodus longibasis, Costiconus ethingtoni, Dapsilodus viruensis, Drepnodus arcuatus, D. reclinatus, Dzikodus tablepointensis, Eoplacognathus suecicus, Panderodus gryphus (An, 1985), Panderodus sulcatus, Periodon hankensis, P. macrodentatus, P. flabellum, P. zgierzensis, Phragmodus polonicus, P. sp., Plectodina onychodenta, Protopanderodus cooperi, P. varicostatus and Spinodus spinatus. Most concurrent species, including all of the biostratigraphically important species, of this Subzone extend from the underlying P. lunnensis Subzone. Compared to P. lunnensis, its proposed descendent P. anitae has a much wider geographic range, being recorded from Baltoscandia (Bergström, Reference Bergström1983; Zhang, Reference Zhang1998d; Zhang and Sturkell, Reference Zhang and Sturkell1998; Bergström, Reference Bergström2007b) , North China (Wang and Luo, 1984; An and Zheng, Reference An and Zheng1990), Argentina (Ottone et al., Reference Ottone, Albanesi, Ortega and Holfeltz1999; Albanesi and Ortega, Reference Albanesi and Ortega2002; Ortega et al., Reference Ortega, Albanesi and Frigerio2007), Russia (Melnikov, Reference Melnikov1999), Tarim (Zhao et al., Reference Zhao, Zhang and Xiao2000) and New Zealand (Zhen et al., Reference Zhen, Cooper, Simes and Percival2011a). The subzonal species stratigraphically ranges from the upper E. suecicus Zone to the lower part of the Y. foliaceus Subzone of the P. serra Zone (Löfgren, Reference Löfgren1978; Zhang and Sturkell, Reference Zhang and Sturkell1998), but does not co-occur with P. serra. The P. anitae-yielding sample HT-1-1 at the base of the Hatuke Creek Section is included into the P. anitae Zone because no Y. foliaceus has been found in it.

Pygodus serra Zone

P. serra (Hadding, 1913), one of key species in global Ordovician conodont biostratigraphy, was recovered from two samples (HT-3-1 and HT-5-2) in the lower part of the Wulalike Formation of the Hatuke Creek Section and is associated with ten other species, including Besselodus semisymmetricus, B. variabilis (Webers, 1966), Coelocerodontus trigonius, Cornuodus longibasis, Costiconus ethingtoni, Drepanodus reclinatus, Periodon aculeatus Hadding, 1913, P. hankensis, P. flabellum and P. zgierzensis. The Pygodus serra Zone was erected by Bergström (Reference Bergström1971) as coinciding with the stratigraphic range of P. serra, and its upper boundary was defined to the FO of P. anserinus Lamont and Lindström, 1957 and the disappearance of P. serra. Bergström (Reference Bergström1971) further subdivided the zone into five subzones, namely Eoplacognathus suecicus, “E.” foliaceus, “E.” reclinatus, “E.” robustus and E. lindstroemi subzones. Löfgren (1978) amended the lower boundary of this zone to be the first appearance of “E.” foliaceus, with the subzones of this zone only being the upper four subzones. When the co-occurrence of P. serra and its descendent P. anserinus was found in Tarim (Bergström and Finney, Reference Bergström and Finney1999) and Baltoscandia (Bergström et al., 2000), the upper boundary of the zone was defined by the appearance of P. anserinus. Accordingly, the P. serra zone is limited into the interval between the FO of Y. foliaceus and the FO of P. anserinus. As summarized by Bergström (Reference Bergström2007b), this zone has an extraordinarily wide geographic range, being known from numerous localities in Baltoscandia, British Isles, North America, Argentina, Australia, China, and other parts of the world. In China, the P. serra Zone has been extensively reported from North China (Wang and Luo, 1984), South China (An, Reference An1987; Ding et al., Reference Ding, Chen, Zhang, Cao and Bao1993; Wang et al., Reference Wang, Bergström and Lane1995) and Tarim (Zhang and Gao, Reference Zhang and Gao1990; Zhou and Chen, Reference Zhou and Chen1992; Wang and Qi, 2000; Zhao et al., Reference Zhao, Zhang and Xiao2000; Zhen et al., Reference Zhen, Wang, Zhang, Bergström, Percival and Cheng2011b; Wang et al., Reference Wang, Wu and Bergström2013d). This zone was correlated to the lower and middle parts of Dw 3 (Bergström et al., Reference Bergström, Chen, Gutiérrez-Marco and Dronov2009).

Yangtzeplacognathus foliaceus Subzone

The assemblage of this zone recovered from sample W-10-3 at 185m above the base of the Wolonggang section. The accompanying conodonts includes 12 species, namely Besselodus semisymmetricus, Coelocerodontus trigonius, Costiconus ethingtoni, Dapsilodus viruensis, Panderodus gryphus, Periodon aculeatus, P. flabellum, Protopanderodus calceatus, P. varicostatus, Pygodus anitae, Spinodus spinatus and Venoistodus balticus Löfgren, 2006. The Y. foliaceus Subzone (An, Reference An1987; Chen and Zhang, Reference Chen and Zhang1984a, Reference Chen and Zhang1984b; Ding et al., Reference Ding, Chen, Zhang, Cao and Bao1993) was initially recognized as a range zone (An, Reference An1981) in South China. Later, the Y. foliaceus Zone was widely used in the Guniutan Formation of South China (e.g., Zhang, Reference Zhang1998a; Wu et al., Reference Wu, Stouge, Li and Wang2010; Heredia, Carlorosi, Mestre, and Soria, Reference Heredia, Carlorosi, Mestre and Soria2013). Considering that the Y. foliaceus Zone was normally recognized based upon the absence of P. serra beneath the Pygodus anserinus Zone, we adopt the zonal scheme of Baltoscandia and discriminate the Y. foliaceus Subzone in the study area. As mentioned above, the co-occurrence of Y. foliaceus and Pygodus anitae, along with the absence of P. serra suggest a horizon in the lower part of the Y. foliaceus Subzone.

Pygodus anserinus Zone

The zonal species, an important age-diagnostic taxon like its ancestor P. serra, was recorded in three samples (HT-5-2, HT-6-1, and TH-6-2) in the middle and upper part of the Wulalike Formation of the Hatuke Creek Section. Associated species are Ansella jemtlandica, Besselodus semisymmetricus, Costiconus ethingtoni, Drepanodus arcuatus, D. reclinatus, Panderodus sulcatus, Periodon aculeatus, P. flabellum, P. hankensis, P. zgierzensis, Protopanderodus cooperi, P. varicostatus, Pseudooneotodus mitratus (Moskalenko, 1973), Pygodus serra, Spinodus spinatus, and Venoistodus balticus. P. anserinus is a morphologically distinctive species with a stratigraphic range spanning the P. anserinus Zone and the lower A. tvaerensis Zone (Sweet, Reference Sweet1988). Like P. serra, P. anserinus is also a world-wide distributed species. Around North China Platform, P. anserinus has a broader distribution than P. serra along the eastern and southeastern margin. The former species has been recovered from Longxian (Li and Yu, Reference Li and Yu1983; An and Zheng, Reference An and Zheng1990), Wuhai (Wang and Luo, 1984; An and Zheng, Reference An and Zheng1990), and Pingliang (An and Zheng, Reference An and Zheng1990; Finney et al., Reference Finney, Bergström, Chen and Wang1999; Wang, Reference Wang2000; Wang, Bergström, Zhan, and Zhang, Reference Wang, Bergström, Zhen and Zhang2013a; Wang et al., Reference Wang, Bergström, Zhen, Chen and Zhang2013b). Bergström et al. (Reference Bergström, Finney, Xu, Pålsson, Wang and Grahn2000) identified an interval in the basal part of the P. anserinus Zone where P. serra and P. anserinus co-occur. Moreover, P. serra ranges well into the upper P. anserinus Biozone in the Saergan Formation at the Dawangou section of Tarim (Bergström and Finney, Reference Bergström and Finney1999; Zhen et al., Reference Zhen, Wang, Zhang, Bergström, Percival and Cheng2011b). Zhang (Reference Zhang1998d) differentiated two morphotypes of P. anserinus: the early form has denticles of the fourth row developed only on the distal part of the platform; the late form has a well-developed fourth row. Most Pa elements (Fig 9:13-16) of the P. anserinus from HT-6-1 and HT-6-2 are late form, only some (Fig 9:11) from HT-5-2 and a few (Fig 9:12) from HT-6-1 represent the early form. After analyzing the symbiotic relationship between conodonts and graptolites from the Yenwashan Formation at the Huangnitang Section (the GSSP section for the base of the Darriwilian), Zhen et al. (Reference Zhen, Zhang and Percival2009) concluded that a mixture of advanced and primitive forms together with the absence of P. serra and P. xinjiangensis Wang and Qi, 2001 suggests a correlation with the upper part of the P. anserinus Zone. Therefore, the sampling horizon of HT-5-2 is referred to the lower part of P. anserinus Zone which correlates with the upper part of Dw3 (Bergström, 2009), while those of HT-6-1 and HT-6-2 are assigned to the upper part of the P. anserinus Zone that corresponds to the lower part of the Sa1 (lower Sandbian) of Bergström et al. (Reference Bergström, Chen, Gutiérrez-Marco and Dronov2009).

Baltoscandia

Southern Sweden is the most intensively studied area in Baltoscandia for research on Ordovician conodonts. The Ordovician conodont biostratigraphy of Southern Sweden was first summarized by Lindström (Reference Lindström1971) and Bergström (Reference Bergström1971), with only minor changes made by later conodont workers (e.g., Löfgren, Reference Löfgren1978, Reference Löfgren2000a, Reference Löfgren2000b; Zhang, Reference Zhang1998c, Reference Zhang1998d). On the basis of modern multielement taxonomy, the vertical ranges and biostratigraphy of the early to middle Darriwilian conodonts from southern Sweden and the middle Darriwilian to Hirnantian conodonts of Siljan region in south-central Sweden were summarized by Löfgren (2004) and Bergström (Reference Bergström2007b), respectively. These two recent summaries are considered as standard references for Baltoscandian conodont biostratigraphy in this paper. The middle Darriwilian to lowermost Sandbian conodont fauna from the Wuhai area and the Baltic area shows a general similarity on the species level, the Coefficient of Similarity (Bergström, Reference Bergström1990) between them is about 0.42. They share several key species of the genera Eoplacognathus, Pygodus, Yangtzeplacognathus, and Dzikodus, and this allows a relatively straightforward and highly resolved biostratigraphic correlation (Fig. 4). Apart from the Dzikodus tablepointensis Zone, other zones and biozones of Wuhai can be correlated directly with the namesakes. Otherwise, the co-occurrence of D. tablepointensis and E. pseudoplanus (Viira, 1972) had been found both in South China (Zhang, Reference Zhang1998a) and Baltoscandia (Löfgren, 2004), but there are still some differences between these two regions. The earliest occurrence of D. tablepointensis is much lower than that of E. pseudoplanus in South China, however, the situation in Baltoscandia is reversed. Both Zhang (Reference Zhang1998a) and Löfgren (2004) correlated the D. tablepointensis Zone of South China with the E. pseudoplanus Zone of Baltoscandia, except that slightly higher stratigraphic position of the base of the former zone was questionably proposed by Löfgren (2004).

South China

Conodonts from the two sections of the present study show a high similarity with those from the Guniutan and Miaopo formations of South China (Zhang, Reference Zhang1998a), and the Coefficient of Similarity (Bergström, Reference Bergström1990) between the faunas of Wuhai and South China is about 0.52. The following middle Darriwilian and earliest Sandbian conodont zones were established for South China by Zhang (Reference Zhang1998a): the Yangtzeplacognathus crassus, Dzikodus tablepointensis, Eoplacognathus suecicus, Y. foliaceus, Y. protoramosus, and Y. jianyeensis–Pygodus anserinus zones, in ascending order (Fig. 4). The D. tablepointensis Zone therein was subdivided into the Microzarkodina hagetiana and M. ozarkodella subzones (Zhang, Reference Zhang1998a), and such a subdivision is the same as that of E. pseudoplanus Zone in Baltoscandia (Löfgren, 2004). Almost all the important age-diagnostic conodonts recovered from the Klimoli and Wulalike formations are in common with the representative zonal species of South China; hence, the biostratigraphic correlations between them are also comparatively simple. The D. tablepointensis and E. suecicus zones between Wuhai and South China are correlated, but comparisons at the subzone level cannot be made. The P. serra Zone of this study corresponds to the Y. foliaceus and Y. protoramosus zones in South China because both Y. foliaceus and Y. protoramosus (Chen et al., 1983) are present in the Pygodus serra Zone of Baltoscandia (Bergström, Reference Bergström1971; Dzik, Reference Dzik1994). The Y. jianyeensis (An and Ding, 1982) occurs normally within the P. anserinus Zone in South China (An, Reference An1987; Ding et al., Reference Ding, Chen, Zhang, Cao and Bao1993; Zhang, Reference Zhang1998a), and therefore the P. anserinus Zone is equivalent to the Y. jianyeensis–P. anserinus Zone in South China.

Tarim

Zhou and Chen (Reference Zhou and Chen1992), Wang and Zhou (1998), and Zhen et al. (Reference Zhen, Wang, Zhang, Bergström, Percival and Cheng2011b) have studied the Ordovician conodonts of the Dawangou Section in Kalpin area, which is a representative Ordovician section in Tarim and is also an global auxiliary stratotype for the base of the Upper Ordovician (Bergström et al., Reference Bergström, Finney, Xu, Pålsson, Wang and Grahn2000). Strong resemblance of the middle Darriwilian and lowermost Sandbian conodont faunas between Kalpin and Wuhai areas is observable and the Coefficient of Similarity (Bergström, Reference Bergström1990) of these two areas is about 0.42. Most of the identified species in the present report are common to the typical coeval species of Tarim, and this allows a confident correlation between them. According to Zhen et al. (Reference Zhen, Wang, Zhang, Bergström, Percival and Cheng2011b), 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 discovered 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 Yangtzeplacognathus crassus, and Dzikodus tablepointensis ranges from the middle H. holodentata Zone to the appearance of H. kristinae (Zhen et al., Reference Zhen, Wang, Zhang, Bergström, Percival and Cheng2011b), 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, Reference Rasmussen2001). 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., Reference Zhen, Wang, Zhang, Bergström, Percival and Cheng2011b), 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., Reference Zhen, Wang, Zhang, Bergström, Percival and Cheng2011b). 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. (Reference An, Zhang, Xiang, Zhang, Xu, Zhang, Jiang, Yang, Lin, Cui and Yang1983), Pei and Cai (Reference Pei and Cai1987), An and Zheng (Reference An and Zheng1990), and Wang et al. (Reference Wang, Bergström, Zhen, Zhang and Wu2014). 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., Reference Wang, Bergström, Zhen, Zhang and Wu2014). 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. Reference An, Zhang, Xiang, Zhang, Xu, Zhang, Jiang, Yang, Lin, Cui and Yang1983), Oslodus semisymmetricus (=Dapsilodus compressus sensu An et al. Reference An, Zhang, Xiang, Zhang, Xu, Zhang, Jiang, Yang, Lin, Cui and Yang1983), and Venoistodus balticus (=Oistodus venustus of An et al. Reference An, Zhang, Xiang, Zhang, Xu, Zhang, Jiang, Yang, Lin, Cui and Yang1983). 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 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., Reference An, Zhang, Xiang, Zhang, Xu, Zhang, Jiang, Yang, Lin, Cui and Yang1983; An and Zheng, Reference An and Zheng1990; Wang et al., Reference Wang, Bergström, Zhen, Zhang and Wu2014). Unfortunately, two stratigraphically important conodonts, Histiodella holodentata and Eoplacognathus pseudoplanus, which were discovered in the platform facies (An et al., Reference An, Zhang, Xiang, Zhang, Xu, Zhang, Jiang, Yang, Lin, Cui and Yang1983; Wang et al., Reference Wang, Bergström, Zhen, Zhang and Wu2014), 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 (Reference Stouge1984, Reference Stouge2012), who established two Periodon-based phylozones and five Histiodella-based subphylozones 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, Reference Stouge2012), 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.

Type species

Polonodus tablepointensis Stouge, Reference Stouge1984.

Remarks

As summarized by Zhen and Percival (Reference Zhen and Percival2004), the derivation of the genus Dzikodus has been taxonomically complex. Zhang (Reference Zhang1998a) established Dzikodus to accommodate the Polonodus tablepointensis species group of Stouge (Reference Stouge1984), the sinistral and dextral Pb elements of Dzikodus are unpaired and do not mutually form mirror images, unlike the situation in Polonodus. For Polonodus (inclusive of Dzikodus), Löfgren (Reference Löfgren1990) proposed a seximembrate species apparatus, one geniculate (M), and three ramiform (Sa, Sb, and Sd) elements in association with the two platform (Pa and Pb) elements. Zhang (Reference Zhang1998a) documented the different growth stages of the Pa elements of Dzikodus, and she indicated that secondary lateral processes will form gradually at the anterolateral and the anterior processes during the growth. However, Rasmussen (Reference Rasmussen2001) nominated a Pa-1 element for the adult form and a Pa-2 element for the juvenile form, and then proposed a septimembrate species apparatus for Polonodus (including Dzikodus) consisting of planate Pa-1, Pa-2, and Pb; geniculate M; alate Sa; tertiopedate Sb; and quadriramate Sd elements. The more reasonable seximembrate apparatus described by Zhang (Reference Zhang1998a) and Löfgren (1990) is used by the present authors.

Dzikodus tablepointensis Stouge, Reference Stouge1984

Figure 6.28–6.36

1998a Dzikodus tablepointensis (Stouge); Zhang, p. 65, pl. 7, figs. 1–12, pl. 8, figs. 1–6. [see for synonymy]

Reference Mestre and Heredia2013bDzikodus tablepointensis; Mestre and Heredia, p. 46, figs. 5a–i. [see for synonymy]

Material

Eighteen specimens (Table 1).

Occurrence

W-2-2, W-3-1, W-4-1, W-6-1, W-10-1.

Remarks

Six elements of D. tablepointensis were documented and described in detail by Löfgren (1990) and Zhang (Reference Zhang1998a). Our specimens show the main features of D. tablepointensis and support a confident species identification, although most of the Pa elements of this species are badly preserved and characterized by missing or broken processes. Branched processes (figure 6.29, 6.30) indicate that they are anterolateral or anterior processes of Pa elements. One sinistral Pb (figure 6.28) and two well-preserved dextral Pb (figure 6.31, 6.32) elements are comparable with the D. tablepointensis figured by Zhang (Reference Zhang1998a) from the Guniutan Formation of South China. The two Sa (figure 6.33, 6.34) and two Sb (figure 6.35, 6.36) elements show close similarity to those figured by Löfgren (1990) and Zhang (Reference Zhang1998a).

Genus Periodon Hadding, 1913

Type species

Periodon aculeatus Hadding, 1913.

Remarks

The long ranging and pandemic genus Periodon appeared first in the Floian Stage, flourished in the Dapingian, Darriwilian, and Sandbian stages and disappeared in the latest Ordovician (Stouge et al., Reference Stouge, Du and Zhao2011). Stouge (Reference Stouge1984) reconstructed this genus as a septimembrate apparatus, and then Stouge and Bagnoli (1988) confirmed that it comprises seven element types (Pa, Pb, M, Sa, Sb, Sc, Sd). In the same paper, Stouge and Bagnoli noted that the evolutionary trend of this genus presents an increase in denticulation, and such a tendency has been demonstrated by many other conodont workers (e.g., Serpagli, 1974; Löfgren, 1978; Zhang and Chen, 1992; Stouge, Reference Stouge2012). However, Rasmussen (Reference Rasmussen2001) found that some specimens of Periodon species from Scandinavian Caledonides exhibit an opposite tendency rather than the general trend. A similar situation as in the Scandinavian Caledonides occurs in our sections, as shown by the fact that some of the stratigraphically older specimens of Periodon species have elements with more denticles than the younger ones. Rasmussen (Reference Rasmussen2001) explained this phenomenon as probably due to the fact that the number of denticles also was influenced by environmental changes. Nevertheless, Stouge (Reference Stouge2012) maintained his viewpoint that the number of free denticles at the front of the cusp of the Pa element is valuable for discriminating the species of Periodon. We follow the concept of Stouge (Reference Stouge2012), and distinguish five species of Periodon in the present paper. In evolutional order, they are: P. flabellum (one denticle on the anterior process of Pa element), P. hankensis (two denticles), P. macrodentatus (three denticles), P. zgierzensis (four denticles), and P. aculeatus (five denticles). If that is the case, two primitive Periodon species (P. flabellum and P. hankensis) extend very high stratigraphically (Figure 2 and 3), and such a circumstance was likely subject to some environmental control as suggested by Rasmussen (Reference Rasmussen2001). Among the five recognized species, P. flabellum, P. hankensis, P. macrodentatus, and P. aculeatus were recently described by Stouge and Bagnoli (1988), Zhen (2009), and Stouge (Reference Stouge2012); therefore, we describe only P. zgierzensis herein.

Periodon zgierzensis Dzik, Reference Dzik1976

Figure 8.12, 8.17–8.24

1976 Periodon aculeatus zgierzensis Dzik, p. 424, pl. 44, figs. 5,6.

1984 Periodon aculeatus zgierzensis; Stouge, p. 82, pl. 16, figs. 1–15.

2001 Periodon macrodentatus (Graves and Ellison); Rasmussen, p. 114, pl. 14, figs. 1–8.

2004 Periodon macrodentatus; Zhen and Percival, p. 168, figs. 10A–N.

2006 Periodon macrodentatus; Chen et al., p. 163, figs. 10J–M.

Material

One hundred and eight specimens (Table 1).

Occurrence

W-6-1, W-9-1, W-9-2, W-9-3, HT-1-1, HT-5-2.

Remarks

Stouge (Reference Stouge1984) described P. aculeatus zgierzensis in detail as a multielement subspecies from the Table Head Formation of western Newfoundland. He differentiated this subspecies from the younger type species P. aculeatus mainly by the M element, whose ratio of length of oral edge to length of cusp is 3:4 for P. aculeatus zgierzensis but 1:1 for P. aculeatus. Otherwise, M element of this subspecies bears fewer anterior denticles than that of P. aculeatus. Rasmussen (Reference Rasmussen2001), Zhen and Percival (Reference Zhen and Percival2004) and Chen et al. (Reference Chen, Zhang, Bergström and Xu2006) referred P. aculeatus zgierzensis to P. macrodentatus. Although Rasmussen (Reference Rasmussen2001) distinguished an early-stage Periodon species as P. zgierzensis, Stouge (Reference Stouge2012) considered it as P. hankensis. Stouge (Reference Stouge2012) proposed as characteristic of P. zgierzensis, there are four denticles on the anterior process of Pa element. In addition, the M element of P. zgierzensis has a more sinuous aboral edge and the S elements have a more developed zone of recessive basal margin than those of the ancestral species. Also, the strong sinuous aboral edge of the M element makes the connection of the cusp and the base show a near quadrate outline (Stouge, personal communication, 2014). On balance, most characteristics of our specimens correspond to descriptions by the above authors of P. zgierzensis.

Genus Phragmodus Branson and Mehl, 1933

Type species

Phragmodus primus Branson and Mehl, 1933.

Remarks

Sweet (Reference Sweet1981) proposed a seximembrate skeletal apparatus for the genus Phragmodus, including dichognathiform or ozarkodiniform Pa, dichognathiform Pb, oistodontiform or cyrtoniodontiform M, phragmodontiform Sa and Sb, phragmodontiform or cordylodontiform Sc elements. The representatives of this genus have been discovered on all the continents in the stratigraphic interval spanning from the uppermost Dapingian to the uppermost Hirnantian (Tolmacheva et al., Reference Tolmacheva, Zaitsev and Alekseev2013). Hitherto, at least ten species are known to be included in the genus Phragmodus, but only five of them are universally recognized: P. polonicus, P. flexuosus, P. inflexux, P. cognitus, and P. undatus.

Phragmodus polonicus Dzik, Reference Dzik1978

Figure 7.12–7.14, 7.18, 7.23, 7.24

1978 Phragmodus polonicus Dzik, p. 63, pl. 14, figs. 1–2,4–5.

1983 Plectodina onychodonta An and Xu; An et al., only pl. 23, figs. 1–5, pl. 25, fig. 7.

1987 Plectodina onychodonta; Pei and Cai, only pl. 7, figs. 5–7,9.

2013 Phragmodus polonicus; Tolmacheva et al., pp. 402–403, pl. 3, figs. 1–12. [see for synonymy]

2014 Plectodina onychodonta; Wang et al., Reference Wang, Bergström, Zhen, Zhang and Wu2014, only pl. 2, figs. 11–12.

Material.—Twenty sepcimens (Table 1).

Occurrence

W-1-2, W-2-1, W-2-2, W-3-1, W-6-1, W-8-2, W-9-1.

Remarks

Dzik (Reference Dzik1978) and Tolmacheva et al. (Reference Tolmacheva, Zaitsev and Alekseev2013) described this species in detail. A septimembrate species apparatus (Pa, Pb, M, Sa, Sb, Sc, and Sd elements) was indicated by Tolmacheva et al. (Reference Tolmacheva, Zaitsev and Alekseev2013). Mature elements of P. polonicus from Poland (Dzik, Reference Dzik1976, Reference Dzik1978, Reference Dzik1994), Estonia (Viira et al., Reference Viira, Aldridge and Curtis2006) and Russia (Tolmacheva et al., Reference Tolmacheva, Zaitsev and Alekseev2013) are all relatively large, the S elements are laterally flattened, the M element has a massive denticle and nondenticulate posterior process, and the P elements bear denticulate posterior and lateral processes. Our specimens possess the essential characters of this species. Based on the P elements, Dzik (Reference Dzik1994) distinguished two morphotypes of P. polonicus, the early form has a smooth anterior costa on the cusp, and a late form that has a serrate anterior costa. In our collections, the Pb elements of P. polonicus (Figure 7.12–7.14) carry slightly undulating margin on the anterior process, and this indicates that they are comparatively advanced as the late form. In addition, we discovered several phragmodontiform (S) elements (Figure 7.15–7.17) from the Wolongang section, which have separated denticles on the posterior processes, and these denticles are mostly circular or ovoid in cross section. Because the characteristics of these elements differ from those of P. polonicus, we tentatively assign them to P. sp.

Genus Plectodina Stauffer, 1935

Type species

Prioniodus aculeatus Stauffer, 1930.

Remarks

A seximembrate apparatus of this genus was reconstructed by Sweet et al. (Reference Sweet, Thompson and Statterfield1975), then Sweet (Reference Sweet1981) revised the six morphologically distinct elements as dichognathiform or prioniodiniform Pa, ozarkodiniform Pb, prioniodiniform M, trichonodelliform Sa, zygognathiform Sb, and cordylodontiform or eoligonodiniform Sc elements. The shallow and warm water living habit of Plectodina (Sweet, Reference Sweet1988) led to much more endemic species than the deep or cold water genera (Zhen and Percival, Reference Zhen and Percival2003). As one of the numerous endemic Plectodina species, P. onychodenta is morphologically prominent and stratigraphically important in North China (An et al., Reference An, Zhang, Xiang, Zhang, Xu, Zhang, Jiang, Yang, Lin, Cui and Yang1983; An and Zheng, Reference An and Zheng1990; Wang et al., Reference Wang, Bergström, Zhen, Zhang and Wu2014).

Plectodina onychodenta An and Xu, 1983

Figure 7.19–7.22

1983 Plectodina onychodonta An and Xu, p. 121, pl. 23, figs. 1–16, pl. 24, figs. 1–15, pl. 25, fig. 4. [partim] non pl. 23, figs. 1–5, pl. 25, fig. 7.

1987 Plectodina onychodonta; Pei and Cai, p. 87, pl. 7, figs. 1–10,12–13,15–16. [partim] non pl. 7, figs. 5–7,9

2006 Plectodina onychodonta; Agematsu et al., Reference Agematsu, Sashida, Salyapongse and Sardsud2006, fig. 7:13.

2014 Plectodina onychodonta; Wang et al., p. 8, pl. 2, fig. 11–13. [partim] non pl. 2, figs. 11–12.

Description

The Pa element is prioniodiniform. In the present study, one adult and two juvenile Pa specimens are illustrated (Figure 7.19–7.21). The adult one (Figure 7.19) is robust, with a procline cusp and a slightly bowed aboral outline; the anterior and posterior processes are of unequal length; denticles on the shorter anterior process are closely spaced and basally confluent; the longer posterior process bears separated denticles; basal cavity is fairly inflated under the cusp, and anteroposteriorly and evenly extends below the two processes. Both juvenile Pa elements (figure 7.20, 7.21) have rather straight aboral outlines and scarcely inflated basal cavities. The Sa element (figure 7.22) is also a juvenile form, the two lateral denticulated processes form a very acutely arched basal outline.

Material

Ten specimens (Table 1).

Occurrence

W-6-1, W-7-1, W-8-1.

Remarks

An and Xu (in An et al., Reference An, Zhang, Xiang, Zhang, Xu, Zhang, Jiang, Yang, Lin, Cui and Yang1983) recognized six elements of this species: prioniodiniform (=Pa), dichognathiform (=Pb), cyrtoniodontiform (=M), trichonodelliform (=Sa), zygognathiform (=Sb), and subcordylodontiform elements. However, their subcordylodontiform elements (pl. 23, Figs. 1–5) and a dichognathiform element (pl. 25, Fig. 7) are identical to the Sc and Pa elements of Phragmodus polonicus respectively, and the present authors prefer to assign them to P. polonicus. Several characteristics of our specimens, such as limited inflation of basal cavity under the cusp and slightly bowed aboral outline of Pa element, agree with the descriptions by An et al. (Reference An, Zhang, Xiang, Zhang, Xu, Zhang, Jiang, Yang, Lin, Cui and Yang1983) and Wang et al. (Reference Wang, Bergström, Zhen, Zhang and Wu2014). These characters also indicate that the specimens differ from the congeneric species in North America (Sweet, Reference Sweet1981), such as P. aculeata (Stauffer, 1930), P. tenuis (Branson and Mehl, 1933), P. aculeatoides Sweet, 1979, and P. florida Sweet, 1979.

Genus Pygodus Lamont and Lindström, 1957

Type species

Pygodus anserinus Lamont and Lindström, 1957.

Remarks

The seximembrate apparatus of Pygodus that consists of pygodontiform (modified stelliscaphate Pa) and haddingodontiform (pastinate Pb) elements and four types of ramiform (alate Sa, tertiopedate Sb, bipennate Sc and quadriramate Sd) elements has been generally accepted (e.g. McCracken, Reference Mccracken1991; Zhang, Reference Zhang1998a; Zhen et al., Reference Zhen, Zhang and Percival2009, Reference Zhen, Wang, Zhang, Bergström, Percival and Cheng2011b). Dzik (Reference Dzik1994) suggested the presence of a possible geniculate M element in the species apparatus of Pygodus, but such an element has not yet been reported in other collections.

Pygodus lunnensis Zhang, Reference Zhang1998c

Figure 9.1, 9.2, 9.10

1998cPygodus lunnensis Zhang, p. 95, pl. 1, figs. 12–16. [see for synonymy]

1999 Polonodus sp. A Ottone, pl. 6, figs. 6,8–9.

2013 Pygodus lunnensis; Agematsu, fig. 4:10.

2013 Polonodus sp. A Wang et al., fig. 14I.

Description

The broken Pa elements (Figure 9.1, 9.2) have anterior, posterolateral, and anterolateral processes. The surface of the platform is ornamented with contoured ridges. The anterior and posterolateral processes are close and confluent. The anterolateral process produces a secondary anterolateral process on the edge of its platform, and is wider than the other processes. The concavity between the postero-lateral and anterior processes is deeper than that between the anterolateral and anterior processes. The Pb element (Figure 9.10) has an erect cusp, and weakly denticulated anterolateral, anterior, and posterior processes.

Material

Three specimens (Table 1).

Occurrence

W-6-1.

Remarks

The two broken Pa and one complete Pb elements in our collections display the main characters of Pygodus lunnensis as described by Zhang (Reference Zhang1998c). Bergström (Reference Bergström1983) discussed the evolution of Pygodus species and regarded P. lunnensis (=P.? n. sp. in Bergström, Reference Bergström1983) as the pioneer species of this genus. Stouge and Bagnoli (1999) postulated that Pygodus might have evolved from Dzikodus. Although Rasmussen (Reference Rasmussen2001) considered that P. lunnensis is more similar to Polonodus, the taxon itself and the eponymous subzone have been widely adopted (e.g. Löfgren, Reference Löfgren2003, Reference Löfgren2004; Bergström, Reference Bergström2007b; Agematsu et al., Reference Agematsu, Sashida and Sardsud2013).

Genus Spinodus Dzik, Reference Dzik1976

Type species

Polygnathus spinatus Hadding, 1913.

Remarks

Dzik (Reference Dzik1994) suggested Spinodus might be closely related to Erraticodon or Erismodus, but distributed in relatively deeper water environments. As noted by Zhang (Reference Zhang1998a), Spinodus is a geographically very widespread genus and its species appear to occur mainly in continental margin and basin deeper-water sediments. Therefore, this genus has been considered as a good depth indicator. Species of Spinodus have been interpreted to consist of a seximembrate (or likely septimembrate) ramiform-ramiform species apparatus (Albanesi et al., Reference Albanesi, Hünicken and Barnes1998; Stouge, Reference Stouge2012) or a trimembrate apparatus (Lindström, Reference Lindström1964; Zhang, Reference Zhang1998a).

Spinodus spinatus (Hadding, 1913)

Figure 9.30–9.34

2011a Spinodus spinatus (Hadding); Zhen et al., p. 310, pl. 15, figs. A–H. [see for synonymy]

Material

Seventeen sepcimens (Table 1).

Occurrence

W-2-2, W-4-3, W-6-1, W-7-1, W-9-2, W-9-3, W-10-3, HT-1-1, HT-6-1.

Remarks

S. spinatus is relatively common in our samples, and its main characters have been fully described by Albanesi et al. (Reference Albanesi, Hünicken and Barnes1998) and Zhang (Reference Zhang1998a). Although elements representing a symmetry transition series (S elements) were widely reported for this species, its apparatus and affinities are still poorly known. Albanesi et al. (Reference Albanesi, Hünicken and Barnes1998) suggested a seximembrate species apparatus for it, in which the P elements were differentiated from other elements by having a prominent anticusp. However, Lindström (Reference Lindström1964) and Zhang (Reference Zhang1998a) interpreted S. spinatus as consisting of a trimembrate apparatus (Sa, Sb and Sc), and considered that the occurrence of the anti-cusp had little significance in differentiating its element types. Zhen et al. (Reference Zhen, Cooper, Simes and Percival2011a) distinguished an Sd element for the apparatus by the feature of prominent anti-cusp. We agree with the latter viewpoint. Our specimens are identical to the corresponding S elements figured by Zhang (Reference Zhang1998a) and Zhen et al., (Reference Zhen, Cooper, Simes and Percival2011a).

Genus Yangtzeplacognathus Zhang, Reference Zhang1998b

Type species

Polyplacognathus jianyeensis An and Ding, 1982.

Remarks

Before Yangtzeplacognathus was proposed by Zhang (Reference Zhang1998b), species of this genus were assigned to Eoplacognathus. Yangtzeplacognathus was erected to accommodate a group of species with unpaired, markedly dissimilar sinistral and dextral stelliplanate Pa and pastiniplanate Pb elements (Zhang, Reference Zhang1998b). Löfgren and Zhang (2003) recognized the geniculate M, alate Sa, quadriramate Sb, bipennate ScA, and tertiopedate ScB elements of the genus. Considering analogies to the other related genera with ramiform-pectiniform apparatus structures, Zhen et al. (Reference Zhen, Cooper, Simes and Percival2011a) reassigned the quadriramate and tertiopedate elements to the Sd and Sb positions, respectively.

Yangtzeplacognathus foliaceus (Fåhræus, 1966)

Figure 9.35, 9.36

1998a Yangtzeplacognathus foliaceus (Fåhræus); Zhang, p. 97, pl. 19, figs. 9–12. [see for synonymy]

2011a Yangtzeplacognathus foliaceus; Zhen et al., p. 311, figs. 16–17. [see for synonymy]

Description

The stelliplanate Pa element (Figure 9.35) has four main processes: a short, distally pointed anterior process; a relatively wide, distally pointed posterior process; a very short, curved and distally blunt posterolateral process; and a bifid, well-developed but narrow antero-lateral process with a extended, blade-like posterior lobe and a short anterior lobe. The anterior lobe of the anterolateral process and the anterior process form a acute angle about 20°, and they are of equal length and look similar to each other. The distal end of the posterolateral process curves anteriorly, although this feature is shielded mostly by debris in the figured specimen. The pastiniplanate Pb element (Figure 9.36) has a relatively long and distally blade-like anterior process, a much shorter posterior process forming an angle of about 160° with the lateral process (distally broken off). All processes of the two specimens bear a central denticle row. Most of the denticles are laterally confluent and show a ridge shape.

Material

Two specimens (Table 1).

Occurrence

W-10-3.

Remarks

Bergström (Reference Bergström1983) and Zhang (Reference Zhang1998b) considered this species to have four types of morphologically different platform elements that do not occur in mirror images, but that are unpaired, markedly dissimilar sinistral and dextral stelliplanate Pa and pastiniplanate Pb elements. Unfortunately, we only obtained two types of them, sinistral Pa (figure 9.35) and sinistral Pb (figure 9.36). Our specimens correspond to the E. foliaceus-E. reclinatus transition of Bergström (Reference Bergström1983) and Yangtzeplacognathus foliaceus of Zhen et al. (Reference Zhen, Cooper, Simes and Percival2011a) in some respects, such as outline of sinistral Pa element and a wide-angle formed by the short posterior process and the lateral process of sinistral Pb element. Following the suggestions of Zhen et al. (Reference Zhen, Cooper, Simes and Percival2011a), both elements described above are tentatively assigned to this taxon.