Cambrian trilobite Ovatoryctocara granulata ...

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Progress in Natural Science 19 (2009) 213–221 www.elsevier.com/locate/pnsc

Cambrian trilobite Ovatoryctocara granulata Tchernysheva, 1962 and its biostratigraphic significance Jinliang Yuan a,*, Yuanlong Zhao b, Jin Peng b, Xuejian Zhu a, Jih-pai Lin c a

Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, East Beijing Road 39, Nanjing 210008, China b College of Resource and Environment Science, Guizhou University, Guiyang 550003, China c School of Earth Sciences, Ohio State University, Columbus, OH 43210, USA Received 28 March 2008; received in revised form 27 May 2008; accepted 14 August 2008

Abstract The genus Ovatoryctocara Tchernysheva, 1962, and its key species Ovatoryctocara granulata Tchernysheva, 1962, are revised. Ovatoryctocara granulata occurs near the base of the Ovatoryctocara Zone and ranges up into the lower portion of the Kounamkites Zone in the Siberian Platform. O. granulata also appears in southeastern Guizhou, South China, but O. granulata in northern Greenland may represent an indefinite species. Specimens of Ovatoryctocara from Newfoundland cannot be identified to species level. Specimens including two cranidia and three pygidia from the lower part of the Aoxi Formation at Yaxi Village, Shizhu Town, eastern Tongren, northeastern Guizhou, were previously assigned to O. granulata, which is now reassigned as a new species O. yaxiensis sp. nov. It bears the following main features: glabella club-shaped, slightly expanded medially, with four pairs of lateral furrows, of which S1–S3 are triangular pits, S4 is shallow, connecting with axial furrow; shorter palpebral lobe situated a little anterior to the midway of facial suture across the fixigenae, longer posterolateral area (exsag.); semielliptical pygidium consisting of seven axial rings with a terminal piece and with eight pairs of marginal tips giving a sawtooth-like shape of the lateral margins in dorsal view. Although O. granulata is a widely distributed species, the FAD of O. granulata, for a global stage boundary, still has some disadvantages. First, its distribution is not as wide as that of Oryctocephalus indicus (Reed, 1910). Second, specimens of O. granulata are common only in Siberia. Third, the stratigraphic range of the species has not been studied in detail in all continents. Fourth, O. granulata is relatively small and easily affected by post-burial distortion. Last but not least, there exists obviously a facies change between the Ovatoryctocara Zone (lower Amgan Stage; deeper water facies) and the underlying Anabaraspis splendens Zone (Toyonian Stage; shallow water facies) in Siberian Platform. Nevertheless, the stratigraphic correlation utility of O. granulata in South China and Siberia is discussed. Ó 2008 National Natural Science Foundation of China and Chinese Academy of Sciences. Published by Elsevier Limited and Science in China Press. All rights reserved. Keywords: Trilobites; Ovatoryctocara granulata Tchernysheva, 1962; Cambrian; Biostratigraphy

1. Introduction Ovatoryctocara Tchernysheva, 1962, was first described as a subgenus Oryctocara (Ovatoryctocara), with the type species Oryctocara ovata Tchernysheva by Kryskov et al. [1] and Tchernysheva [2] from the Amgan strata of eastern *

Corresponding author. Tel.: +86 25 86082532; fax: +86 25 83357026. E-mail addresses: [email protected], [email protected] (J. Yuan).

Siberia. It is a fairly common trilobite in the lower Amgan strata (Ovatoryctocara Zone) of the Yudoma-Olenek region, where Oryctocara (= Ovatoryctocara) was used to define the base of Amgan Stage, namely the base of traditional Middle Cambrian in Siberia. Oryctocara Zone is here replaced by Ovatoryctocara Zone because of lack of any species of Oryctocara in this Zone. Ovatoryctocara granulata Tchernysheva, 1962, was proposed as a key species to define a global Cambrian stage boundary [3,4], and the FAD of O. granulata defining the base of Ovatoryctocara

1002-0071/$ - see front matter Ó 2008 National Natural Science Foundation of China and Chinese Academy of Sciences. Published by Elsevier Limited and Science in China Press. All rights reserved. doi:10.1016/j.pnsc.2008.08.002

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J. Yuan et al. / Progress in Natural Science 19 (2009) 213–221

Zone coincides with the base of the Amgan Stage in the Kuonamka Formation along the Molodo River in the north of the Siberian Platform, Russia. Since 1977, extensive discussions and reinvestigations on Oryctocara granulata Tchernysheva, 1962, or on O. granulata Tchernysheva, 1962, and its stratigraphic significance have been carried out [5–13]. However, the definitions of both Ovatoryctocara Tchernysheva, 1962, and the species O. granulata Tchernysheva, 1962, are still problematic; therefore, the main aim of this study is to revise the genus Ovatoryctocara Tchernysheva, 1962, and its species. 2. Revised diagnosis of Ovatoryctocara Tchernysheva, 1962 The revised diagnosis of Ovatoryctocara Tchernysheva, 1962: small oryctocarid trilobite with subcylindrical glabella, slightly expanded forward, widest anteriorly; lateral glabellar furrows with S1–S3 developed as deeper circular pits near but isolated from the axial furrows, and S4 merely as very shallow pits; preglabellar field absent; palpebral lobe short situated about at the midlength of fixigenae; eye ridge narrow and distinct, inclined backward; facial suture proparian; librigenae small, without genal spine; posterior lateral limb very wide (tr.) and moderately long (exsag.); thorax of four segments, with pleural tips subtruncate; semielliptical to semicircular pygidium with axis of seven to eleven rings and a terminal piece; pleural regions

with deep pleural furrows and distinct interpleural furrows; border furrow and border not defined; surface covered with or without granules. Known species: O. ovata Tchernysheva, 1960, O. granulata Tchernysheva, 1962, O. angusta Tchernysheva, 1962, O. yaxiensis sp. nov. The Ovatoryctocara sp. in the upper portion of the Amecephalus arrojosensis Zone of western North America [14] may be a juvenile specimen of Euarthricocephalus sp. or Arthricocephalus sp. because of the absence of developed interpleural grooves on pleural regions of pygidium as in Ovatoryctocara Tchernysheva, 1962. Based on the characteristics of the type species Oryctocara ovata Tchernysheva [1], Tchernysheva [2] (Fig. 1a and aa0 ) and Sovorova [15] elevated the subgenus Oryctocara (Ovatoryctocara) Tchernysheva, 1962. Later, Whittington [16] diagnosed the genus Ovatoryctocara as characterized by ‘‘the proparian facial suture, the genal angle rounded and lacking a spine, four thoracic segments, the relatively long pygidium with 10 – 12 axial rings and 10 – 1; pleural ribs. The cheeks and pleurae are not granulated, but bear a network of fine ridges” (Fig. 1a and aa0 ). However, these features with the exception of the relatively shorter and less segmented pygidium and different patterns of lateral glabellar furrows are also shared by the Arthricocephalus Bergeron with A. chauveaui Bergeron, 1899, as the type species [17] (see also Lane et al. [18] (Fig. 1c and cc0 ).

Fig. 1. Ovatoryctocara Tchernysheva, 1962, and its related genera based on their type species. a and aa0 : Ovatoryctocara ovata (Tchernysheva in Kryskov et al., 1960) from Tchernysheva, 1962; b and bb0 : Oryctocara geikiei Walcott, 1908, from Yuan et al., 2002; c and cc0 : Arthricocephalus chauveaui Bergeron, 1899, from Yuan, Zhao et Yang, 2006, assigned to Arthricocephalites xinzhaiheensis Chien et Lin in Lu et al., 1974; d and dd0 : Shergoldiella vincenti Geyer, 2006, from Geyer, 2006, Scale bars are 1 mm.


Arthricocephalus chauveaui Bergeron, 1899, is characterized by slightly expanded forwardly subrectangular glabella, with four pairs of lateral pits, of which S1–S3 are transglabellar furrows, palpebral lobes situated anteriorly, eight thoracic segments, semicircular pygidium with narrow border and distinct narrow border furrow. It differs from Ovatoryctocara ovata (Tchernysheva, 1960) in the presence of three transglabellar furrows, much wider fixigenae between palpebral lobes, eight thoracic segments (instead of four thoracic segments in the latter), shorter semicircular pygidium with narrow pygidial border, distinct border furrow, and with five axial rings and five pleural ribs (instead of 10–12 axial rings and 10 pleural ribs in the latter). Therefore, a few figured specimens that were previously assigned to Arthricocephalus granulus Chien et Lin in Zhou et al. [19] from the early Cambrian Dachenling Formation of Hetang, Jiangshan, western Zhejiang [20,21] should belong to Ovatoryctocara. On the other hand, a few illustrated specimens that were previously assigned to Arthricocephalus chauveaui Bergeron, 1899, from the Dachenling Formation of Dongzi, southern Anhui [22] should be reinterpreted as Shergoldiella cf. S. sequei (Linãń et Gozalo, 1999) [23], because it has the slightly expanded forwardly glabella with four pairs of lateral glabellar furrows, of which S1–S3 developed as circular pits near the axial furrows, shorter palpebral lobes situated anteriorly, very long and moderately wide (exsag.) posterior lateral limb (tr.); eight thoracic segments, with subtruncate pleural tips; semielliptical to semicircular pygidium with axis of 5–6 rings and a terminal piece; pleural regions with deep pleural furrows and distinct interpleural furrows; pygidial border furrow and border not defined. In comparison with other species of Shergoldiella, the specimens of Arthricocephalus chauveaui mentioned in Zhu et al. [23]

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are most similar to Shergoldiella sequei (Linãń et Gozalo, 1999), which was previously described either as Tonkinella sequei or as a specimen of ‘‘Hamatolenus (Myopsolenus)? jilocanus” in Linãń et Gozalo [24] from Lower Cambrian of Spain. Oryctocara Walcott [25] and its type species O. geikiei Walcott, 1908 (Fig. 1b and bb0 ), from the Spence Shale (lower portion of Cambrian Series 3 or traditional early Middle Cambrian) in Idaho, USA, can be easily distinguished from Ovatoryctocara by its rather broader cylindrical glabella consisting of four pairs of glabellar furrows as rounded pits, far from the axial furrows and two pairs of transglabellar furrows (S1–S2), gonatoparian facial suture, 12 thoracic segments and a shorter pygidium with seven segments. In addition, Oryctocara lata Tchernysheva, 1952 [26], from Amgan strata of Siberia (Tchernysheva [2]) should belong to Euarthricocephalus Ju, 1983 [20], as having rod-shaped glabella consisting of four pairs of lateral glabellar furrows as rounded pits, of which S1 is bifurcated axially and connected laterally to axial furrows to form a pair of small lateral basal glabellar lobes, wider fixigenae between palpebral lobes, and 8–9 thoracic segments with much wider pleural lobes. Shergoldiella Geyer, 2006, and its type species Shergoldiella vincenti Geyer [27] (Figs. 1d and 2d) from the base of Cephalopyge notabilis Zone of Jbel Wawrmast Formation in the eastern Anti-Atlas of Morocco is mostly similar to Ovatoryctocara, because both genera have subcylindrical glabella, with four pairs of lateral glabellar furrows as circular pits, short palpebral lobe, proparian facial suture, but it differs from the latter in having slightly expanded glabella medially, widest across S2 or S3, four pairs of lateral circular furrows, of which S1 is transglabellar furrow, shorter palpebral lobe situated slightly anterior to the midway of

Fig. 2. Ovatoryctocara granulata Tchernysheva, 1962, and its related form from South China. (a and b) Ovatoryctocara granulata Tchernysheva, 1962, from the Bathynotus holopygus–Ovatoryctocara granulata Zone of the Kaili Formation, southeastern Guizhou; of which (a) (GK6003, Yuan et al., 2002) from Wuliu-Zengjiayan section and (b) from (CD 1000-89; GK6801) Jianshan section, Chuandong Village, Jianhe County. (c) Arthricocephalus sp. from the Bathynotus holopygus–Ovatoryctocara granulata Zone of the Kaili Formation, (GK6803) from Jianshan section, Chuandong Village, Jianhe County, southeastern Guizhou. Scale bars are 1 mm.

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facial suture across the fixigenae, wider (exsag.) posterior lateral area of fixigenae, thorax of 6–8 segments (instead of four segments in Ovatoryctocara); pygidium with axis of six rings and a terminal piece (instead of 7–11 rings plus a terminal piece in Ovatoryctocara). The differences between Ovatoryctocara and other genera mentioned above are illustrated in Fig. 1. 3. Ovatoryctocara granulata (Tchernysheva, 1962) Tchernysheva (1962) selected one cranidium as the holotype of O. granulata [2]. This species is distinct from the type species Ovatoryctocara ovata (Tchernysheva, 1960) in having an expanded glabella medially, S1–S3 connected with shallow longitudinal furrows, a relatively longer palpebral lobe, a semicircular pygidium with only 6–7 axial rings and the surface with granules. O. granulata is not a junior synonym of Arthricocephalus chauveaui Bergeron, 1899 [28], because it has distinctly different patterns of glabellar furrows. In addition, specimens (Fig. 2a and b) from the Bathynotus holopygus–O. granulata Zone of the Kaili Formation, southeastern Guizhou are nearly identical to the type material from Siberia with the exception of slightly shorter palpebral lobes situated more anteriorly and wider posterolateral area (exsag.). Other specimens of O. granulata have been reported from Avalon Peninsula, southeastern Newfoundland [3]. The cranidium of O. granulata from Newfoundland (Fletcher, 2003 [3]), shown in Fig. 3d, shows (1) rather short palpebral lobes situated a little anterior to the midway of facial suture across the fixigenae, (2) S1–S3 connected without shallow longitudinal furrows, wider (exsag.) posterior lateral area of fixigenae, (3) the pygidium (Fletcher, 2003 [3]) shown in Fig. 3e, with eight axial rings and a terminal piece and (4) sculpture on the surface of cranidium and pygidium being not as strong as Siberian type material. Therefore, the assignment of these Newfoundland specimens to O. granulata is questionable, and these specimens may represent an indefinable species of Ovatoryctocara. In addition, two cranidia and seven pygidia formerly assigned to Ovatoryctocara sp. A. from North Greenland

[28] were considered as O. granulata Tchernysheva, 1962, by Geyer [8] (Fig. 3f and g). However, these Greenland specimens are rather small (about less than 1.35 mm in length) and apparently represent meraspid specimens. The cranidium of Greenland specimens has four pairs of lateral furrows connecting with axial furrow and without longitudinal furrows, shorter palpebral lobe situated a little anterior to the midway of facial suture across the fixigenae, wider posterolateral area (exsag.), and semielliptical pygidium (Fig. 3g) consists of eight narrow axial rings and a terminal piece. The differences between O. granulata and other related forms mentioned above are shown in Fig. 3. 4. Ovatoryctocara yaxiensis sp. nov. and associated species Ovatoryctocara yaxiensis sp. nov. (Fig. 4a–e; aa´ and bb´) is described here based on cranidium (a and aa´) as the holotype from the Aoxi Formation at Yaxi, Shizhu Town, Tongren County, eastern Guizhou. The new species is characterized by the following features: cranidium trapezoidal; glabella club-shaped, slightly expanded medially, with four pairs of lateral furrows, of which S1–S3 are triangular pits, S4 is shallow, connecting with axial furrow; narrower fixigenae (tr.), shorter palpebral lobe situated a little anterior to the midway of facial suture across the fixigenae, wider posterolateral area (exsag.); semielliptical pygidium with seven axial rings and a terminal piece and with seven pairs of marginal tips giving a sawtooth-like shape of the lateral margins in dorsal view; pleural region with seven pairs of deep pleural furrows and distinct narrow interpleural furrows; exoskeleton with a few coarser and fine granules. It differs from O. granulata mainly in having clubshaped glabella with triangular glabellar furrows (S1–S3), shorter palpebral lobe situated a little anterior to the midway of facial suture across the fixigenae, longer posterolateral area (exsag.), and semielliptical pygidium with eight pairs of marginal tips giving a sawtooth-like shape of the lateral margins in dorsal view. The new species occurs together with Barklyella sp. (Fig. 4f), Deiradonyx sp. (Fig. 4g) and Xiaofangshangia sp. (Fig. 4h–m). Barklyella Shergold, 1969 [29], with its type species B. expansa Sher-

Fig. 3. Reconstruction of Ovatoryctocara granulata Tchernysheva, 1962, and its related forms from Siberia, South China, SE Newfoundland and North Greenland. (a and b) Ovatoryctocara granulata Tchernysheva, 1962, from Siberian Platform; (c) Ovatoryctocara granulata Tchernysheva, 1962, from South China; (d and e) O. sp. from southeastern Newfoundland; (f and g) O. sp. A from North Greenland. Scale bars are 1 mm.


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Fig. 4. Ovatoryctocara yaxiensis sp. nov. and associated species from the Aoxi Formation,Yaxi, Shizhu Town, Tongren County, eastern Guizhou. (a–e), aa0 and bb0 : Ovatoryctocara yaxiensis sp. nov. from Yuan et al., 2002 as O. granulata (pl. 31, Figs. 10–13, NIGP 133180–133180, cranidium (a and aa’), holotype) and as Barklyella sp. (pl. 28, Fig. 5, NIGP133178); (f) Barklyella sp. from Yuan et al., 2002 (pl. 28, Fig. 4, NIGP133177); (g) Deiradonyx sp. (NIGP 133184); (h–m) Xiaofangshangia? sp. (NIGP 133185–133190). Scale bars are 1 mm.

gold, 1969 [28], was originally described from the DinesusXystridura fauna of the Beetle Creek Formation, Dingo Creek, Northwest Queensland, which is considered as early Middle Cambrian in age. However, Laurie [30] assigned the Xystridura negrina assemblage and Redlichia forresti assemblage to the Early Cambrian Ordian Stage. Recently, Laurie [31] reported the occurrences of Pagetia aff. edura ¨ pik, ptychoparioid sp.1, ArthricoJell, Xystridura altera O cephalus sp. nov. and Itagnostus sp. from the Jigaimara Formation, Arafura Basin, Northern Territory. Among them, Itagnostus sp. is similar to ‘‘Peronopsis” normata from the early Templetonian Beetle Creek Formation. Thus, the Beetle Creek Formation may include partly traditional, latest Early Cambrian and earliest Middle Cambrian based on trilobite correlations. In South China, the LAD of Arthricocephalus Bergeron, 1899, is below the Oryctocephalus indicus, which is the key species for defining the base of Cambrian Series 3 (Yuan et al.) [32], therefore, the Ordian Stage and the lower portion of the Templetonian Stage in Australia may be time-equivalent to the Longwangmiaon or late Duyunnian in South China. ¨ pik, 1982 [33], is ranging from the Ordian Deiradonyx O to early Templetonian Stages in Australia. Figured Deirad¨ pik [33] onyx sp. (Fig. 4g) is similar to Deiradonyx toddi O from the Northern Territory, Australia, except for having

a narrower anterior border. Furthermore, the illustrated Xiaofangshangia sp. (Fig. 4h–m) is quite similar to Xiaofangshangia divergens Qiu [34], which comes from the upper part of the Mantou Formation (the upper part of Redlichia chinensis Zone) in Dananzhuang, Tongshan County, northern Jiangsu. Xiaofangshangia sp. differs from Xiaofangshangia divergens in having a less tapering glabella forwardly with four pairs of deeper lateral glabellar furrows. Comparing with older literatures on the related forms of Ovatoryctocara, O. yaxiensis sp. nov. may occur elsewhere. For example, Ovatoryctocara sp. (Fig. 3d and e) at the top of the Branch Cove Marl Member of the Brigus Formation in the Easter Cove of Branch, St. Mary’s Bay, southeastern Newfoundland (upper Cephalopyge Zone) is more similar to O. yaxiensis sp. nov. (Fig. 3a–e; aa´ and bb´) than O. granulata Tchernysheva, 1962, because of lack of longitudinal furrows on glabella, and having shorter palpebral lobes, much wider posterolateral area (exsag.), and rather elevated relief of cranidium and pygidium. 5. Intra- and inter-cratonic biostratigraphic correlations In China, the oldest representative of Shergoldiella, cf. S. sequei (Linãń et Gozalo, 1999) [23], is from the early Cambrian Dachenling Formation of Dongzi, southern

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Anhui [22]. The Dachenling Formation is time-equivalent to the Tsinghsutung Formation in northeastern and southeastern Guizhou. Based on the reported ranges of associated species mentioned above, Ovatoryctocara yaxiensis sp. nov. occurs relatively higher than Shergoldiella cf. S. sequei. Thus, the Ovatoryctocara yaxiensis assemblage from the Aoxi Formation may be older than the B. holopygus–O. granulata Zone in the lower part of the Kaili Formation, Guizhou (Table 1). Based on the ranges of key traditional Early Cambrian trilobite taxa from southwestern China and North China [12,20,21,34–37], the Early Cambrian trilobite successions in the platform facies include the Chittidilla plana-Paragraulos kunmingensis Zone, Redlichia guizhouensis Zone, R. murakamii–Hoffetella Zone, Megapalaeolenus Zone, Palaeolenus Zone, Paokannia–Ushbaspis Zone, and Drepanuroides Zone in descending order. The trilobite zonations in the slope facies consist of the B. holopygus–O. granulata Zone, Protoryctocephalus wuxunensis Zone, Arthricocephalus taijiangensis Zone, A. chauveaui Zone, A. jiangkouensis Zone, Protolenella conica Zone and Drepanuroides Zone. The stratigraphic correlation of upper portion of the Cambrian Series 2 between platform and slope facies in China is summarized in Table 1. In Siberia platform, the base of the Amgan Stage coincides with the base of the Schistocephalus Zone. In other words, the beginning of the Amgan Stage is determined

by the first occurrence of typical Middle Cambrian trilobites, including Paradoxides, Schistocephalus, and Oryctocephalidae (Oryctocara = Ovatoryctocara), and associated agnostoids [38]. However, Schistocephalus was considered as a synonym of Palaeolenus Mansuy, 1912 [39], and Palaeolenus antiquus (Tchernysheva, 1956) [40] is interpreted as a cospecies with Megapalaeolenus deprati (Mansuy, 1912) [39]. Therefore, the base of the Amgan Stage can roughly correlate with the upper part of the Megapalaeolenus Zone or Arthricocephalus chauveaui Zone in South China. If this correlation is correct, the upper range of O. granulata occurs in the lower portion of both Kounamkites Zone in Siberia and B. holopygus–O. granulata Zone in South China. In other words, the lower portion of Kounamkites Zone in Siberia is a time-equivalent of the upper Duyunian Stage in South China. In Newfoundland, the Hupeolenus Zone consists of many key correlatable trilobite taxa, including Protolenus (Hupeolenus), Orodes, Chelediscus, Tannudiscus, Condylopyge, Cobboldites and Ovatoryctocara [3]. Besides Ovatoryctocara sp. that is similar to O. yaxiensis sp. nov. in South China, Protolenus (Hupeolenus) mckillopi [3] is similar to Sichuanolenus nanjiangensis from the upper part of the Yanwangbian Formation, as mentioned in Zhang et al. [35]. Based on both accounts, the base of the Hupeolenus Zone (Agdzian Stage) in Newfoundland should be correlatable to the base of Protolenella conica Zone (Nangaoan) in South China.

Table 1 Correlation chart of the upper portion of the Cambrian Series 2 between platform and slope facies in China based on key early Cambrian trilobite species (data of ranges of key genera and species from Yin et Lee, 1978; Zhang et al., 1980; Qiu, 1980; Yuan et al., 1999, 2002, 2006).


The correlation between South China and Laurentia is achieved based on redlichiid trilobites in South China and olenellid trilobites in Greenland and western United States. The interval of Ovatoryctocara sp. A. [28] (Fig. 4f and g) from the upper part of the Henson Gletscher Formation, North Greenland, may be correlatable to the A. taijiangensis Zone in South China. The stratigraphic correlation of the upper portion of the Cambrian Series 2 between China and selected regions is summarized in Table 2. 6. Conclusions Based on the review of Ovatoryctocara Tcherysheva, 1962, a new species O. yaxiensis sp. nov. is erected in the present paper. The new species bears the following main features: glabella club-shaped, slightly expanded medially, with four pairs of lateral furrows, of which S1–S3 are triangular pits, S4 is shallow, connecting with axial furrow; shorter palpebral lobe situated a little anterior to the midway of facial suture across the fixigenae, wider posterolateral area (exsag.); semielliptical pygidium with seven axial rings and a terminal piece, and with eight pairs of marginal tips giving a sawtooth-like shape of the lateral margins in dorsal view. O. granulata was previously regarded as a widespread species, however, the FAD of O. granulata as a key horizon for defining a global Cambrian stage bound-

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ary is not being supported by this study. First, the distribution of O. granulata is not as wide as that of Oryctocephalus indicus, which is another candidate species for defining a global Cambrian stage boundary. Second, the specimens of O. granulata are only common in Siberia, only a few specimens have been found in southeastern Guizhou, South China. Third, taxonomic identifications of known Ovatoryctocara are sometimes questionable. The Ovatoryctocara sp. in the upper portion of the Amecephalus arrojosensis Zone of western North America [14] may be a juvenile specimen of Euarthricocephalus sp. or Arthricocephalus sp. because of the absence of developed interpleural grooves on pleural regions of pygidium as in Ovatoryctocara. Fourth, correlation utility of O. granulata has not been validated. In South China, Arthricocephalus has a long stratigraphic range from the base of the Balang Formation (A. jiangkouensis Zone, Nangao Stage) into the top of the Duyunian (O. granulata–B. holopygus Zone), and the LAD of the Arthricocephalus provides additional evidence to strengthen the coeval level of the FAD of O. indicus. In the same way, the LAD of O. granulata provides evidence about the synchronicity of the FAD of O. indicus, an additional potential criterion for international correlation. Last but not least, there exists obviously a facies change between Ovatoryctocara Zone (lower Amgan Stage; deeper water facies) and Anabaraspis splendens Zone

Table 2 Correlation chart of the upper portion of the Cambrian Series 2 between China and selected regions.

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(Toyonian Stage; shallow water facies) in Siberian Platform, however, where Ovatoryctocara has a relatively longer range (from the base of the Ovatoryctocara Zone into the lower portion of the Kounamkites Zone). Based on all the factors mentioned above, the LADs of O. granulata in both South China and Siberia can be correlated to each other, but not precisely elsewhere. Acknowledgements This study is supported by the National Natural Science Foundation of China (Grant Nos. 40672018, 40762001), the Foundation of the Mobile Project of Science and Technology of Guizhou Province (Grant No. Gui Ji.2007-4004), the Foundation of the Governor of Guizhou Province (Gui E. 2006-7), the Key Project of International Cooperation of Guizhou Science and Technology (Gui.co.G.2008-700110) and the Major Basic Research Projects of MST of China (2006CB80640). Many thanks are given to Gerd Geyer (Institut fu¨r Palaöntologie, Bayerische Julius-Maximilians-Universita¨t Wu¨rzburg, Germany), who provided photographs of relevant trilobite specimens from the Jbel Tarhoucht area in the eastern Anti-Atlas of Morocco; to Zhu Xuejian (Nanjing Institute) for his technical assistance. Peng Shanchi, Zhu Maoyan (Nanjing Institute) and Loren E. Babcock (The Ohio State University, USA) reviewed the submitted manuscript and largely improved the quality of the present paper. References [1] Kryskov LN, Lazarenko NP, Ogienko LV, et al. New early Palaeozoic trilobites of eastern Siberia and Kazakhstan. New species of prehistoric plants and invertebrates of the USSR, vol. 2. Moscow: VSEGEI; 1960. [2] Tchernysheva N Ye. Cambrian trilobites of the Family Oryctocephalidae. In: Problems of oil and gas occurrence in the Soviet Arctic, palaeontology and biostratigraphy. 3 Trudy nauchnoissledovatel’skii geologii Institut Arktiki (NIIGA) 1962; 127, Leningrad, p. 188 [in Russian]. [3] Fletcher TP. Ovatoryctocara granulata: the key to a global Cambrian stage boundary and the correlation of the Olenellid, Redlichiid and Paradoxidid realms. Special Papers in Palaeontology, vol. 70, 2003; p. 73–102. [4] Fletcher TP. The base of Cambrian Series 3: the global significance of key oryctocephalid trilobite ranges in the Kaili formation of South China. Memoir of the association of Australasian palaeontologists, vol. 33, 2007; p. 29–33. [5] Robison RA, Rosova AV, Rowell AJ, et al. Cambrian boundaries and divisions. Lethaia 1977;10:257–62. [6] Geyer G, Malinky JM. Middle Cambrian fossils from Tizi N’Tichka, The High Atlas, Morocco. Part 1. Introduction and trilobites. J Paleontol 1997;71(4):620–37. [7] Geyer G. Intercontinental, trilobite-based correlation of the Moroccan early Middle Cambrian. Can J Earth Sci 1998;35:374–401. [8] Geyer G. The base of a revised Middle Cambrian: are suitable concepts for a series boundary in reach? Geosci J 2005;9(2):81–99. [9] Geyer G, Shergold JH. The quest for internationally recognized divisions of Cambrian time. Episodes 2000;23(3):188–95. [10] Geyer G, Landing E. Middle Cambrian of Avalonian Massachusetts: stratigraphy and correlation of the Braintree trilobites. J Paleontol 2001;75(1):116–35.

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