2. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China;
3. Natural Resources Canada, Geological Survey of Canada, Vancouver, BC, V6E4S6, Canada
Since 1960s, the International Commission on Stratigraphy has worked to establish the global stratotype section and point (GSSP) for each stage for stratigraphic division and correlations around the world. Conodont biostratigraphic study established the age of strata and enabled intercontinental stratigraphic correlations through their global distribution, rapid evolution, and abundant occurrence in marine strata. Conodonts therefore are widely used as an important index fossil in stratigraphic division and correlations from the Cambrian to Triassic. In particular, conodonts also have a crucial role in providing a precise Triassic timescale (Orchard, 2016), and their zonation has been well accepted as a guide to determine stratigraphic resolution for the Triassic.
The Triassic contains three series and seven stages. The Lower Triassic includes the Induan and Olenekian stages. The base of the Induan is also the Permian-Triassic boundary (PTB) (Yin et al., 2001). Orchard (2010) reviewed the distribution of conodont faunas across each of the stage boundaries within the system. Of these, the GSSP for the base of the Induan (also the base of the Triassic) was selected in the Meishan Section, Changxing County, Zhejiang Province, South China (Yin et al., 2001), in which the first appearance of conodont Hindeodus parvus marks the PTB, which is calibrated to the base of Bed 27c (Chen et al., 2015b; Yin et al., 2001). In contrast, the Induan-Olenekian boundary (IOB), the second GSSP of the Lower Triassic series, remains undefined. Growing evidence shows that conodonts are very abundant across this stage boundary, and may potentially define the IOB (Orchard, 2007b). In particular, conodont elements of Novispathodus waageni group occur in the boundary beds worldwide (Fig. 1), and they are a product of rapid evolution, and thus are strong candidate index fossils for precisely defining the IOB.
The conodont segminate element Nv. waageni was first established from the Mianwali Formation of Salt Range, western Pakistan (Sweet, 1970), and later has been widely reported from around the world, as shown in Figs. 1 and 2.
As a result of its global distribution, the first appearance of Nv. (previously Neospathodus) waageni sensu lato has been widely accepted as a stratigraphic mark defining the IOB (Tong and Zhao, 2011; Krystyn et al., 2007; Zhao et al., 2007; Tong et al., 2004, 2003). Two candidate sections for the GSSP of the IOB have been proposed on this basis: the West Pingdingshan Section in Chaohu, Anhui Province, China where the first appearance datum (FAD) of Nv. waageni eowaageni has been proposed to define the boundary (Zhao et al., 2013, 2008a, b, 2007; Tong and Zhao, 2011; Tong et al., 2004), and the Mud Section in Spiti, India (Krystyn et al., 2007, 2005; Orchard, 2007a; Orchard and Krystyn, 2007). A third candidate section, in Nammal, Pakistan, has recently been proposed on the basis of ammonoids (Ware et al., 2017).
To date, many subspecies or variant forms of Nv. waageni have been proposed and illustrated, and this has led to taxonomic uncertainty within the group, masked evolutionary traits of those important taxa, and thus prevented precise definition of the IOB. Recently, Goudemand (2014) assigned variant forms to Nv. waageni and Nv. waageni eowaageni, and questioned the utility of the latter subspecies in defining the IOB. Accordingly, a reappraisal of taxonomy and evolutionary lineage of Nv. waageni group is essential to provide better understanding of conodont evolutions across the IOB, and to offer a reliable definition for its GSSP.1 GEOLOGICAL AND STRATIGRAPHIC SETTINGS
The restudied conodont specimens were extracted from the samples collected from both the West Pingdingshan Section of Chaohu area, Anhui Province and the Jianshi Section of western Hubei Province, South China (Fig. 2). The Chaohu area was located in a deep part of carbonate ramp on the northern margins of the Lower Yangtze Platform, while the Jianshi area was situated on the relatively shallower part of a carbonate ramp on the northern margin of the Yangtze Platform during the Permian-Triassic transition (Feng et al., 1997; Chen et al., 2011; Lyu et al., 2018; Fig. 2). Both sections yield abundant conodont fossils throughout the entire Lower Triassic (Lyu et al., 2018; Zhao et al., 2008a, b, 2007). The Chaohu Section has been extensively studied since 1980s (Zhao et al., 2008a, b, 2007, 2005a, b, 2004, 2003; Tong et al., 2005, 2004; Ding, 1983). Zhao et al.(2007, 2005a) established eight conodont zones for Lower Triassic succession in Chaohu. The continuous conodont zonation has become an important reference for the Lower Triassic stratigraphic correlations around the world. The Uppermost Permian through Lower Triassic successions of the Jianshi Section has been well studied by systematically sampling for conodont biostratigraphy, and a total of nine conodont zones has been established (Lyu et al., 2018; Fig. 3).2 CONODONTS ACROSS THE INDUAN-OLENEKIAN BOUNDARY
Novispathodus waageni has been reported worldwide after its first report from Salt Range, western Pakistan (Sweet, 1970) (Fig. 1). Later, two subspecies: waageni waageni and waageni eowaageni were established from Nv. waageni species population based on materials obtained from Chaohu area (Zhao et al., 2008a, b, 2007, 2005a, 2004). Of these, the first appearance datum (FAD) of Nv. waageni eowaageni occurs beneath the FAD of Nv. waageni waageni. These two subspecies therefore were nominated to represent the Nv. waageni eowaageni and Nv. waageni waageni subzones within the Nv. waageni Zone (Zhao et al., 2008a, 2007, 2005a). The first occurrence of Nv. waageni eowaageni (also marking the base of the Nv. waageni Zone) was nominated as the marker defining the IOB (Zhao et al., 2008a, b, 2007, 2005a).
Besides the high-resolution conodont biostratigraphy (Zhao et al., 2008a, b, 2007, 2005a), carbon and oxygen isotopes (Liang et al., 2011; Zhao et al., 2008a; Zuo et al., 2006; Tong et al., 2005), magnetostratigraphy (Sun et al., 2009), and astronomical cyclostratigraphy (Li et al., 2016) have also been well studied for the Lower Triassic of the Chaohu Section. The Chaohu conodont zones correlate well with their counterparts reported from North America and other areas of the world, and these proxies provide a supplementary basis for defining the GSSP of the IOB in Chaohu (Zhao et al., 2008a, b, 2007, 2005a).
With respect to the spatial-temporal occurrence of Nv. waageni eowaageni, this subspecies was recognized from Bed 24-16, 24-20, 24-21, and 24-22, Bed 25-26, 25-27, 25-29, and 25-30, Bed 26, and Bed 27-1 in the West Pingdingshan site (Fig. 4). Later, this subspecies has been frequently reported from the IOB beds elsewhere in China. For instance, Zhao et al. (2013) recognized both Nv. waageni eowaageni and Nv. waageni waageni subzones from the IOB succession of the Daxiakou Section, western Hubei Province, South China. The IOB was placed to the base of Bed 86a in Daxiakou based on the first occurrence of Nv. waageni eowaageni, which is clearly beneath the ammonoid Flemingites-Euflemingites Zone (Zhao et al., 2013). Nv. waageni eowaageni has also been reported from the Jiarong Ⅲ Section of the southern Guizhou, southwestern China (Chen et al., 2015a), where this subspecies is below Nv. waageni waageni subzone, and its first occurrence, and the IOB, was calibrated to the horizon 3.6 m above the base of the section.
Both Nv. waageni eowaageni and Nv. waageni waageni subzones have also been established from the IOB succession of the Jianshi Section, western Hubei Province, South China (Lyu et al., 2018). In Jianshi, the IOB was placed at Bed 225+40 cm based on the first appearance of Nv. waageni eowaageni. More recently, additional specimens of Nv. waageni eowaageni have also been extracted from Bed 225 to Bed 232 in the same section, and they are illustrated here.
Outside South China, Nv. waageni eowaageni, together with Ns. posterolongatus, has also been reported from the IOB succession of the Mud Section, Spiti, India (Fig. 1). Orchard and Krystyn (2007) distinguished five morphotypes of Nv. waageni from the IOB succession of Mud Section. Of these, Morphotype 3 is identical to Nv. waageni eowaageni Zhao and Orchard, which is characterized by round basal cavity, and has a length to height ratio of about 1:1. Goudemand (2014) also illustrated some specimens similar to Nv. waageni group from Mud (Spiti, Himashal Pradesh, Northern India), and these specimens are re-illustrated here (Fig. 4).
In western Australia, Metcalfe et al. (2013) also reported Nv. waageni elements from the Senecio-1 core, Perth Basin. Therein the FA of Nv. waageni eowaageni occurs slightly before the FA of Nv. waageni waageni, and the IOB was placed at a depth of 2 719.25 m based on the FA of Nv. waageni eowaageni (Metcalfe et al., 2013).3 A TAXONOMIC RE-ASSESSMENT OF THE NOVISPATHODUS WAAGENI GROUP 3.1 Novispathodus Waageni and Its Differentiation from Other Species
The waageni species was first described by Sweet (1970) based on specimens collected from the Lower Triassic of Salt Range, Pakistan, and originally assigned to Neospathodus. This Pakistani species was later re-assigned to Novispathodus Orchard, 2005 by Goudemand and Orchard (in Goudemand et al., 2012).
Sweet (1970) defined the waageni species as bladelike skeletal elements, in which the height to length ratio is about 1:1 in all stages of growth, but the ratio of width to length (or height) changes from 1:3 in early stages to 1:2 in late stages. Denticulate margin is arcuate in lateral profile, with greatest height in posterior half of element. The basal margin is deflected conspicuously upward beneath posterior half of element. The holotype of Sweet's (1970) species is re-illustrated here (Fig. 4.1).
Since Sweet's (1970) proposal, a wide range of morphologically similar specimens has been assigned to the waageni species. In order to better understand the waageni species and its subspecies, a total of 63 new specimens (P1 element) of Nv. waageni group from the Chaohu and Jianshi sections, and several illustrated type specimens of the species are restudied here. Their length, height, area, angle of the upturning basal cavity margin, and denticle numbers are shown in Table 1 (also see Figs. 4–7). These biometrics enable the distinction between Nv. waageni and other species, and between various subspecies within Nv. ex gr. waageni. The latter usually possesses 7–12 denticles, and the angles of the upturning of the basal cavity margin vary mostly from 5° to 30° (Figs. 8–10).
It is true that the waageni species exhibits considerable variation, particularly in its denticulation, which has led to the recognition of several subspecies (Zhao et al., 2004, 2005a) and morphotypes (Orchard and Krystyn, 2007). Of these, Nv. waggeni posterolongatus has been upgraded to species rank based on its relative length and particularly its elongated basal cavity. A flange-bearing Morphotype 1 has also been re-assigned to Nv. latiformis Orchard and Zonneveld, 2009. After removing these species from the waageni species, Nv. waageni (sensu lato) is confined to those specimens with a P1 element that is a relatively equi-dimentional blade-like element, with a distinctive arcuate upper profile, and a posterior edge that bears smaller denticles descending towards the basal margin.3.2 Subspecies within Nv. ex gr. Waageni
Currently, three subspecies: Nv. waageni waageni (Sweet), Nv. w. eowaageni (Zhao and Orchard), and Nv. w. n. subsp. A Goudemand are proposed.
Measurements of a great number of specimens enable the distinction of Nv. w. eowaageni from Nv. w. waageni. The angle of the upturning basal cavity margin of Nv. w. eowaageni remains relatively small, < 20° (Figs. 8, 10), in contrast, the same angle in Nv. w. waageni is much greater, > 20°. There are 10–11 denticles in Nv. w. eowaageni, while 7–12 denticles (Figs. 8–9) occur in Nv. w. waageni. Moreover, Nv. w. eowaageni has the length/height ratio of 1.03–1.34 (with mean value of 1.21), and the angle of the upturning basal cavity of 3°–18°(with mean value of 11.6°). The same values of Nv. w. waageni are 1.06–1.48 (with mean value of 1.26), and 12°–34° (with mean value of 24°), respectively (Fig. 10).
A re-assessment of the Nv. w. eowaageni type material reveals both large (younger holotype, Zhao et al., 2005a, pl. 8, Fig. 4) and small (op. cit., pl. 8, Fig. 6) specimens with the essential attributes of that species, and a small specimen (op. cit., pl. 8, Fig. 7) that is identical in its length to height ratio but with a longer posterior edge with fewer small denticles. The latter specimen may represent the earliest growth stage in which the introduction of posterior denticles is incomplete, whereas the later stages display this feature regardless of their size. Size alone is not considered grounds for separating them, and all these specimens are retained in Nv. w. eowaageni. In recognition of the variation in posterior denticulation, the conodont element from Chaohu lacking stronger posterior denticulation
(Zhao et al., 2005a, pl. 8, Fig. 7; also Fig. 4.3) is differentiated from other forms of the subspecies. A very similar form is newly recognized from the specimen derived from Bed 225 of the Jianshi Section (Fig. 5.23). These primitive individuals therefore are assigned to Morphotype A of Nv. w. eowaageni (Figs. 4.3, 5.23).
The third subspecies, Nv. w. n. subsp. A, was introduced by Goudemand (2014, p. 49) who regarded it as synonymous with the oldest representatives of Nv. w. eowaageni reported from Chaohu by Zhao et al.(2008a, 2005a), but unlike the younger holotype of the subspecies. However, Goudemand (2014) did not describe these specimens, instead, illustrated a few specimens (Goudemand, 2014, Figs. 1A–1D) that are re-figured here (Figs. 4.8–4.10). Nv. w. n. subsp. A (Goudemand, 2014) should not be assigned to Nv. w. eowaageni and nor does it belong to the Nv. waageni group as described in taxonomic notes. Such an assignment is reinforced by diagrams of various parameters (i.e., length/height ratio, denticle number, and the upturning basal cavity angle) of these conodont specimens that show no relationship between Nv. w. n. subsp. A and Nv. w. eowaageni (Figs. 8–10).4 ONTOGENY OF NV. W. EOWAAGENI, EVOLUTIONARY LINEAGE, AND DEFINITION OF THE IOB
Abundant specimens allow recognition of ontogenic variations of the waageni species that occur around the IOB. For instance, conodont specimens derived from Bed 230 (Figs. 5.9–5.20) and Bed 232 (Figs. 5.7–5.10), respectively of the Jianshi Section show that the size variation in Nv. w. eowaageni specimens is accompanied by little change in morphology, and implies that Nv. w. eowaageni is rather stable near the IOB.
In addition, conodont evolutionary lineage still remains unclear despite extensive efforts. This is because most studies on conodont lineage are based principally on the morphology of the P1 elements (Orchard, 2007b), and lack multielement data that are essential in revealing the true evolution of the clade. It should be noted that Goudemand (2014) illustrated a characteristic P2 element, and suggested that Novispathodus originated in the IOB interval. Orchard and Krystyn (2007) first proposed one possible evolutionary lineage: Ns. Pakistanensis-Ns. Posterolongatus-Ns. spitiensis, across the IOB. Later, Zhao et al. (2008a) also suggested two conodont evolutionary lines across the IOB: Neospathodus dieneri Morphotype 3-Nv. w. eowaageni-Ns. n. sp. R (=Nv. posteowaageni, in Zhao et al., 2008b) lineage, and Ns. dieneri Morphotype 3-Nv. w. waageni-Nv. pingdingshanensis lineage (Zhao et al., 2008a). Here, we propose a new evolutionary lineage based on morphologic variations of Nv. w. eowaageni and its allies from Bed 225 to Bed 232 in the Jianshi Section (Fig. 11).
In Jianshi Nv. w. eowaageni Morphotype A seems to be evolved from Ns. dieneri M 3 because both forms share similar features in both lateral and lower views (Fig. 12). Morphologic variations from Ns. dieneri M 3 to Nv. w. eowaageni is characterized by (1) changes from relative few and discrete denticles to more numerous and fused denticles; (2) descending denticles gradually developing in the posterior, which lead to an arcuate upper profile, with denticle height descending in both directions finally (Fig. 12.4); and (3) outline variations from oval basal cavity to subrounded to rounded basal cavity gradually. These morphologic variations suggest the Ns. dieneri Morphotype 3-Nv. w. eowaageni Morphotype A-Nv. w. eowaageni evolutionary lineage across the IOB. Within this lineage, we propose the first appearance datum of Nv. w. eowaageni to define the IOB.5 TAXONOMIC NOTES
Genus Novispathodus Orchard, 2005
Type species: Novispathodus abruptus (Orchard, 1995)
Neospathodus waageni waageni Sweet, 1970
1970 Neospathodus waageni Sweet, pp. 260–261, pl. 1, Figs. 11–12.
1976 Neospathodus waageni Sweet, Wang and Wang, pl. Ⅲ, Figs. 6–7.
1981 Neospathodus waageni Sweet, Chhabra and Sahni, pl. 1, Figs. 9–10, 14, 16, 20.
2005a Neospathodus waageni Sweet, Zhao et al., pl. 9, Figs. 1–8.
2007 Neospathodus waageni waageni Sweet, Zhao et al., pp. 36–37, pl. 1, Fig. 10.
2007 Neospathodus waageni Sweet, Orchard and Krystyn, pl. 1, Figs. 12–16.
2009 Novispathodus waageni (Sweet), Orchard and Zonneveld, Figs. 13.1–13.10, 14, 15.
2013 Novispathodus waageni waageni (Sweet), Zhao et al., Figs. 9C, 10D–H, 10J–K, 11F, 11P–R).
2016 Novispathodus waageni waageni (Sweet), Liang et al., Fig. 4.8.
2018 Novispathodus waageni waageni (Sweet), Lyu et al., Figs. 7.9, S3.11, S4.1–S3.3.
Remark: The subspecies Novispathodus w. waageni is based upon the holotype of Nv. waageni described by Sweet (1970). The new collections of this (sub)species from Chaohu, Jianshi, Mud, and other sections fit well with those described by Sweet (1970) although there is variation in size, numbers of denticles, and the angles of the upturning of the basal cavity margin (Table 1). Besides the biometrics, Nv. w. waageni is also distinguishable from Nv. w. eowaageni in having a mostly thicker blade, sometimes with medial thickening and fewer (broader) denticles per unit length, highest denticles closer to posterior, generally recurved denticles, not straight and upright, highest denticles closer to posterior, common differentiation of a posterior cusp, and more sinuous basal profile, with increased posterior upturning, which is one of the most important criteria. Many mature elements of this (sub)species bear a distinct lateral rib (revised after Zhao et al., 2005a).
Novispathodus waageni eowaageni (Zhao and Orchard, 2005a)
2005a Neospathodus waageni eowaageni Zhao and Orchard (nov. subsp.), pl. 8, Figs. 4–7.
2007 Neospathodus waageni eowaageni Zhao and Orchard, Orchard and Krystyn, pl. 1, Fig. 12.
2013 Novispathodus waageni eowaageni (Zhao and Orchard), Zhao et al., 2013, Fig. 10B.
2013 Neospathodus waageni eowaageni Zhao and Orchard, Metcalfe et al., p, 1142, Fig. 7.1–7.2.
Diagnosis: The P1 element is characterized by blade-like elements with an arcuate upper profile and fan-shaped denticles (in mature elements); the length/height ratio ranges from 1.0–1.4 with about 10–12 denticles fused for most of their length. Denticles are blunt or pointed. In lateral view, the basal margin is relatively straight throughout (with an angle of the upturning of the basal cavity margin less than 18°). The greatest height is located in the middle part. The lower surface has a broadly expanded oval to subround basal cavity around a moderate pit (revised after Zhao et al., 2005a).
Remark: Measurements of a great number of specimens enable the distinction of Nv. w. eowaageni from Nv. w. waageni. Besides, limited illustrated material of Nv. w. n. subsp. A (Goudemand, Fig. 1A, 1C, 1D) shows a P1 element that is relatively long, with a relatively straight upper profile, and lacking progressively lower denticles on its posterior edge. These features are not seen in the holotype or in any of other Chaohu specimens assigned to Nv. w. eowaageni. In fact, these characteristics of the Goudemand (2014) material also do not agree with any of the undifferentiated species of Nv. waageni.
Novispathodus waageni eowaageni Morphotype A
2005a Novispathodus waageni eowaageni Zhao and Orchard, in Zhao and Orchard, pl. 8, Fig. 7.
Diagnosis: The P1 element is characterized by blade-like elements with 9–11 pointed denticles of which the highest one is post-median in position; the length/height ratio ranges from 1.1–1.2, in lateral view, the basal margin is almost straight with only a slight upturning (the angle of the upturning basal cavity ~10°).
Remark: This morphotype is lacking stronger posterior denticulation, bearing relatively fine and pointed denticles with almost straight basal margin, and smaller upturning angle of the basal cavity, which are differentiated from other forms of Novispathodus waageni eowaageni.6 CONCLUSION
A taxonomic re-assessment on Nv. waageni group proposes that Nv. ex gr. waageni be defined by the following diagnostic features: (1) an approximately equi-dimentional P1 blade element, (2) an arcuate upper profile with denticle height descending in both directions, (3) a denticulated posterior edge (lower denticles posterior of the highest denticle), and (4) a round basal cavity outline. Concerning the two subspecies, Nv. w. waageni differs clearly from Nv. w. eowaageni in having (1) a slightly higher length/height ratio (holotype=1.30:1.23), (2) a thicker blade, sometimes with medial thickening, (3) fewer (broader) denticles per unit length, (4) generally recurved denticles, not straight and upright, (5) highest denticles closer to posterior, (6) common differentiation of a posterior cusp, and (7) more sinuous basal profile, with increased posterior upturning. Limited published information of Nv. waageni n. subsp. A of Goudemand suggests that this new subspecies is representative of neither early Nv. w. eowaageni, nor of the Nv. waageni group. Abundant specimens of Nv. w. eowaageni from higher horizons in the studied sections demonstrate a clear ontogenic process of this subspecies, indicating that Nv. w. eowaageni is rather stable. Moreover, the new materials of small, early individuals (from Bed 225 in Jianshi), referred to as Nv. w. eowaageni Morphotype A, likely evolved from Ns. dieneri Morphotype 3, and leads to typical mature elements of Nv. w. eowaageni more common in higher horizons. The first appearance datum of Nv. w. eowaageni therefore is an ideal mark defining the IOB.ACKNOWLEDGMENTS
This study was supported by four NSFC grants (Nos. 41473006, 41572091, 41673011, 41772007), and one grant from the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan). This is a contribution to the IGCP 630: Permian-Triassic climatic and environmental extremes and biotic responses. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0795-3.
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