Hangenberg Black Shale with cymaclymeniid ammonoids in the terminal Devonian of South China
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The Hangenberg Crisis at the end of the Devonian is marked by a sudden global mass extinction (main Hangenberg Event), which was especially severe for ammonoids. Among the order Clymeniida, only the cymaclymeniids survived for a short time. We report the first discovery of Postclymenia cf. evoluta in South China in equivalents of the Hangenberg Black Shale (the regional Changshun Shale) at the Jiarantang section in Guizhou. The South China plate was far away and completely different from the Euramerica continent, where the Hangenberg Event/Crisis was first recognised. The presence of similar ammonoids as in contemporaneous beds of the Rhenish Massif, Germany, suggests close faunal relationship through the Palaeotethys Ocean. It agrees with a sudden spread of opportunistic extinction survivors with the initial Hangenberg Transgression. The regional facies and faunal succession at Jiarantang confirms previous concepts of a eustatically driven, significant transgressive-regressive couplet in the lower/middle crisis interval. The near-global distribution of cymaclymeniid survivors shows that their extinction at the end of the extended crisis interval must have been caused by a so far neglected, small-scale global extinction event in the open marine realm.
KeywordsHangenberg Crisis Changshun Shale Ammonoids Guizhou Famennian
The end-Devonian bio-crisis, which has often been referred to as the Hangenberg Event, represents one of the major Phanerozoic global extinction intervals (e.g. Walliser 1984; Caplan and Bustin 1999). It shows extinction patterns and magnitudes as in the scale of the well-known “Big Five” (Kaiser et al. 2016). During the polyphase Hangenberg Biocrisis (see Kaiser et al. 2011), ammonoids suffered severe extinctions, especially during its initial and main phase (e.g. Price and House 1984; Becker 1993a, b, 1996; Korn 1993, 2000; House 1993). A species-level extinction rate of 87% was noted at the base of the Hangenberg Black Shale and its global equivalents (Kaiser et al. 2016) and no species at all is known to have survived through all of the extended biocrisis time (Becker et al. 2016a). Nevertheless, three lineages of goniatites (Mimimitoceras s.l., Sporadoceras, one tornoceratid genus) and two cymaclymeniid genera, Postclymenia and Cymaclymenia, are currently known to have survived the initial, anoxic Hangenberg Event (e.g. Kaiser et al. 2016; Klug et al. 2016). For overviews of D/C boundary biozonations, extinctions, sea-level changes, glaciations, isotope excursions, and many other aspects of the global Hangenberg Crisis, the reader is referred to the recent reviews by Becker et al. (2016a), Kaiser et al. (2016), and Lakin et al. (2016).
Global record of Hangenberg Crisis survivor cymaclymeniids
Cymaclymeniids of the transgressive and often anoxic pulse of the Lower Crisis Interval (sensu Kaiser et al. 2016) have first been recorded from the Ardennes (Frech 1897; Delepine 1929; Austin et al. 1970) and then from many localities of the northern Rhenish Massif of Germany, such as Riescheid near Wuppertal-Barmen (Paeckelmann 1922), Wuppertal-Elberfeld (Mirker Hain and Am Haken Brickwork Quarry, Paeckelmann 1922; Schmidt 1924; Schindewolf 1926), Oberrödinghausen (Schindewolf 1937; Paproth and Streel 1970; Luppold et al. 1994; Becker et al. 2016a), the Bilstein Cave near Warstein (Korn 1981), Apricke (Paproth and Streel 1982), Oese (Paproth and Streel 1982; Korn 1991; Luppold et al. 1994; Becker et al. 2016a), Hasselbachtal (Luppold et al. 1994), and Drewer (Becker et al. 2016b). These occurrences were reported under different names, from cf. Pseudoclymenia (in Schmidt 1922) to Postclymenia evoluta Schmidt, 1924, Cymaclymenia camerata Schindewolf, 1923a (in Schindewolf 1923b, 1923c, 1926) to Cyma. euryomphala Schindewolf, 1937 (see also Korn 1981), Striatoclymenia euryomphala (in Paul 1939), and Cyma. evoluta (in Korn 1988, 1991). At present, all these Rhenish occurrences are thought to belong to just one crisis opportunist, Postclymenia evoluta. Unfortunately, the poor preservation prevents a detailed morphometric analysis (Price and Korn 1989). It has also been widely overlooked that Paeckelmann (1922, p. 284) emphasised the presence of two Postclymenia species in the Am Haken Brickwork Quarry and that Schindewolf (1926, p. 97) re-assigned one of these, the more involute form, to Cyma. striata. Therefore, the survival of two cymaclymeniid groups was postulated very early in research history but not really documented.
The extinction of most ammonoid groups at the base of the Hangenberg Black Shale defines the Cymaclymenia (now Postclymenia) evoluta Zone sensu Becker (1988) or Postclymenia Genozone (Upper Devonian = UD VI-E, Becker 1993a; Becker et al. 2016a). However, the type-level of Postclymenia evoluta Schmidt, 1924, lies at its Drewer type-locality below, in the slightly older Drewer Sandstone, not in the Hangenberg Black Shale (Schmidt 1922; Schindewolf 1923b, 1923c; Korn 1981, 1988, 1991; Price and Korn 1989). The Drewer Sandstone falls in the higher part of the pre-event Wocklumeria Genozone (UD VI-D), just below the entry of Epiwocklumeria applanata (Wedekind, 1918), which defines in the Rhenish Massif and Poland an upper or applanata Subzone (Becker and House 2000; UD VI-D2).
Postclymenia occurs also widely (in more than a dozen localities) in the more neritic uppermost Famennian of the northwestern Rhenish Massif, in crinoidal limestone and grey or greenish marls at the top of the Velbert Formation (“Etroeungt”; Schmidt 1924; Bärtling and Paeckelmann 1928; Paul 1938, 1939; Conil and Paproth 1968; Price and Korn 1989; Brauckmann 1990). The correlation of these faunas with the more eastern occurrences is not straight forward. Since “Striatoclymenia euryomphala” has been recorded from several levels in the Ratingen-Velbert region, and partly in association with a very rich and fully oxygenated benthic assemblage (e.g. Paul 1939), it is likely that there are regional Postclymenia-bearing equivalents of both the Drewer Sandstone (upper UD VI-D) and Hangenberg Black Shale (UD VI-E) levels. There is evidence for the presence of forms with different umbilical width at different localities (e.g. Paul 1938: pl. 4, fig. 2 versus Paul 1939: pl. 40, fig. 3). Unfortunately, most of the old outcrops have disappeared and cannot be re-sampled.
The Rhenish Hangenberg Black Shale faunal level (UD VI-E) with survivor cymaclymeniids has subsequently been traced in a range of other countries but not yet in South China. There are records from the uppermost Cleveland Shale (0.6–1.5 m below its top) of Ohio (House et al. 1986: localities 3-5), Thuringia (Bartzsch et al. 1998: Breternitz East), the Montagne Noire (southern France, Kaiser et al. 2009: p. 113), the Maider Basin of southern Morocco (Klug et al. 2016), and recently from the Urals (based on collections kindly shown by Y. Gatovsky in 2016). According to supposedly associated conodonts and miospores (Xu et al. 1990), a corresponding richer fauna occurs in the distant, palaeogeographically isolated Junggar Basin of Xinjiang (Zong et al. 2015: Cymaclymenia-“Mimimitoceras” Assemblage from Emuha, Unit 2). This fauna and its precise age require further research.
A younger Postclymenia species, Post. nigra (Korn, 1991) (see discussion below for the inclusion of this species in Postclymenia), occurs at Drewer together with Post. evoluta above the Hangenberg Black Shale and Hangenberg Sandstone equivalent (Korn 1991). Both were collected as three-dimensionally preserved specimens from black nodules in the main Acutimitoceras (Stockumites) Genozone (UD VI-F). They were associated with early protognathodid conodonts (Pr. meischneri and Pr. collinsoni in Bed 97; Pr. kockeli in Beds 99/100; Korn et al. 1994), and, therefore, correlate partly with the Lower Stockum Limestone sensu Becker et al. (2016a, p. 359). Youngest (UD VI-F) cymaclymeniids are also known from other Rhenish units and localities, such as the main oolites of the Seiler Conglomerate near Iserlohn (Gallwitz 1928, p. 498), Upper Stockum Limestone of Müssenberg sensu Becker et al. (2016a, p. 361) (Korn 1989: Cyma. striata (Münster, 1832)), and the greenish-grey Hangenberg Shale of Oese (Becker et al. 2016a: Postclymenia sp.). They correlate partly with occurrences in Lower Stockum Limestone equivalents of southern Morocco (Lalla Mimouna South, kockeli Zone, Post. evoluta in Korn et al. 2004, 2007; subsequently re-named as Post. calceola Korn in Klein and Korn, 2014). Cymaclymeniids were also mentioned (but not described) together with acutimitoceratids from the oncolitic marker unit within the Leatham Member (Pilot Shale) of Utah (Feist and Petersen 1995: Cymaclymenia sp.). Elsewhere in the Great Basin, this unit lies above the “Conchostracan Shale”, a clear Hangenberg Black Shale equivalent with mass occurrences of the bivalve Guerichia (field observations by RTB, see Becker et al. 2016a). Cymaclymeniids from dark-grey, laminated shales of the uppermost Famennian Tercenas Formation of South Portugal (Oliveira et al. 1986) are probably not from the Hangenberg Crisis Interval but older, since a second locality of that unit yielded pre-extinction clymeniid genera, such as Linguaclymenia and Cyrtoclymenia (Korn, 1997).
Our new record from South China widely increases the geographic record of cymaclymeniid ammonoid survivors during the Hangenberg Crisis. It underlines a rapid spread triggered by the short-term, eustatic Hangenberg Transgression (e.g. Becker 1993b; Bless et al. 1993; Kaiser et al. 2016), which was previously evident from North African and North American records, where postclymeniids do not occur in immediate pre-crisis beds. The near-global, low to middle palaeolatitude distribution of Postclymenia in the terminal Devonian makes its subsequent sudden disappearance at the end of the extended Hangenberg Crisis Interval, when the other ammonoid survivor group, the Prionoceratidae (Goniatitida) re-diversified and flourished, enigmatic. The Cymaclymeniidae and Prionoceratidae co-existed since early in the upper Famennian (UD IV-B) and without evidence of general differences in their distribution and abundance, or for direct competition between both groups. This emphasises our current poor understanding of survivor extinctions at the end of first-order global biocrises.
The Jiarantang Devonian–Carboniferous boundary interval
The Jiarantang section lies approx. 100 km southeast of the well-known Muhua area (Fig. 1). It was located in the transition zone between the semi-restricted platform and the intraplatform basin facies. Feng (1977) first reported the section, on the basis of a previous geological survey, and with a description of the first Syringothyris brachiopod fauna discovered in South China.
Nomenclatural note: The Changshun Shale of Wang and Yin (1984), which is a marl and shale layer in the Muhua area, corresponds to the lower part of the Gedongguan Bed of Hou et al. (1985). The upper part of the Gedongguan Bed of Hou et al. (1985) is composed of marl/shale and lenses of limestones. The use of the term Gedongguan Bed is restricted here to the upper part of its original definition.
Two conodont samples were collected. Sample XSJ-1, from a level 0.1 m above the top of the Gedongguan Bed, includes the conodonts Bispathodus aculeatus aculeatus, Protognathodus kockeli, Neopolygnathus communis communis, and others. Protognathodus kockeli is a post-Hangenberg Event (Black Shale) species and has a stratigraphic range from the Pr. kockeli Zone (= Upper Si. praesulcata Zone; Kaiser et al. 2009) up to the lower part of the Lower Si. crenulata Zone (Corradini et al. 2017); the two Pr. kockeli specimens obtained from this sample are both morphologically less advanced than the holotype. One specimen possesses four nodes in a row on one side and two nodes on the other side that are not restricted to one strict row (Fig. 2(e)). In light of the underlying (Devonian) Postclymenia-bearing Changshun Shale, this sample is probably within a range from the Pr. kockeli Zone to the Si. sulcata Zone.
Sample XSJ 15-1, from a level ca. 0.75 m above the top of the Gedongguan Bed, includes an advanced Siphonodella praesulcata with a platform curvature of ca. 13.5° that is strongly transitional towards Si. sulcata (Fig. 2(d-2), see morphometrics in Flajs and Feist 1988), Protognathodus meischneri (Fig. 2(d-1)), Bispathodus aculeatus aculeatus, “Pandorinellina” n. sp. (sensu Hartenfels, 2011), and Neopolygnathus communis communis. This assemblage is not reliably age diagnostic in the Devonian–Carboniferous boundary interval (Si. praesulcata to Si. sulcata zones). The spathognathodids and polygnathid crossed the Devonian–Carboniferous boundary. However, Pr. meischneri occurs in South China mostly in the Pr. kockeli Zone, e.g. in the Gedongguan and Limushan sections (Hou et al. 1985: associated with the first occurrence of Pr. kockeli in the former), in the Lali section (Ji and Ziegler 1993: in the upper part of the 2 m thick Tangkou Mbr right below the first occurrence of Si. sulcata), in the Nanbiancun II, III, and IV sections (Yu 1988: associated with or above the first occurrence of Pr. kockeli, whereas Ji et al. (1989) assigned the same interval to the basal Carboniferous), or in the basal Carboniferous, e.g. in the Huangmao section (Bai et al. 1994, fig. 6-2: 0.2 m above the D-C boundary). The species was only rarely found below the Hangenberg Black Shale equivalent, e.g. in the Muhua II (Hou et al. 1985) and Dapoushang (= Daposhang) sections (Ji et al. 1989). Considering the general occurrences of Pr. meischneri in South China and the morphological nature of Si. praesulcata, this sample may have a similar time range to that of sample XSJ-1.
Overall, the above fossil data support the ammonoid evidence that the Jiarantang succession represents a Devonian–Carboniferous boundary sequence and that the Changshun Shale in this section is an equivalent of the Hangenberg Black Shale of Europe.
Macrofauna of the Changshun Shale in the Jiarantang section
Postclymenia Schmidt, 1924 (emend.)
Discussion: Giving credit to H. Schmidt, the genus Postclymenia was first mentioned and briefly explained in a footnote by Paeckelmann (1922, p. 284). However, in the absence of a type-species designation, the name remained a nomen nudum. The genus was then formally erected by Schmidt (1924). He assumed that the species name Clymenia evoluta, which had only appeared without any explanation in a footnote in Frech (1897, p. 179), was valid. This is not the case (Price and Korn 1989) and, therefore, Schmidt became the author of Postclymenia evoluta. The assumed distinction between Postclymenia and Cymaclymenia by Schmidt (1924) was rejected by Schindewolf (1926) based on the analysis of a topotype collection. Therefore, the genus was abandoned for many subsequent decades. Eventually Postclymenia was resurrected by Korn and Klug (2002), who briefly justified this by its supposedly much more asymmetric flank lobe (see also Korn et al. 2004). However, this feature is only distinctive in the Moroccan Post. calceola, while the flank lobe in the Rhenish type-species does not differ from that in many Cymaclymenia species. This is clearly visible in the suture illustrations of Schmidt (1924: pl. 8, fig. 21), Schindewolf (1926: fig. 2b), Korn (1981: fig. 21), Price and Korn (1989: fig. 12), Korn et al. (1994: fig. 16A), and Becker et al. (2016a: fig. 5.2). Therefore, Zong et al. (2015) drew attention to differences in the growth ornament between Postclymenia and Cymaclymenia. The first is here re-defined by concavo-convex growth lines with a lateral sinus that extends in full curvature (without interruption by a low salient or straight/radial interval) right to the umbilical seam. In median-sized to mature Cymaclymenia (= Striatoclymenia and Miroclymenia) there is an incipient to well-developed growth line salient on the lower flanks (see illustration of holotype of the type-species, Cyma. striata, in Korn 1981: fig. 11a, and the ornament in many other forms in Korn 1981, Nikolaeva and Bogoslovskiy 2005, and Klein and Korn 2014). In this revised sense, Postclymenia includes also Cyma. camerata (= Cyma. warsteinensis Korn in Clausen et al. 1979), Cyma. curvicosta Lange, 1929, Cyma. nigra Korn, 1991, and Post. calceola. The genus originated long before the Hangenberg Biocrisis in the upper Famennian (Dasbergian of German nomenclature, UD V; Schindewolf 1923c). It probably gave rise to some Russian and Chinese genera. These share the same course of growth ornament but differ in the presence of an additional median ventral lobe (Laganoclymenia, Kazakhoclymenia), ventral band (Laminoclymenia), or marked ventrolateral furrows (Auritoclymenia). Procymaclymenia differs from Postclymenia in its simpler, not pointed flank lobe and fully biconvex, Cymaclymenia-type ornament. Rodachia is characterised by the reduction of the ventral sinus of growth ornament.
Stratigraphic range: Upper Devonian (UD) V-B to VI-F (zonal key after Becker and House 2000).
Postclymenia cf. evoluta Schmidt, 1924
Description: Three Jiarantang specimens show sutures, but each with a somewhat different morphology. The dorsolateral saddle is short and the ventral one is very broad in PUM 15031 (Fig. 4(g, h, m)), while the dorsolateral saddle has a longer, straight (not curved) inner prong in PUM 15027 (Fig. 4(b, c)), where the ventral saddle is not as wide. This seems to be related to different compaction during flattening and may explain why the sutures look rather different in PUM 15035 (Fig. 3(c)), variably with or with only an incipient dorsolateral saddle, creating sutures resembling Genuclymenia, the middle Famennian ancestor of cymaclymeniids. PUM 15035 was probably partially corroded on the seafloor before it was buried and flattened. Based on PUM 15031 (Fig. 4(h)), the adventitious flank lobe is only moderately asymmetric. Fig. 3(b) shows the coarse wrinkle layer in PUM 15034 as it is known from some Moroccan postclymeniids (see Klug et al. 2016: fig. 4h). PUM 15034 (Fig. 3(a)) and PUM 15033 (Fig. 4(j–l)) display the concavo-convex growth ornament (but without distinctive lirae). The umbilical width/diameter (uw/dm) ratio varies at 10–15 mm dm (PUM 15033 and PUM 15031) between 0.34 and 0.37 and decreases to 0.30 (PUM 15029) to 0.33 (PUM 15030) between 20 and 25 mm dm. The estimated whorl expansion rate (WER) fluctuates between 1.9 and 2.0, based on PUM 15029 and PUM 15030.
Discussion: Based on the poorly preserved type population from the Drewer Sandstone, Post. evoluta is characterised by a uw/dm ratio of 0.33 to 0.35 between 20 and 40 mm dm and WER of ca. 1.9. This is confirmed by better preserved material from the overlying prorsum Zone (Korn 1991), which additionally give a whorl width/whorl height (ww/wh) ratio of ca. 0.70 at ca. 20 mm dm. Specimens from the Hangenberg Black Shale of Oese and Oberrödinghausen partly show lower uw/dm ratios of only ca. 0.30 to 0.32 between 10 and 35 mm dm (e.g. Paproth and Streel 1970: pl. 24, fig. 4; Becker et al. 2016a: figs. 5.2 and 5.3; additional unpublished specimens), which may reflect their strong flattening and distortion. But it is best to identify such forms as Post. cf. evoluta. As emphasised by Korn (1991), all Sauerland specimens are characterised by very weak growth ornament.
Postclymenia nigra differs from Post. evoluta in its wider whorls (ww/wh ratio near 0.90 at ca. 20 mm dm), steep umbilical wall, and decreasing uw/dm ratios between 20 and 30 mm dm (from near 0.40 to 0.33; see measurements in Korn 1991). It has also smooth whorls without growth lirae. The lectotype of Post. camerata (here designated as the original of Schindewolf 1923a: pl. 17, fig. 6) is slightly more evolute than Post. evoluta (uw/dm = ca. 0.37 at 26 mm dm) and its whorls are somewhat higher (WER = ca. 2.2). In addition there are dense and regular growth lirae, while its ww/wh ratio (ca. 0.69) is not distinctive. The holotype of Cyma. warsteinensis, which also shows the characteristic lirae, is very similar to the Post. camerata lectotype with respect to uw/dm (ca. 0.35), ww/wh (0.69), and WER values (ca. 2.3). Therefore, the species is regarded as a subjective senior synonym (see also Korn and Klug 2002). Morphometric analyses will have to clarify whether other specimens that have been assigned to Cyma. camerata really belong to that species. This applies to Moroccan representatives (Becker et al. 2002; Post. cf. camerata of Becker et al. 2018a, 2018b) or additional material from Franconia (Price 1982). A cross-section figured by Korn (1981) from Reigern Quarry in the Rhenish Massif is too involute (uw/dm = 0.32 at ca. the same size as the camerata lectotype) and has too wide whorls (ww/wh = 0.87). It may present a related but thicker-whorled species with similarly high whorls (WER = 2.15) as in Post. camerata. Specimens from the pre-Hangenberg limestones of Nanbiancun (Ruan 1988), identified as Cyma. warsteinensis, have somewhat intermediate uw/dm (0.33 to 0.34) and ww/wh values (0.80 at ca. 25 mm dm) and show no evidence of regular growth lirae. This suggests relationships with Post. evoluta but the most complete specimen (Ruan 1988: pl. 65, fig. 4), may have a higher WER resembling Post. camerata. The presence of a possibly additional taxon in the more neritic facies of Guangxi has to be tested based on additional material and cross-sections. In any case, alleged Cyma. camerata from Russia (Nikolaeva and Bogoslovskiy 2005) are too involute for that species (uw/dm = 0.30 to 0.31) and they display a Cymaclymenia-type ornament that is straight on the lower flanks.
Postclymenia curvicosta is characterised by umbilical ribbing, the large-sized Post. calceola by its strongly asymmetric flank lobe. Mature individuals of the latter (> 40 mm dm) show a slight increase of WER (towards 2.1) and decrease of uw/dm ratios towards 0.30 to 0.35 (Klein and Korn 2014).
With respect to the lack of distinctive growth lirae and the shape of the flank lobe, the Jiarantang specimens are closest to the Rhenish Post. evoluta. The uw/dm ratio of only 0.30 to 0.33 at 22–25 mm dm and WER values agree with the flattened, contemporaneous Hangenberg Black Shale specimens of the Sauerland that should be called Post. cf. evoluta. Another match is the unusually short dorsolateral saddle (compare Becker et al. 2016a: fig. 5.2) but this may be related to similar distortion during the flattening.
The probably contemporaneous but poorly preserved cymaclymeniids from the top of the Cleveland Shale of Ohio are also similar. The most complete specimen (House et al. 1986, fig.4.14) displays Postclymenia-type concavo-convex growth ornament without lirae and an uw/dm ratio of ca. 0.31. However, the whorls seem to be slightly higher, with an estimated WER of ca. 2.2.
Specimens from the more neritic facies of the northwestern Rhenish Massif are partly very similar to Post. cf. evoluta (e.g. Paul 1939: pl. 40, fig. 3 and pl. 41, fig. 3). However, Paul (1938: p. 4, fig. 2) illustrated as Cyma. euryomphala a large form (without sutures) that has more than 70 mm dm and a wider umbilicus (uw/dm ca. 0.40) than any other illustrated postclymeniid. Such specimens indicate that our knowledge of the group is still incomplete.
Hangenberg Black Shale equivalents in South China
The presumed Hangenberg Black Shale equivalent is also present in the cherty basin facies of Guangxi, e.g. in the Shijia (= Bancheng) section (Bai et al. 1994, p. 54; Fig. 1). The D–C boundary is composed of Devonian Liujiang Fm. and Carboniferous Shijia Fm. (Fig. 2(f)). However, the whole Upper Devonian through Lower Carboniferous strata in the Shijia section was assigned to the Shijia Fm. (Yin 1997, p. 113) due to their uniform lithology (bedded cherts). The uppermost Liujiang Fm. consists of medium-bedded cherts. The Changshun Shale is a siliceous black shale unit of 25 cm thickness, yielding the bivalve “Posidonia” (now Guerichia; Bai et al., 1994). The “Gedongguan Bed” is composed of (dark) grey, thin-bedded silty cherts or siliceous siltstones intercalated with medium-bedded siliceous rock (Fig. 2(f)). The basal part of the Shijia Fm. consists of thin- (to medium-) bedded cherts.
Wang and Ji (2013) described a very thick succession of the Changshun Shale (in a wide sense) from the Huohua section in Ziyun County, Guizhou. A thin (20 cm) basal black shale with “Posidonia” (= Guerichia) is overlain by ca. 20 meters of siliceous shales and thin limestone bands. According to an anonymous reviewer, the limestone bands are restricted to the lowermost part of this 20-meter unit; the 20 m “shale” is in fact mainly composed of thin-bedded cherts, intercalating with very thin (1–2 cm) black shale, which is overlain by ca. 60 m of calcareous siltstones and calcarenites of the early–middle Tournaisian. In light of the above data, we can correlate the basal part of the 20 m thick unit with the Gedongguan Bed.
In the shallow water mixed carbonate and siliciclastic facies, such as the Malanbian section, Hunan (Fig. 5), the Hangenberg Crisis Interval may be present in the uppermost Menggong’ao Fm., a coarsening-upwards sequence of ca. 3.2 m, although its precise age cannot be determined due to the lack of index conodonts. Benthic fossils may be used as auxiliary markers; the brachiopod Cyrtospirifer (as well as related forms) most probably became extinct at the base of Si. praesulcata–Pr. kockeli Interregnum sensu Kaiser et al. (2009), e.g. in the Nanbiancun section (except for a single specimen in the Pr. kockeli = Upper Si. praesulcata Zone, see Yu 1988 and also Ji et al. 1989, which is here interpreted to be a reworked specimen) and the Xiakou section (Wang et al. 1987). The rugose coral Cystophrentis is commonly associated with cyrtospiriferid brachiopods in the uppermost Devonian in South China (e.g. in the Dushan area of Guizhou). Therefore, the disappearance of both Cyrtospirifer and Cystophrentis at the approximate same level is suggestive of the main Hangenberg Extinction level at the top of the praesulcata Zone (sensu Kaiser et al. 2009). The spore data give an inconsistent or contradictory result. Tan et al. (1987) found a number of spores in Unit 36 (Fig. 5), including Verrucosisporites nitidus, the index species of the LN Zone, whereas Steemans et al. (1996) suggested the upper part of the LV Zone (below the (Lower) Si. praesulcata Zone) based on size measurements of Retispora lepidophyta in Unit 36. However, they indicate in the meantime that the D–C boundary lies within the same unit or at the top of Unit 36. In the shallow water carbonate facies, the Hangenberg Crisis Interval clastic deposits (such as shale and siltstones/sandstones) may be missing, for example in the Xiakou section (Wang and Yin 1985; Wang et al. 1987: the “Middle Si. praesulcata Zone limestone” is directly overlain by “Si. sulcata Zone limestone”) and the Sujiaping section (Tan et al. 1987; Bai et al. 1994: the Devonian Cystophrentis- and Quasiendothyra-bearing grey bio-intraclastic calcisiltitic limestone is directly overlain by Carboniferous grey bio-intraclastic calcisiltitic limestone bearing the foraminifer Vicinesphaera angulata-Bisphaera malevkensis Assemblage of Wang 1987).
Apart from South China, Hangenberg Black Shale equivalents are also well developed in Tibet (lower part of Zhangdong Formation, Tulong Section, Liu et al. 2018), which belonged in the Devonian and Carboniferous to a different terrain/microcontinent.
Sea level changes during the Hangenberg Crisis interval
There are different opinions regarding the sea level changes of the Hangenberg Crisis Interval, including a (dramatic) sea level fall (e.g. Bai et al. 1994; Sandberg et al. 2002; Ji 2004) or a dramatic initial sea level rise and subsequent major sea level fall (e.g. Bless et al. 1993; Kaiser et al. 2011, 2016). The latter interpretation is supported by palynofacies data, with a significant rarity or restriction of miospores in the Hangenberg Black Shale and a subsequent rapid increase of terrestrial input (e.g. Higgs and Streel 1994; Marynowski and Filipiak 2007). The transgressive nature of the Hangenberg Black Shale was questioned by Bábek et al. (2016), who, however, did not clearly separate the different siliciclastic intervals within the polyphase Hangenberg Crisis Interval and who interpreted the increasing detrital proxies in a too simple way as a sign of general regression. However, it is long known that in offshore, pelagic carbonate settings, the decrease of bottom turbulence with sea-level rise enables the settling of detrital clay and fine silt, resulting in the deposition of transgressive shales and marls.
The lithological succession in the Jiarantang section is consistent with the pattern of a distinct sea level rise and subsequent fall. According to GBG (1965, p.38), the uppermost Rongxian Fm. in the study area is composed of grey medium- to thick-bedded limestones intercalated by dolomitic and oolitic limestones, which apparently represents deposits of a platform margin setting, probably with episodic shallowing. The Changshun Shale, with its sudden influx of abundant pelagic ammonoids and reduced turbulence that enabled the settling of fine, organic-rich mud, undoubtedly suggests a rapid sea level rise corresponding to the transgressive pulse of the Hangenberg Black Shale (Kaiser et al. 2016). The overlying Gedongguan Bed, mostly bioclastic limestones, probably deposited during a shortly following sea level fall, corresponding to the regressive phase of the Hangenberg Shale and Sandstone (Becker et al. 2016a; Kaiser et al. 2016). The Hangenberg Crisis Interval in the Lali section and in the Muhua area have a nearly identical lithological succession and reflect a similar process of sea level changes.
In the shallow water mixed carbonate and siliciclastic facies, the Hangenberg Crisis Interval is represented by a coarsening-up clastic sequence that ends with medium- to thick-bedded sandstones, e.g. in the Xikuangshan (Coen et al. 1996; our own observation) and Malanbian sections. Evidence for a sea level rise of the initial Hangenberg Crisis Interval is generally not visible or less evident, because of the general lack of index fossils. More importantly, the often thin, transgressive Lower Hangenberg Crisis Interval (represented by shale or marl units) may be missing in shallow water carbonate facies due to subsequent erosion during the subsequent sea level lowstand, for example in the Xiakou section of Wang and Yin (1985) and Wang et al. (1987), and in the Sujiaping (= Lijiaping) section of Tan et al. (1987) and Bai et al. (1994).
The discovery of relatively abundant Postclymenia cf. evoluta in Hangenberg Black Shale equivalents (Changshun Shale) of South China (Guizhou) has significant implications for our understanding of ammonoid palaeobiogeography and evolution, of regional patterns of the Hangenberg Crisis, and for the reconstruction of D–C boundary eustatic developments.
1. Since South China belonged to a completely different continent far away from Euramerica (Laurussia) and NW Gondwana (Morocco), the new record documents a global Postclymenia distribution in low to middle palaeolatitudes of the Hangenberg Crisis Interval.
2. The strong similarity with contemporaneous German Post. cf. evoluta indicates an event-controlled, very fast west-east spread of this opportunistic disaster species within the Palaeotethys, whilst differences between the closer southern Laurussia and northern Gondwana areas were more significant, since these regions harboured related but different Postclymenia species.
3. The intercalation of the Changshun Shale with pelagic ammonoids at Jiarantang between shallow-water, organic poor neritic limestones proves its transgressive nature, in an area without high terrestrial input.
4. The disappearance of globally widespread survivor clymeniids suggests that a so far neglected small-scale global extinction occurred at the end of the extended Hangenberg Crisis Interval (Pr. kockeli Zone). Since the recovery from the main Hangenberg Extinction had only just started, this event could not produce high extinction rates and can only be recognised if studies are done at the finest available time resolution.
This paper benefited from various topical discussions with Hou Hongfei (Institute of Geology, CAGS) and Liao Weihua (Nanjing Institute of Geology and Paleontology). Yin Bao’an (Guangxi Regional Geological Survey) and Tan Zhengxiu (Hunan Regional Geological Survey) were helpful during our fieldwork. Sven Hartenfels (WWU Münster) commented on the conodonts. This manuscript benefited from comments by Dieter Korn (Museum für Naturkunde, Berlin) and two other anonymous reviewers. The support of the National Natural Science Foundation of China (Grant 41290260) is also acknowledged.
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Conflict of interest:
The authors declare that they have no conflict of interest.
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