Late Devonian–early Carboniferous ostracods (Crustacea) from South China: taxonomy, diversity and implications

The impact of the late Devonian Hangenberg Event on ostracods is quantified for the first time from newly acquired data from the Blue Snake section, Guizhou Province, South China. Ninety–eight species belonging to 31 genera are identified and figured. Four new species are described: Clavofabella? lanshella sp. nov., Sansabella gelaohensis sp. nov., Cytherellina caerulea sp. nov., Sulcella baisuzhena sp. nov. The ostracod associations from the Blue Snake section document a significant drop in specific diversity as well as major changes in taxonomic composition through the Hangenberg Event. We here report the reduction of the proportion of Palaeocopida and the increase in that of Podocopida between the latest Famennian and the Tournaisian. The specific extinction and renewal rates are estimated at 44% and 62%, respectively. The main factor of the post–crisis renewal of ostracod faunas in the Blue Snake section appears to be the progressive diversification of the family Bairdiidae and was probably related to palaeoenvironmental changes. The characteristics of associations point to an increase of the water depth over the studied area, from a near–shore shallow environment during the latest Famennian to a deeper and more open offshore environment during the Tournaisian.


Introduction
The late Devonian witnessed two particularly intense biotic events: the Kellwasser event close to the Frasnian-Famennian boundary (F-FB) and the Hangenberg Event (HE) close to the Devonian-Carboniferous boundary (D-CB). The Kellwasser event particularly affected marine ecosystems of the warm intertropical zone and caused the disappearance of 97% of shallow-water rugose corals, 60% to 70% of deeper species (Pedder 1982) and 75% of ostracods species (Lethiers & Casier 1999a). The Hangenberg Event, sometimes considered as severe as the Kellwasser event, impacted both marine and terrestrial ecosystems. It particularly impacted marine ecosystems and caused the disappearance of 85% of ammonoid genera (Becker 1993), about 30% to 50% of benthic neritic ostracod species (Kaiser et al. 2016 after data from Casier et al. 2004Casier et al. , 2005, about 60% (Holy Cross Mountains, estimated from data from Olempska 1997) to 66% (Thuringia, estimated from data from Blumenstengel 1993) of deeper species and about 50% of pelagic species (Walliser 1996). However, the impact of the Hangenberg Event on the Thuringian mega-assemblage is varying among the different locations in which it has been studied. For example, the specific extinction rate of these deep faunas is lesser in Puech de la Suque (Montagne Noire, France) with a value between 25% and 31.5 % (Casier et al. 2001) and in Avesnois (North of France) than in the Holy Cross Mountains and Thuringia (Casier et al. 2002). It also impacted terrestrial ecosystems such as the Archaeopteris forests (Kaiser et al. 2016) and Retispora lepidophyta flora (Streel et al. 2000).
Ostracods are bivalved micro-crustaceans, which are among the most diversified arthropod taxa in marine ecosystems and are present from the intertidal zone to the deepest zones of the oceans. In benthic marine ecosystems, they are known since the early Ordovician (e.g., Salas et al. 2007;Siveter 2008; cavusformis Assemblage Zone which is considered as equivalent to the standard upper S. praesulcata Zone and that has already been recognized in Central Hunan and Southern Guangdong (Ji 1987;Qin et al. 1988). The high δ 13 C values documented in Qie et al. (2015) for this part of the Tangbagou Formation support this correlation. Consequently, the D-CB is placed in the lowermost part of the Tangbagou Formation (between units B and C, Qie et al. 2015: fig. 2).
Miospores (Gao 1981) and foraminifers (Wang 1987) indicate a latest Devonian age (Liao 2003) for the Zhewang and Gelaohe formations. Gao (1981) placed the D-CB within the Gelaohe Formation on the basis of miospores assemblages but others rather placed it within the lower part of the Tangbagou Formation where a maximum flooding surface occurred (Wang & Wang 1996;Feng et al. 2010). Song & Gong (2019) studied the faunal variations of ostracod assemblages at the Devonian-Carboniferous transition in Baihupo section (= Blue Snake section, Dushan, Guizhou) and placed the D-CB at the base of the Tangbagou Formation (Song & Gong 2019: fig. 3).
Doubts remain as to the location of the different sections named Baihupo and their datations have not been recently revised. Consequently, we choose to place the D-CB in the lowermost part of the Tangbagou Formation at the top of the succession of thick beds of massive limestones. This part of the formation may correspond to the Unit B of the Qilinzhai section in Qie et al. (2015) and the position of the bed of yellowish shales at the top of the Gelaohe Formation must correspond to the Hangenberg Event which occurred in the middle S. praesulcata Zone.

Material and methods
For the present investigation, 88 samples (labelled 19BAI xxx) spanning the Devonian-Carboniferous boundary were collected: 30 from the Gelaohe Formation and 58 from the Tangbagou Formation (Fig. 3). Ostracods were extracted by hot acetolysis technique (Lethiers & Crasquin-Soleau 1988;Crasquin-Soleau et al. 2005). About 2150 specimens were pictured using the Hitashi FlexSEM1000 Scanning Electron Microscope and the Zeiss Axio Zoom V.16 microscope of the CR2P. 98 species belonging to 31 genera are identified and figured. Most of the specimens are relatively poorly preserved (recrystallized), particularly brittle and thus difficult to move once prepared for the SEM work: most of them are only shown in lateral view. Following Fürsich & Wendt (1977), Nützel & Kaim (2014) and Haussmann & Nützel (2015), the number of specimens of each species was counted by adding articulated carapaces and higher number of left or right valves.
Many species are kept in open nomenclature because of a poor preservation and/or availability of specimens that don't allow a precise attribution. Species left in open nomenclature or referred to already known species are only discussed when necessary but the complete taxonomic list is available (Table 1) and all species are figured to allow future comparison. In order to make descriptions as objective as possible, characters are quantified as much as possible and the length convention of carapaces/valves is as follows: < 0.40 mm: very small, 0.40-0.50 mm: small, 0.50-0.70 mm: medium, 0.70-1.00 mm large, >1.0: very large. The stratigraphic distribution of all species is available in Table 2.

Material repository
All type and figured specimens are temporarily in the collections of Sorbonne Université, Paris, France under catalogue numbers P6M 3837 to P6M 3975.

Remarks
This taxon is rare in the studied material and is represented by poorly preserved specimens that do not allow for a specific attribution. It is morphologically close to Phlyctiscapha? xiangzhouensis Wang, 1983 from the Lower Devonian of the Sipai Formation, Guangxi, South China (Wang 1983a). The specimen illustrated in Wang (1983a) lacks the small velate ridge parallel to the marginal ridge characteristic of the genus Phlyctiscapha. Its outline, the absence of velar structure or teeth along free margin and the carapace slightly compressed along free margin could rather pointed to the genus Fellerites Gründel, 1962. Fellerites sp. 1 differs from Phlyctiscapha? xiangzhouensis by its higher carapace, slightly stockier morphology and its smaller size (L = 1060-1360 µm, H = 660-920 µm, H/L = 0.62-0.76 for Phlyctiscapha? xiangzhouensis). 19BAI 67,19BAI 68,19BAI 74,19BAI 80, Gelaohe Formation, Blue Snake section, Famennian, late Devonian (this work).

Preliminary remark
The original diagnosis of Clavofabella by Martinsson (1955) is questionable because it does not include characters shared by all specimens: "Dolonate flanges do not meet posteriorly. Dolon that continues forwards in a velar ridge or a velar bend" (Martinsson 1955: 23). These structures are absent in non-dolonate specimens described or mentioned by Martinsson in the same paper in Clavofabella multidentata Martinsson, 1955, Clavofabella incurvata Martinsson, 1955 and Clavofabella reticristata (Jones, 1888). We follow the diagnosis of Clavofabella provided in Moore (1961) because it is based on characters shared by all forms: "inequivalved, RV overlapping LV; hinge consisting of median groove with corresponding ridge and lateral elongate pits and sockets; unisulcate […]; surface pitted or reticulate, marginal structure tuberculate" (Moore 1961: 174). Moore (1961) furthermore indicates that velar structures are developed as ridge but that in heteromorphs they are wider and form posterior flanges that do not meet.  Henningsmoen, 1953Suborder Beyrichiocopina Scott, 1951Superfamily Aparchitoidea Jones, 1901Family Aparchitidae Jones, 1901 Genus Aparchites Jones, 1889 Table 1 (continued). List and systematic placement of species occurring in the Blue Snake section.

Diagnosis
A new species questionably attributed to Clavofabella with elongate outline and laterally compressed spinose free margin at LV.

Description
Carapace large, preplete, ovoid/rectangular (adults) to inverted triangular (young juveniles) in lateral view, lenticular with laterally compressed anterior and posterior margins in dorsal view. Lmax slightly above mid-H, Hmax in front of mid-L, Wmax around mid-H and slightly in front of mid-L. RV overlaps LV all around, with maximum along free margin. Hinge line slightly sinuous with gentle concavity immediately posterior to ACA, slightly invaginated all along its length. Obtuse cardinal angles, ACA about 145°, PCA about 130°. Surface coarsely reticulate with round reticulae. LV with a spinose free margin that could indicate the presence of a velar structure. Pit relatively large (H about 32% of Hmax, L about 14% of Lmax), ovoid, elongated dorso-ventrally, located slightly in front of mid-L, slightly above mid-H, at Wmax.

Remarks
This species is doubtfully attributed to the genus Clavofabella Martinsson, 1955 described from the Late Wenlock (Silurian) of Gotland, Sweden (Martinsson 1955) because of RV overlapping LV, surface reticulated, sulcal pit and spinose free margin that could indicate the presence of a velar structure and that points to heteromorphs after Moore (1961). However, the tubercles in all known species belonging to Clavofabella are short and rounded and are absent at DB where the dolon is located in heteromorphs. In our material, these structures are spines rather than tubercles and are located along the entire free margin and are larger than those described from all known species belonging to this genus. Clavofabella? lanshella sp. nov. also differs from all other species of this genus by its laterally compressed morphology, more elongate lateral outline, larger sulcal pit, coarse reticulation and straighter DB. The H/L scatter plot of Clavofabella? lanshella sp. nov. (Fig. 4A) allows to discriminate four ontogenetic stages (A-3 to Ad).
The ontogenetic development of this species is marked by the elongation of the carapace, increasing compression of the free margin, enlargement of the pit and growth of spines along free margin. 19BAI 63,19BAI 67,19BAI 68,19BAI 74

Remarks
Our material is morphologically close to non-dolonate specimens of Clavofabella multidentata Martinsson, 1955 from the Late Wenlock, Silurian of Mulde, Parish Fröjel, Gotland, Sweden (Martinsson 1955) but differs by its more rounded cardinal angles, deeper sulcus and larger adductorial pit. It is also close to Libumella? sp. from the Famennian, late Devonian of the Yjid-Kamenka River section, Pechora Uplift, Russia (Sobolev 2020) but our material differs from this species by its more laterally compressed free margin, its deeper sulcal pit and its more developed lobation. This species may be new to science but the 16 specimens discovered are not sufficient to establish its generic attribution and to reasonably describe its diagnostic characters. Six different ontogenetic stages are present in our material (from A-5 to Ad). The ontogenetic changes through development cannot be precisely observed because of preservation but it seems that the outline does not show major changes, the sulcus apparently becomes deeper and the anterior lobe more developed. 19BAI 68,19BAI 69, Gelaohe Formation, Blue Snake section, Famennian, late Devonian (this work). Suborder Kloedenellocopina Scott, 1961Superfamily Kloedenelloidea Ulrich & Bassler, 1908 Family Beyrichiopsidae Henningsmoen, 1953 Genus Kloedenellitina Egorov, 1950 Type species

Remarks
Knoxiella cf. oblonga Wang, 1978in Song & Gong (2019  outline with AB and PB that are in the same plane. It is morphologically similar to Kloedenellitina sincera Tschigova, 1960 from our material and we consider that they are conspecific. The adult carapace in our material is bigger than the holotype (L = 795 µm, H = 422 µm in Tschigova 1960) but the H/L ratio is similar (H/L = 0.53). The other specimens from our material are smaller and may represent juveniles of this species. The specimen shown in Song & Gong (2019: fig. 4p) is larger than our material and the holotype (L = 970 µm, H = 530 µm) but the H/L ratio is also similar (H/L = 0.55). The posterior part is inflated and shows a vertical sulcus opened ventrally which suggests that it may be a female (Ellis & Messina 1964). All our specimens are less inflated in the posterior part so they may correspond to males. The size variations clearly indicate that several stages are present in the assemblages studied but it is impossible to discriminate them.

Remarks
In the genus Kloedenellitina, females have an inflated posterior carapace, which corresponds to a brood pouch (Ellis & Messina 1964). This character allows us to distinguish females ( Fig. 6B) from males ( Fig. 6C) in the studied material. The lateral surface of the specimens preserving their carapace appears as reticulate (Fig. 6C), it is rather smooth in steinkern but the reticulation remains visible in certain areas (Fig. 6B). Our material is smaller than the type material (L = 760-930 µm, H = 440-490 µm in Gurevitch 1972) with H/L between 0.53 and 0.58 while it is between 0.45 and 0.59 in our material (both valves taken together). The specimen shown in Tschigova (1977: pl. 61 fig. 13) is also larger with L = 1200 mm, H = 556 mm but the H/L ratio is similar (H/L = 0.46). In the present material, this species may mainly be represented by juvenile specimens corresponding to at least two different ontogenetic stages that we cannot discriminate because of the wide distribution of their dimensions.    (Tschigova 1977). Samples 19BAI 60,19BAI 74,19BAI 80, Gelaohe Formation, Blue Snake section, Famennian, late Devonian (this work).
Family Geisinidae Sohn, 1961 Preliminary remark This family has been revised by Adamczak (2006). Following these recommendations, the family name Knoxitidae Egorov, 1951 is here not adopted because "no generotype was explicitly designated" (Adamczak 2006: 277) and we chose to use Geisinidae Sohn, 1961.
Several genera belonging to this family have very similar morphologies (Adamczak 2006): Geisina Johnson, 1936, Hypotetragona Morey, 1935, Jonesina Ulrich & Bassler, 1908, and Knoxiella Egorov, 1950. This observation has led some authors to consider these genera as synonyms (e.g., Sohn in Moore 1961;Adamczak 2006;Jones 2011). Pollard (1966) considered that Geisina and Jonesina are both valid genera and indicated that Jonesina differed from Geisina by valves strongly and deeply bisulcate, S2 deep and ending by a pit, hinge not depressed in the posterior half and overlap absent along the hinge except at cardinal angles. Lethiers (1981) distinguished Knoxiella from Hypotetragona essentially by the invagination of the hinge and weak variations of the reticulation (Lethiers 1981: fig. 17). Here, we consider that these characters are not sufficient for generic discrimination, more specifically owing that the weak reticulation variations may be related to the preservation state. Thus, the morphologic differences observed by Lethiers (1981) could rather be differences at specific level.
Adamczak considered Knoxiella and Jonesina as "waste-basket taxon(s)" (Adamczak 2006: 299) and indicated that Hypotetragonais is preferred over Knoxiella in recent work. For Jonesina, it seems that "most of the species referred to this genus belong to Geisina and Hypotetragona" (Moore 1961: 413). However, Hypotetragona and Knoxiella are still being used (e.g., Casier 2017 for Knoxiella; Nazik et al. 2021 for Hypotetragona; Olempska 1999 used both genera) and an in-depth revision of all species is necessary to clarify their generic status.
As this revision is out of the scope of the present contribution and material: -We here consider Knoxiella and Hypotetragona as synonyms, Knoxiella being a junior synonym of Hypotetragona. We therefore attribute all the present species to Hypotetragona, for which we can fully appreciate the morphological characters; -We keep the genus name Knoxiella for the species that have already been attributed to this genus until they are thoroughly revised (Table 1).

Remarks
The adults of this species are represented by poorly preserved specimens that do not allow for a precise specific attribution. Indivisia minata Wei, 1988  At least 4 ontogenetic stages are present in our material. The ontogenetic development of this species is marked by the outline becoming more rectangular, the sulcus becoming deeper and lobes more developed.
Genus Knoxina Coryell & Rogatz, 1932 Preliminary remark Adamczak (2006) revised the family Geisinidae and considered that Knoxina Rogatz, 1932 andChesterella Croneis &Gutke, 1939 are synonyms, the latter being junior synonym of the former. Thus, both have typical Geisinidae outline, deep adductorial sulcus, prominent costa along ventral margin and one or two longitudinal costae. He also considered Knoxina as an "unclear [genus] to be investigated in details" (Adamczak 2006: 294-295). As this revision is out of the scope of the present contribution and material, we choose to keep the genus Knoxina in Geisinidae until its family position is thoroughly revised.

Remarks
The present species is rare in the studied material and is represented by poorly preserved specimens that do not allow for a specific attribution. It is attributed to the genus Knoxina because it has a sulcus opened dorsally just anterior to mid-L, a node in antero-dorsal part of the carapace and longitudinal ridges, which are diagnostic of the genus. It is morphologically close to Knoxina costata (Zaspelova, 1959) from the Frasnian of the Russian Platform by its rectangular outline, presence and location of anterior, ventral and dorsal ridges and node located in front of sulcus. However, the specimens found here have a more rounded morphology, a strong overlap all around free margin and are smaller (based on the dimension of the figured holotype of Knoxina costata shown in Zaspelova (1959: pl. 8 fig. 9): L = 700 µm, H = 400 µm and H/L around 0.57). The presence of pit and ornamentation cannot be verified because of the poorly preservation of our material. The specimen attributed to Knoxiella subreticulata Wang, 1983 shown in  (2019) is a steinkern which would explain these differences so that we still consider it as conspecific.

Diagnosis
Smooth Sansabella species with a convex free margin, strong ventral overlap, a postero-ventral spine at LV, hinge line straight, occupying entire dorsal margin.

Etymology
The specific epithet refers to the Gelaohe Formation in which all the specimens were found.

Description
Carapace large, subcircular in lateral view, slightly preplete, with Lmax above mid-H. RV larger than LV, overlapping it all around with maximum along VB and interruption at DB directly posterior to ACA. Hinge line straight, invaginated, occupying entire dorsal margin and equivalent to about 70% of the Lmax. Dorsal margin straight. Cardinal angles blunt, ACA about 135°, PCA about 130°. Ventral margin largely convex at both valves. Left valve with a short latero-ventral spine, slightly posterior to mid-L. AB largely convex with maximum of convexity around mid-H. PB from narrowly rounded in juveniles to close to vertical in adults, with maximum of convexity around the upper ⅓ rd of H. Shallow sulcus opened dorsally sometimes visible, located in front of mid-L, never extending below mid-H. Node sometimes visible, located in the antero-dorsal part of carapace, just in front of the sulcus. Juveniles morphologically similar to adults except for the size, morphology of PB and less developed ventral spine. Carapace smooth.

Remarks
This species, identified as Sansabella sp., was only briefly described by Coen (1989) from a sample from the Famennian, late Devonian, Gelaohe Formation of Baihupo section, Dushan County. Sansabella gelaohensis sp. nov. differs from the majority of species belonging to Sansabella Roundy, 1926 by its subcircular lateral outline. More precisely, it differs from Sansabella amplectans Roundy, 1926 from the Mississippian of Texas (Roundy 1926) in which the larger valve is the left one and has a parallelogram lateral outline. Sansabella gelaohensis sp. nov. also differs from Sansabella polonica Błaszyk & Natusiewicz, 1973 from the Tournaisian of Poland (Błaszyk & Natusiewicz 1973) which has a longer trapezoidal carapace, a reticulate surface with a more distinct sulcus at both valves which begins in the medial part of the DB. Sansabella sinuoventralis Lethiers, 1981 from the Famennian of Western Canada (Lethiers 1981) has a more convex VB. Sansabella? curiosa Stewart & Hendrix, 1945 from the Upper Devonian of Ohio (Stewart & Hendrix 1945) has LV larger than RV, H practically uniform throughout its L, a less convex VB, a granulate or reticulate ornamentation. The H/L scatter plot (Fig. 4B) does not allow to discriminate different ontogenetic stages but the wide distribution of the dimensions of the specimens clearly indicates that several stages are present in the assemblages studied.

Etymology
The specific epithet refers to the latinization of the Chinese Bai Suzhen (also known as Baishe Niangniang which means White Snake Empress), which is a snake spirit in the Chinese "Legend of the white snake".

Description
Carapace of medium size, elongate with Hmax around to slightly anterior to the ⅓ rd of Lmax, at the junction between DB and ADB; Lmax at mid-H. RV larger than LV, overlapping it all around, sometimes absent or reduced at PVB and AB, stronger along dorsal margin and VB. Dorsal margin with a very long and straight to slightly arched DB, short PDB and uniformly convex ADB. VB concave; AB largely rounded with maximum around mid-H at both valves; PB weakly rounded, PVB slightly truncated. ACA about 155°, PCA close to 90°. Stragulum absent. Surface smooth.

Diagnosis
A new species of Cytherellina with carapace compressed along AB and PB, more or less pointed anterior end, RV with bipartite DB and a strong angulation, ADB straight and more inclined than PDB.

Etymology
By apposition, in reference to the blue colour (Latin: caeruleus) of the snake found during the fieldwork which gave its name to the studied section.

Description
Carapace large, subreniform and elongate. Hmax around to slightly posterior to mid-L; Lmax slightly below mid-H; dorsal margin arched on both valves. LV larger than RV, overlapping it all around, sometimes absent or reduced at PVB, stronger along dorsal margin and VB. DB tripartite on LV, bipartite and with a strong obtuse angle at mid-L on RV. VB very gently convex at LV, with a tenuous oral concavity around mid-L at RV. RV with very straight ADB which is longer and more inclined than PDB. The anterior extremity is more or less rounded, particularly on LV. Maximum curvature slightly under mid-H at AB and about mid-H at PB. Wmax in the posterior part of the carapace, slightly after mid-L. Laterally compressed anterior and posterior margins. Carapace smooth.

Remarks
This species was only briefly described, identified as Cavellina sp., from an assemblage from the latest Famennian Gelaohe Formation, Baihupo section, Dushan County (Coen 1989). The specimen shown in Coen (1989: pl. 2 fig. 10) is smaller than our material with L = 850 µm, H = 533 but H/L = 0.62 while it is between 0.5 and 0.6 in our material. It has a more rounded outline and could be a juvenile of Cytherellina caerulea sp. nov. Cavellina prona Wei, 1988 shown in Song & Gong (2019: fig. 4s) is removed from Cavellina prona, which has a less elongated outline and an overlap less developed along free margin, and absent at PDB. The specimen shown in Song & Gong (2019) is slightly tilted anteriorly and the posterior extremity is broken. This specimen is considered as Cytherellina caerulea sp. nov. because it is morphologically very close to our materiel which has a rounded extremity at RV and its size is consistent with our material with L = 916 µm, H = 596 µm, while H/L is greater with 0.65. Wang, 1983 in Song & Gong 2019 is morphologically similar and its size is consistent with our material with L = 1157 µm, H = 673 µm, H/L = 0.58. Although, this species was originally described from the Ertang Formation, early Devonian of Guangxi (Wang 1983b). Raup (1978) showed that the mean species duration among invertebrates is about 11 Ma. For the ostracod genus Puriana Coryell & Fields 1953, common in Cenozoic shallow marine fossil assemblages from the Americas and many Caribbean islands, the mean species duration of most species is between 5 and 8 Ma (Cronin 1987). Consequently, it is unlikely to find Cytherellina subclara in the late Famennian because the duration of this species would then be at least 30 Ma. The taxa duration of ostracod species is out of the scope of this paper: the reader is refered to Forel et al. (2021) for further discussion. It is here considered as Cytherellina caerulea sp. nov.

Cytherellina subclara
The new species differs from Cytherellina obusa Lethiers, 1976in Casier (1989 Jones, 1968in Lethiers (1981 from the Upper Frasnian-Lower Famennian of Western Canada (Lethiers 1981) but this species has a shorter and less convex AVB, a shorter ADB, a more rounded anterior extremity and the overlap stronger at AVB and along dorsal margin. The H/L scatter plot of all Cytherellina caerulea sp. nov. specimens from this work (Fig. 4D) documents a relatively narrow dispersion of the specimens as regard to their size, with the impossibility to discriminate ontogenetic stages.

Remarks
This species is rare in the studied material. The specimens are morphologically close to Healdianella subdistincta Wang, 1983 from the Emsian, early Devonian of Guangxi, South China (Wang 1983a) but they have a more elongated morphology, a straight DB with Hmax developed all along DB and a straight PDB with distinct AD angulation. Healdianella subdistincta is larger, with L ranging from 1440 to 1760 µm, H from 880 to 1000 µm and H/L between 0.57 and 0.61, while it is between 0.47 and 0.56 here. Our material has a small size and may correspond to juveniles so that we only compare them to Healdianella subdistincta until more material is found to clarify this issue. 19BAI 02,19BAI 03,19BAI 08,19BAI 23 Preliminary remark Sohn (1960) erected 2 genera from the study of specimens previously attributed to Bairdia McCoy, 1844: Cryptobairdia Sohn, 1960 andRectobairdia Sohn, 1960. The first differs from Bairdia by the "dorsoanterior margin being not distinct" (Sohn 1960: 47) and the second by "a straight to very gently curved dorsal margin" (Sohn 1960: 52). However, Becker (1965) demonstrated that the morphology of the dorsal outline is "rather variable (even partly superficial) and, therefore only useful to described subgenera" (Becker 2001: 288). We consider that these differences are not sufficient to consider them as valid genera and we choose to follow Becker (1965) and use Cryptobairdia and Rectobairdia as subgenera.

Remarks
This species is rare in the studied material. Bairdia beichuanensis Wei, 1983in Song & Gong (2019 from the Famennian and the Tournaisian of the Baihupo section, Guizhou, South China (Song & Gong 2019) does not belong to Bairdia beichuanensis as shown by the strong ventral and dorsal overlap and stocky morphology. Based on these characters, it is here reattributed to Bairdia nanbiancunensis Wang, 1988. Coen (1989 reported Bairdia sp. from the Gelaohe Formation, Baihupo section, Guizhou, South China, Famennian. Following Olempska (1999), we consider that it belongs to Bairdia nanbiancunensis. The specimens shown in Coen (1989: pl. 2 gigs 2b, 3b) and in Song & Gong (2019: fig. 5a) have a size similar to our material with respectively L = 1000-1100 µm, H = 650-675 µm, H/L = 0.59-0.67 and L = 1120 µm, H = 706 µm, H/L = 0.63. The specimen shown in Olempska (1999: fig. 29h) is as long as our biggest specimen and higher (L = 2310 µm, H = 1540 µm, H/L = 0.67) but we consider that these differences are intraspecific variability. Three ontogenetic stages (A-2 to Ad) are present in our material; they only differ by the size without major changes throught ontogeny.

Remarks
Our material is smaller than the holotype shown in Buschmina (1970: pl: 5 fig. 2) with L = 625 µm, H = 325 µm, H/L = 0.52. Specimens shown in Kotschetkova & Janbulatova (1987: pl. 24 fig. 1) from Mugodzhary, Western Kazakhstan and Sobolev (2020: figs 5, 9-10)) from Pechora Uplift, Russia, have a similar size to our material with respectively L = 532 µm, H = 250 µm and H/L = 0.47 and L = 580 µm, H = 280 µm, H/L = 0.48. However, the specimen shown in Zhuravlev & Sobolev (2019) from North Ural is slightly larger than the present material with L = 580 µm, H = 275 µm and H/L = 0.47 but still smaller than the holotype in Buschmina (1970). The size variations in our material indicate that at least two stages are present and may correspond to juveniles, compare to the holotype in Buschmina (1970), but the wide distribution of the dimensions of the specimens does not allow to clearly discriminate them.
Five species have been mentioned as Bairdianella aff. cuspis in papers discussing Devonian-Carboniferous ostracod faunas, they are summarized in Table 3. Based on specimens shown in these different contributions, the main differences between these species and Bairdianella cuspis Buschmina, 1970 seem to be weak variations of the length of the carapace, concavity at VB, degree of postero-dorsal angulation, posterior end more or less rounded and AVB sometimes truncated (visible only on Bairdianella sp. A, aff. cuspis Buschmina, 1970in Casier et al. 2004). Specimens of Bairdianella aff. cuspis Buschmina, 1970sensu Kotschetkova & Janbulatova 1987in Casier et al. (2002, Bairdianella sp. A, aff. cuspis Buschmina, 1970in Casier & Préat (2003 and Bairdianella sp. B, aff. cuspis Buschmina, 1970in Casier et al. (2004 are also larger than the holotype but others had sizes similar to our material so may also correspond to juveniles. These differences do not seem sufficient to reasonably distinguish these forms from the species described by Buschmina (1970). However, only one specimen is shown in each of these papers with no discussion on possible other specimens, ontogenetic and intraspecific variations within the material or comparisons between these different species. Only a complete review of these taxa, based on close observations of all these specimens, can clarify this major issue. For this reason, they are kept as distinct species.

Remarks
This species is rare in the studied material. Its outline is typical of Podocopida and its lateral surface is characterized by 4 large nodes along the free margin and 3 other large nodes along the dorsal margin. This species is morphologically close to some ornamented Bairdiidae like the Triassic genus Nodobairdia Kollmann, 1963 which has large nodes in the dorsal part on the carapace but these forms of Bairdiidae are unknown from the Devonian and Carboniferous. Moreover, the overlap is reversed compared to most of genera among Bairdiidae, which is rare in this family. We cannot discuss further the systematic position of these specimens, even at family level, until more material will be available.

Diversity and taxonomic composition of ostracod assemblages
On the 88 samples collected from the Blue Snake section, Dushan County, Guizhou province, South China, 23 yielded identifiable ostracods. In total, 98 species have been identified, belonging to 31 genera and 13 families. 50 species are already known from the Devonian and/or early Carboniferous, 4 are new and 44 are kept in open nomenclature due to poor preservation and/or paucity of material. The generic diversity in Blue Snake section ranges from 3 in sample 19BAI 16 to 17 in sample 19BAI 01 and the species diversity from 5 in sample 19BAI 16 to 36 in sample 19BAI 03. Species and genera both document comparable diversity variations with an important increase in the lowermost part of the section followed by a plateau throughout the Gelaohe Formation and an important drop at the top of the Table 3. Occurrences of species attributed to Bairdianella aff. cuspis Buschmina, 1970. Bairdianella sp. A, aff. cuspis Buschmina, 1970Dinant Basin, Belgium Casier et al. 2004 pl. 4 fig. 9 Bairdianella sp. B, aff. cuspis Buschmina, 1970 pl. 4 fig. 10 Bairdianella sp. A, aff. cuspis Buschmina, 1970  Gelaohe Formation. It is followed by an increase and a plateau at the base of the Tangbagou Formation and then a progressive increase towards the top of the Tangbagou Formation.

Species
Individual rarefaction and Shannon-Wiener and Simpson diversity indices were calculated with PAST (Hammer et al. 2001). Rarefaction curves (Fig. 12A) show the expected number of taxa as a function of the number of specimens in each sample from the Blue Snake section. Rarefaction curves from samples 19BAI 08,19BAI 39,19BAI 44,19BAI 54,19BAI 58,19BAI 60,19BAI 63,19BAI 74 and 19BAI 80 reach a plateau, indicating that they provided assemblages representative of the original faunal diversity. On the opposite, rarefaction curves for samples 19BAI 05,19BAI 16,19BAI 23,19BAI 37,19BAI 41,19BAI 43 and 19BAI 55 show that assemblages from these samples are not representative and that a more complete sampling would be useful to have a better idea of the original faunal diversity. In particular,samples 19BAI 16,19BAI 23, 37 display only few identifiable specimens compared to other samples. Samples 19BAI 01-03 recorded the maximum of diversity with 35 to 36 species belonging to 12 to 17 genera and 5 to 7 families. Sample 19BAI 58 recorded the minimum of diversity among representative assemblages, with 6 species belonging to 5 genera and 3 families.
Diversity for each sample was calculated ( Fig. 12B -C) using Shannon-Wiener (Shannon & Weaver 1949) and Simpson indices (Simpson 1949). The first is weakly affected by the sample size and allows to analyse species richness considering the number of specimens of each species. The second allows to estimate the dominance in the assemblages: it is close to 1 if species are uniformly distributed and close to 0 if the assemblage is dominated by one or few species (Forel et al. 2020). Diversity indices in the Blue Snake section show patterns comparable to the generic and specific variations described above ( Fig. 12B-C). A decrease in diversity paralleled by an increase in dominance between samples 19BAI 80 to 19BAI 74. Both indices have low value in sample 19BAI 74 with Simpson index which reach its lowest value throughout the section with D-1 = 0.57 and Shannon-Wiener index reach one of its lowest values with H = 1.5.
The general trend across the Gelaohe Formation (samples 19BAI 80 to 19BAI 60) shows an increase in specific diversity and dominance with 1 major decrease in sample 19BAI 74 (H = 1.5, D-1 = 0.57) and a weaker at sample 19BAI 63 (H = 2.5, D-1 = 0.86). In this formation, assemblages are dominated by Sansabella gelaohensis sp. nov. that represents 64% of collected specimens in sample 19BAI 74. A strong reduction of diversity and increase of dominance occur between samples 19BAI 60 and 19BAI 58 at the contact between the Gelaohe and Tangbagou Formations, and the value for both indices is low in sample 19BAI 58 (H = 1.3, D-1 = 0.65). This assemblage is dominated by Bairdiacypris subcyliпdrica Buschmina, 1984 that represents half of the collected specimens. This drop is followed by an increase of both indices between samples 19BAI 58 and 19BAI 55 and an interval between samples 19BAI 55 and 19BAI 44  In the Blue Snake section, 3 different associations are recognized based on their composition, diversity (H), and dominance (D-1): their number of species, genera and families are summarized in Table 4 and the relative abundance and diversity of each family are shown on Fig. 13. The association A is characteristic of the Gelaohe Formation and corresponds to pre-Hangenberg Event assemblages (samples 19BAI 60 to 19BAI 80). It is composed of 53 species that belong to 24 genera and 12 families and is the most diversified in the Blue Snake section at genus and family levels. It is dominated by Palaeocopida (62% of collected specimens), with the family Sansabellidae represented by the single species Sansabella gelaohensis Guillam & Forel sp. nov. which accounts for 22% of the collected specimens, Beyrichiopsidae (15%), Geisinidae (11%), Primitiopsidae (10%), Aparchitidae (4%) and Hollinellidae (less than 1%). Podocopida represent less than one-third of the collected specimens (32%), Bairdiidae being the most abundant (20%) and essentially represented by Bairdia and Acratia. The families Bairdiocyprididae (9%) and Pachydomellidae (4%) are also part of this association, essentially represented by Healdianella and Microcheilinella respectively. Platycopida (5%) all belong to the family Cavellinidae with Sulcella (4%) and Cavellina (1%). They are represented only in this association.
The association B occurs in the lowermost Tangbagou Formation (sample 19BAI 58 to 19BAI 54) and corresponds to the assemblages in the direct aftermath of the HE. It is composed of 26 species belonging to 11 genera and 6 families and is the less diversified in this section. It is dominated by Podocopida (88% of the collected specimens), Bairdiidae being the most abundant (67%) with Bairdiacypris (44%) and Bairdia (19%) being the most representative genera. Bairdiocyprididae are well represented but only with the single genus Healdianella (19%). Pachydomellidae (2%) are also represented. Palaeocopida are poorly represented (12%), Paraparchitidae being the most abundant (9%) but only represented by the genus Shishaella. Geisinidae (2%), represented by Hypotetragona and Knoxiella, and Aparchitidae (1%), represented by the single genus Fellerites, are also part of this association. The family Primitiopsidae is absent only in this association. In terms of species, this association is the less diversified in the Blue Snake section.

Impact of the Hangenberg Event on ostracod faunas
In the upper Famennian Gelaohe Formation, before the shally bed corresponding to the Hangenberg Event, 53 species belonging to 24 genera and 12 families are present. The lowermost Tangbagou Formation yielded only 26 species belonging to 11 genera and 6 families, and record the lowest value in the Blue Snake section. Moreover, the assemblage from sample 19BAI 58 is very poor with only 6 species belonging to 5 genera and 3 families: Bairdiidae represents 61% of the species and Bairdiacypris subcylindrica accounts for half of specimens. The Shannon-Wiener and Simpson indices are also very low in the assemblage from sample 19BAI 58 which confirms a very low diversity (H = 1.3) and the dominance of the species Bairdiacypris subcyliпdrica in this assemblage (D-1 = 0.65). Based on these data, the bed of yellowish shales (sample 19BAI 59) that yielded no ostracods certainly corresponds to the Hangenberg Event as hypothesized above. Twenty-eight of the 53 species found in our material from the Gelaohe Formation actually cross the Hangenberg Event and are present in the Tangbagou Formation. Kloedenellitina sincera, Cytherellina caerulea Guillam & Forel sp. nov, and Bairdia nanbiancunensis are not represented in our post-Hangenberg Event assemblages but they were found by Song & Gong (2019) in the Tournaisian Tangbagou Formation from the Blue Snake section. Cytherellina caerulea sp. nov is only represented in the first half of the Tangbagou Formation (Song & Gong 2019: fig. 3). Two species that are represented only in our post-event associations (Tangbagou Formation) was also found in the Gelaohe Formation by Song & Gong (2019): Microcheilinella infradominica in Song & Gong (2019) and Fabalicypris sundarijanata Wang & Cao, 1997. Consequently, 31 species of the 55 cross the Hangenberg Event and record a specific extinction rate of 44% but it is worth noting that in our material only 13 species are represented in the lowermost Tangbagou Formation (association B). The others are present only in the rest of this formation (association C). These lasts could have either temporarily disappeared from the Blue Snake section, survived at another yet unidentified location and recolonized this area when the conditions became better or their presence in the latest Famennian (lowermost Tangbagou Formation) has not been recorded here, perhaps because of low abundance (i.e., lazarus taxa). The value of specific extinction rate is slightly lower than that estimated from the data from Song & Gong (2019). In this study, 17 species among the 35 species present in the Gelaohe Formation became extant, that correspond to a specific extinction rate of about 49%. Kaiser et al. (2016) estimated that 30 to 50% of neritic benthic ostracod species disappeared at the Hangenberg Event from data published by Casier et al. (2004Casier et al. ( , 2005 from the Dinant Basin. The values in the present work and Song & Gong (2019) are consistent with this estimation. Crasquin et al. (1986) documented a more severe impact of the Hangenberg Event on neritic benthic ostracod faunas from Western Canada with only 5 surviving species among 148 which correspond to a specific extinction of about 96%. This discrepancy may relate to different environmental conditions. Among species that disppeared in the Blue Snake section with the Hangenberg Event, 2 have been documented elsewhere in the Tournaisian: Kloedenellitina spinosa in the Russian platform (Gurevitch 1972), and Bairdia nanbiancunensis in Guilin (Wang 1988b) and Guizhou, China (Olempska 1999) and Canning Basin, Australia (Jones, unpublished data, fide Olempska 1999). At the genus level, the extinction rate is about 46% with the disappearance of 11 genera among the 24 represented in the pre-event assemblages.
Palaeocopida have been more impacted by the Hangenberg Event in the Blue Snake section with a specific extinction rate of about 76% while it is about 25% in Podocopida. All species of Platycopina disappeared so this order is not present in post-event associations (B and C).

Recovery of ostracod faunas after the Hangenberg Event
In the lowermost Tangbagou Formation (association B), 13 species among the 26 recorded are new, corresponding to a specific turnover rate of 50%. When considering the whole Tangbagou Formation (association B and C), 45 species among the 72 that are present after the HE are new which correspond to a specific turnover rate of about 62%. This value is for example higher than in the Yjid-Kamenka river section, Pechora Uplift in which it is only of 20% (data from Sobolev 2020). However, Crasquin et al. (1986) documented a severe and almost complete renewal of neritic benthic ostracod faunas from Western Canada with only 5 species that survived from the Famennian and 205 new species in the Lower Carboniferous Banff Formation. This is a sharp renewal which is unique in Palaeozoic ostracods; in most of the case the recovery is more gradual with species represented in pre-event and post-event  assemblages ). Acutiangulata Buschmina, 1968, represented by the single species Acutiangulata acutiangulata (Tschigova, 1959) and Shishaella, represented by Shishaella hastierensis Crasquin-Soleau, 1988 and Shishaella porrecta (Zanina, 1956), are the only genera that appeared in the lowermost Tangbagou Formation of the Blue Snake section. No new family has been found in this part of the Tangbagou Formation.
In the rest of the Tangbagou Formation (association C), 32 of the 72 species are new. Six genera appeared, all represented by a single species (Table 1): Blessites, Amphissites, Bairdianella, Coryellina, Aparchites and Knoxites. Of species only represented after the HE in the Blue Snake section, 6 have been recorded elsewhere before the Hangenberg Event (Table 5). Most of the families represented in the association C count for between 1 and 14 % of the specific diversity except Bairdiidae which represent 54.5% of the species, in particular the genus Bairdia which counts for 38% of the total number of species all families included. This family recorded the greatest diversification among all families throughout the Tournaisian with 16 species in the lowermost Tangbagou Formation (associations B) and 40 species in the rest of this Formation (association C). After the HE, Podocopida became more numerous and diversified and dominate the assemblages, in particular the family Bairdiidae. The abundance of Palaeocopida did not change between lowermost Tangbagou Formation (association B) and the rest of this Formation (association C) but they became slightly more diversified at genus level with the appearance of Aparchites in Aparchitidae, Knoxites in Geisinidae, and Paraparchites in Paraparchitidae. At the family level, the association C from the Tournaisian of the Tangbagou Formation is also more diversified with the appearance of Amphissitidae and Primitiopsidae. All palaeocopid genera represented in the lowermost Tangbagou Formation are also represented in the Tournaisian. However, at specific level, Geisinidae and Paraparchitidae are the only families among Palaeocopida which become more diversified with both 2 species in the lowermost Tangbagou Formation (association B) and respectively 6 and 4 species in the rest of this Formation (association C).
Consequently, the recovery of ostracod faunas after the HE in the Blue Snake section is here linked essentially to the diversification of Podocopida, more particularly Bairdiidae with the genus Bairdia, and a turnover between Palaeocopida and Podocopida, these latter becoming dominant in post-event assemblages.

Palaeoenvironmental implications
Before discussing palaeoenvironmental interpretations, the autochthonous or allochthonous nature of ostracod assemblages must be discussed. It is generally determined by studying the ratio between complete carapaces with both articulated valves and isolated valves and the demographic structures of populations (e.g., Oertli 1971;Boomer et al. 2003). In all assemblages from the Blue Snake section, complete and articulated valves are dominant. Moreover, most of species are represented by adults and juvenile stages, although adults and last ontogenetic stages are often more represented in our material than the earlier stages ( In the Palaeozoic, 3 ostracod mega-assemblages (Fig. 14) are classically recognized (recently summarized in Casier 2017): 1. Eifelian Mega-assemblage (= "sandigkalkiger Fazies Typ" from Blumenstengel 1973 and Eifelian ecotype from Becker in Bandel & Becker 1975) that indicate benthic neritic marine environments (0 to III on Fig. 14). 2. Thuringian Mega-assemblage (= "Thüringischer Typus" from Zagora 1968 and the "kalkigtoniger Fazies Typ" from Blumenstengel 1973 and Thuringian ecotype from Becker in Bandel & Becker 1975) that indicates deeper benthic environments (IV on Fig. 14). 3. Myodocopida Mega-assemblage (= "toniger, hemipelagischer Fazies Typ" from Blumenstengel 1973 and Entomozoacean ecotype from Becker in Bandel & Becker 1975) indicates poorly oxygenated marine environments, V on Fig. 14).
For the Palaeozoic of China, Wang (1988a) defined 5 ostracod associations that could be compared with the mega-assemblages as follow: leperditiid association (nearshore lagoonal environments, 0 on Fig. 14), palaeocopid association (more open shallow nearshore environments, I and II on Fig.  14), smooth-podocopid association (slightly deeper and more distal offshore environments, III on Fig. 14), spinose-podocopid association (deep environments at the base of the slope, IV on Fig. 14), and entomozoacean association, (poorly oxygenated marine environment, V on Fig. 14).
The regressive pulse that occurred in the middle praesulcata zone (late Famennian), interpreted as a minor sea-level fall associated with a minor glacial phase (Kaiser et al. 2011(Kaiser et al. , 2016 and often considered as one of the possible causes of the HE (e.g., Walliser 1996) is here difficult to highlight. The study of ostracod faunas from the Famennian Zhewang Formation which was deposited before the Gelaohe Formation could bring more information about this regression trend during the late Famennian.
In the Blue Snake section, the association A is dominated by Palaeocopida (62% of collected specimens) and is the only one from the Blue Snake section that contains Platycopida. These characteristics point to the Assemblage II of the Eifelian Mega-assemblage, pointing to a nearshore shallow water environment. The presence and dominance of the genus Sansabella that occurred in shallow-waters and is rather typical of nearshore conditions (Coen 1989) also support these interpretations. However, Podocopida are more diversified (60% of species), in particular the family Bairdiidae which indicate a relatively strong marine influence and an open environment. The association Β of the Blue Snake section is dominated by Podocopida, particularly Bairdiidae with the genus Bairdiacypris: it therefore corresponds to the Assemblage III of the Eifelian Mega-assemblage and the Smooth-Podocopid Association of Wang (1988a). However, contrary to the Assemblage III, the association Β is poorly diversified which is probably linked to the HE that occurred slightly earlier. Consequently, the environmental signal is here weak and unclear but seem to correspond to a slightly deeper and opened marine environment but we cannot be sure that this corresponds to the beginning of a transgressive phase as the extinction signal may be overprinting. The association C is largely dominated by Podocopida, particularly Bairdiidae and the genus Bairdia. It totally corresponds to the Assemblage III of the Eifelian Mega-assemblage and the Smooth-Podocopid Association of Wang (1988a). Both indicate a shallow and open offshore marine environment. The high diversity and abundance of ostracods in the Tangbagou Formation, in particular in the top of the section, indicate an offshore environment between fair-weather and storm wave base (Casier et al. 2005).
Thus, the study of ostracod assemblages from the Blue Snake section documents the increasing waterdepth from the late Famennian-Tournaisian interval, in line with the transgressive trends that have already been reported from the study of ostracod faunas from France (e.g., Lethiers & Feist 1991;Casier et al. 2002), Belgium (e.g., Casier et al. 2004Casier et al. , 2005, Canada (e.g., Lethiers et al. 1986), Urals (e.g., Zhuravlev & Sobolev 2019;Sobolev 2020) and South China (e.g., Song & Gong 2019). Blessites feluyensis, Knoxiella complanata, Praepilatina adamczalki and Bairdianella cuspis were also reported together from a succession recording a transgressive trend through the Devonian-Carboniferous transition from North Urals (Sobolev 2020). Blessites feluyensis and Bairdia feliumgibba, were represented together in the Devonian-Carboniferous of the Vangyr River section from the east of the Pechora plate in Russia (Zhuravlev & Sobolev 2019). Bairdia feliumgibba, only present in the Tournaisian in our section, were represented in lagoonal environment in the late Famennian but became extinct a little after the Hangenberg event. Blessites feluyensis, which is also only present in the Tournaisian in our section, was represented in shoal in marine open context and in transgressive trends in the Tournaisian (Zhuravlev & Sobolev 2019). This species seems to be represented in the same palaeoenvironmental settings in the Tournaisian of the Blue Snake section in South China. The increasing number of species and specimens belonging to Bairdiidae, in particular the genus Bairdia, throughout the Tangbagou Formation indicates a progressively opening and deepening of the environment during the Tournaisian. Because the renewal of ostracod faunas in the Blue Snake section was mainly linked to this diversification, we propose that the recovery was depending on the palaeoenvironmental changes.

Conclusion
Ninety-eight species belonging to 31 genera have been retrieved from the Blue Snake section spanning the nov. The analysis of the diversity of ostracod assemblages shows that in the studied area, the Hangenberg Event caused a significant diversity fall with the disappearance of 44% of the species. This event also influenced the taxonomic composition of the assemblages, with the reduction of the proportion of Palaeocopida and the increase in that of Podocopida between the latest Famennian and the Tournaisian. The main factor of the renewal of ostracod faunas in the Blue Snake section appears as the progressive diversification of the family Bairdiidae during the early Tournaisian, in particular the genus Bairdia, and was probably constrained by the palaeoenvironmental changes at the Devonian-Carboniferous transition. Thus, the study of the composition and characteristics of assemblages indicates that the Tangbagou Formation was deposited in a transgression trend, with a marine shallow environment that become deeper and more open during the Tournaisian.