New Katian bivalves from the Upper Ordovician Xiazhen Formation, Jiangxi Province, China

Yong Wang, Ren-bin Zhan, and Zong-jie Fang

Wang, Y., Zhan, R.-B., and Fang, Z.-J. 2026. New Katian bivalves from the Upper Ordovician Xiazhen Formation, Jiangxi Province, China. Acta Palaeontologica Polonica 71 (1): 133–153.

A bivalve association reported herein was recently discovered from the Xiazhen Formation (upper Katian, Upper Ordovician) at Zhuzhai, southern Yushan County, northeastern Jiangxi Province, East China. Nine genera and ten species are systematically documented including four new species: Palaeoneilo rectus sp. nov. (most abundant), Palaeoneilo cf. constricta, Goniophorina volvens, Modiolopsis elegantulus, Cyrtodontula sp., Cuneamya? sp., Trigonoconcha brevis sp. nov., Similodonta minor sp. nov., Concavodonta varius sp. nov., and Paulinea cf. parva. The association is mainly composed of Protobranchia (Nuculiformii and Nuculaniformii) and subordinate Pteriomorphia, suggesting a relatively low palaeolatitudinal setting. The dominance of infaunal protobranch bivalves is notable, and against the trend of rapid diversification of pteriomorphs during the Late Ordovician. This discrepancy is likely attributable to a cold water tongue extending from then South Pole to South China palaeoplate, and the ecological pressure from those much more abundant brachiopods on the substrate surface, both hindering the development of epifaunal pteriomorphs in the region. Significantly, this bivalve association shares several common genera with its coeval bivalve fauna from the Oslo region, Norway.

Key words: Bivalvia, Protobranchia, Pteriomorphia, epifauna, infauna, Katian, Late Ordovician, Jiangxi, South China.

Yong Wang [yongwang@nigpas.ac.cn; ORCID:https://orcid.org/0000-0002-3253-2242], and Ren-Bin Zhan [rbzhan@nigpas.ac.cn; ORCID: https://orcid.org/0000-0003-3541-9023], State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China; University of the Chinese Academy of Sciences, Beijing 100049, China.

Zong-Jie Fang [zjfang@nigpas.ac.cn; ORCID: https://orcid.org/0000-0002-0254-5077], State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China.

Received 11 August 2025, accepted 11 January 2026, published online 26 March 2026.

Copyright © 2026 Y. Wang et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License (for details please see http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Introduction

During the Sandbian (Late Ordovician), the radiation of pteriomorph bivalves took place as part of the Great Ordovician Biodiversification Event (GOBE) (Cope 2004; Fang 2006). The Late Ordovician bivalve faunas were taxonomically diverse especially at low to middle palaeolatitudes (Bretsky 1969, 1970a, b; Toni 1975; Frey 1987; Tunnicliff 1982; Cope 1996; Ebbestad et al. 2022).

The bivalve association reported in this paper occurs in the middle part of the Xiazhen Formation (upper Katian, Upper Ordovician) at the Zhuzhai section of southern Yushan County, northeastern Jiangxi Province, East China (Fig. 1B, C), which was palaeogeographically near the Cathaysian Block (Fig. 1A). The Xiazhen Formation at this section comprises limestone, mudstone, and calcareous mudstone interbeds (Fig. 2). Abundant shelly fossils, such as brachiopods, corals, bryozoans, stromatoporoids and trilobites, have been found and systematically studied by various experts (e.g., Hu 1986; Zhan and Rong 1994, 1995a, b; Zhan et al. 2002; Lee 2013; Zhang et al. 2018; Jeon et al. 2022). Bivalves found in this formation are quite species-rich, although not as many as brachiopods. The bivalve association from the Xiazhen Formation provides an important insight into the Late Ordovician bivalve fauna of the South China palaeoplate, and is a valuable case study for this time interval.

Yushan is located in the Jiangshan-Changshan-Yushan district (abbreviated as JCY area). This is one of the classical areas for the Palaeozoic research and includes two GSSPs, i.e., the base of the Jiangshanian Stage (upper Cambrian) at Duibian of Jiangshan County and the base of the Darriwilian Stage (Middle Ordovician) at Huangnitang of Changshan County respectively. The Zhuzhai section (Fig. 1, GPS of collection JYZ-10: 28°34′23″N, 118°20′14″E) exposes the upper Katian (Upper Ordovician) Xiazhen Formation, one of the best representatives of this formation in JCY area. Zhan and Fu (1994) summarized three contemporaneous lithostratigraphic units for the upper Katian in JCY area, including the Changwu Formation (on the Zhexi slope), the Sanqushan Formation (along the platform margin, or in the transition belt between the Zhe-Gan platform and the Zhexi slope), and the Xiazhen Formation (on the Zhe-Gan platform). The sediments cropping out at Zhuzhai section were deposited in the nearshore side of the Zhe-Gan platform. Zhan and Rong (1995a) recognized four brachiopod dominated communities from the Xiazhen Formation at this site, such as the Protospirifer praecursor Community, the Deloprosopus jiangshanensis Community, the Altaethyrella-Ovalospira Community, and the Sowerbyella-Antizygospira Community. Four communities alternately recur several times throughout the Xiazhen Formation at this section, indicating the fluctuating environmental conditions.

Institutional abbreviations.—NIGP, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, China.

Other abbreviations.—ES, eastern part of the section JCY, Jiangshan-Changshan-Yushan; JYZ, Jiangxi-Yushan-Zhuzhai.

Nomenclatural acts.—This published work and the nomenclatural acts it contains, have been registered in Zoobank: urn:lsid:zoobank.org:pub:E42CE936-29B9-450D-8997-B16F9C00358E.

Material and methods

The bivalves studied here were collected from the mudstone of the middle part of the Zhuzhai section, together with the brachiopods of the Sowerbyella-Antizygospira Community, which were thought to inhabit a nearshore, shallow water environment corresponding to muddy substrate with benthic assemblage 2–3 (Rong and Han 1986; Rong and Chen 1987; Zhan and Rong 1995a; Zhan et al. 2002). Bivalves were collected from four horizons: ES-a, ES-b (JYZ-9), JYZ-11 and ES-c (JYZ-10). Among these, ES-c yields the most abundant and diverse bivalve fossils. The Xiazhen Formation at this site is biostratigraphically assigned to the Dicellograptus complexus Biozone (upper Katian) (Chen et al. 1987; Zhan and Cocks 1998; Zhan et al. 2002). Most of the bivalves are disarticulated, and those articulated are mainly from the collection ES-c (JYZ-10). The valves are commonly complete and have no distinct orientation or sorting, indicating minimal transportation. All figured specimens are stored at Nanjing Institute of Geology and Palaeontology (NIGP), Chinese Academy of Sciences. The systematic palaeontology in this work primarily follows Carter et al. (2011).

Systematic palaeontology

Class Bivalvia Linnæus, 1758

Subclass Protobranchia Pelseneer, 1889

Superorder Nuculiformii Dall, 1889

Order Nuculida Dall, 1889

Superfamily Nuculoidea Gray, 1824

Family Praenuculidae McAlester, 1969

Subfamily Praenuculinae McAlester, 1969

Genus Paulinea Cope, 1996

Type species: Paulinea parva Cope, 1996; Ogof Hên Formation, Lower Ordovician, South Wales, UK.

Paulinea cf. parva Cope, 1996

Figs. 3, 4.

cf. 1996 Paulinea parva gen. et sp. nov.; Cope 1996: 985, pl. 1: 11–22.

Material.—Five internal and three external moulds of right valves, two internal and two external moulds of left valves (NIGP208861–208867). All from ES-c (JYZ-10), Yushan County, Jiangxi Province (China); Katian (Late Ordovician).

Description.—Small sub-rectangular shell; equivalve and inequilateral. The length/height ratio (L/H) is approximately 1.5 (Fig. 3; Table 1). Anterior margin round; posterior margin inclined truncate; ventral margin broadly convex, with a slight sinus near posteroventral corner; posteroventral angle approximately 90°. Umbo broad and situated at third anterior of the shell, slightly prosogyrate and projected above the hinge line. Anterior adductor scar small and round with short myophoric buttress developed posteriorly. Hinge line with graded series of peg-like taxodont teeth, at least four anterior and eight posterior, increasing slightly in size distally. Ligament opisthodetic, ligament groove visible (Fig. 4C2). Outer shell surface smooth (Fig. 4A2–E2).

Remarks.—According to Sánchez’s (1999) definition of the Praenuculinae, most of the genera have convexodont teeth except for Pensarnia (Cope, 1996) and Paulinea. Pensarnia has a more distinct adductor scar and ornamentation than those observed in our specimens. Compared with the type species Paulinea parva, our specimens share a similar shell outline with some individuals of the type species (Cope 1996: pl. 1: 16–17). However, our specimens have a more significantly truncated posterior margin. As the dentition in our specimens is unclear, it seems reasonable to assign our specimens to Paulinea cf. parva, rather than to erect a new species.

Subfamily Concavodontinae Sánchez, 1999

Genus Concavodonta Babin & Melou, 1972

Type species: Nucula ponderata Barrande, 1881, Upper Ordovician, Czech Republic.

Concavodonta varius sp. nov.

Figs. 5, 6.

Zoobank LSID: urn:lsid:zoobank.org:act:9DE10739-775D-4D93-A160- 77A4669542BC.

Etymology: From Latin varius, various, indicating the changes in shell forms within the ontogeny.

Type material: Holotype, NIGP208868 (Fig. 5A), internal and external moulds of articulated valves. Paratypes: NIGP208869 (Fig. 5B); NIGP208870 (Fig. 5C); NIGP208873 (Fig. 5D); NIGP208871 (Fig. 5E); NIGP208872 (Fig. 5F).

Type locality: Yushan County, Jiangxi Province, East China.

Type horizon: Katian, Upper Ordovician.

Material.—Seven internal moulds (four right valves, two left valves, one conjoined valve), one external mould of conjoined valve (NIGP208868–208873). From JYZ-11 and ES-c (JYZ-10), Yushan County, Jiangxi Province (China); Katian (Late Ordovician).

Diagnosis.—Concavodonta with sub-rectangular shell in adults but oval in juveniles.

Description.—Small to medium sub-rectangular shell; equivalve and inequilateral. Anterior margin rounded; posterior margin truncated; ventral margin convex; anterior dorsal margin concave; posterior dorsal margin straight. The L/H varies from 1.24 to 1.86, with juvenile shells being less elongated posteriorly and having a smaller L/H (Fig. 6; Table 2). Broad umbo situated in anterior third of the shell, prosogyrate. Adductor scars ill-defined, a faint oval anterior adductor scar partially in one specimen (Fig. 5F). Taxodont dentition with concavodont teeth: six–seven anterior teeth and 11 posterior teeth in adult shells; three–four anterior teeth and eight posterior teeth in juvenile shells. An edentulous area between anterior and posterior teeth below the umbo. Anterior and posterior dorsal borders meet at angle of 160°. Shell ornamentation of fine commarginal growth lines.

Remarks.—In comparison with the type species Concavodonta ponderata, C. varius sp. nov. has a more elongate outline, a straighter dorsal margin, and a greater angle between the anterior and posterior dorsal borders. Concavodonta ovalis (Sánchez, 1999) from the Upper Ordovician of Argentina is similar to our specimens in outline, but its umbo does not project above the hinge line as in our specimens, and is located more centrally on the shell. Concavodonta imbricata (Portlock, 1843) from the Upper Ordovician of Ireland has more posterior teeth, more distinct concentric ornamentation, and a more contracted posterior margin than our specimens.

Stratigraphic and geographic range.—Middle part of the Xiazhen Formation, upper Katian, Upper Ordovician; Yushan County, China.

Superorder Nuculaniformii Carter et al., 2000

Order Cardiolaritida Cope, 2000

Superfamily Tironuculoidea Babin, 1982

Family Similodontidae Carter & Pojeta in Carter et al., 2011

Genus Trigonoconcha Sánchez, 1999

Type species: Trigonoconcha acuta Sánchez, 1999, from the Don Braulio Formation, Upper Ordovician, Argentina.

Trigonoconcha brevis sp. nov.

Figs. 3, 7.

Zoobank LSID: urn:lsid:zoobank.org:act:AE78B449-4021-4736-B87D-3B246010B765.

Etymology: From Latin brevis, short, suggesting the shorter height and bigger L/H ratio than those of the type species T. acuta.

Type material: Holotype, NIGP208881 (Fig. 7H), an internal mould of right valve and its cast. Paratypes: NIGP208874 (Fig. 7A), NIGP208875 (Fig. 7B), NIGP208876 (Fig. 7C), NIGP208879 (Fig. 7E), NIGP208878 (Fig. 7F), NIGP208877 (Fig. 7G).

Type locality: Yushan County, Jiangxi Province, East China.

Type horizon: Katian, Upper Ordovician.

Material.—Two internal moulds and one external mould of right valves, three internal moulds and one external mould of left valves (NIGP208874–208881). Most from JYZ-11, few from ES-c (JYZ-10), Yushan County, Jiangxi Province (China); Katian (Late Ordovician).

Diagnosis.—Trigonoconcha with L/H ratio about 1, anterior and posterior dorsal margins meeting at an angle of around 70°.

Description.—Small to large triangular shell; equivalve and inequilateral. Umbo acute and situated subcentral, orthogyrate or slightly prosogyrate. Anterior adductor scar well marked, elliptical. Anterior and posterior hinge lines meeting below the beak at an angle approximately 70°. Taxodont dentition, containing five–seven posterior teeth and six–nine anterior convexodont teeth in juvenile shells, increasing to up to 12 posterior teeth and 14 anterior teeth in adult shells; anterior teeth slightly larger than posterior teeth. Amphidetic ligament visible (Fig. 7A, C1). Shell ornamentation of concentric lines.

Remarks.—The type species was determined by Sánchez (1999) primarily on the base of the small angle between its anterior and posterior dorsal margins (usually less than 70°). Compared with T. acuta, our specimens have fewer anterior and posterior teeth, a higher L/H ratio of approximately 1 (Fig. 3; Table 3), and a greater angle between the anterior and posterior dorsal margins.

Stratigraphic and geographic range.—Middle part of the Xiazhen Formation, upper Katian, Upper Ordovician; Yushan County, China.

Genus Similodonta Soot-Ryen, 1964

Type species: Tellinomya similis Ulrich, 1892, Richmond Group, Upper Ordovician, Minnesota, USA.

Similodonta minor sp. nov.

Figs. 6, 8.

Zoobank LSID: urn:lsid:zoobank.org:act:030A0A6E-177D-4F14-B300-C780E46DC3D2.

Etymology: From Latin minor, less, indicating fewer teeth than other species of Similodonta.

Type material: Holotype, NIGP208882 (Fig. 8A), an internal mould of right valve. Paratypes: NIGP208883 (Fig. 8B), NIGP208923 (Fig. 8D), NIGP208885 (Fig. 8E), NIGP208887 (Fig. 8F).

Type locality: Yushan County, Jiangxi Province, East China.

Type horizon: Katian, Upper Ordovician.

Material.—Three internal moulds of right valves, four internal moulds of left valves (NIGP208882–208887, NIGP208923). Most from JYZ-11, few from ES-c (JYZ-10) or ES-b (JYZ-9), Yushan County, Jiangxi Province (China); Katian (Late Ordovician).

Diagnosis.—Similodonta with less than 30 teeth and an opisthogyrate umbo.

Description.—Small sub-triangular shell; equivalve and inequilateral. Length 2.2–3.4 mm, height 2.2–3.2 mm (Fig. 6; Table 4). Anterior, posterior and ventral margin rounded; anterior and posterior dorsal margin meet at an angle of about 90°. Umbo broad, sub-central to anterior, slightly opisthogyrate. Taxodont dentition, consisting of 13–15 anterior and 10–11 posterior teeth; teeth grading from convexodont teeth away from the umbo to peg-like teeth close to the umbo; anterior teeth slightly larger than posterior teeth. Anterior adductor scar rounded. Posterior adductor scar elliptical, slightly larger than anterior adductor scar.

Remarks.—Our specimens are similar in their triangular outline to Similodonta collina (Reed, 1946) from the Upper Ordovician of Scotland, S. spjeldnaesi (Soot-Ryen & Soot-Ryen, 1960) from the Upper Ordovician of Norway, and the type species S. similis. However, the present new species has significantly fewer teeth. Compared with S. minor, S. ceryx (Cope, 1999) from the Middle Ordovician of Wales can be distinguished by its prosogyrate umbo and more elongate posterior. Similodonta wahli (Isakar, 1991) from the Upper Ordovician of Estonia differs from our species in its smaller angle between anterior and posterior hinges.

Stratigraphic and geographic range.—Middle part of the Xiazhen Formation, upper Katian, Upper Ordovician; Yushan County, China.

Order Nuculanida Carter et al., 2000

Superfamily Malletioidea Adams & Adams, 1858

Family Cucullellidae Fischer, 1886

Subfamily Palaeoneilinae Babin, 1966

Genus Palaeoneilo Hall & Whitfield, 1869

Type species: Nuculites constricta Conrad, 1842, Hamilton Group, Middle Devonian, New York, USA.

Palaeoneilo rectus sp. nov.

Figs. 9–11.

Zoobank LSID: urn:lsid:zoobank.org:act:1598E00A-4CF5-454C-9B9B-CEC8561B7BED.

Etymology: From Latin rectus, straight, suggesting the sub-straight hinge line.

Type material: Holotype, NIGP208907 (Fig. 9H), internal and external moulds of left valve. Paratypes: NIGP208888–208906, NIGP208908–208910, NIGP208924–208926 (Figs. 9A–G, I–K, 10A–O).

Type locality: Yushan County, Jiangxi Province, East China.

Type horizon: Katian, Upper Ordovician.

Material.—52 internal moulds and 15 external moulds of right valves, 32 internal moulds and 11 external moulds of left valves, 23 internal moulds and 13 external moulds of conjoined shells (NIGP208888–208911, NIGP208924–208926). From all collections, but mostly from JYZ-11, Yushan County, Jiangxi Province (China); Katian (Late Ordovician).

Diagnosis.—The Palaeoneilo with a sub-straight hinge line and tilted posterior orthodont or slightly concavodont teeth.

Description.—Small to medium elongated-elliptical shell; equivalve and inequilateral. Average L/H ratio around 1.8 (Fig. 11, Table 5), Anterior margin rounded, with an auricle near the anterior dorsal margin; posterior margin elongated and slightly contracted; ventral and dorsal margins convex. The umbo situated in anterior third of the shell, projected above the hinge line, orthogyrate or prosogyrate. Ovate anterior adductor scar well preserved in some specimens, close to the umbo, with a small pedal retractor scar dorsally (Figs. 9D2, H1, 10C, L). Hinge line nearly straight, half length of the dorsal margin, with 4 concavodont anterior teeth and 14 peg-like posterior teeth in adult shell (8 in juvenile shell). The posterior teeth tilted towards the umbo. No distinct discontinuity between the anterior and posterior teeth series, and the teeth beneath the umbo are smaller. Outer shell surface smooth.

Remarks.—The umbonal-posteroventral depression was cited as a key diagnostic feature of Palaeoneilo (Babin 1966; McAlester 1969). However, because the depression is not preserved in all assigned species, it may not be considered as the diagnostic feature for the genus.

Compared with the type species Palaeoneilo constricta, P. rectus has a more rounded and elongate posterior margin. This feature distinguishes it from P. smithi (Pojeta & Gilbert-Tomlinson, 1977) from the Middle Ordovician of Australia. The new species differs from Palaeoneilo filosa (Conrad, 1842) and P. muta (Hall, 1870) from the Devonian of USA in having shorter hinge and weaker ornamentations. Other species, such as P. emarginata (Conrad, 1841), P. angusta (Hall, 1885), P. perplana (Hall & Whitfield, 1869) and P. truncata (Hall, 1883) from the Devonian of USA could be distinguished in having much more distinct umbonal posterior depressions.

Many species of Palaeoneilo have convexodont teeth in both anterior and posterior hinge lines, such as P. constricta, P. smithi, P. hopensacki (Verneuil & Barrande, 1855) and P. beirensis (Sharpe, 1853). Some others have orthodont teeth beneath the umbo, like P. fecunda. The dentition of P. rectus consists of concavodont anterior teeth and orthodont posterior teeth. The posterior teeth tilt toward the umbo, thicken and grade into concavodont teeth posteriorly (Fig. 9K2).

Stratigraphic and geographic range.—Middle part of the Xiazhen Formation, upper Katian, Upper Ordovician; Yushan County, China.

Palaeoneilo cf. constricta Conrad, 1842

Figs. 11, 12.

cf. 1842 Nuculites constricta sp. nov.; Conrad, 1842: 249, pl. 15: 8.

Material.—Two right valve internal moulds, four left valve internal moulds, five incomplete external moulds (NIGP208912–208914). From ES-b (JYZ-9), ES-c (JYZ-10) and JYZ-11, Yushan County, Jiangxi Province (China); Katian (Late Ordovician).

Description.—Small nuculaniform shell; equivalve and inequilateral. Anterior and ventral margin convex; posterior dorsal margin concave; posterior margin forming a short rostrum. A ventral sinus is situated at the posterior ventral margin. The umbo broad, situated subcentral to anterior, orthogyrate or slightly opisthogyrate. Adductor scars well preserved (Fig. 12B). Anterior adductor scar rounded, smaller than elongated elliptical posterior adductor scar, and positioned closer to the dorsal margin. Taxodont dentition, including 8 anterior teeth and 12 posterior teeth; anterior and posterior teeth show similar size. The shell surface ornamented with commarginal lines. The intervals between the lines are much wider than lines themselves.

Remarks.—The distinct ventral sinus and the constricted posterior margin suggest the specimens are close to the type species Palaeoneilo constricta. The shape is variable on the L/H ratio (Hall 1885; McAlester 1962, 1968). In our specimens, the L/H ratio is around 1.5 (Table 6), similar to those figured by Conrad (1842) and Hall (1885: pl. 48: 2, 4). The ornamentation differs slightly from that of previously documented specimens by having much wider intervals between the concentric lines.

This represents the first record of material closely comparable to P. constricta from the Ordovician. However, all previously confirmed occurrences of P. constricta are from the Devonian, representing a significant stratigraphic gap with our Ordovician material. Therefore, we propose the more conservative designation of P. cf. constricta for these specimens.

Subclass Autobranchia Grobben, 1894

Infraclass Pteriomorphia Beurlen, 1944

Order Mytilida Férussac, 1822

Superfamily Modiolopsoidea Fischer, 1886

Family Goniophorinidae Sánchez, 2006

Genus Goniophorina Isberg, 1934

Type species: Goniophorina volvens Isberg, 1934, Kallholn Formation, Silurian, Dalarna, Sweden.

Goniophorina volvens Isberg, 1934

Figs. 13, 14A, B.

1934 Goniophorina volvens sp. nov.; Isberg 1934: 205, pl. 27: 6a–b; pl. 28: 1a–c.

1969 Goniophorina volvens Isberg, 1934; LaRocque and Newell 1969: N395, figs. D2, 10.

1975 Goniophorina volvens Isberg, 1934; Toni 1975: 150, fig. 3K.

Material.—One internal and one external mould of a right valve, one internal and one external mould of a left valve (NIGP208915–208916). All from ES-c (JYZ-10), Yushan County, Jiangxi Province (China); Katian (Late Ordovician).

Description.—Small to medium sized shell, posteriorly elongate sub-elliptical, equivalve, inequilateral. Anterior margin rounded; posterior margin broad and elongated ventrally, with the posteroventral angle less than 90°. Ventral margin straight, with a faint sinus centrally. Broad and prosogyrate umbo is situated in anterior fourth of the shell, strongly curved and projecting above the hinge line. Well-defined carina extending from the beak to the posteroventral angle, slightly curving dorsally and delimiting the posterior slope. Edentulous. Ornamentation consisting of fine commarginal lines.

Remarks.—The carina in Goniophorina volvens described by Isberg (1934) curves ventrally, differing from our specimens. In contrast, the specimen described by Toni (1975) possesses a straight or slightly curved carina. It is worthy to note that the carina of Goniophora contraria (Mansuy, 1912) from the Silurian of China is similar to that of our specimens, curved toward dorsal margin. However, as the internal details of Goniophora contraria are not preserved, it is difficult to confirm whether it is edentulous. Compared with other species, our specimens differ from G. tenuicostata (Harrington, 1938) from the Lower Ordovician of Argentina and G. extensa (Cope, 1996) from the Upper Ordovician of UK in lacking the radial ornamentation. Some species reported in China, such as G. mangbuensis (Guo, 1985) from the Lower Ordovician, G. dianensis (Guo, 1985) from the Silurian and G. contracta (Fang & Cope, 2004) from the Middle Ordovician differ from our specimens in having a weaker carina.

Stratigraphic and geographic range.—Upper Ordovician, Oslo, Norway, Dalarna, Sweden, and Yushan County, China.

Family Modiolopsidae Fischer, 1886

Genus Modiolopsis Hall, 1847

Type species: Pterinea modiolaris Conrad, 1838, Pulaski beds, Upper Ordovician, New York, USA.

Modiolopsis elegantulus Sharpe, 1853

Figs. 13, 15A, B.

1853 Modiolopsis elegantulus sp. nov.; Sharpe 1853: 152, pl. 9: 15a, b.

1991 Modiolopsis? elegantulus Sharpe, 1853; Babin and Gutiérrez-Marco 1991: 123, pl. 5: 1.

2003 Modiolopsis elegantulus Sharpe, 1853; Babin and Beaulieu 2003: 201, pl. 4: 4–6, 8.

Material.—One internal mould and two external moulds of right valves, two internal moulds and one external mould of left valves (NIGP208917–208919). All from ES-b (JYZ-9), Yushan County, Jiangxi Province (China); Katian (Late Ordovician).

Description.—Small to middle modioliform shell; equivalve and inequilateral. Anterior margin rounded; posterior elongated; a sinus in the anterior third of the ventral margin. Umbo situated anterior fourth of the shell, prosogyrate. A posterior umbonal ridge distinct near the umbo, fading towards the postero-ventral angle. Edentulous. Anterior adductor scar small and ill-defined, elliptical (Fig. 15A1). Shell thin, the fine commarginal lines clearly visible on the internal moulds.

Remarks.—Our specimens are very similar to those from the Upper Ordovician of Portugal (Sharpe 1853). In contrast, the specimens from the Middle Ordovician of Spain (Babin and Gutiérrez-Marco 1991) have a thinner shell and stronger ornamentations than ours. Our specimens differ from the type species Modiolopsis modiolaris and other species, such as M. drabovensis (Barrande, 1881) from the Upper Ordovician of Czech Republic, M. pojetai (Jokobsen et al., 2016) from the Middle Ordovician of Australia, and M. cuyana (Sánchez, 1990) from the Upper Ordovician of Argentina, in having a distinct ventral sinus. M. aragonensis (Babin & Hammann, 2001) from the Lower Ordovician of Spain has a stronger umbonal ridge than our specimens.

Stratigraphic and geographic range.—Darriwilian (Middle Ordovician)–lower Silurian. Bussaco, Portugal, Iberian Cordillera, Spain, Maine-et-Loire, France, and Yushan County, China.

Order Cyrtodontida Scarlato & Starobogatov in Nevesskaja et al., 1971

Superfamily Cyrtodontoidea Ulrich, 1894

Family Cyrtodontidae Ulrich, 1894

Subfamily Cyrtodontinae Ulrich, 1894

Genus Cyrtodontula Tomlin, 1931

Type species: Whitella obliquata Ulrich, 1890, from the Cincinnati Group, Upper Ordovician, Ohio, USA.

Cyrtodontula sp.

Fig. 14C, D.

Material.—One internal mould and one external mould of the right valve (NIGP208920) from ES-c (JYZ-10), Yushan County, Jiangxi Province (China); Katian (Late Ordovician).

Description.—Shell large, subquadrate, tumid, and inequilateral, with dimensions of approximately 61 mm in length and 49 mm in height. Anterior and posterior margins rounded; dorsal margin straight; ventral margin broadly rounded. Umbo strongly projected above the dorsal margin, situated in anterior third of the shell, prosogyrate. Shell surface ornamented with fine growth lines (Fig. 14C, D1). A possible elongated posterior lateral tooth visible on the internal mould (Fig. 14C, D2). Ligament and adductor scars not preserved.

Remarks.—The large subquadrate shell with its prominent umbo and a possible posterior lateral tooth suggest the affinity to Cyrtodontula. Due to its solitary occurrence, we propose to assign the specimen under open nomenclature. This specimen is similar to the type species C. obliquata, but its dentition is unclear. Compared with C. hadzeli (Pojeta & Gilbert-Tomlinson, 1977) from the Lower Ordovician of Australia, the posterior part of our specimen is not elongate. Furthermore, compared with C. sohli (Pojeta & Gilbert-Tomlinson, 1977) from the Upper Ordovician of Australia and C. scofieldi (Ulrich, 1890) from the Upper Ordovician of USA, our specimen lacks the prominent umbonal ridge.

Infraclass Heteroconchia Hertwig, 1895

Cohort Cardiomorphi Férussac, 1822

Order Pholadomyida Newell, 1965

Superfamily Pholadomyoidea King, 1844

Family Grammysiidae Miller, 1877

Subfamily Cuneamyinae Morris et al.1991

Genus Cuneamya Hall &Whitfield, 1875

Type species: Cuneamya miamiensis Hall &Whitfield, 1875, Trenton Limestone, Upper Ordovician, New York, USA.

Cuneamya? sp.

Figs. 13, 15C, D.

Material.—One internal mould and one external mould of a right valve, one external mould of a left valve (NIGP208921–208922). All from ES-b (JYZ-9), Yushan County, Jiangxi Province (China); Katian (Late Ordovician).

Description.—Medium sized elongated elliptical shell; equivalve and inequilateral. Anterior margin short and rounded; posterior margin elongated and rounded; ventral margin slightly convex with a sinus central to anterior of the margin; posterior dorsal margin concave. The umbo situated in anterior fourth of the shell, strongly incurved and projected above the dorsal margin, orthogyrate or prosogyrate. A broad umbonal ridge extended from the umbo, but faded toward the posteroventral angle. Dentition should be edentulous. Shell thin, ornamented with strong commarginal wrinkles.

Remarks.—The strongly incurved umbo and uniform concentric ornamentation may suggest an affinity to Cuneamya. The type species C. miamiensis has two lateral sulci: one prominent, extending from the umbo to central of the ventral margin, and another one shallower, located behind the edge of the lunule. These sulci are also well shown in C. grandis (Hind, 1910) from the Upper Ordovician of United Kingdom. In contrast, other Ordovician species lack the second shallow sulcus (Runnegar 1974), such as the species from the Upper Ordovician of Norway (Toni 1975), C. multistriata (Soot-Ryen & Soot-Ryen, 1960) from the Middle Ordovician of Norway, and the specimens described in this paper. Compared with Cuneamya multistriata, our specimens have much more elongate posterior. C. truncatula (Ulrich, 1894) and C. oblonga (Ulrich, 1894) from the Upper Ordovician of USA are both similar to our specimens in having subequal anterior and posterior heights, but C. truncatula and C. oblonga have much narrower anterior part. Cuneamya catilloides (Barrande in Perner, 1903) from the Upper Ordovician of Bohemia differs from our specimens in its oval shell shape.

Discussion

The bivalve association recognized here consists of 10 species dominated by Palaeoneilo rectus; therefore, it can be named as the Palaeoneilo rectus association. On the basis of Stanley’s (2015) conclusion that nuculoids are deposit feeders, we interpret Paullinea, Trigonoconcha, Similodonta, and Concavodonta as the infaunal deposit feeders. Palaeoneilo with elongate constricted shells also might be an infaunal deposit feeder (Hodges 2000; Fang 2006; Stanley 2015). Cuneamya with strong commarginal wrinkles is likely a slow infaunal burrower (Pojeta 1971; Polechová 2019). Modiolopsis, Cyrtodontula and Goniophorina are probably semi-infaunal endobyssate bivalves (Pojeta 1971; Liljedahl 1994; Fang 2006; Stanley 2015). Therefore, the Palaeoneilo rectus association recognized here primarily includes infaunal burrowers with few epifaunal or semi-infaunal filter feeders. The bottom of the epicontinental sea was predominantly occupied by epifaunal brachiopods and their dominance likely inhibited the diversification of epifaunal bivalves owing to the competition for the same eco-space.

Cope and Kříž (2013) proposed that the composition of Ordovician bivalve faunas shows a close correlation with the palaeolatitudes. Generally, Ordovician heteroconch bivalves preferred higher palaeolatitudes, while pteriomorph bivalves prefer lower palaeolatitudes, and Ordovician protobranchs (Nuculiformii and Nuculaniformii) have much higher diversities in lower palaeolatitudes. However, owing to the lack of data on high palaeolatitude bivalve faunas, the correlation between the palaeolatitude and the composition of the Late Ordovician bivalve fauna is complicated (Polechová 2022). Nevertheless, based on their known composition, three different Katian bivalve faunas can be recognized:

– The fauna dominated by pteriomorphs (Fig. 16A–C), such as the faunas from Morocco (Ebbestad et al. 2022), Norway (Toni 1975), and USA (Bretsky 1970a). However, the Moroccan fauna is an exception among these faunas, because it occurs in high latitudes during the Katian, where heteroconch bivalves typically prevail. This suggests that the dominance of pteriomorphs might have been affected by the Boda Warming Event (Fortey and Cocks 2005; Loi et al. 2010; Colmenar et al. 2018), which allowed the lower latitude faunas, to spread to higher latitudes during the Late Ordovician.

– The fauna dominated by nuculiform bivalves (Fig. 16D, E), including those from Northern Ireland and Scotland (Tunnicliff 1982; Cope 1996). These Nuculiformii bivalves are interpreted as inhabiting in a deeper-water environment, which is also confirmed by their contemporaneous brachiopods (Harper 2001; Candela 2006). Polechová (2022) suggested that protobranch bivalves (Nuculiformii and Nuculaniformii) can be dominant in the deeper shelf environments regardless of palaeolatitudes.

– The fauna dominated by nuculaniform bivalves (Fig. 16F), which known only from the Palaeoneilo rectus association in the Xiazhen Formation. The palaeoenvironment of the Xiazhen Formation at Zhuzhai was interpreted as nearshore shallow-water setting (Zhan et al. 2002). This interpretation was based on the dominance of brachiopods and the presence of associated taxa like corals, trilobites, bryozoans, and stromatoporoids (Zhan and Rong 1995a; Lee 2013; Joen et al. 2022). Within the Palaeoneilo rectus association, the pteriomorph bivalves are rare in abundance and low in diversity, despite the Yangtze Platform being in low latitude during the Late Ordovician, which is a setting typically favorable for them. The equatorial cold-water tongue during the Late Ordovician (Jin et al. 2018) affected South China palaeoplate, as it transported the cold-water mass from then South Pole to the subequatorial (or even equatorial) Yangtze Platform and created an unsuitable environment for pteriomorph bivalves, which might be the main reason for the rarity of pteriomorph bivalves in the Palaeoneilo rectus association. The cold-water tongue enabled the cool-water favoring Foliomena brachiopod fauna to be widespread on the Yangtze Platform during the Sandbian and Katian (Late Ordovician). Conversely, Foliomena brachiopod fauna was almost absent in the epicontinental seas of Laurentia, where warm-water brachiopods were flourishing (Jin et al. 2018). A similar difference is also evident in the bivalve associations, and Late Ordovician warm-water pteriomorph bivalves are primarily found in Laurentia but rare in South China (Fig. 16C, F). The dominance of nuculiform bivalves in Ireland and Scotland may also be attributed to the similar cold-water mass, as these regions lay directly in the path of the cold-tongue.

The Palaeoneilo rectus association has some elements in common (Similodonta, Modiolopsis, Cyrtodontula, Goniophorina, Cuneamya) with its contemporaneous bivalve fauna from the Oslo region, Norway (Toni 1975). However, the Oslo fauna exhibits a significantly higher diversity. The lecithotrophic larvae of bivalves are thought to have appeared in the Ordovician (Babin 1995; Sánchez and Babin 2003), and could migrate along deep shelves or between separate terranes (Havlíček et al. 1994; Fatka and Mergl 2009). Both the poleward faunal migrations associated with the Boda Warming Event (Cocks and Torsvik 2002) and the migration corridor from pole to equator provided by High-Latitude Peri-Gondwana Cold Current (Jin et al. 2018), enabled interchange between these two bivalve associations.

Conclusions

• The late Katian bivalve association from the Xiazhen Formation at Zhuzhai contains nine genera and ten species, including four new species: Goniophorina volvens, Modiolopsis elegantulus, Cyrtodontula sp., Cuneamya? sp., Palaeoneilo rectus sp. nov., Palaeoneilo cf. constricta, Trigonoconcha brevis sp. nov., Similodonta minor sp. nov., Concavodonta varius sp. nov., and Paulinea cf. parva. This association is named as the Palaeoneilo rectus association, with the eponymous species being the most abundant one. This well-defined bivalve association will significantly support the investigation on the bivalve GOBE in South China.

• Most of the species in the Palaeoneilo rectus association are infaunal bivalves. The development of epifaunal bivalves was significantly suppressed by the cold-water tongue from then south-pole and the prevalence of brachiopods.

• The dominance of protobranch bivalves (Nuculiformii and Nuculaniformii), with rare pteriomorphs in the Palaeoneilo rectus association is consistent with a low-palaeolatitude setting, based on the faunal composition patterns (Cope and Kříž 2013). However, because the correlation between palaeolatitude and composition of various Katian bivalve faunas is more complicated (Polechová 2022), caution is needed in interpretation of palaeogeographical distribution based on these faunal patterns. In addition, the prevalence of other epifaunal benthic animals such as brachiopods affects the composition and palaeoecological distribution of the co-occurring bivalves.

Acknowledgements

We express our sincere thanks to Chi Xiangri, Liu Congying, Gao Jiahua, Li Fengyan, Ruan Ting, Song Siyu, Yang Jingwen, Yu Xinmiao, Yue Chaosheng, Wang Ting, Wu Shouhan, and Zhu Xiuping (all NIGP) for their kind help in the field, to Li Wenjie (NIGP) for his generous sharing of his samples, and to David A.T. Harper (Durham University, UK) for his help in linguistic check of our manuscript. We are deeply grateful to Marika Polechová (Czech Geological Survey, Czech Republic) and an anonymous reviewer for their insightful comments and suggestions which have substantially improved the manuscript. Financial support for this study was provided by the National Key Research and Development Program of China (2023YFF0803602), the Ministry of Science and Technology of China (2021FY200102) and the State Key Laboratory of Palaeobiology and Stratigraphy (NIGP). This is also a contribution to the IGCP 735: Rocks and the rise of Ordovician life.

Editor: Krzysztof Hryniewicz

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Acta Palaeontol. Pol. 71 (1): 133–153, 2026

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