Molecular clock studies have suggested that major modern Bivalve lineages are likely to have diverged early in the Phanerozoic, although the poor fossil record of early Bivalves makes this difficult to assess. Notably, the Archiheterodonta and Palaeotaxodonta have been predicted to have diverged from one-another during the Cambrian, although there is little evidence of this; perhaps unsurprisingly, given the rarity of Cambrian and Early Ordovician Bivalves.
In a paper published in the Journal of Molluscan Studies on 15 October 2024, Ren-jie Zhang of the Hubei Key Laboratory of Paleontology and Geological Environment Evolution and the Wuhan Center of the China Geological Survey, Paul Johnston of the Department of Earth and Environmental Sciences at Mount Royal University, Zhi-jun Niu, also of the Hubei Key Laboratory of Paleontology and Geological Environment Evolution and the Wuhan Center of the China Geological Survey, Chu-an Li of the Guangdong Marine Geologic Survey, Zhi-hong Wang, again of the Hubei Key Laboratory of Paleontology and Geological Environment Evolution and the Wuhan Center of the China Geological Survey, Kun Hu of the Guangdong Geologic Survey Institute, Fang Song, Yao-yan He, and Jin-lan He, again of the Hubei Key Laboratory of Paleontology and Geological Environment Evolution and the Wuhan Center of the China Geological Survey, Xiao-ming Lin, again of the Guangdong Geologic Survey Institute, and Wen-qiang Yang, once again of the Hubei Key Laboratory of Paleontology and Geological Environment Evolution and the Wuhan Center of the China Geological Survey, describe a new species of Heteroconch Bivalve from the Middle Ordovician Dongchong Formation of Guangdong Province, China.
The new species is named Yunfuconcha bimenta, where 'Yunfuconcha' means 'Yunfu-shell', in reference to the city of Yunfu in Guangdong Province, and 'bimenta' means 'double-state', in reference to the combination of Archiheterodont-like and Palaeotaxodont-like features seen in the species.
Yunfuconcha bimenta, Ordovician, Dongchong Formation, near Gankeng Village, Yunan County, Guangdong Province, China. (A)–(E) Holotype, Ng525, right valve. (A) Incomplete internal mould. (B)–(D) Latex cast. (B) Internal view; arrow marks possible inner edge of posterior adductor scar. (C) Dorsal oblique view showing subdued umbo. Arrows mark short opisthocline ridges, better seen in (E). (D) Ventral oblique view showing angular contact of ligament area and anterior hinge plate. Long arrow marks prominent ligament ridge. An apparent second prominent ridge to the right may be owing to breakage developed on the mould during casting. Short arrow indicates one of several short oblique ridges on anterior portion of ligament area. (E) Close-up of hinge area on internal mould of holotype. Arrows show short opisthocline ridges (grooves on mould). Vertical white lines show inferred lateral extent of ligament area. (F)–(H) Paratype, Ng526, right valve. (F) Silicone rubber cast; ventral oblique view; long arrow marks ligament ridge; short arrows mark short opisthocline ridges near boundary of the ligament area. (G), (H( Incomplete internal mould. (G) Anterodorsal portion of valve. White box is detailed in (H). (H) Detail from (G); arrows mark subtle radial ornament (this portion of mould is composite). (A), (E), (G), and (H), unwhitened. Scale bars are 5 mm. Abbreviations: la, ligament area; pa, posterior adductor scar. Zhang et al. (2024).
Yunfuconcha bimenta has a subequilateral shell with a distinctive dentition on the anterior part of the hinge plate (in Bivalves 'dentition' refers to protrusions on the articulations of the shells, which generally fit into sockets on the opposing shell, rather than actual teeth).
Zhang et al. note that Yunfuconcha bimenta shows some similarities to the Early Ordovician Palaeotaxodont Afghanodesma, although, curiously, they also challenge the validity of Afghanodesma on the basis that the type material for the species is a series of wax casts made in the field, rather that the actual fossil material, in breach of the rules of the International Commission on Zoological Nomenclature. This is technically correct, but seems unhelpful in this instance, and seems a strange point to raise given that they describe their own material as 'moulds were with latex or silicone rubber for study'.
Zhang et al. suggest that Yunfuconcha bimenta is either a stem-group Heteroconch (i.e. a species more closely related to the Heterochomchia than any other group, but not decended from the last common ancestor of all extant members of the group) or an early member of the Archiheterodonta, thought to be one of the earliest branching groups within the Heterochomchia.
Hypothesized phylogenetic relationships of Yunfuconcha as a stem Archiheterodont or a stem Heteroconch. Solid vertical lines denote known paleontological ranges of lineages. Dashed vertical lines denote inferred ranges. Horizontal grey lines mark late, middle and early intervals of respective period. The relative width of the Ordovician Period is expanded. Images from left to right: Yuexiconcha duplicata, Praeleda subtilis, Yunfuconcha bimenta, Crassatellopsis yongei, Carminodonta crossi, Paracyclas alleni, Cypricardella inflata, and Sanguinolites? concentrirugosa. Zhang et al. (2024).
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