Cytherellina caerulea Guillam & Forel & Song & Crasquin 2022, sp. nov.

Cytherellina caerulea Guillam & Forel sp. nov. urn:lsid:zoobank.org:act: 094CDF1E-56D9-4459-8A8C-367F44755F7B Fig. 8H–L Cavellina sp. Coen, 1989: 312, pl. 2 fig. 10. Cavellina prona Wei, 1988 – Song & Gong 2019: fig. 4s. Cytherellina subclara Wang, 1983 – Song & Gong 2019: fig. 5o. non Cavellina prona Wei, 1988: 304, pl. 112, fig. 14-15. non Cytherellina subclara Wang, 1983: 190, pl. 5, fig. 20-24. 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. Material examined Holotype CHINA • 1 complete carapace (Fig. 8H); Blue Snake section, Gelaohe Formation, sample 19BAI 68; Famennian, late Devonian; P6M 3903. Paratypes CHINA • 1 complete carapace (Fig. 8I); Blue Snake section, Gelaohe Formation, sample 19BAI 68; P6M 3904 • 1 complete carapace (Fig. 8J); Blue Snake section, Gelaohe Formation, sample 63; P6M 3905 • 1 complete carapace (Song & Gong 2019: fig. 5o); Blue Snake section, Gelaohe Formation, bed 17 (Song & Gong 2019: fig. 3); GBL2014007 (palaeontological collections of the Museum of the China University of Geosciences, Wuhan, China). All from the Famennian, late Devonian. Other material CHINA • 1 complete carapace (Fig. 8K); Blue Snake section, Gelaohe Formation, sample 19BAI 63; P6M 3907 • 1 complete carapace (Fig. 8L); Blue Snake section, Gelaohe Formation, sample 19BAI 69; P6M 3907 • 7 complete carapaces; Blue Snake section, Gelaohe Formation, samples 19BAI 60, 19BAI 63, 19BAI 68, 19BAI 69. All from the Famennian, late Devonian. Dimensions See Fig. 4D. 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. Cytherellina subclara 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. The new species differs from Cytherellina obusa Lethiers, 1976 in Casier (1989) from the Frasnian of Belgium by its smaller size (holotype: L = 1.19 mm, H = 0.80 mm, W = 0.40 mm), the absence of overlap at PVB and PDB, its more arched DB with a stronger angle at mid-L and its pointed posterior end. Cytherellina caerulea sp. nov. is also different from Cytherellina sp. 3 in Olempska & Chauffe 1999 from the Famennian of Iowa (Olempska & Chauffe1999) by its smaller size (figured specimen in Olempska & Chauffe 1999: L = 960µm, H = 480 µm), its straighter PDB, its stockier morphology and the overlap which is really weak in Cytherellina sp. 3 in Olempska & Chauffe 1999. It is morphologically close to Sulcella cf. altifrons Jones, 1968 in 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. Occurrence Gelaohe Formation, Baihupo section, Dushan, Guizhou, South China, Famennian, late Devonian (Coen 1989). Gelaohe and Tangbagou Formations, Blue Snake section, Guizhou, South China, Famennian–Tournaisian, late Devonian–early Carboniferous (Song & Gong 2019). Samples 19BAI 60, 19BAI 63, 19BAI 68, 19BAI 69, Gelaohe Formation, Blue Snake section, Famennian, late Devonian (this work).

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