Rafinesquina (Mesogeina)
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publication ID |
https://doi.org/ 10.4202/app.00102.2014 |
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persistent identifier |
https://treatment.plazi.org/id/DE6FAC27-FFC8-625E-FCD6-5226FDF2BBB4 |
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treatment provided by |
Felipe |
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scientific name |
Rafinesquina (Mesogeina) |
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Rafinesquina (Mesogeina) ? sp. A
Fig. 19 View Fig .
Material.—Internal and external moulds of 5 ventral valves and 4 dorsal valves (MPZ 2015/1398–1402) from alternating marlstone and bioclastic limestone beds of the lower part of the Gabian Formation, Montagne Noire ( France), middle– upper Katian (Ka2–4), Ordovician.
Description.—Shells of large size (largest specimen 48 mm wide), lateral profile concavoconvex, semicircular in outline; maximum valve width at hinge line in early growth stages and at hinge line or near to it in adult stages; cardinal angles acute in early growth stages and right to obtuse in adult stages. Ventral valve approximately 92% as long as wide and approximately 7% as deep as long, lateral profile slightly convex. Ventral interarea planar, apsacline, 5% as long as valve length; delthyrium partially covered by apical pseudodeltidium. Dorsal valve with slightly concave lateral profile; notothyrium completely covered by medially grooved, convex chilidium.
Radial ornament unequally parvicostellate with 5–6 ribs per 2 mm at 10 mm anteromedially from umbo, all of them with angular crests; between two costae up to 6 costellae. Concentric ornamentation consisting of poorly marked, fine fila; well marked growth lines sometimes occur related to evidences of shell damage ( Fig. 19A View Fig 3).
Ventral interior with strong teeth, dental plates extending forward approximately 12% of valve length and with average dental plates divergence angle of 85° (range 75–93°; n = 4); muscle field large, triangular, poorly impressed anteriorly, surrounded posterolaterally by outwardly curved muscle bounding ridges. Adductor scars lanceolate, about 20% as wide as muscle scars, located between diductor scars but not surrounded anteriorly.
Dorsal interior with thin and divergent socket ridges (96°) bounding triangular dental sockets; bifid cardinal process with ridge-like, subparallel, medially convergent lobes. It is located on a very reduced or even absent notothyrial platform, continuous anteriorly with very thin and low median ridge halving poorly impressed dorsal muscle field. Pseudopunctae thin and poorly developed in both valve interiors.
Remarks.—All the external characters as well as dorsal internal characters suggest the inclusion of the studied material within Rafinesquina (Mesogeina) , but the ventral internal features display some similarities with some leptostrophiids. It includes the triangular muscle field outline and the outwardly curved muscle bounding ridges, although the absence of denticles in the hinge line, a priori, discards the assignment of this material to the family Leptostrophiidae . As the studied material displays mixed features between the two families, it could belong to a new genus with an uncertain family position. On the other hand, the dorsal interior of the studied shells displays some features in common with Rafinesquina (Mesogeina) , as the subparallel, plate like cardinal process lobes. To confirm the generic assignment and ascertain the family position of these shells, more material, especially adult dorsal valves, is needed.
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