Plectambonitoidea Jones, 1928

Gutiérrez-Marco, Juan Carlos & Villas, Enrique, 2007, Brachiopods from the uppermost Lower Ordovician of Peru and their palaeogeographical significance, Acta Palaeontologica Polonica 52 (3), pp. 547-562 : 549-550

Superfamily Plectambonitoidea Jones, 1928 Plectambonitoidea gen. and sp. indet.

Fig. 3C–F .

Material.—Internal and external mould of one ventral valve (MGM 5967), internal moulds of three ventral valves (MGM 5968X, 5970X, 5971X) and external moulds of two shells with conjoined valves (MGM 5969X, 5972X) from Carcel Puncco section; horizons D, E, and F of the San José Formation.

Description.—Shell convexoplane to convexoconcave, up to 9 mm long, transverselly elongate, 52–58% as long as wide, auriculate, with slightly invaginated anterior margin. Multicostellate to slightly parvicostellate, with accentuated ribs only in central part, 10 ribs per 2 mm counted five mm anteromedianly from umbo; strong and irregular growth striae accumulated on margin of shell; subtle impersistent rugae on posterolateral areas. Ventral valve plane to gently concave, except in the lateral extremities which are almost planar, with narrow and low median fold. Ventral interarea catacline, flat, 13–14% as long as valve, transversally striated, with apically perforated pseudodeltidium occupying delthyrium posterior half; delthyrium anterior half filled by cardinal process. Dorsal valve convex, with maximum depth posteriorly, subplanar lateral extremities and narrow sulcus. Dorsal interarea apsacline, flat, shorter than ventral interarea, with wide chilidium totally covering notothyrium and welded to possibly trifid cardinal process.

Ventral interior with strong teeth and conspicuous crural fossettes, very short dental plates converging onto valve floor and with bases diverging anteriorly; muscle field short, 30% as long as valve, 18–25% as wide as valve, bilobed, with diductor scars not enclosing anteriorly to adductor scars. Very strong platform, 5–7 mm long, subtending angle of about 110 ° with underlying marginal area, but not related to any kind of geniculation of valve exterior. Vascula genitalia digitated very close to their origin at dental plate bases. Vascula terminalia strongly impressed on margin of platform and on valve margins.

Discussion.—It is not possible to precisely determine this plectambonitoid without knowing its dorsal interior, on which all the diagnostic characters for family and subfamily classification are based ( Cocks and Rong 1989). An apparently trifid cardinal process, welded to a well developed chilidium, can be observed in the collected shells exteriors, but it is not possible to ascertain if it is undercut or not. With only this information on their dorsal interior these shells can only be discriminated from the bimuriids, plectambonitids and taffiids, with simple cardinal processes, but could be assigned to any of the other seven families recognised within the superfamily ( Cocks and Rong 1989). The resupinate profile of the studied shells, although not very common in any of these seven families, is not enough to discriminate among them.

Figures

Fig. 3. Taffiid brachiopod Ahtiella sp. A. MGM 5965X, internal mould (A1) and latex cast of interior (A2) of dorsal valve. B. MGM 5966X, latex cast of exterior in ventral view (B1) and postero−ventral view (B2) of shell with conjoined valves. C–F. Plectambonitoidea gen. et sp. indet. C. MGM 5967X, internal mould (C1) and latex cast of exterior (C2) of ventral valve. D. MGM 5969X, latex cast of exterior in postero−ventral view (D1) and ventral view (D2) of shell with conjoined valves. E. MGM 5972X, latex cast of exterior in postero−ventral view (E1) and ventral view (E2) of shell with conjoined valves. F. MGM 5968X, latex cast of interior (F1) and internal mould (F2) of ventral valve. Scale bars 5 mm.