Sivorthis calatravaensis Reyes−Abril and Villas, 2010

Reyes-Abril, Jaime, Villas, Enrique & Gutiérrez-Marco, Juan Carlos, 2010, Orthid brachiopods from the Middle Ordovician of the Central Iberian Zone, Spain, Acta Palaeontologica Polonica 55 (2), pp. 285-308 : 300-302

Sivorthis calatravaensis Reyes−Abril and Villas sp. nov.

Figs. 9, 10A–D.

Etymology: After the Campo de Calatrava region (Ciudad Real), with important Ordovician fossiliferous localities.

Type material: Holotype: MGM−6199−O, internal and external moulds of ventral valve. Paratypes: twenty one internal moulds and fifteen external moulds of ventral valves, eighteen external moulds and thirteen internal moulds of dorsal valves, and internal and external moulds of four ventral valves and two dorsal valves, with numbers: MGM−6170−O to MGM−6247−O.

Type locality: NE−IIIA, 5600 m SE of Navas de Estena , Ciudad Real Province, Spain. UTM coordinates:lat 39 ° 27'33''N , long 4 ° 28'31''W. Type horizon: 130 metres above the base of the Navas de Estena Formation , Didymograptus artus Biozone , lower Oretanian , middle Darriwilian Stage of the Middle Ordovician Series .

Diagnosis.— Sivorthis species of variably auriculate outline, acute cardinal angles, 60–85% as long as wide and sulcate anterior commissure; radial ornamentation irregularly costellate, with 15–25% of costae ramified and 23–36 ribs in valve margin, capillate and filate; subtriangular ventral muscle field, 20–40% as long as valve; quadripartite dorsal muscle field, with anterior pair larger than posterior, and median ridge widest at its anterior end by mid valve length.

Description.—Shell ventribiconvex, up to 25 mm long, of variably auriculate outline, maximum width along hinge line, acute cardinal angles, 60–85% as long as wide (m = 72%, v = 1%, n = 36) and weakly sulcate anterior commissure, more accentuated in deformed valves. Ventral valve convex, with weak median fold and subplanar posterolateral areas, 20–46% as deep as long (m = 30, v = 1%, n = 13); ventral interarea planar to slightly curved, apsacline, 5–11% as long as valve (m = 7%, v = 0. 04%, n =13), with wide open delthyrium. Dorsal valve slightly convex, with shallow and wide median sulcus; dorsal interarea shorter than ventral interarea, with open notothyrium. Radial ornamentation irregularly costellate, with 15% of costae ramified by 10 mm growth stage and 25% ramified in largest valves; high ribs with rounded crests, numbering 23–36 at valve margins and 6–9 per 5 mm at 5 mm anteromedially from umbo; capillate and filate.

Ventral interior with pedicle callist weakly developed,

Fig. 9. Orthid brachiopod Sivorthis calatravaensis Reyes−Abril and Villas sp. nov. A. Holotype, MGM−6199−O, internal mould (A 1), latex cast of interior H (A 2) and latex cast of exterior (A 3) of ventral valve. B. MGM−6203−O, internal mould (B 1) and latex of the interior (B 2) of ventral valve. C. MGM−6211−O, latex cast of exterior of ventral valve. D. MGM−6242−O, internal mould (D 1) and latex cast of interior (D 2) of dorsal valve. E. MGM−6219−O, latex cast of exterior of ventral valve. F. MGM−6239−O, internal mould (F 1), latex cast of interior (F 2) and latex cast of exterior (F 3) of dorsal valve. G. MGM−6241−O, (NEII), internal mould (G 1) and latex cast of interior (G 2) of dorsal valve. H. MGM−6247−O, latex cast of exterior of dorsal valve. Scale bars 5 mm.

doi:10.4202/app.2009.0032

triangular teeth and crural fossettes; dental plates short and perpendicular to valve floor, diverging anteriorly and continuous with muscle bounding ridges; subtriangular muscle field with anterior margin slightly rounded, 20–40% as long as valve (m = 29%, v = 0.2%, n = 24), 16–55% as wide as valve (m = 24%, v = 0.1%, n = 24), with adductor track halved by weak median ridge, 10–22% as wide as muscle field (m = 17%, v = 0.001%, n = 6), diductor scars about as long as adductor scars, not enclosing them anteriorly; ventral vascula media parallel and adjacent, arising from adductor track and diverging at anterior third of valve.

Notothyrial platform short and high, excavated anteriorly by posterior adductor scars and continuous with median ridge, widening forward and reaching about mid length of valve in adult stages (see Fig. 9D 1, D 2); cardinal process ridge−like, narrow and short; brachiophores rod−like, short, welded to notothyrial platform; dental sockets broad, excavated on secondary shell deposits; muscle field quadripartite, 34–63% as long as valve (m = 48%, v = 0.5%, n = 15), 16–35% as wide as valve (m= 26%, v = 0.2%, n = 20), with anterior adductor scars at least double in width and length than posterior adductor scars, separated by curved ridges that diverge anteriorly.

Radial ornamentation impressed on interior of both valves, with internal margins crenulated with deep and narrow grooves, corresponding to crest of ribs, and wide eminences with shallow sulci on intercostal spaces.

Discussion.—The weakly sulcate anterior commissure, costellate ornamentation and parallel and adjacent ventral vascula media of these orthids are amongst the most distinctive diagnostic features of Sivorthis . But these specimens can be discriminated from all the known species of this genus by their auriculate outline, thus a new species is proposed.

The irregularly costellate ornamentation of the new species, with 23–36 ribs, most closely resembles that of Sivorthis tenuicostata (Cooper, 1956) , with 30–40 ribs, and Sivorthis friendsvillensis (Cooper, 1956) , with 32–36 ribs; they can be distinguished by the acute cardinal angles of the new species, the obtuse cardinal angles of S. tenuicostata and the rectangular to obtuse cardinal angles of S. friendsvillensis . The new species has more numerous ribs than in Sivorthis ardmillanensis ( Reed, 1917) , which has notably coarser ribs. Its auriculate outline distinguishes it from Sivorthis eucharis (Schuchert and Cooper, 1938) that has a rounded outline and obtuse cardinal angles. The relatively coarse costellation of the new species is very different from the finely costellate type species, Sivorthis filistera Jaanusson and Bassett, 1993 , with 40–46 ribs. Its ribbing and weakly sulcate anterior commissure allows simple distinction from Sivorthis noctilio ( Sharpe, 1849) , with 36–52 observed ribs and strongly sulcate shell, occurring in the overlying horizons and described below.

Stratigraphic and geographic range.—Known from the Navas de Estena syncline (locality NE−IIIA), in the lower horizons of the Navas de Estena Formation, and from eastern Sierra Morena (localities CC−IIIA, Calzada de Calatrava; VM−I, Viso del Marqués, Ciudad Real Province; and AQ−II, Aldeaquemada, Jaén Province), in the lower part of the Río Formation.