Sivorthis noctilio ( Sharpe, 1849 )

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 : 302-305

Sivorthis noctilio ( Sharpe, 1849)

Figs. 10E–I, 11.

1849 Orthis noctilio sp. nov.; Sharpe 1849: 151, pl. 6: 2a–2c.

1955 Orthis vespertilio Sow. ; Ramírez y Ramírez 1955: 421, 422, pl. 71: B.

1956 Orthis ( Harknessella ?) cf. noctilio Sharpe ; Thadeu 1956: 22, pl. 9: 1.

1984 Monorthis noctilio (Sharpe) ; Gutiérrez−Marco et al. 1984: 315, 316.

1994 “ Orthis ” noctilio Sharpe ; Mayoral et al. 1994: pl. 1: 1–6.

Lectotype: Internal and external moulds of a dorsal valve, figured by Sharpe (1849: pl. 6: 2a, b) and stored at NHM with registration numbers NHM 82902 and NHM 82904.

Type locality: The lectotype was reported by Sharpe (1849) to have been “found in slate near Valongo” (north, south, or west of that city according with those stated in the original paper). The more accessible outcrop exposed today near Valongo is found about 2 km southeast of the city, along the road to San Pedro da Cova in the western flank of the Valongo anticline, where S. noctilio has been determined in one of the studied localities (grid reference lat 41 ° 10'45''N, long 8 ° 30'09''W) .

Type horizon: Lower part of the Valongo Formation, lower Oretanian, middle Darriwilian Stage of the Middle Ordovician Series.

Material.—Twenty nine internal moulds and twenty three external moulds of ventral valves, nine internal moulds and thirteen external moulds of dorsal valves, internal and external moulds of two ventral valve and three dorsal valves, and two shells with conjoined valves, with numbers: MGM−6248−O to MGM−6328−O.

Emended diagnosis.— Sivorthis with mucronate outline in young stages and semicircular outline in adults, cardinal angles acute to rectangular and strongly sulcate anterior commissure; radial ornamentation ramicostellate, with up to three generations of costellae in adult shells, and 36 to 52 ribs at valve margin.

Description.—Shell ventribiconvex, up to 28 mm long and 52 mm wide, of mucronate outline in young stages changing to semicircular in adults, maximum width along hinge line, cardinal angles acute to rectangular, rarely slightly obtuse in very large shells, 51–84% as long as wide (69% in lectotype), with highest values in largest shells; anterior commissure

Fig. 10. A–D. Orthid brachiopod Sivorthis calatravaensis Reyes−Abril and Villas sp. nov. A. MGM−6213−O, internal mould (A 1) and latex cast of interior H (A 2) of dorsal valve. B. MGM−6236−O, latex cast of exterior of dorsal valve. C. MGM−6228−O, internal mould (C 1) and latex cast of interior (C 2) of dorsal valve. D. MGM−6226−O, latex cast of exterior of dorsal valve. E–I. Sivorthis noctilio ( Sharpe, 1849) . E. MGM−6269−O, internal mould (E 1) latex cast of interior (E 2) and latex cast of exterior (E 3) of dorsal valve. F. MGM−6266−O, internal mould (F 1) and latex cast of interior (F 2) dorsal valve. G. MGM−6303−O, latex cast of exterior of ventral valve. H. MGM−6285−O, internal mould (H 1) and latex cast of interior (H 2) of ventral valve. I. MGM−6322−O, latex cast of exterior of ventral valve. Scale bars 5 mm.

doi:10.4202/app.2009.0032

strongly sulcate, more accentuated with deformation. Ventral valve convex, with strong median fold and subplanar posterolateral areas, 11–39% as deep as long (29% in lectotype), with highest values in deformed shells; ventral interarea planar to slightly curved, apsacline, 5–11% as long as valve, with wide and open delthyrium. Dorsal valve weakly convex with accentuated median sulcus, occupied by 4 to 6 ribs; dorsal interarea plane, anacline, 5–8% as long as valve, with radial striae parallel to open notothyrium. Radial ornament ramicostellate with high and narrow ribs on rounded crests, numbering 36–52 in adult valve margins and 3–8 per 5 mm at 5 mm anteromedially from umbo; up to three generations of costellae in adult shells, 25–30 fila per mm and 2–3 strong growth striae in adult shells.

Ventral interior with triangular teeth and crural fossettes; dental plates short, continuous with slender muscle bounding ridges and well developed pedicle callist; muscle field subcordate, 24–43% as long as valve (m =34%, v = 0.2%, n = 21), 18–32% as wide as valve (m = 24%, v = 0.2%, n = 20), with diductor scars longer than adductor scars, separated by slender median ridge, partially enclosing them anteriorly, and adductor track 9–16% as wide as muscle field. Ventral mantle canal system saccate, with parallel and adjacent vascula media, arising from adductor track and diverging at anterior third of valve.

Dorsal valve with short notothyrial platform, posteriorly sloping and continuous with wide, short median ridge; cardinal process ridge−like, thick and high, flanked by two notothyrial ridges; brachiophores rod−like, welded to notothyrial platform, and narrow dental sockets excavated on secondary shell deposits; muscle field with poorly impressed adductor scars, 18–30% as wide as valve.

Shell with radial ornamentation well impressed on interiors of both valves, often obscuring muscle fields; internal margins crenulated with deep and narrow grooves, corresponding to crest of ribs, and wide eminences with shallow sulci on intercostal spaces, not reaching valve margin.

Discussion.— Orthis noctilio Sharpe, 1849 was described originally from the Darriwilian horizons within the Valongo Formation, close to Valongo ( Portugal). The description was based on the internal and external moulds of a dorsal valve and the internal mould of a ventral valve ( Sharpe 1849: pl. 6: 2a–2c) and although referred to in several stratigraphic papers (e.g., Delgado 1908; Gutiérrez−Marco et al. 1984; Sá 2005), there has not been a taxonomic update until this study. In spite of the scarcity of the type material and the brief original description of O. noctilio , its features are distinctive enough to assign it with certainty the above described taxon. The study of the large Spanish sample and the revision of the Portuguese type material, stored at NHM have allowed an emended diagnosis and the proposal of a generic assignment for the species. The dorsal valve figured by Sharpe (1849: pl. 6: 2a, b) has been selected as the lectotype of the species. Another of the species defined by Sharpe (1849) from the Middle Ordovician of Valongo, Orthis duriensis , is similar externally to O. noctilio , and could be conspecific. Nevertheless, its revision has not been possible, since the dorsal valve illustrated by Sharpe (1849: pl. 6: 4a, b), could not be found among the other Valongo types, stored at NHM (Sarah Long, personal communication 2007).

The radial ornamentation of O. noctilio , finely costellate and filate, with one or two generations of costellae close to the umbo, besides its ventral vascula media, parallel and adjacent until the anterior third of the valve, allows assignment to Sivorthis . Within the genus, it can be discriminated from the other known species by its strongly sulcate anterior commissure and its high rib number. Most Sivorthis species are coarsely costellate, and even S. filistera Jaanusson and Bassett, 1993 , with the finest ribs, numbering 40–46 at the valve margin, has a lower rib number than Sivorthis noctilio , with counts up to 52. Another clearly distinguishing feature from congeneric species is the outline, with obtuse cardinal angles for most shells, while they are acute to mucronate in S. noctilio .

Stratigraphic and geographic range.—Known from Valongo and Arouca localities, northern Portugal, in the middle part of the Valongo Formation ( Sharpe 1849); from the locality VPA (Ventas con Peña Aguilera, Toledo Province, central Spain), in the lower part of the Navas de Estena Formation; and from Sion−les−Mines and “Laillé high” east from Guichen (south of Rennes, Armorican Massif, western France), in the lower part of the Traveusot Formation; all of them of early Oretanian age. All of these localities are of early Oretanian (mid Darriwilian) age.