Paralenorthis estenaensis 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 : 296-298

Paralenorthis estenaensis Reyes−Abril and Villas sp. nov.

Fig. 7.

Etymology: After the type locality at Navas de Estena, Toledo Province.

Type material: Holotype: MGM−6068−O, internal mould of ventral valve. Paratypes: eighteen internal moulds and twelve external moulds of ventral valves, fourteen external moulds and twelve internal moulds of dorsal valves, and internal and external moulds of two ventral valves and three dorsal valves, with numbers: MGM−6056−O to MGM−6116−O.

Type locality: NE−IIIA, 5600 m SE of Navas de Estena , Province, Spain. UTM coordinates: lat 39 ° 27'33''N , long 4 ° 28'31''W.

Type horizon: About 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.— Paralenorthis species of subquadrangular outline, with acute cardinal angles in young stages, changing to rectangular to slightly obtuse in adult stages; ventral valve strongly convex, radial ornamentation costate with high ribs, numbering 26–38; bilobed ventral muscle field, deeply impressed, extending forward 28–44% of valve length; strong notothyrial platform with thick, ridge−like cardinal process, flanked by slender notothyrial ridges.

Description.—Shell ventribiconvex, up to 19.5 mm long, of subquadrangular outline, with maximum width at hinge line, acute cardinal angles in young stages and rectangular to slightly obtuse in adults, 63–84% as long as wide (m =75%, v = 0.5%, n = 43), with rectimarginate anterior commissure. Ventral valve strongly convex, 14–45% as deep as long (m = 24%, v = 1%, n = 27), with maximum depth at mid length of valve; ventral interarea apsacline, flat to slightly curve, 7–19% as long as valve (m = 12%, v = 0.1%, n = 25), with wide and open delthyrium. Dorsal valve slightly convex, 61–87% as long as wide (m = 71%, v = 0.5%, n = 43), with shallow median sulcus progressively widening forward; dorsal interarea flat, steeply anacline to catacline, 5–10% as long as valve, with open notothyrium, flanked by radial striae in adult shells. Radial ornamentation costate, with few postero−lateral ribs that run subparallel to hinge line, arising beyond umbo; ribs high, subtriangular in section, becoming rounded in latest growth stages, of similar width to intercostal spaces, numbering 26–38 at valve margin, with 29 the commonest number, and 4–6 per 5 mm counted 5 mm anteromedially from umbo; 10–12 capillae per mm, 5 fila per mm and up to 2 strong growth lines in largest shells.

Ventral interior with triangular teeth and strong crural fossettes; dental plates short, divergent onto valve floor, continuous with muscle bounding ridges, curved inwardly to bound diductor scars; muscle field large, bilobed and scalloped, with diductor scars longer and wider than adductor scar, not enclosing it anteriorly, 28–44% as long as valve (m = 35%, v = 0.5%, n = 28), 24–40% as wide as valve (m = 32%, v = 0.2%, n = 22), with highest values in adult stages; adductor scars progressively widening forward, 9–30% as wide as muscle field (m = 16%, v = 0.3%, n = 22); divergent vascula media, arising from anterior end of diductor scars and not observable beyond mid length of valve.

Notothyrial platform high, sloping posteriorly and continuous anteriorly with strong median ridge, lowering and occasionally narrowing forward; cardinal process blade−like and robust, flanked by slender notothyrial ridges in adult stages; brachiophores rod−like, divergent, welded to notothyrial platform; dental sockets excavated in secondary shell deposits, bisected posteriorly by short ridges; quadripartite muscle field, with anterior adductor scars lobate (see Fig. 7I 1) and longer

Fig. 7. Orthid brachiopod Paralenorthis estenaensis Reyes−Abril and Villas sp. nov. A. MGM−6068−O, internal mould (A 1) and latex cast of interior (A 2) of H ventral valve. B. MGM−6077−O, latex cast of exterior of ventral valve. C. Holotype, MGM−6074−O, internal mould (C 1) and latex cast of interior (C 2) of ventral valve. D. MGM−6058−O, latex cast of exterior of ventral valve. E. MGM−6070−O, internal mould (E 1) and latex cast of interior (E 2) of ventral valve.

F. MGM−6114−O, latex cast of exterior of dorsal valve. G. MGM−6097−O, internal mould (G 1) and latex cast of interior (G 2) of dorsal valve.

H. MGM−6091−O, latex cast of exterior of dorsal valve. I. MGM−6098−O, internal mould (I 1) and latex cast of interior (I 2) of dorsal valve. J. MGM−6104−O, latex cast of exterior of dorsal valve. Scale bars 5 mm.

doi:10.4202/app.2009.0032

that posterior adductor scars, separated by straight ridges, diverging anteriorly, 22–61% as long as valve (m = 49%, v = 2%, n = 19) and 18–42% as wide as valve (m = 33%, v = 0.4%, n = 23).

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

Discussion.—The costate, capillate and filate ornaments, in addition to the ventribiconvex profile and the divergent ventral vascula media, indicate an affinity to Paralenorthis Havlíček and Branisa, 1981 . The new species differs from other congeneric species in its subquadrangular outline, with changing cardinal angles, acute in young stages to rectangular in adults, the ponderous ventral muscle field and high and strong cardinal process, and more numerous ribs.

The studied shells display similar external features to Orthis miniensis Sharpe, 1849 (Middle Ordovician of Valongo, Portugal), which is based on two single external moulds. Its precise location within the Valongo Formation is not known and it has not been revised since its erection. Consequently, despite the fact that their ribs and intercostal spaces have a similar width to the Spanish sample, and their rib number ranges partly overlap, 20–30 in Orthis miniensis and 26–38 in the Spanish shells, it is not possible to make a precise comparison between the two. Actually, it is not even possible to make a generic assignment of this Portuguese species because a lack of internal features. In consequence, it is proposed herein to restrict the use of the name O. miniensis to the two specimens illustrated by Sharpe (1849: pl. 6: 3a, b), and stored at NHM with registration numbers NHM 82896 and NHM 82897. Additional Portuguese specimens of “ Orthis ” miniensis have been recently illustrated (fragments of valves) from the base of the Brejo Fundeiro Formation (Cacemes Group) of Bussaco ( Romano et al. 1986) and from the middle part of the Moncorvo Formation of Trás−os−Montes ( Rebelo and Romano 1986). Sá (2005) described as Paralenorthis? miniensis ( Sharpe, 1849) the abundant material from 6 different fossil localities in the Moncorvo Formation, some of which could belong to P. estenaensis . The older records of O. miniensis from the Armorican Massif are based on incomplete descriptions ( Tromelin and Lebesconte 1876) or illustrations ( Péneau 1950), impossible to currently place within Sharpe’s species because of the inconsistencies in its original description.

The rib number of this new Paralenorthis , up to 38 in adult shells, is much higher than in most of its co−generic species characterised by high rib numbers, like the type species, Paralenorthis immitatrix Havlíček and Branisa, 1980 , with up to 28 ribs in the Peruvian specimens ( Gutiérrez−Marco and Villas 2007). Nevertheless it is extraordinarily similar in rib number to P. alata, with 29–38 ribs, occurring in the higher horizons of the same Navas de Estena Formation; in any case, the mucronate outline of P. alata in all growth stages allows an easy distinction from the new species, with obtuse cardinal angles and subquadrangular outline in adult stages. In young stages, with acute cardinal angles and small ears, the new species is similar to Paralenorthis subalata (Ulrich and Cooper, 1938) , but the rib number of this species, up to 29, is never so high as in Paralenorthis estenaensis Reyes−Abril and Villas sp. nov.

Stratigraphic and geographic range.—Known from both flanks of the Navas de Estena syncline (localities SP−II, south of San Pablo de los Montes, Toledo Province, and NE−III and IIIA, southeast of Navas de Estena, Ciudad Real Province), in the lower part of the Navas de Estena Formation. Also the species was recognised from the locality AD−I, (south of Almuradiel, Ciudad Real Province) in eastern Sierra Morena, from the lower part of the Río Formation.