Paralenorthis, Havlicek and Branisa, 1980

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 : 294-296

Paralenorthis alata (J. de C. Sowerby in Murchison, 1839)

Fig. 6 .

1839 Spirifer alatus sp. nov.; J. de C. Sowerby in Murchison 1839: 49, pl. 22: 7.

1849 Orthis alata (J. de C. Sowerby); Salter in Murchison 1849: 55, pl. 5: 3.

1869 Orthis carausii Davidson ; Davidson 1869: 229, pl. 33: 1–7.

1869 Orthis alata (J. de C. Sowerby); Davidson 1869: 232, pl. 33: 17–21.

1912 Orthis carausii Davidson ; Matley 1912: 78.

1918 Orthis calligramma var. alata Sow. ; Born 1918: 336, pl. 24: 2a, b.

1969 Lenorthis alata (J. de C. Sowerby); Bates 1969: 10, pl. 2: 14; pl. 3: 1–7; pl. 4: 1–10; pl. 5: 1–6.

1993 Paralenorthis alata (J. de C. Sowerby); Jaanusson and Bassett 1993: 34.

Material.—Eleven internal moulds and eight external moulds of ventral valves, six external moulds and five internal moulds of dorsal valves, besides internal and external moulds of two ventral valves and four dorsal valves, with numbers MGM−6018−O to MGM−6055−O.

Emended diagnosis.— Paralenorthis with mucronate outline, acute cardinal angles, allometric growth, lowest values of length relative to width corresponding to largest valves, and sulcate anterior commissure; dorsal interarea steeply anacline to catacline; ornamentation costate with up to 38 ribs, of which some subcentral ribs have slightly delayed origin; ventral muscle field of subrhomboidal outline.

Description.—Shell ventribiconvex, up to 19 mm long, of mucronate outline, maximum width along hinge−line, acute cardinal angles, 47–69% as long as wide, corresponding lowest values to largest valves, and sulcate anterior commissure. Ventral valve convex, with variably developed median fold, subplanar postero−lateral areas, 24–38% as deep as long; ventral interarea apsacline to orthocline, curved, 6–11% as long as valve, with wide and open delthyrium. Dorsal valve slightly convex, with weak median sulcus, widening and deepening anteriorly, 50–73% as long as wide; dorsal interarea steeply anacline to catacline, 6–9% as long as valve, with open notothyrium. Radial ornamentation costate with 29–38 ribs, of which only 4 to 6 posterolateral ribs arise beyond umbo, and run subparallel to hinge line, besides 2 to 5 subcentral ribs with delayed origin along first 7 mm of growth; low ribs of subtriangular section in young stages, becoming more rounded in latest stages, numbering 4–7 per 5 mm counted 5 mm anteromedially from umbo, with intercostal spaces of similar width than ribs; fila and capillae poorly preserved (see Fig. 6F 2 , F 3); up to 3 strong growth lines in largest shells.

Ventral interior with pedicle callist, triangular teeth and crural fossettes; short dental plates diverging onto valve floor and continuous with anteriorly converging muscle bounding ridges; bilobed muscle field, of subrhomboidal outline, 28–45% as long as valve, 19–26% as wide as valve, with longer and wider diductor scars than triangular adductor scar, not enclosing it anteriorly; adductor scar 8–17% as wide as muscle field; divergent vascula media (see Fig. 6D ), arising from anterior end of diductor scars and not observable beyond mid length of valve.

Dorsal valve with high notothyrial platform, sloping posteriorly and continuous anteriorly with strong median ridge, widening forward and fading by mid length of valve; ridge−like, high and narrow cardinal process; rod−like brachiophores, welded to notothyrial platform; dental sockets excavated on secondary shell deposits; quadripartite muscle field, 34–56% as long as valve, 17–32% as wide as valve, with quadrangular adductor scars separated by transverse ridges.

Shell with radial ornamentation variably impressed on valve interiors, with grooves corresponding to crest of ribs as wide as intercostal impressions, divided by weak sulci at marginal areas.

Discussion.— Spirifer alatus J. de C. Sowerby in Murchison, 1839 was thoroughly re−described based on material from doi:10.4202/app.2009.0032

South Wales, and assigned to Lenorthis Andreeva (in Nikiforova and Andreeva, 1961) by Bates (1969). Jaanusson and Bassett (1993), after the rejection of Lenorthis by Havlíček and Branisa (1980), re−ascribed the species to Paralenorthis , erected by these authors to replace Lenorthis . The strongly mucronate outline of the Iberian Paralenorthis , besides its allometric growth, with progressively higher values of valve length relative to width, and a very high number of ribs, with up to 38 ribs in adult shells, indicate clear assignment to P. alata. The main difference of the Iberian sample from the Welsh collection is in its lower variability of rib number, with 29 to 38 ribs on adults, versus 18–36 ribs on the Welsh shells. This difference could be due to the inclusion in the collections studied by Bates (1969) of some specimens of Paralenorthis proava (Salter, 1866) , a species co−occurring at the same localities as P. alata, but with a much lower number of ribs: 16 to 20 ( Bates 1968). The fine preservation of the Iberian sample allows the proposal of a more precise, diagnosis for P. alata, extracting its more clearly differentiating features from the lengthy diagnosis of Bates (1969).

Stratigraphic and geographic range.—Moridunian (Lower Arenig) beds of South−west Wales (Carmarthen, Pembrokeshire and Ramsey Island; see Bates 1969). Blue Schists of Carmarthen Mountains; Yellow Quartzites of North−East Tremanhire; Ogof Hên Formation of Ogof Hên Bahia and Ramsey Island. All from the Lower Arenig (Floian Stage, uppermost Lower Ordovician Series) of Wales ( Bates 1969).

Lower half of the Navas de Estena Formation in the localities VPA (Ventas con Peña Aguilera, Toledo Province), NE−IV (Navas de Estena, Ciudad Real Province), and lower part of the Río Formation in the locality CHI−I 10E (north of Chillón, Ciudad Real Province). All lower Oretanian (middle Darriwilian Stage, Middle Ordovician Series).

Figures

Fig. 6. Orthid brachiopod Paralenorthis alata (J. de C. Sowerby in Murchison, 1839). A. MGM−6021−O internal mould (A1), latex cast of interior (A2) and latex cast of exterior (A3) of ventral valve. B. MGM−6030−O, internal mould (B1) and latex cast of interior (B2) of ventral valve. C. MGM−6038−O, latex cast of exterior of ventral valve. D. MGM−6037−O, internal mould of ventral valve. E. MGM−6048−O internal mould (E1) and latex cast of interior (E2) of dorsal valve. F. MGM−6027−O, latex cast of ventral exterior (F1), latex cast of dorsal exterior (F2), and detail of the latter (F3) of a shell with conjoined valves. G. MGM−6042−O, internal mould (G1), latex cast of interior (G2) and latex cast of exterior (G3) of dorsal valve. Scale bars 5 mm.

Fig. 2. Generalised stratigraphical columns of the Middle Ordovician rocks in the southern Central Iberian Zone, representatives of the three main areas (A–C) represented on Fig. 1, to show the vertical distribution of the studied brachiopod taxa. The left−hand column indicates the equivalence between the global chronostratigraphy and the north−Gondwanan regional scheme. Abbreviations: AQ, Armorican Quartzite; GF, Guindo Formation; Fe, thin oolithic ironstone bed. Note that the stratigraphic range of the graptolite biozones are directly annotated on the left side of each column.