Euorthisina orthiformis Havlíček and Branisa, 1980

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 : 557

Euorthisina orthiformis Havlíček and Branisa, 1980

Fig. 7 .

1980 Euorthisina orthiformis sp. nov.; Havlíček and Branisa 1980: 27–28, pl. 4: 4, 9–12.

Material.—23 ventral valves and 19 dorsal valves preserved as internal and/or external moulds, with collection numbers MGM 5888X–5924X and 5997X, from Carcel Puncco section; horizons J, K and L in the of the San José Formation.

Description.—Shell up to 23 mm wide, equibiconvex, with transversally oval outline, maximum width at posterior third of shell, hinge line 77–97% as wide as valve, with rounded slightly obtuse cardinal angles and rectimarginate anterior commissure. Ventral valve gently and evenly convex, with very slightly developed umbo, 64–91% as long as wide, ventral interarea attitude and delthyrium not observable due to flattening. Dorsal valve gently and evenly convex, 62–87% as long as wide, with narrow sulcus in early growth stages; dorsal interarea attitude and notothyrium not seen due to flattening.

Radial ornament ramicostellate and filate, with very irregular branching, asymmetrical on both sides of valve; ribs triangular in cross−section, although rounded in flattened shells, especially on anterior and lateral margins of valve. Irregular ribbing with ventral median rib deviating from the median line in its forward growth; correspondingly, one of the two dorsal median ribs occupies the median line after early growth stages. With fine fila, numbering about 15 per mm, arranged in rugose irregular bands.

Ventral interior, with subvertical dental plates, continuous anteriorly with muscle bounding ridges, very slightly divergent anteriorly in early stages and then subparallel, extending anteriorly to 16–17% of valve length. Undifferentiated triangular muscle field.

Dorsal interior with brachiophore plates converging to form v−shaped septalium, 4–7% as long as valve, supported by median septum that extends anteriorly 14–21% of valve length. Muscle field not observed. Ribs and rugose bands well marked on interior of both ventral and dorsal valves.

Discussion.—In spite of the strong flattening of most of these Euorthisina shells, several peculiar features, diagnostic of the Bolivian species Euorthisina orthiformis Havlíček and Branisa, 1980 , can be seen, such as the strong irregularity of its ramicostellate ornamentation and the triangular section of the ribs. The latter feature is most evident in the less flattened valves, especially on their posterocentral region, while they have a rounded section on the most flattened marginal areas. According to Havlíček and Branisa’s (1980) description of E. orthiformis it differs from the Peruvian specimens by the absence of concentric lines and in displaying maximum width at the hinge line. Nevertheless, in Havlíček and Branisa’s (1980: pl. 4: 12; pl. 5: 20) illustrations well preserved fila are seen, as well as the maximum width of shell well anterior to hinge line, as it occurs in the studied shells. Actually, an oval outline and a filate ornament are features very typical of the European species of Euorthisina and, in spite of Havlíček and Branisa’s (1980) descriptions of the Bolivian E. orthiformis and E. kobayashii , they seem to be characteristic of them, too.

Figures

Fig. 7. Euorthisinid brachiopod Euorthisina orthiformis Havlíček and Branisa, 1980. A. MGM 5913−X3, latex cast of ventral exterior. B. MGM 5997X, latex cast of dorsal exterior. C. MGM 5915X, latex cast of dorsal exterior. D. MGM 5898X, internal mould of ventral valve. E. MGM 5897X, internal mould of ventral valve. F. MGM 5902X, internal mould of ventral valve. G. MGM 5913X−2, latex cast of exterior (G1) and internal mould (G2) of dorsal valve. H. MGM 5896X, internal mould of ventral valve. I. MGM 5899X−1, internal mould of dorsal valve. J. MGM 5913X−1, internal mould of ventral valve. K. MGM 5899X−2, internal mould of dorsal valve. Scale bars 5 mm.