Paralenorthis carlottoi Villas, 2007

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 : 555-557

Paralenorthis carlottoi Villas sp. nov.

Fig. 6 .

Derivation of the name: After Dr. Víctor Carlotto for his studies on the Peruvian Palaeozoic.

Holotype: MGM 5973 X−1. External mould of ventral valve.

Type locality: Cuesta Blanca village, Inambari River, 20 Km NW of San Gabán, Eastern Cordillera, Peru.

Type horizon: Horizon A of the San José Formation, upper Floian Stage, uppermost Lower Ordovician Series.

Material.—About a hundred ventral and dorsal valves from the horizon A of the San José Formation, preserved as internal or external moulds, including numbers MGM 5973X–5984X.

Diagnosis.— Paralenorthis species with rectangular cardinal angles, costate and capillate, with 18–20 costae in adult dorsal valves, only two median costae occupying median sulcus, ventral muscle field 26–33% as long as valve, notothyrial platform restricted to notothyrial cavity and 14–22% as long as valve, blade−like divergent brachiophores and low median ridge.

Description.—Shell ventri−biconvex, up to 10 mm long, of semicircular outline, with maximun width on hinge line, rectangular cardinal angles and slightly sulcate anterior commissure. Ventral valve convex, 20–30% as deep as long, maximun depth at umbonal region, 71–98% as long as wide; ventral interarea curve, apsacline, 10–15% as long as valve, with open delthyrium. Dorsal valve weakly convex, 70–90% as long as wide; dorsal interarea curve, anacline, nearly as long as ventral interarea, with open notothyrium.

Radial ornamentation costate and capillate, with a single ventral valve showing ramification of one costae (see Fig. 6B ); ribs counts of 17–21 in ventral valves and 18–20 in dorsal valves, with only the two dorsal median costae occupying median sulcus.

Ventral interior with apical collar, triangular teeth, short but high dental plates, triangular muscle field 26–33% as long as valve and 20–30% as wide as valve, with adductor scar 40–50% as wide as muscle field, and nearly as long as diductor scars, not enclosed anteriorly by these. Vascula media poorly impressed, divergent anteriorly.

Dorsal interior with blade−like cardinal process borne on long notothyrial platform restricted to notothyrial cavity, 14–22% as long as valve; blade−like divergent brachiophores, deep dental sockets excavated on thickening of valve floor, wide and low median ridge, bisecting a quadripartite muscle field, about 50% as long as valve and 25–37% as wide as valve, with anterior adductor scars larger than posterior.

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

Discussion.—The divergent ventral vascula media, typical of Paralenorthis Havlíček and Branisa 1980 , are poorly developed in a few of the studied valves ( Fig. 6L ), but their small size, besides the main external and internal features also points to an assignment to that genus, thoroughly revised by Jaanuson and Bassett (1993). These specimens can be discriminated from all the known species of Paralenorthis and thus the erection of a new species is proposed.

Paralenorthis carlottoi Villas sp. nov. displays a much lower number of ribs than P. immitatrix , the type species of the genus which also occur in the studied succession as well as in the Arenig of Bolivia ( Havlíček and Branisa 1980). Its ornamentation is not distinguishable from that of the Argentine P. altiplanicus Benedetto, 1998 , although its notothyrial cavity is much broader and longer: 14–22% as long as valve in P. carlottoi while 10–13% as long as valve in P. altiplanicus ( Benedetto 1998b: 13) . The new species is also close to the British P. proava ( Salter, 1866) in shape, number and wave length of ribs, but it displays only two ribs on the dorsal sulcus while there are four ribs in P. proava ; they also differ in the relative size of the dorsal median ridge and the length of the ventral muscle field that is shorter in P. carlottoi . This can be also distinguished from P. parvicrassicostatus ( Cooper, 1956) , which is close to P. proava by the non capillate external surface of P. parvicrassicostatus (see Williams 1974). The Baltoscandian P. orbicularis ( Pander, 1830) displays a very variable radial ornament that is occasionally similar to that of the new species; nevertheless, its ventral interarea is longer and the ventral valve is less strongly convex than in P. parvicrassicostatus . The Peruvian form has an ornamentation very similar to the North American P. buttsi ( Schuchert and Cooper, 1932) , but they differ in their brachiophore bases, strongly convergent towards the bottom of the notothyrial cavity in the latter, as well as in the median septum, higher and narrower in P. buttsi . The new species is also very close in its ornamentation to Orthis serica Martelli, 1901 from South China, assigned to Paralenorthis by Jaanusson and Basset (1993); nevertheless can be distinguished by its more prominent ventral umbo, better developed dorsal median sulcus and longer ventral muscle field than in P. serica (see Xu and Liu 1984: pl. 2: 15, 16, 19–29).

The lack of auriculation in the new species allows ready distinction from the British P. alata (J. de C. Sowerby, in Murchinson, 1839), the Argentine P. riojanus ( Levy and Nullo, 1973) and P. suriensis Benedetto, 2003 , as well as the North American P. marshalli ( Wilson, 1926) , P.? minusculus ( Phleger, 1933) and P.? angulata ( Cooper, 1956).

The semicircular outline of the Peruvian Paralenorthis , with its maximum width at the hinge line, allows discrimination from the Argentine P. vulgaris ( Herrera and Benedetto, 1989) and the North American P. robusta ( Neuman,1964) , whose outline is rounded, cardinal angles obtuse and the maximum width occurs in front of the hinge line.

Stratigraphic and geographic range.— It is only known from its type locality and horizon, at the Cuesta Blanca village , Inambari River , 20 km NW of San Gabán, Eastern Cordillera , Peru., in the horizon A of the San José Formation , upper Floian Stage, uppermost Lower Ordovician

Series.

Figures

Fig. 6. Orthid brachiopod Paralenorthis carlottoi Villas, sp. nov. A. MGM 5973X−1, latex cast of exterior of ventral valve, holotype.B. MGM 5977X, latex cast of exterior of ventral valve. C. MGM 5975X, latex cast of exterior of ventral valve. D. MGM 5973X−6, latex cast of exterior of dorsal valve. E. MGM 5981X−2–3, latex casts of exteriors of two dorsal valves. F. MGM 5978X−2, latex cast of exterior of dorsal valve. G. MGM 5981X−1, internal mould (G1) and latex cast of interior (G2) of dorsal valve. H. MGM 5973X−4, internal mould (H1) and latex cast of interior (H2) of dorsal valve. I. MGM 5973X−2, internal mould (I1) and latex cast of interior (I2) of ventral valve. J. MGM 5973X−5a, internal mould of dorsal valve. K. MGM 5974X, internal mould (K1) and latex cast of interior (K2) of dorsal valve. L. MGM 5980X, internal mould (L1) and latex cast of interior (L2) of ventral valve. M. MGM 5979X, internal mould (M1) and latex cast of interior (M2) of ventral valve. Scale bars 2 mm.