Ahtiella zarelae 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 : 551

Ahtiella zarelae Villas sp. nov.

Fig. 4.

Derivation of the name: After Dr. Zarela A. Herrera, for her studies on South American Ordovician brachiopods.

Types: Holotype MGM 5935 X. External mould of ventral valve.

Type locality: Carcel Puncco section, Inambari River , Cuesta Blanca village, 20 km NW of San Gabán, Eastern Cordillera, Peru .

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

Material.—Nearly a hundred internal or external moulds of ventral and dorsal valves, including numbers MGM 5925X– 5964X, from the horizons F and G of the San José Formation.

Diagnosis.— Ahtiella with rectangular to slightly alate cardinal angles, pronounced carinate ventral fold and dorsal sulcus, fine ramicostellate ribbing and weak corrugation.

Description.—Shell resupinate, up to 15 mm wide, with maximum width at hinge line, rectangular to acute cardinal angles, some slightly alate; anterior commissure plicate. Ventral valve concave and slightly geniculate in adult stages, with high angulose and narrow, carinate, median fold, mean length relative to width of 0.54 (number [n] = 33, variance [v] = 0.004), up to 37% as deep as long, with trail slightly differentiated at anteriormedian region from disc 4.5–7 mm long; ventral interarea flat, catacline to steeply apsacline, about 15% as long as valve, with apical pseudodeltidium. Dorsal valve strongly convex in adult stages, with deep median sulcus, having v−shaped cross−section, mean length relative to width of 0.60 (n = 21, v = 0.003), up to 33% as deep as long, with anacline to orthocline dorsal interarea, 3–7% as long as valve, with poorly preserved chilidium. Radial ornamentation uniformly ramicostellate, with 10–12 ribs, very rarely 13, per 2 mm counted 5 m anterolaterally from umbo, intersected by filae, with counts of about 20 per mm. Occassionally with 2–3 rugae on posterior margins, strongly oblique to subparallel to hinge line.

Ventral interior with triangular teeth, supported by short dental plates, continuous anteriorly with very low lateral muscle bounding ridges; muscle field subpentagonal, with mean length relative to valve length of 0.25 (n =17, v = 0.002), mean width relative to valve width of 0.18 (n = 17, v = 0.002), adductor scar as wide as each diductor scar and not enclosed anteriorly by them. Ribbing strongly impressed on interior of trail, as well as on margins of disc of large valves, with ridges corresponding to external intercoastal furrows accentuated by pustules. Conspicuous narrow and deep sulcus−like impression of external median fold.

Dorsal interior with simple cardinal process, short notothyrial platform continuous anteriorly with septum−like impression of external median sulcus; median sulcus is highest at its anterior end, at margin of platform, where it is strenghtened by elongated pustules. Socket ridges small bounding triangular sockets; quadripartite muscle field only poorly impressed in largest valves. Subperipheral rim low, emerging from cardinal angles, bounding a platform 5–7 mm long. Ribbing also strongly impressed on internal margin of dorsal valves, accentuated by pustules.

Discussion.—This plectambonitoidean can clearly be ascribed to Ahtiella based on its convexo−concave profile, simple geniculation, impersistent rugae and dorsal median septum. It differs from the congeneric Baltic species A. arenaria Öpik, 1933 , A. baltica Öpik, 1932 and A. lirata Öpik, 1932 by its well developed ventral fold and dorsal sulcus, and from A. jaanussoni Hessland, 1949 for its much less developed rugae. It coincides with the Welsh A. concava Bates, 1968 in outline and the prominent fold and sulcus but lacks its typical strong lateral ridges radiating from posterior dorsal muscle scars. It differs from A. quadrata , also from Wales and the oldest known species of the genus, for the parvicostellate and much thicker ribbing of the latter, with only 6 ribs per 2 mm. The Peruvian species can be distinguished from A. argentina Benedetto and Herrera, 1986 , from the Argentine Precordillera, by the strongly mucronate outline, not so accentuated fold and sulcus, more prominent rugae and parvicostellate ribbing of the Argentine species. It compares with the North American A. paucirugosa Neuman, 1977 in the weak rugosity and pronounced fold and sulcus, but differs from it in its ramicostellate ornamentation, in contrast to the parvicostellae in the North American species, and its much shorter ventral muscle field.

The new species is close to Ahtiella sp. ( Benedetto 2003b: 210), from the Arenig Suri Formation of Famatina (Argentina) , in its ventral and dorsal interiors, as well as in its uniformly ramicostellate ornamentation, which is only slightly finer in the Peruvian shells. Nevertheless, these can be discriminated from the Argentine Ahtiella sp. for their much more pronounced ventral fold and dorsal sulcus.

Stratigraphic and geographic range.—It is only known from its type locality, at the Carcel Puncco section, in the horizons F and G of the San José Formation.