Orechina violetae, Salas & Vaccari, 2012

Salas, María José & Vaccari, N. Emilio, 2012, New insights into the early diversification of the Ostracoda: Tremadocian ostracods from the Cordillera Oriental, Argentina, Acta Palaeontologica Polonica 57 (1), pp. 175-190 : 184

Orechina violetae sp. nov.

Fig. 8A–C, E .

Etymology: For one of my (MJS) twins Violeta.

Holotype: CEGH−UNC 24029 , right valve ( Fig. 8A ).

Type locality: Purmamarca area, Quebrada de Chalala section, northwest of Argentina.

Type horizon: Ch2 sample, Coquena Formation, Late Tremadocian.

Diagnosis.—Species of Orechina with a semicircular and amplete outline. Adductorial sulcus short, narrow and U−shaped. Valves reticulate, with rectangular cells. The dorsal area is smooth. Lateral surface of valves is convex.

Description.—The valves are sub−elliptical to semicircular and amplete in outline. The maximum length is situated at the mid−height of the valve. The average L:H is 1.54. The dorsal margin is straight and shorter than the maximum length. Lateral and ventral margins are rounded, the posterior one showing a sharper outline. The cardinal corners are both well defined, and obtuse (cardinal corners are ca. 140°). Valves unisulcate, the adductorial sulcus (S2) is in the antero−dorsal half of the valve. S2 is short, narrow, U−shaped, and poorly to well defined depending on the specimens. An oval muscle spot is present in the ventral end of the sulcus, in some specimens it is indistinct. In some exemplars, there is a poorly defined and small preadductorial node. The lateral surface of the valves is rather convex, with the maximum width in the posterior half of the valves. The lateral surface is reticulate with elongated cells (rectangular). The reticulation follows the free margin of the valve, and the cells are smaller towards the cardinal corners. The dorsal area is smooth as the marginal surface. In the internal moulds there is a well defined and short sulcus. Immediately in front of the sulcus is an oval node. The surface of the mould is smooth.

Material.— Over 20 specimens of valves and internal moulds, including the figured material CEGH−UNC 24029–31 , 24036 . Specimens we have examined range from 0.68 mm to 1.12 mm in length .

Discussion.—The studied material shared several features with the genus Orechina , i.e., the presence of an adductorial sulcus S2, with a rounded muscle spot in its ventral part, the rounded outline of the valves, and the reticulated lateral surface, except for the dorsal area. The new species does not have a bend between the lateral and marginal surface of the valves, however, Schallreuter and Krůta (1987), in the emended diagnosis of the genus, include the bend as a normally present feature, and not as a diagnostic character. The convexity of the valves and the large angle (ca. 90°) between the marginal surface and the plane of the separation of the valves are not common features in the genus and in the family Circulinidae . However, the other characters of this form resemble the genus Orechina .

The genus Orechina has several species widespread in Bohemia, Baltic region, and Poland during the Middle to Late Ordovician. The presence of Orechina in the Upper Tremadocian levels of northwest Argentina would be the first mention of the genus in Gondwana and would extend its range to the first stage of diversification of ostracods. This new species is comparable with Orechina procera Schallreuter, 1980 ( Schallreuter 1980: figs. 8.1–8.3; Schallreuter and Krůta 1987: figs. 2.1–2.2) from the Sandbian of Sweden, and with Orechina krutai Olempska, 1994 from the Middle Ordovician of Poland, based mainly on the lack of a bend between the lateral and marginal surface of the valves and on a ridge at the dorsal margin, as well as for the outline of the valves. However, the new species is larger, S2 is less defined (feature shared with O. krutai ), and has a muscle spot in its ventral end, the valves are more convex, and the lateral surface is reticulate with rectangular cells. With respect to the Bohemian species, Orechina punctata Krůta, 1968 and Orechina densipuncta ( Schmidt, 1941) (in Schallreuter and Krůta 1987: figs. 1–4), both from the Late Ordovician, the main difference is the presence of the bend between the lateral and marginal surfaces, and of the ridge in the dorsal margin.

The studied material is comparable too with Mytoa klabava Schallreuter and Krůta, 1988 , from the Arenig of Bohemia, in the presence of a rounded muscle spot and in the indistinct sulcus. However, the Bohemian species is elongate, the lateral surface is flat, and the reticulation is not defined.

Orechina violetae sp. nov. differs from the other Argentinian species, Orechina catalinae sp. nov., in that it has wider valves, which are more convex, the reticulation more defined with rectangular cells and mainly in the presence of a muscle spot, absent in O. catalinae sp. nov.

Stratigraphic and geographic distribution.—Known only from the type locality.

Figures

Fig. 8. Late Tremadocian binodicopids ostracods from the Northwest Basin of Argentina. A–C, E. Orechina violetae sp. nov. Coquena Formation in Chalala section, Purmamarca area. A. Holotype CEGH−UNC 24029, right valve in lateral view. B. CEGH−UNC 24030, left valve in lateral view. C. CEGH−UNC 24031, right valve, lateral (C1) and ventral (C2) views. E. CEGH−UNC 24036, internal mould of right valve, lateral view. D, F–H. Orechina catalinae sp. nov. Humacha Member of the Santa Rosita Formation, lower part of the Humacha section. D. CEGH−UNC 24017, left valve in lateral view. F. CEGH−UNC 24027, left valve in lateral view. G. Holotype CEGH−UNC 24037, left valve in lateral view. H. CEGH−UNC 24028, right valve in lateral view. Scale bars 100 µm.