Apheoorthina aff. ferrigena Havlíček, 1949

Muñoz, Diego F. & Benedetto, Juan L., 2016, The eoorthid brachiopod Apheoorthina in the Lower Ordovician of NW Argentina and the dispersal pathways along western Gondwana, Acta Palaeontologica Polonica 61 (3), pp. 633-644 : 635

Apheoorthina aff. ferrigena Havlíček, 1949

Figs. 2A–M, 3 .

Material.—Seven ventral valves and six dorsal valves, CEGH-UNC 26596–26604, 26915–26918 from the upper Santa Rosita Formation (levels CA1, CA4, CA4-5), upper Tremadocian; road from Santa Victoria to Acoyte, Salta Province, NW Argentina.

Description.—Shell medium sized, up to 17 mm wide, ventribiconvex, transversely semielliptical in outline (average length/width ratio 0.72); maximum width near 1/5 valve length; cardinal extremities obtuse to slightly rounded. Ventral valve moderately convex with a well defined carina on the anterior half. Ventral umbo rounded; beak weakly curved up to the hinge. Dorsal valve slightly convex with a broad sulcus originating at the umbo, corresponding to ventral carina. Dorsal umbo low, inconspicuous. Ornament coarsely fascicostellate consisting of 12−14 subangular, unequal-sized primary costellae originated at the beak, increasing in number mostly by lateral branching on both the medial (slope) and lateral sides of parental costae, sporadically by dichotomy; size and height of costellae diminishes towards the posterolateral areas. Interespaces subangular, of similar width to primary costellae bearing 2−5 finer unbranched ribs. Entire shell surface covered by fine radial striae with 8−10 in 100 μm; concentric ornamentation consists of evenly spaced filae and growth lamellae sporadically developed near the valve margins.

Ventral interior with thin dental plates supporting small, bluntly triangular teeth extending anteriorly into low convergent ridges bounding almost completely a subpentagonal muscle field extended for 20−25% valve length. Muscle field divided longitudinally by a thin septum; adductor and diductor scars undifferentiated. Interior of dorsal valve with subrectangular, concave, anteriorly bilobed notothyrial platform, slightly elevated anteriorly above valve floor extending anteriorly for 15−20% valve length. Cardinal process a simple rounded ridge slightly enlarged anteriorly. Muscle field and vascular system not impressed.

Remarks. —The Santa Victoria specimens are very similar to the type material of Apheoorthina ferrigena of Havlíček (1949) and additional material illustrated by Havlíček (1977) from the Třenice Formation of the Prague Basin. In size, outline and ornamentation the Argentinean material is indistinguishable from the Bohemian species. Internally both species are also very similar, excepting that in the Andean specimens the notothyrial platform is less elevated on the valve floor and the ventral muscle bounding ridges are less prominent. In addition, in our material the muscles scars are not impressed and internal ribbing is marked on the entire valve. We consider that such differences may be attributed to preservation since the Bohemian material is preserved in haematitic sandstone and the Andean material comes from mudstone and calcareous siltstone. Our specimens also resemble Apheoorthina bohemica Havlíček, 1949 , which is from the same haematite lenses at the base of the Třenice Formation yielding A. ferrigena . The dorsal interiors of the two Bohemian species are nearly identical; the ventral interiors of A. bohemica are not known and thus cannot be compared. According to Havlíček (1977) A. bohemica can be distinguished from A. ferrigena in having more numerous ribs in the interspaces. However, the ornamentation pattern of both species shows a broad overlap (compare ventral exteriors of A. bohemica and A. ferrigena illustrated by Havlíček 1977: pl. 4: 3 and pl. 4: 8, respectively), so this feature is taxonomically indecisive. It is noteworthy that the Argentinean material displays a similar range of variation, some specimens having ornamentation of A. bohemica type and others of A. ferrigena type (cf. Fig. 2B 2 and I). Because the strong variation in the ornamentation and in absence of substantial internal differences A. ferrigena and A. bohemica should be considered as conspecific.

Figures

Fig. 3. Eoorthid brachiopod Apheoorthina aff. ferrigena (Havlíček, 1949), CEGH-UNC 26918, upper Tremadocian, road from Santa Victoria to Acoyte, Salta Province, NW Argentina. Dorsal valve (illustrated on Fig. L) showing detail of embrionary (ep) and larval protegulum (lp). Highresolution confocal laser scanning microscopy positive image (A) and negative image (B). Scale bar 400 μm.