Eostropheodonta cf. schmalenseei ( Bergström, 1968 )

Hints, Linda & Harper, David A. T., 2015, The Hirnantian (Late Ordovician) brachiopod fauna of the East Baltic: Taxonomy of the key species, Acta Palaeontologica Polonica 60 (2), pp. 395-420 : 403-404

Eostropheodonta cf. schmalenseei ( Bergström, 1968)

Fig. 7.

Material.—11 variably preserved specimens, embedded in rock.Porkuni Regional Stage, Kuldiga Formation, Hirnantian (Upper Ordovician); western Latvia, southwestern Estonia. Drill core and depth (in meters) of sample intervals with brachiopods: collection GIT 542: Engure, 882.9; Mežmaļi-16, 913.0, 913.4; Piltene-1, 1017.1; Ruhnu-500, 616.9; collections LDM G: Cicere (depth is unknown); Kronauce, 1055.1; Talsi-55, 866.7–867.1; Pāvilosta,1097.0–?1097.5; Remte-3, 965–966.

Description.—Large oval, thin-shelled specimens. Ventral valve is very weakly convex in posterior part, dorsal valve is almost flat with weak concavity in the middle. The largest specimen is 52.3 mm wide and over 37 mm long. Ornamentation parvicostellate, with about 50 ribs at the 10 mm growth stage along the valve margins, 28 of which are accentuated; 5 ribs occur per 2 mm at 20 mm from umbo; 2–3 stronger ribs with 1–2 finer costellae between them occur per 2 mm on the anterior margin. Concentric growth lines very fine with 8–10 lines per 1 mm.

Ventral interarea is flat, 0.5 mm high. The denticulate teeth diverge at 110°. Dental plate with about 12 denticles capping the teeth and with crenulations on the antero-medi- an faces. The denticles and crenulations continue along the edge of the delthyrium as small protuberances ( Fig. 7A 2 , A 3 , B 1). Dental plates very short. Muscle scars not visible. The external ornament is impressed on the interior valve surface; tubercles (pseudopunctae) are sporadically developed.

Two dorsal valves have low interareas with notothyrium covered by convex, non-grooved chilidium ( Fig. 7D 1); interior is unknown.

Remarks.— Eostropheodonta cf. schmalenseei differs from E. h. hirnantensis by its larger size and less well-differentiated ornament. The ventral valves of E. cf. schmalenseei differ from E. h. hirnantensis in having teeth, developed as oblique plates with about 12 denticles on the anterior margin of the interarea. The latter species has antero-laterally directed teeth with 4–6 denticles ( Temple 1965: pl. 17: 4, 5; Bergström 1968, pl. 6: 7; Rong and Cocks 1994). The lack of a median groove on the chilidium ( Fig. 7D 1) in the East Baltic species confirms the close relationship with E. schmalenseei from Sweden.

Eostropheodonta cf. schmalenseei is similar to Eostropheodonta luna from the Boda Limestone ( Cocks 2005: pl. 9: 11–15) and to the Estonian species Pirgumena (= Eostropheodonta by Cocks 2005: 269) martnai ( Rõõmusoks 2004: pl. 15: 8–10; pl. 16: 1–5) by the shell size. However, E. cf. schmalenseei has a more uniform ornament with wider interspaces between costellae, especially on the postero-lateral parts of valves. E. luna and the Estonian species have ornaments bearing accentuated ribs. There are insufficient data on the interiors of related species to permit their clear comparison.

Figures

Fig. 2. A. The facies belts in the Baltic Basin and the study area. B. The location of drill core sections. C. Enlarged Jurkalne area. Black dots mark the locality of the drill core with brachiopods of the Hirnantia Fauna; empty dots indicate occurrences of the Hirnantia Fauna (from Paškevičius 1997, 2000). Drill cores: 1, Stirnas-18; 2, Pliekalni-14: 3, Dizrungi-17, 4, Vilcini-19; 5, Adze-6; 6, Dreimaņi-11; 7, Riekstini-15; 8, Mežmaļi-16; 9, Mežvagari-13, 10, Ēdole-60; 11, Anši. Distribution of the Hirnantian rocks: a, outer limit the the Porkuni Stage, including of the Saldus Formation on the NE areas; b, outer limit of the Saldus Formation in northern part of region; c, outer limits of the distribution area of the Kuldiga Formation.

Fig. 3. Leptaenin brachiopods from the Kuldiga Formation of the Porkuni Regional Stage, Hirnantian (Upper Ordovician), East Baltic, western Latvia A–D, F–G), southwestern Estonia (E). A, B, D, F, G. Leptaena (L.) rugosa (Dalman, 1828). A. Shell, LDM G 273-1, Ēdole-60 drill core, depth 840.7 m, ventral (A1) and ventro-lateral (A2) views. B. Shell, GIT G 542-2, Vilcini-15, 910.35 m (figured in Kaljo et al. 2008), ventral (B1) and posterior (B2) views. D. Shell, GIT 542-17, Stirnas-18, 908.6 m, dorsal exterior view. F. Shell, GIT 542-18, Stirnas-18, 909.4 m, ventral view. G. Dorsal valve, GIT 542-197, Sturi-8, 941.8 m, view of interior (G1) and cruralium (G2). C, E. Leptaena sp. C. Dorsal valve, GIT 542-189, Vilcini-15, 909.3 m, exterior view. E. Incomplete ventral valve, GIT 542-192, Ikla, 536.5 m, exterior view. Scale bars 10 mm.