Leptaena (Leptaena) Dalman, 1828

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 : 398-399

Subgenus Leptaena (Leptaena) Dalman, 1828 Leptaena (Leptaena) rugosa Dalman, 1828

Fig. 3A, B, D–F .

1828 Leptaena rugosa Dalman ; Dalman 1828: 106, pl. 1: 1.

1968 Leptaena rugosa Dalman, 1828 ; Bergström 1968: 14–15, pl. 5: 8, 9; text-fig. 7.

2008 Leptaena (Leptaena) rugosa Dalman ; Cocks 2008: 59 (see synonymy therein).

Material.— 30 specimens (among them 15 fragmentary); mostly 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. Depth of fragmentary specimens is marked by “?”. Collection GIT 542 : Adze-6 ,

844.5–845.2; Aizpute-41, 997.1–?997.8; Engure, 882.9–

884.4; Ikla,?536.5; Mežmaļi-16,?912.8–?913.2; Riekstini-15,?857.3; Ruhnu-500, 610.1–?617.25; Stirnas-18, 908.6–909.4; Sturi-8, 941.8; Vilcini-15, 909.3–910.35; collections LDM G: Anši-12, 922.5; Cicere (from the sample with Hirnantia sagittifera ; depth unknown); Ēdole-60, 840.7; Kandava-52, 932.5; Kuili-9, 948.0–?948.2; Priekule-23, 1363.0; Pliekalni-14,?884.8; Talsi-55,?866.5.

Remarks.—The East Baltic specimens of Leptaena are almost identical with L. (L.) rugosa from Sweden and Norway ( Bergström 1968: text-fig. 7; Spjeldnaes 1957: pl. 7: 1–2, 4; Cocks and Rong 2000: fig. 150, 1d) having a transversely subquadrate shell outline, weakly convex ventral disc with up to 11 concentric rugae, multicostellate ornament with few accentuated ribs on median fascicle and 5–9 costellae per 2 mm at anterior part of the disc; the exterior of the dorsal valve is similar to the material from Norway and Sweden. The shell size, up to 45 mm wide at alate hinge line, seems to be more similar to the Norwegian specimens of the species. However, the Baltic specimens possibly have a shorter trail and less geniculated profile. The Baltic specimens have ventral interarea about twice as high as the dorsal interarea; foramen opening apical. Chilidium, up to 4 mm wide, triangular, convex with the median groove, which is characteristic for the Swedish specimens ( Bergström 1968). The single dorsal interior with trapezoidal notothyrial platform and median septa on anterior half of disc ( Fig. 3G 1, G 2 ) does not differs from the Swedish specimen ( Bergström 1968: text-fig. 7). An exception is the cardinal process of the Baltic specimen where the lobes seem to be less divergent.

The species affiliation of one ventral and one dorsal valve ( Leptaena sp. ; see Fig. 3C, E ) is unclear due the discontinuous nature of the rugae. Nevertheless, the latter valve has a peripheral ridge, similar to L. (L.) rugosa , but the ventral valve has smooth transition from the disc to trail, similar to the specimens of L. (L.) rugosa from the Killey Bridge Formation (Katian) of Ireland ( Mitchell 1977).

The Baltic specimens differ from the older representatives of L. (L.) rugosa from Pomeroy, N. Ireland ( Candela 2003) in having a more transverse disc and fewer rugae.

The early Hirnantian leptaenids Leptaena acuteplicata ( Schmidt, 1908) and L. friedrichi ( Rõõmusoks, 2004) (assigned to the genera Schmidtomena and? Similoleptaena in Rõõmusoks 2004) in the reef complex (Ärina Formation) of North Estonia differ markedly from L. (L.) rugosa . Both Estonian species have much smaller shells; L. acuteplicata differs in having less prominent rugae; the L. friedrichi has a more robust ornament. A comparison of several Hirnantian species of Leptaena in Baltoscandia was presented by Cocks (2005).

Stratigraphic and geographic range.—This widespread species is mainly restricted to the Hirnantian Stage, Upper Ordovician, in Europe ( Czech Republic, France, Ireland, and Wales), China ( Rong and Harper 1988; Rong et al. 2002), and in North and South America. In Baltoscandia besides Norway and Sweden, it occurs in Estonia and Latvia (this paper) and also in Lithuania ( Paškevičius 1997). Closely related forms occur in the highest Katian in, for example, Northern Ireland and Scotland.

Figures

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.

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.