Eostropheodonta hirnantensis hirnantensis ( M’Coy, 1851 )

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 : 400-402

Eostropheodonta hirnantensis hirnantensis ( M’Coy, 1851)

Figs. 5 , 6A 1.

1851 Orthis Hirnantensis M’Coy, 1851: 395 .

2008 Eostropheodonta hirnantensis hirnantensis ( M’Coy, 1851) ; Cocks 2008: 68 (see synonyms therein).

Material.—Variably preserved specimens in 129 samples with one or more specimens in each. Porkuni Regional Stage, Kuldiga Formation, Hirnantian (Upper Ordovician); western Latvia, southern Estonia. Drill core and depth (in meters) of sample intervals with brachiopods.Depth of fragmentary preserved material is marked by “?”. Collection GIT 542: Adze- 6, 840.9–844.2; Aispute-41, 987.7–1000.5; Engure, 884.3– 884.4; Ikla,?531.0–?535.7; Mežmaļi-16, 906.56–?913.4; Priekule-20, 1357.3–?1363.4; Riekstini-15, 846.15–?860.4; Ruhnu-500,?616.3–617.4; Stirnas-18, 899.0, Taagepera, 413.7–413.9; Vilcini-19, 895.7–909.6; collections LDM G: Adze-6,?838.1–?844.8, Dizrungi-17,?893.8, Dreimaņi-11,?953.6–953.75; Kandava-52,?930.3–?931.5; Mežmali-16,?917.8–921.35 (the revised depths are published by Gailite et al. (1989) and Hints et al. (2012) (the lower boundary of the Porkuni Stage is at a depth of 916 m); Mežvagari-13,?871.2; Pliekalni-14,?883.7; Priekule-20,?1355.5–1363.4; Priekule-23,?1392.1–1395.6; Remte-3, 958.0–958.8; Talsi-55,?866.7–?867.1.

Description.—Plano- to concavo-convex shell, transversely subquadrate in outline, length-width ratio 0.6–0.8, maximum width commonly less than 20 mm at hinge line. Cardinal angles obtuse on smaller and acute on larger shells. Ventral valve slightly convex, maximum convexity in posterior part. Specimens less than 3 mm long have a small beak extending backward; small depression begins anterior of beak. Ventral interarea up to 0.5 mm high, delthyrium with small protegular apical deltidium. Dorsal valve commonly has small drop-shaped protegulal node covered by concentric filae, on some valves it continues anteriorly as median costae. Radial ornament parvicostellate in most specimens, becoming multicostellate with faint short rugae in postero-lateral parts of shell; accentuated costae divide the ornament into 7 or more sectors, 6–12, on average 9, costellae per 2 mm at 5 mm from beak. The interspaces between ribs are densely covered by concentric filae ( Fig. 5B 1).

Ventral interior ( Fig. 5D ) has small teeth with minute crenulations on upper side; crural fossettes strong, dental plates short, divergent.

Remarks.—The material is insufficient for precise differentiation between the subspecies E. hirnantensis hirnantensis ( M’Coy, 1851) and E. hirnantensis siluriana ( Davidson, 1871) , which are defined mainly by the ribbing. The latter subspecies ( Hiller 1980) has coarser, more fascicostellate ribbing than E. h. hirnantensis . Ornament of a few specimens reminiscent of E. h. siluriana ( Fig. 6A 2 ) co-occurring in some samples with specimens more similar to H. h. hirnantensis ( Fig. 6A 1).

The Baltic specimens are similar to E. h. hirnantensis from Poland and England ( Temple 1965) in shape and size and in the radial ornament, showing a strong median costa on the ventral valve and in the arrangement of ribs into the sectors between the stronger ribs. In many samples, indeterminate strophomenoid brachiopods are represented by incomplete valves and fragments having an ornament similar to Eostropheodonta and, more specifically, to E. h. hirnantensis .

Stratigraphic and geographic range.—This key taxon occurs mainly in the Hirnantian Stage, Upper Ordovician. The species is a characteristic component of the globally distributed Hirnantia brachiopod fauna in Africa, Canada, China, the Czech Republic, England, and Ireland ( Rong and Harper 1988). In Baltoscandia it occurs in Norway ( Cocks 1982), Sweden ( Bergström 1968), Estonia, Latvia (this paper), and Lithuania ( Paškevičius 1997).

Figures

Fig. 5. Leptostrophiid brachiopod Eostropheodonta hirnantensis (M`Coy, 1851) from the Kuldiga Formation of the Porkuni Regional Stage, Hirnantian (Upper Ordovician), East Baltic, western Latvia. A. Ventral valve, GIT 542-316, Riekstini-15, depth 850.3 m, ventral exterior view. B. Ventral valve, GIT 542-333, Riekstini-15, 854.0 m, fragment of valve with ornament (B 1) and exterior view (B 2). C. Juvenile specimen, GIT 542-319, Mežmali-16, 906.56 m, ventral exterior view. D. Ventral valve, GIT 542-385, Aispute-41, 1000.5 m, interior view. E. Bedding plane with numerous casts of valves, GIT 542- 360, Vilcini-19, 906.5 m.

Fig. 6. Leptostrophiid brachiopods from the Kuldiga Formation of the Porkuni Regional Stage, Hirnantian (Upper Ordovician), East Baltic, western Latvia A–E, G) and southwestern Estonia (F). A. Eostropheodonta hirnantensis (M`Coy, 1851), Taagepera, depth 413.9 m, moulds of ventral valves, GIT 542-337/1 A1) and GIT 542-337/2 (A2), exterior views. B, E. Eostropheodonta cf. parvicostellata Rong, 1984. B. Shell, GIT 542-381, Vilcini-19, 906.4 m, exterior view of dorsal valve (B1) and view on interarea (B2). E. Mould of dorsal valve, LDM G328-12, Priekule-33, 1395.6 m, exterior view. C, D, F, G. Coolinia sp. C. Mould of ventral valve, LDM G328-26, Dižrungi-17, 984.5 m, exterior view. D. Ventral valve, LDM G328-26, Dizrungi-17, 984.5 m, exterior view. F. Fragment of dorsal valve, GIT 542-364, Ruhnu-500, depth 612.9 m, view of cardinalia (F1) and exterior view (F2). G. Incomplete ventral valve, GIT 542 48-2, Stirnas-18, 899.0 m, interior view. Scale bars 2 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.