Chapinella striata, Baliński, 2002

Baliński, Andrzej, 2002, Frasnian-Famennian brachiopod extinction and recovery in southern Poland, Acta Palaeontologica Polonica 47 (2), pp. 289-305 : 298

Chapinella striata sp. nov.

Figs. 5A, B , 6 .

Colophragma ? sp.; Baliński 1995a: 45–47, pl. 4: 6–7, fig. 11B.

Holotype: ZPALBp 46/33a, a complete shell illustrated in Fig. 5A .

Type locality: Trench Z−17 near the village of Dębnik, Cracow Region, southern Poland.

Type horizon: Platy Limestone Unit, Lower Famennian, the Early Palmatolepis triangularis Zone.

Etymology: After very delicate striae on the shell surface.

Diagnosis.—Shell medium sized, usually up to 21 mm in width, wider than long, strongly dorsibiconvex, the greatest thickness situated at two−third of the shell length from the umbo or slightly further anteriorly; anterior commissure strongly uniplicate; usually three costae in fold and two in sulcus, up to three costae on flanks; medial costae originate near mid−length of the shell, lateral costae are distinct near the anterior margin; shell surface weakly striate. Interior of the dorsal valve with extremely short medial myophragm that supports the base of the hinge plates. The new species differs from C. bucareliensis Savage et al., 1978 by the more inflated posterior region of its dorsal valve.

Material.—Ten complete or almost complete shells and 45 shell fragments, all from trench Z−17. In addition, 33 specimens were recovered and described by Baliński (1995) from slightly younger horizons at the Z−2 and Z−9 outcrops (see synonymy).

Description.—Shell medium sized for the genus, outline subelliptical, wider than long, width averages 102 to 139% of shell length, strongly dorsibiconvex; cardinal margin curved, lateral margins rounded, anterior margin truncate or gently arched; anterior commissure strongly uniplicate.

Ventral valve distinctly less convex than the dorsal one, with gently convex flanks; sulcus distinct, begins one−third to one−half of the valve length from umbo, deep, with concave to slightly convex bottom; beak suberect to erect, protuberant.

Dorsal valve regularly convex in lateral profile, with swollen posterior region and slightly concave near both ends of the cardinal margin; the valve thickness from 51 to 71% of the total shell thickness; fold originates at mid−valve, moderately elevated, with rounded top in transverse profile.

Eighty−one per cent of the specimens with three costae on fold and two costae in sulcus. Four and five costae, respectively, occur in 15% of the specimens. One shell with two costae on fold and two shells with five costae. Lateral costae are shorter and markedly weaker than the medial costae; with one to as many as three lateral costae on each flank of the dorsal valve and two to four costae on the ventral valve. Very weak and fine radial striae are observed on well preserved specimens; their number averages 8 to 9 per mm.

Ventral interior with distinct subparallel dental plates and distinct umbonal chambers ( Fig. 6 ). Dorsal interior with thin hinge plates unsupported by median septum; in umbonal region a very short median thickening or myophragm occurs in sectioned specimens ( Fig. 6 ), although it cannot be regarded as a true median septum.

Remarks.—The new species characterizes the earliest Famennian brachiopod assemblage found in trench Z−17. This species was identified earlier as Colophragma ? sp. in nearby Z−2 and Z−9 outcrops with limestones that represent slightly younger horizons of Pa. triangularis Zone ( Baliński 1995a: 45–47, pl. 4: 4–5, fig. 11B). Chapinella striata sp. nov. shares many external characters with the type species C. bucareliensis Savage et al., 1978 . The latter was described from the Middle to Late Famennian of Port Refugio Formation, southeastern Alaska ( Savage et al. 1978). The Polish species differs mainly by having more inflated posterior region of the dorsal valve as it is seen in a lateral profile of the shell. Small individuals of C. striata resemble very much “ Pugnax” rara Sartenaer, 1969 (= Chapinella rara , according to Savage et al. 1978) from the Lower Famennian of western Canada ( Sartenaer 1969). The main difference between these species is in the position of the greatest thickness of the shell that in the latter is situated at the top of always recurved posteriorly tongue. The Polish species differs from Chapinella kovalevi Cherkesova, 1997 of the Late Famennian of Taimyr ( Cherkesova 1997) by having weaker costae, more convex ventral valve, shorter tongue, and lower dorsal fold.

Savage et al. (1978) attributed Parapugnax bactriense Johnson et al., 1969 from the Famennian West Range Limestone of Nevada, USA, to Chapinella . The holotype and the three of the illustrated paratypes of the Nevada species differ markedly from C. striata . The fourth paratype ( Johnson et al. 1969, pl. 157: 14–18) externally is close to the Polish species on one hand, and to Colophragma ellipticum Cooper and Dutro, 1982 on the other. It is possible that all three species are closely related although systematically they are currently attributed to different genera. The differences between Chapinella and Colophragma are not great, if not problematic, and the presence of a short median septum in the latter genus seems to be overemphasized.

Occurrence.— Chapinella striata sp. nov. occurs in dark−grey micrites in trench Z−17 near the village of Dębnik, Cracow Region, southern Poland. It was described earlier as Colophragma ? sp. in slightly younger horizons sampled in exposures in quarry Z−2 and trench Z−9 ( Baliński 1995a).

Figures

Fig. 5. Early Famennian brachiopods, trench Z−17, Dębnik anticline, Poland. A, B. Chapinella striata sp. nov. A. Holotype Bp 46/33a in dorsal, ventral, lateral, and anterior views; A5 shows surface microornamentation. B. Shell Bp 46/33b in ventral view. C. Orbiculatisinurostrum laeve (Gürich, 1903); shell Bp 46/28a in dorsal and anterior views. D–J. Pampoecilorhynchus geniculatus sp. nov. D. Juvenile shell Bp 46/52a in dorsal and ventral views. E. Shell Bp 46/52b in dorsal and anterior views. F, G. Two shells Bp 46/52c and Bp 46/52d in dorsal, ventral, lateral, posterior, and anterior views (G, holotype). H. Expanded laterally shell Bp 46/52e in dorsal and posterior views.I. Elongated shell Bp 46/52f in dorsal, posterior, and anterior views. All × 1.5 except A5 × 4.

Fig. 6. Transverse serial sections of shell Bp 46/33c of Chapinella striata sp. nov. Numbers refer to distances in mm from the ventral apex.