Pygmis jablonensis ( Biernat, 1983 )

Baliński, Andrzej, 2002, Frasnian-Famennian brachiopod extinction and recovery in southern Poland, Acta Palaeontologica Polonica 47 (2), pp. 289-305 : 301-302

Pygmis jablonensis ( Biernat, 1983)

Figs. 7 , 8C–E .

Nuclespira jablonensis n. sp.; Biernat 1983: 142, 149–150, pl. 4: 1, pl. 5: 1, pl. 6: 1–2.

Emended diagnosis.—Shell small (rarely exceeding 8 mm in length), as wide as long, widest at midlength, subcircular to subpentagonal in outline, rather thick, ventribiconvex to subequal, weakly uniplicate, fold and sulcus poorly developed.

Material.—Three complete and one slightly damaged shell, and 25 shell fragments. All shells interiors infilled by sparry calcite that conceals some details of internal shell structures.

Dimensions in mm: Description.—Shell rarely exceeds 8 mm in length, about as wide as long, shell thickness averages 45–69% of the shell length, ventral valve slightly more or equally convex as the dorsal valve; hinge line short, slightly angular; lateral margins regularly arched, anterior margin almost straight, anterior commissure weakly uniplicate.

Ventral valve with erect beak and very shallow medial sulcus, which appears about midvalve; area high but narrow and concave; delthyrium open. Dorsal valve with slightly swollen posterior region; median fold lacking or weakly developed at the anterior margin only.

Interior of the ventral valve with distinct subparallel dental plates and cavities, teeth massive. Dorsal valve interior with massive cardinal plate, posteriorly embedded in shell substance, expanded anteriorly as thin, conjunct inner hinge plates ( Fig. 7A, B ). Spiralia with at least four volutions; lateral branches of jugum long, arise in posterior one−third to two−fifths of the shell length. Other details of the internal shell structure are difficult to study due to recrystalization of the shell interior.

Shell surface smooth except very faint growth lines.

Remarks.—The Dębnik specimens are identical externally with Pygmis jablonensis ( Biernat, 1983) described from the early part of the Famennian of Jabłonna. At Dębnik, P. jablonensis occurs a few meters above the last occurrence of helenathyrin Biernatella polonica . The similar general appearance of the shell of both forms raises the question whether they are phylogenetically related. On close comparison it is found that externally P. jablonensis differs from all species of Biernatella in having a smooth uniplicate shell lacking concentric lamellae. Careful sectioning of several specimens did not confirm the presence of diplospiralium found in all of the helenathyrins.

Occurrence.—The species is uncommon in the topmost 3 m of dark−grey micrites (set III, Middle Pa. triangularis Zone ) in trench Z−17 near the village of Dębnik, Cracow Region, southern Poland. It occurs here usually in small nest−like groupings composed of a few complete shells. The species was described originally by Biernat (1983) from the early part of the Famennian of Jabłonna (Holy Cross Mountains, Central Poland) where it is documented in trench III in the intervals of the Middle Pa. triangularis, Late Pa. crepida or Early Pa. rhomboidea to Early Pa. marginifera Zones.

Figures

Fig. 7. Transverse serial sections of three shells Bp 46/57e, 46/58, and 46/57f of Pygmis jablonensis (Biernat, 1983). Numbers refer to distances in mm from the ventral apex.

Fig. 8. Early Famennian brachiopods, trench Z−17, Dębnik anticline, Poland. A, B. Crinisarina angelicoides (Merriam, 1940). A. Complete shell Bp 46/49a in dorsal, lateral, and anterior views. B. Complete shell Bp 46/49b in dorsal, ventral and anterior views. C–E. Pygmis jablonensis (Biernat, 1983); three shells Bp 46/57a, b, and c in dorsal, ventral, lateral, posterior, and anterior views. F. Cyrtospirifer minor (Gürich, 1903); adult shell Bp 46/1a in dorsal, lateral, and anterior views. G. Warrenella sp.; incomplete shell Bp 46/16 in ventral view. All × 1.5.