Pygmis, Baliński, 2002

Baliński, Andrzej, 2002, Frasnian-Famennian brachiopod extinction and recovery in southern Poland, Acta Palaeontologica Polonica 47 (2), pp. 289-305 : 299-301

Superfamily Athyridoidea Davidson, 1881 Family Athyrididae Davidson, 1881

Family?Didymothyridinae Modzalevskaya, 1979 Genus Pygmis gen. nov.

Type species: Nucleospira jablonensis Biernat, 1983 .

Etymology: Combination of letters of Gr. pygmaios (a dwarf) and Athyris .

Diagnosis.—Shell small, smooth, ventribiconvex to almost biconvex, weakly uniplicate; dorsal fold very low, begins at anterior third of shell, or absent; ventral sulcus shallow to very shallow; ventral beak erect; small concave ventral area present, delthyrium open; dental plates distinct, subparallel; cardinal plate short, partly embedded in the shell substance; spiralia directed laterally with at least of four volutions; jugum well developed. Pygmis differs from majority of athyridoids by its small, smooth shell. From equally small helenathyrins it differs by the absence of diplospiralium.

Remarks.— Biernat (1983) described the type species of Pygmis gen. nov. as a species of Nucleospira Hall, 1859 . Nucleospira jablonensis Biernat, 1983 from Jabłonna and Dębnik lacks the microspinose shell ornament, dorsal median septum and the cardinal process characteristic of Nucleospira . Pygmis jablonensis ( Biernat, 1983) has distinct dental plates, that are lacking in Nucleospira .

Pygmis gen. nov. is easily distinguished from majority of the athyridoids by its very small shell (rarely exceeds 8 mm in length) and smooth shell surface lacking growth lamellae. Externally the new genus is most similar to the Upper Devonian (Frasnian) helenathyrin Biernatella Baliński, 1977 . Both forms are characterized by smooth and small to very small rostrate shells. Biernatella differs from Pygmis by its well developed diplospiralium not observed in Pygmis ( Fig. 7 ). Pygmis also lacks the densely spaced growth lamellae characteristic of Biernatella (Baliński 1995: 138, figs. 4N, 7S, 9U).

The problematic small and smooth shelled Glassia drevermanni Maillieux (1936) from the Late Frasnian Matagne Formation tentatively assigned to Peratos Copper, 1986 by Copper (1986: 858) has been demonstrated to represent immature shells of the rhynchonellide Ryocarhynchus tumidus (Kayser, 1872) by Godefroid and Helsen (1998: 263–264, fig. 17). Several small poorly known Late Devonian athyridids may belong to the new genus, including: Athyris (?) meristelloidea Nalivkin, 1930 from the Late Devonian of Turkestan ( Nalivkin 1930: 146–147, pl. 8: 12, 13); and Athyris minuta Klähn, 1912 from the Frasnian of Germany ( Klähn 1912). The latter species most likely represents a species of Biernatella .

Pygmis differs from Leptathyris Siehl, 1962 by the absence of a bisulcate shell, its less pronounced dorsal umbo, and lack of a depressed cardinal plate. Externally Pygmis resembles the small Famennian athyridid Johnsonathyris Savage, Eberlein, and Churkin, 1978 , although the shells of the former are less convex and feature concentric growth lamellae and radial costellae lacking in Pygmis .

Stratigraphic and geographic range.—As for the type species given below.

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.