Discradisca cf. polonica ( Radwańska et Radwański, 1984 )

Dulai, Alfréd, 2017, First record of Discinidae brachiopods from the Miocene of Hungary, Fragmenta Palaeontologica Hungarica 34, pp. 63-74 : 67-71

Discradisca cf. polonica ( Radwańska et Radwański, 1984)

( Figs 2–22 )

1984 Discinisca polonica sp. n. – RADWAŃSKA & RADWAŃSKI, pp. 254–258, text-figs 1–3, pl. 1, figs 1–6, pl. 2, fig. 1.

2015 Discradisca polonica (Radwańska & Radwański) – DULAI, pp. 192, pl. 1, figs 5–10.

Material – Várpalota, Szabó sand pit, Pusztamiske Formation (2 fragments); Nyirád, gravel pit, Pusztamiske Formation (200 fragments), Leitha Limestone Formation (1 dorsal valve, 7 fragments).

Remarks – Except for one specimen, the studied brachiopods are rather fragmentary, but some of them give useful information on the larval shell and the ornamentation of the dorsal valve ( Figs 2–21 ). The only complete dorsal valve from the Leitha Limestone Formation of Nyirád is fully covered by a celleporid bryozoan colony. Therefore, the larval shell and the ornamentation is not observable, however, this specimen is informative for the size, the outline and general shape of the dorsal valve ( Fig. 22 ). After combining these available data, the studied material reliably can be identified as D. cf. polonica .

The organo-phosphatic shells are nearly black in the case of the Várpalota specimens, while reddish brown in the Nyirád samples. The outline of the complete shell is nearly circular ( Fig. 22 ), but irregular or broken at some places. The irregularly circular outline is also suggested by some of the illustrated fragments (e.g., Figs 2, 13, 15 ). RADWAŃSKA & RADWAŃSKI (1984) mentioned some rectangular or even polygonal specimens; however, in the case of the Hungarian material the fragmentary preservation prevents to confirm this feature. The diameter of the complete dorsal valve is about 5.5 mm (similar to the largest one mentioned by RADWAŃSKA & RADWAŃSKI 1984). According to the complete valve and the more or less concentric growth lines of the fragments (e.g., Figs 2, 20 ), the apex is in central or nearly central position. Both the complete specimen and the fragments refer to low-conical shape. The diameter of the completely smooth, circular larval shell is about 0.47–0.53 mm ( Figs 3, 9, 20 ), slightly larger than in the case of Korytnica specimens (both RADWAŃSKA & RADWAŃSKI 1984 and DULAI 2015 mentioned 0.4 mm; however, in the latter case it is 0.45 mm on Pl. 1, Fig. 6 ). In some cases three different growth stages can be differentiated on the dorsal shells: the completely smooth larval shell, the post-larval shell characterized by dense growth lines and the adult part of the shell, where the radial ribs appeared (e.g., Figs 2, 8, 20 ). The external ornamentation of dorsal valves is characterized by numerous radial ribs. The number of ribs cannot be identified exactly on the fragments or on the Bryozoa-covered complete valve. However, the density of ribs is well comparable with the Korytnica specimens. The characters of ribs are also very similar to RADWAŃSKA & RADWAŃSKI’s (1984) description. Well defined swells formed at the intersections of ribs and growth lines which give a beaded appearance for the ribs (e.g., Figs 2, 4, 5, 7, 13 , and 14). In some cases the beaded character of ribs is especially strong, even without definite growth lines (e.g., Figs 10–12 ). The ribs never bifurcate, but some new ribs can be seen intercalated at various growth stages (see Fig. 10 ). Interspaces are wider than ribs ( Fig. 11 ). Unfortunately, the interior of the single complete valve is not well-preserved enough to identify precisely the muscle scars (see horseshoe-shaped white part on Fig. 22 ).

Although the specimens are rather fragmentary in the coarse-grained Pusztamiske Formation, the number of fragments is considerable, as it was not very common in the Korytnica Basin either. Despite the systematic washing, RADWAŃSKA & RADWAŃSKI (1984) mentioned only a few dozens of dorsal valves, while the three samples of Naturalis Biodiversity Center in Leiden contained nearly 200 fragments ( DULAI 2015).

This is the first record of this species outside of Poland, as it was originally described from the Korytnica Basin by RADWAŃSKA & RADWAŃSKI (1984) and recently it was mentioned from the same area by DULAI (2015). A similar ribbed discinid was described also from the Vienna Basin by DREGER (1889) on the basis of a single, fragmentary dorsal valve ( Discinisca scutellum , see also in KROH 2003). It is smaller in size and has fewer ribs than polonica , but the fragmentary, even more damaged condition than originally illustrated by DREGER (1889) make the real revision and comparison very difficult. The third ribbed discinid from the Miocene of the Central Paratethys is Discinisca carpathica described by ČTYROKÝ & FEJFAR (1963); however, it is easily distinguishable on the basis of the significantly larger size, and much numerous fine ribs. The fourth and most widely distributed Central Paratethyan discinid is Discinisca leopolitana , which is ornamented only by concentric growth lines ( ŁOMNICKI 1897; FRIEDBERG 1921; SCHMID et al. 2001; RADWAŃSKI & WYSOCKA 2004; DULAI 2015). A similar discinid species, Discradisca multiradiata (de Morgan, 1915) is known also from the Miocene of France (DOLLFUSS & DAUTZENBERG 1901; DE MORGAN 1915; DULAI 2013; BITNER & CAHUZAC 2013); however, it has more numerous (80–100) fine, beaded ribs, and its apex is situated more regularly in central position. The generic attribution of the species polonica to Discradisca was discussed in detail by DULAI (2013) and BITNER & CAHUZAC (2013).

D. polonica seems to be restricted to shallow water environments: it was described from the littoral deposits of the Korytnica Basin ( RADWAŃSKA & RADWAŃSKI 1984), and both Hungarian localities also refer to shallow habitats in the palaeoenvironmental reconstructions (e.g., MOISSETTE et al. 2007; SELMECZI 1996). The shape variability and irregularity was interpreted by RADWAŃSKA & RADWAŃSKI (1984) as adaptation to the high-energy environment of the shore zone.

Distribution – Middle Miocene (Badenian): Korytnica ( Poland), Várpalota and Nyirád ( Hungary).

Figures

Figs 2–21.Discradisca cf. polonica (Radwańska et Radwański, 1984). – 2–4. Várpalota, Szabó sand pit, 680–690 cm, INV 2018.31. – 2. External view of dorsal valve. – 3. Detail of the smooth larval shell.– 4. Detail of the ribbed ornamentation.– 5. Várpalota,Szabó sand pit,520–530 cm;External view of dorsal valve fragment, INV 2018.32.– 6–7. Nyirád, Pusztamiske Formation, INV 2018.33. – 6. External view of dorsal valve. – 7. Detail of growth lines and radial ribs. – 8–9. Nyirád, Pusztamiske Formation, INV 2018.34. – 8. External view of dorsal valve. – 9. Detail of the smooth, circular larval shell and the post-larval shell ornamented by dense growth lines. – 10–12. Nyirád, Pusztamiske Formation, INV 2018.35. – 10. External view of dorsal valve. – 11. Detail of ornamentation of beaded radial ribs.– 12. Beads within a radial rib and radial micro-ornamentation on and around beads. – 13–14. Nyirád, Pusztamiske Formation, INV 2018.36. – 13. External view of dorsal valve. – 14. Detail of external ornamentation with capillate micro-ornamentation.– 15–17. Nyirád, Pusztamiske Formation, INV 2018.37. – 15. External view of dorsal valve. – 16. Detail of larval shell. – 17. Detail of the rare and irregular ornamentation. – 18–19. Nyirád, Pusztamiske Formation, INV 2018.38. – 18. External view of dorsal valve. – 19. Detail of the larval and postlarval shell. – 20–21. Nyirád, Pusztamiske Formation, INV 2018.39. – 20. External view of dorsal valve. – 21. Detail of post-larval shell with dense and irregular concentric growth lines. Scale bars represent 1 mm (5, 6, 8, 10, 13, 15, 18, 20); 0.5 mm (2, 11, 19); 0.25 mm (9, 16); 0.2 mm (3, 7, 14, 17, 21); 0.1 mm (4, 12)

Fig. 22. Discradisca cf. polonica (Radwańska et Radwański, 1984). Internal view of a more or less complete dorsal valve covered by celleporid bryozoan colony. Nyirád, Leitha Limestone Forma- tion, INV 2018.40. Scale bar: 3 mm