Discradisca multiradiata (de Morgan, 1915 )

Dulai, A., 2013, Sporadic Miocene brachiopods in the Naturalis Biodiversity Center (Leiden, the Netherlands): Records from the Mediterranean, the North Sea, and the Atlantic Ocean, Fragmenta Palaeontologica Hungarica 30, pp. 15-51 : 29-32

Discradisca multiradiata (de Morgan, 1915)

( Figs 25–36)

Material – Amberre (34 partly fragmented dorsal valves). Size (mm) –

Length 6.5 2.0 3.8 8.9 7.6 5.0 5.0 5.9 2.9 5.5

Width 6.0 2.0 3.5 8.8 6.5 4.6 3.8 4.9 2.6 4.8

Notes – Discinidae brachiopods are common in some recent environments, however, generally rare in fossil assemblages. The French Atlantic Miocene discinide material in the NBC collection is relatively numerous, but contains only dorsal valves. The thin and low conical valves are ornamented by numerous (100– 120) fine radial ribs. The position of the apex is variable between subcentral and subposterior. In the interior side the muscle scar area stretches from the adapical region towards the posterior part, and the anterior adductor scars slightly overlap the apex ( Figs 29, 32). The anterior adductor scars are relatively small and narrow.

The species name multiradiata was first used by DOLLFUS & DAUTZENBERG (1901) for the small French Miocene specimens with numerous radial ribs (faluns of Touraine), but their short description did not contain any illustrations. Therefore, according to the ICZN rules this name was a nomen nudum until the adequate description by DE MORGAN (1915) (see also RADWAŃSKA & RADWAŃSKI 1984). Both DOLLFUS & DAUTZENBERG (1901) and DE MORGAN (1915) mentioned the striking similarity between the Miocene D. multistriata and the recent D. stella (size, general form, ornamentation). However, they are different in the number of ribs and the elevation of the apex.

Discradisca was introduced by STENZEL (1964) as the subgenus of Discinisca . During description of Danian (Paleocene) Discinisca (Discradisca) littigensis, Stenzel wanted to distinguish those species having radial costae on the dorsal valve from those lacking costae. COOPER (1977) elevated Discradisca to generic rank. The new, revised Treatise ( HOLMER & POPOV 2000) distinguished Discinisca and Discradisca on the basis of the ventral valve: “similar to Discinisca , but with wide, transversely suboval pedicle track”.

Although the name Discradisca was introduced for a fossil species, and now it is widely used for recent brachiopods and several species were transferred to this genus ( Discradisca antillarum , D. cumingi , D. indica , D. sparselineata , D. stella , D. strigata ; EMIG 1997), this genus name did not take root in fossil brachiopod literature. One of the main reasons may be probably the fact that recently used definition of Discradisca concentrates on the characteristics of the ventral valve and fossil materials generally contain only dorsal valves without any ventral ones (some exceptions are known mainly from the Mesozoic and these were summarised by RADWAŃSKA & RADWAŃSKI 2003). However, if we return to Stenzel’s original purpose (to distinguish discinides with radial costae on the dorsal valve) several fossil species should be transferred to Discradisca . One of these species is D. multiradiata , which is comparable also with recent Discradisca indica , both having about 120 ribs. The same systematic results were given very recently by BITNER & CAHUZAC (2013) on the basis of an Early Miocene material from the Aquitaine Basin ( France).

Drilling predation has a very long history ( KOWALEWSKI et al. 1998), but Neogene discinide brachiopods were very rarely recorded as victims of gastropod attacks. LESPORT & CAHUZAC (2005) illustrated a drilled discinide specimen without specific identification (although they realised the affinity of its outer ornamentation to D. multiradiata and the Late Oligocene D. steiningeri described by RADWAŃSKA & RADWAŃSKI 1989). Their material is derived from the Lower Burdigalian layers from Martillac (Northern Aquitaine, SW France). The shallow marine Mactra sand yielded a very rich benthic assemblage including a discinide brachiopod. Drilling predation of naticid gastropods was rather frequent here on different bivalve species, and the only discinide brachiopod was also affected ( LESPORT & CAHUZAC 2005: Plate 3, fig. 1). An intense drilling predation was reported from here on the basis of a larger material by BITNER & CAHUZAC (2013): 12% of the specimens were affected in the Early Miocene assemblage of Martillac. Additionally to these French records, just recently has been found a drilled D. polonica Radwańska et Radwański, 1984 specimen from the Middle Miocene of the Central Paratethys ( Poland) (DULAI submitted). In some earlier papers RADWAŃSKA & RADWAŃSKI (1984, 1989) have studied relatively extensive collections of D. polonica (a few dozens of dorsal valves) and D. steiningeri (about 70 specimens) from the Central Paratethys, but they did not mention gastropod drill holes on their discinide brachiopods.

The studied Middle Miocene material from Amberre confirms that French Miocene discinide brachiopods were frequent preys of drilling gastropods ( Figs 25, 28, 31, 33, 35–36). Eight out of 34 specimens bear drill holes of predatory gastropods (23.5%). This drilling frequency is very high not only in discinides, but generally in Neogene brachiopods. The average drilling predation is 2–4% (e.g. DULAI 2007; TADDEI RUGGIERO & BITNER 2008) and higher values up to 39% are exceptional (e.g. BAUMILLER & BITNER 2004; HARPER 2005; BAUMILLER et al. 2006). Bryozoan epibionts are also common on D. multiradiata specimens. Nine out of 34 specimens bear encrusting Bryozoa, which are always situated on the external side of the dorsal valves and sometimes cover nearly the entire external surface of the valve ( Fig. 27). It may suggest that bryozoan larvae settled on the living brachiopods attached to the hard surfaces of shallow marine environments.

Geochemical composition of French Miocene discinides was discussed by KOCSIS et al. (2012). The δ 18 O PO4 values of brachiopods from the Middle Miocene of Western France are similar to that of analyzed from the Central Paratethys indicating comparable temperature regime, but the seawater Ε Nd value here is identical to that of the contemporaneous Atlantic Ocean ( KOCSIS et al. 2012).