Discradisca Stenzel 1964

Abstract

Genus Discradisca Stenzel, 1964 Discradisca Stenzel, 1964: 627. TYPE SPECIES. — Orbicula antillarum d’Orbigny, 1845: by original designation (Stenzel 1964: 627). REMARKS Discradisca was introduced by Stenzel (1964) as a subgenus of Discinisca Dall, 1871 when he wanted to distinguish those species having radial costellae on the dorsal valve from those lacking costellae. Later Cooper (1977) elevated Discradisca to generic rank. In the revised Treatise, Holmer & Popov (2000) distinguished Discradisca and Discinisca on the basis of the ventral valve:“similar to Discinisca, but with wide, transversely suboval pedicle track, closed apically by semicircular, gently concave listrium”. However, fossil assemblages generally contain mostly dorsal valves (some exceptions were mentioned by Radwańska & Radwański 2003) and ventral valves are not always known even in the case of recent materials. Therefore, it is expedient to return to Stenzel’s original definition that distinguishes Discradisca based on radial costellae on the dorsal valve, especially if only dorsal valves are available (e.g., Dulai 2013; Bitner & Cahuzac 2013; Ishizaki & Shiino 2023, 2024). Regarding the radial ridges on discinide valves, Williams et al. (1997) distinguished primary radiated ridges (costae) from secondarily branched ridges (costellae). In this paper we follow the work of Ishizaki & Shiino (2023) who did not distinguish between primary and secondary ridges, but referred to them uniformly as costellae. Discinide species are traditionally subdivided into three types (A, B, and C) based on dorsal valve characteristics (Dall 1920; Radwańska & Radwański 1984; Ishizaki & Shiino 2024: fig. 10): Type A: large, lamellose shell without radial costellae; Type B: large, weakly lamellose shell with faint irregular costellae; Type C: small, non-lamellose shell, with regular radial costellae. Taking into account the abovementioned criteria, species of type B and C are classified in the genus Discradisca (although it may be argued that type B might deserve to be placed in a separate genus). The painful lack of knowledge about the ventral valve of the fossil species of Discinisca and Discradisca is excellently illustrated by the carefully compiled summary figure by Ishizaki & Shiino (2024: fig. 10). They show that among the nine known Palaeogene and Neogene Discradisca species, we have no information at all on the ventral valve, and even among the Discinisca species, only three of the twelve species have a fossilised ventral valve: D. singewaldi (Scuchert in Berry 1917); D. elslooensis (Radwańska & Radwański 2003) and D. messii (Pérez et al. 2023). The ventral valve was also described and illustrated by Hertlein & Grant (1944) for the species D. perrini Loel & Corey, 1932 from Miocene strata in western North America. Currently six extant Discradisca species are known characterised by a warm-temperate to tropical, cosmopolitan distribution mainly over the continental shelves (Emig 1997a, b): D. antillarum (d’Orbigny, 1845): the Atlantic coast of America from Cuba to Brazil; the Caribbean Sea at the coasts of Mexico, Cuba, Nicaragua, and Jamaica (d’Orbigny 1841- 1853; Reeve 1862; Davidson 1888; Dall 1920; Cooper 1977; Tunnell 1982; Zezina 1985, 2010; Emig 2017: p. 33). D. cumingii (Broderip, 1834): the Pacific coasts of America from California Peninsula to 40°S (Mexico, Guatemala, Panama, Ecuador, Peru), Antarctic Peninsula, from the low tide line to 15 m of depth (Broderip 1834; Reeve 1862; Davidson 1888; Dall 1920; Zezina 1985, 2010; Zezina & Pakhnevich 1998; Gaspard 2001; Álvarez in Álvarez et al. 2016; Emig 2017). D. indica (Dall, 1920): India, Sri Lanka, Philippines, the Persian Gulf, and Indonesia (as Discinisca keiensis by Jackson & Stiasny 1937) at a depth of 51-68 and (?)329 m (Dall 1920; Cooper 1973; Zezina 1985, 2010; Bitner et al. 2008). D. sparselineata (Dall, 1920): the coasts of Japan from Hakodate to Tokyo Bay (unknown depth) and French Polynesia (96-300 m) (Dall 1920; Hatai 1940; Kato 1983; Zezina 1985, 2010; Bitner 2014; Ogiso et al. 2014, 2019). D. stella (Gould, 1862): the Pacific coasts of Asia from Tokyo Bay to Hong Kong and to New Caledonia; Northern Australia and the seas of the Malay Archipelago at a depth of 31-141 m (Reeve 1862; Davidson 1888; Dall 1920; Hatai 1940; Zezina 1985, 2010; Richardson et al. 1989; Harper 1997; Bitner 2010; Emig 2017: 33; Otani 2019; Ogiso et al. 2019; Ishizaki & Shiino 2023). D. strigata (Broderip, 1834): the Pacific coasts of America from California to Panama at a depth of 5-33 m (Broderip 1834; Dall 1920; LaBarbera 1985; Zezina 1985, 2010; Emig 2009). So far, no Discradisca remains are known from Mesozoic formations (Ishizaki & Shiino 2024: fig.1). The oldest extinct species of the genus is D. littigensis Stenzel, 1964 from the Lower Palaeocene (Danian) of Texas, USA (Stenzel 1964). Several extinct species of the genus are known from the Eocene to Miocene of Europe (e.g., Dreger 1889; Dollfus & Dautzenberg 1901; de Morgan 1915; Muir-Wood 1939; Čtyroký & Fejfar 1963; Radwańska & Radwański 1984, 1989; Popiel-Barczyk 1996; Kroh 2003; Kocsis et al. 2012; Bitner & Cahuzac 2013; Dulai 2013, 2015, 2017). Also, the genus is known from the Palaeogene (Hertlein & Grant 1944) and Neogene (Dall 1909; Portell & Harper 2005) of North America and from the Pleistocene, and possibly the Pliocene of Japan (Yabe & Hatai 1935; Hatai 1940; Hatai & Hayasaka 1965).

Authors 2

1. Alfréd Dulai

   Hungarian Natural History Museum · Hungarian National Museum

   Affiliation as printed

   Department of Palaeontology and Geology, Hungarian National Museum Public Collection Centre, Hungarian Natural History Museum, H- 1083, Budapest, Ludovika Square 2 (Hungary)

2. J. G. M. Raven Aachen Research associate Naturalis Biodiversity Center

   Naturalis Biodiversity Center

   Affiliation as printed

   Research Associate, Naturalis Biodiversity Center, P. O. Box 9517, NL- 2300 RA Leiden (The Netherlands)