Cryptoplax margitae Dulai, 2001

Cryptoplax margitae Dulai, 2001 View in CoL

Fig. 151 View FIGURE 151

Cryptoplax margitae Dulai, 2001, p. 45 View in CoL , pl. 4, figs 1–6; Kroh 2003, p. 136; Dulai 2005, p. 44, pl. 8, figs 13–16; Schwabe 2005, p. 98; Dell’Angelo et al. 2007a, p. 47; Ruman & Hudácková 2015, p. 166; Dell’Angelo et al. 2016, p. 95; Dell’Angelo et al. 2018b, p. 54, tab. 17; Dell’Angelo et al. 2020b, p. 48, fig. 31.

Type material. Holotype: HNHM M.99.113, intermediate valve, width 1.16 mm.

Type locality. Borehole Szokolya-2, 92.7-93 m, Börzsöny Mts ( Hungary) .

Type stage. Middle Miocene, marly sandstone.

Material examined. Lower Miocene (Burdigalian): France: Carrière Vives: 1 head valve ( MZB 50586, Fig. 151A View FIGURE 151 ); Gamachot: 3 intermediate valves ( BD 1141, MZB 50587, Fig. 151G View FIGURE 151 ). Middle Miocene: Romania: Kostej: 1 valve ( NHMW 1869/0001/0797, as Cryptoplax weinlandi , Figs 151B–C View FIGURE 151 ); Lăpugiu de Sus: 1 valve ( NHMW 2010/0256/0031), Figs 151D–F View FIGURE 151 ; Hungary: Bánd: 2 intermediate valves ( BD 1142, Fig. 151H View FIGURE 151 ). Maximum width of the valves: 1.3 / 1.7 / -.

Description. Tail valve unknown. Valves longer than wide. Head valve horseshoe shaped, very small, anterior margin rounded, posteriorly straight, with oval area near apex, smooth and only covered by an irregular arrangement of megalaesthete and micrasthetes. Intermediate valve of pentagonal shape, very small, posterior margin obtusely angled. Jugal area smooth, rectangular, with edges slightly undulated.

Tegmentum strongly porous, HV sculptured with irregularly disposed granules, roundish near the posterior margin, becoming larger, more elongate, and tending to coalesce towards anterior margin; PLA ornamented by 4–5 longitudinal rows of rounded granules, parallel to each other and sides of JA, granules roundish, elevated, well separated near apex, and tending to coalesce towards anterior valve margin. Each granule with a central megalaesthete and several micraesthetes (6-8 or more irregularly spaced all along in HV; many in PLA irregularly arranged, mainly on same side, some micraesthete can be seen also in grooves between rows of granules; an irregular arrangement of micraesthetes is present on JA too).

Articulamentum with apophyses relatively short, ca ¼ of whole length of valve, of triangular shape, slit formula 3 / 0 / 0.

Remarks. Cryptoplax margitae Dulai, 2001 was described from two intermediate valves of the Middle Miocene (Badenian) of Hungary ( Dulai 2001, 2005). Other 4 valves (one head, partly eroded, and 3 intermediate) from the lower Miocene (Burdigalian) of the Aquitaine Basin were attributed to this species by Dell’Angelo et al. (2020b); two valves (1 head, well preserved, and 1 intermediate) were identified in the Šulc collection deposited at NHMW; two intermediate valves from the Middle Miocene of Bánd ( Hungary) are added here. These latest findings allow to include the description of the head valve, while the tail one is still unknown.

The valves of Cryptoplax margitae are also very similar to Choneplax indica Odhner, 1919 , an extant species of the family Acanthochitonidae , known from the western Indian Ocean: Mozambique Channell, Madagascar, Réunion, Mauritius, Rodrigues, and Arabian Sea (Dell’Angelo et al. 2004; Dinapoli & Janssen 2009). The analysis of figures of this species (Kaas 1986: figs 67–72; Dell’Angelo et al. 2004: pl. 6, figs 8–13; Schwabe 2004: fig. 11A–D; Dinapoli & Janssen 2009: pl. 16, figs b–c) confirms indeed their resemblance. The small dimensions, shape of head and intermediate valves, the smooth JA, the sculpture and shape of granules irregularly disposed in HV and arranged in 4–5 longitudinal rows in PLA are really very similar, if not identical. This similarity was already noticed by Dulai (2005: p. 42), while discussing intermediate valves II of Cryptoplax weinlandi , which does not present the typical granulated sculpture characteristic of C. margitae .

The genus Choneplax Carpenter in Dall, 1882, includes four Recent species ( Sirenko 2003): C. lata ( Guilding, 1829) from the Caribbean, C. hastata ( Sowerby, 1840) , habitat unknown ( fide Sirenko 2003), C. indica Odhner, 1919 from the W. Indian Ocean, and C. littlerorum Sirenko, 2003 , from American Samoa. Choneplax is known from the Pleistocene of the Red Sea ( Dell’Angelo et al. 2020a). Before the question of a reassignment of Cryptoplax margitae to Choneplax is taken into consideration, the relation between extant Cryptoplax and Choneplax needs to be clarified. Consequently, we prefer to adhere to Dulai’s attribution of margitae to Cryptoplax . Choneplax (attributed to the family Acanthochitonidae , see Sirenko 2006 and Irisarri et al. 2014) differs from Cryptoplax in the strong imbrications of all the valves, and most importantly in the different characters of girdle. Entire live collected individuals of both species are easily separable, while it is more difficult to separate loose valves. Sirenko (2003) suggests that the evolution of the superfamily Cryptoplacoidea (which includes three families: Acanthochitonidae Pilsbry, 1893 , Cryptoplacidae H. & A. Adams, 1858, and Hemiarthridae Sirenko, 1997 ) can be connected with a reduction of the tegmentum and, as a result, a reduction of slits of the insertion plates.

Comparisons. Cryptoplax lanceolatus Laghi, 1977 is the species whose sculpture most closely resembles that of Cryptoplax margitae (see above). However, the sculpture in C. lanceolatus is more rugged, consisting of weak, lumpy ribs, in contrast to the rows of roundish, elevated, and more regular granules seen in C. margitae . Moreover, the head valve differs— C. lanceolatus displays three arcuate ribs—and so does the outline of the intermediate valves (compare Figs. 150C View FIGURE 150 with 151D and 151G).

Distribution. Lower Miocene: northeastern Atlantic (Burdigalian): Aquitaine Basin, France: Carrière Vives, Gamachot ( Dell’Angelo et al. 2020b); Middle Miocene: Central Paratethys (Langhian-Serravallian): Hungary: Bánd, Szokolya-2 borehole ( Dulai, 2001, 2005; this study), Romania: Kostej, Lăpugiu de Sus (this study).