Acanthochitonidae Pilsbry, 1893

Family Acanthochitonidae Pilsbry, 1893 View in CoL

Genus Acanthochitona Gray, 1821 View in CoL

= Pseudoacanthochitona ( Šulc 1934) View in CoL

Type species. Chiton fascicularis Linnaeus, 1767 , by monotypy.

Distribution. Acanthochitona is known from the Oligocene to the Recent, the latter almost worldwide, except polar waters. The fossil record includes the Oligocene of New Zealand ( Lee et al. 2014; Wu & Lee 2024), the Miocene of the Mediterranean Basin ( Dell’Angelo et al. 2016, 2018a, 2018b), Paratethys ( Šulc 1934; Bałuk 1984), West Pacific Islands ( Ladd 1966), Australia ( Cotton 1964), Pliocene to Pleistocene of the Mediterranean Basin ( Dell’Angelo et al. 2001a, 2013; Garilli et al. 2005), Australia ( Cotton 1964) and New Zealand ( Sutherland et al. 1995), Pleistocene of the Red Sea ( Dell’Angelo et al. 2020a), Holocene of Japan ( Kuroda et al. 1980).

Remarks. Acanthochitona is a confusing group; the standard chiton shell characters (e.g., shape of the valves, shape of granules and appearance of the jugal area) have a limited diagnostic value to easily set apart species ( Ferreira 1983). Moreover, intraspecific variability within Acanthochitona species is high, rendering identification problematic (e.g., Leloup 1941, 1968). Six species of Acanthochitona are known as living along European waters [ A. barbarae Dell’Angelo, Sosso & Taviani, 2024, A. crinita ( Pennant, 1777) , A. discrepans ( Brown, 1827) , A. fascicularis ( Linnaeus, 1767) , A. oblonga ( Leloup, 1981) and A. pilosa Schmidt-Petersen, Schwabe & Haszprunar, 2015 ]; three of them are also known as fossil ( A. crinita , A. fascicularis and A. oblonga ) and considered in this study, whilst the occurrence of A. discrepans is doubtful (see appendix regarding the species of unclear taxonomic position). The main characteristcs of the species here discussed are reported in Tab. 22.

Šulc (1934: p. 16) described the new subgenus Pseudoacanthochiton for species characterized by “ The larger number of cuts on the tail valve, of which two, however, that, based on their position, correspond to the incisions in the genus Acanthochiton , are much better developed than the other ones located in between them… This structure closely approaches [that of] the genus Acanthochiton , but the presence of a larger number of incisions forces us to place them in the genus Cryptoconchus ” (translation by A. Kroh). The subgenus Pseudoacanthochiton ( Šulc, 1934) was subsequently synonymized with Acanthochitona Gray, 1821 ( Smith 1960; Van Belle 1981; Bałuk 1984; Dell’Angelo et al. 2013), mainly highlighting that some tail valves of Acanthochitona with additional slits on the insertion plate had been reported [see above for A. fascicularis ( Linnaeus, 1767) ]. Anyway, all these reports illustrate valves that correspond in all respects to the valves of Acanthochitona , with the exception of the additional slits, which are, however, similar to the two slits typical of the genus Acanthochitona .