An unusual onychochilid mollusc from the Ordovician (Tremadocian) Fezouata Formation, Morocco
Résumé
Pelecyogyra fezouataensis nov. gen., nov. sp. is described from the Lower Ordovician (Tremadocian, A. murrayi Biozone) Fezouata Formation at Oued Beni Zoli (locality Z-F5), in the central Anti-Atlas, Morocco. This is the first Onychochilidae Koken in Koken and Perner, 1925 reported from the Ordovician of Morocco. Its stratigraphical position bridges the gap between the late Cambrian and Middle to Upper Ordovician records. This new taxon draws together large specimens, reaching 27 mm in the greatest diameter, with 3 rapidly expanding whorls. All specimens are invariably preserved as dorsoventrally compressed moulds although preservation suggests that the degree of compaction was not great and that the species was generally low spired. A characteristic axe-shaped apertural lip and an ornamentation consisting of fine and dense lirae serve to distinguish the new genus from other onychochilids. The species appears to have been gregarious. In some specimens, small scalloped injuries are preserved along the apertural margins but these are not considered to reflect failed predation. Up to now, twenty-two species and eight genera of Ordovician onychochilids have been documented, being distributed in eight different terranes. The composition of the Onychochilidae is briefly reviewed and updated from the literature, to include Helicotis? Koken in Koken and Perner, 1925, Hyperstrophema Horný, 1964, Invertospira Horný, 1964, Kobayashiella Endo, 1937, Laeogyra Perner, 1903, Matherella Walcott, 1912, Matherellina Kobayashi, 1937, Onychochilus Lindström, 1884, Pelecyogyra nov. gen., Pervertina Horný, 1964, Scaevogyra Whitfield, 1878, Sinistracirsa Cossmann, 1908, Versispira Perner, 1903, and Voskopiella Frýda, 1992. Onychochilidae and Clisospiridae are placed in Mimospirida Dzik, 1983, regarding Hyperstrophina Linsley and Kier, 1984 as a junior synonym.