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The Lower Greensand Group on the Isle of Wight in southern England forms the basis for the Aptian ammonite biostratigraphy in western Europe mainly established by the work of Raymond Casey between 1960 and 1980. It thus became a world reference succession, due to many ammonite finds from excellent outcrops along the coastline of the Island exposing the fossiliferous Atherfield Clay and Ferruginous Sands formations. Nevertheless, some of the members of these formations only rarely contain ammonites: an example is the horizon containing Walpenites (Group X according to the classical nomenclature of Fitton), in the upper Aptian Epicheloniceras martinioides Zone. Hitherto, this horizon has yielded only very rare Walpenites, a cryptic micromorphic ammonite only known from the type specimen. We report additional specimens of this rare ammonite, unequivocally demonstrating that Walpenites is a heteromorphic ammonite with only a part of the early whorls coiled in a planispiral with whorls attached to each other. Furthermore, the new material allows us to regard Luppovia as a junior subjective synonym of Walpenites, a genus based on material from Kazakhstan and Turkmenistan. Additionally, a new fauna of small ammonites from the Walpenites horizon is described; it contains Epicheloniceras martinioides, E. subnodosocostatum, E. tschernyschewi and the heteromorph cf. Toxoceratoides. This has improved the biostratigraphical dating and correlation of the stratigraphical level herein called the Walpenites horizon and, in contrast to earlier ideas, it contains ammonites that are distributed almost globally in all major faunal realms. |
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