The sonniniid ammonite Euhoploceras from the Middle Jurassic of south‐west England and southern Spain

Abstract

A biostratigraphical and taxonomic revision of the genus Euhoploceras is presented using new material collected from the Upper Aalenian–Lower Bajocian of south‐west England and the Betic Cordillera (Spain). Euhoploceras is shown to range from the uppermost Aalenian (Concavum Zone, Limitatum Subzone) to the Lower Bajocian (Laeviuscula Zone). S. S. Buckman's 69 species of Sonninia are grouped into three macroconch morphospecies: Euhoploceras acanthodes (Buckman), strongly ornamented, with tubercles and strong ribs throughout ontogeny; Euhoploceras marginatum (Buckman) with tuberculate inner whorls and relatively strong ribs persisting to the outer whorls, and Euhoploceras modestum (Buckman) with ribbed, sometimes slightly tuberculate inner whorls, becoming smooth or only slightly ornamented in later stages of coiling. Euhoploceras adicrum (Waagen) is a younger species with a type horizon in the Laeviuscula Zone. Dimorphism and possible polymorphism are present in both English and Spanish faunas. Euhoploceras subspinosum (Buckman) probably represents dwarf forms of E. acanthodes[M] and is included as a junior synonym of the latter; likewise, E. subdecoratum (Buckman) may belong with E. marginatum[M], although they are described separately herein. Some of the microconchs of Euhoploceras[M] are probably represented by Nannoceras. Similarities between English and Betic Euhoploceras confirm that during late Aalenian–earliest Bajocian times, the English and Spanish palaeobiogeographical regions were connected. The ammonite assemblages revised here come from the Concavum to Ovalis zones.