The Sonniniid ammonite Euhoploceras from the Middle Jurassic of south-westEngland and southern Spain

A biostratigraphical and taxonomic revision of the genus Euhoploceras is pr esented using new material collected from the Upper Aalenian-Lower Bajocian of south-west England and the Betic Cordillera (Spain). Euhoploceras is sh own to range from the uppermost Aalenian (Concavum Zone, Limitatum Subzone) to the Lower Bajocian (Laeviuscula Zone). S. S. Buckman's 69 species of So nninia are grouped into three macroconch morphospecies: Euhoploceras acanth odes (Buckman), strongly ornamented, with tubercles and strong ribs through out ontogeny; Euhoploceras marginatum (Buckman) with tuberculate inner whor ls and relatively strong ribs persisting to the outer whorls, and Euhoploce ras modestum (Buckman) with ribbed, sometimes slightly tuberculate inner wh orls, becoming smooth or only slightly ornamented in later stages of coilin g. Euhoploceras adicrum (Waagen) is a younger species with a type horizon i n the Laeviuscula Zone. Dimorphism and possible polymorphism are present in both English and Spanish faunas. Euhoploceras subspinosum (Buckman) probab ly 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 t he microconchs of Euhoploceras [M] are probably represented by Nannoceras. Similarities between English and Betic Euhoploceras confirm that during lat e Aalenian-earliest Bajocian times, the English and Spanish palaeobiogeogra phical regions were connected. The ammonite assemblages revised here come f rom the Concavum to Ovalis zones.