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

A biostratigraphical and taxonomic revision of the genus<E1>Euhoploceras</E1>is presented using new material collected from the Upper Aalenian-Lower Bajocian of south-west England and the Betic Cordillera (Spain).<E1>Euhoploceras</E1>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<E1>Sonninia</E1>are grouped into three macroconch morphospecies:<E1>Euhoploceras acanthodes</E1>(Buckman), strongly ornamented, with tubercles and strong ribs throughout ontogeny;<E1>Euhoploceras marginatum</E1>(Buckman) with tuberculate inner whorls and relatively strong ribs persisting to the outer whorls, and<E1>Euhoploceras modestum</E1>(Buckman) with ribbed, sometimes slightly tuberculate inner whorls, becoming smooth or only slightly ornamented in later stages of coiling.<E1>Euhoploceras adicrum</E1>(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.<E1>Euhoploceras subspinosum</E1>(Buckman) probably represents dwarf forms of<E1>E. acanthodes</E1>[M] and is included as a junior synonym of the latter; likewise,<E1>E. subdecoratum</E1>(Buckman) may belong with<E1>E. marginatum</E1>[M], although they are described separately herein. Some of the microconchs of<E1>Euhoploceras</E1>[M] are probably represented by<E1>Nannoceras</E1>. Similarities between English and Betic<E1>Euhoploceras</E1>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. [ABSTRACT FROM AUTHOR]