A Revision of the Lower Jurassic Ammonite Genus Eoderoceras Spath and its Immediate Descendants and Other Relatives

Abstract

The ammonite genus Eoderoceras Spath is reviewed in the light of recent discoveries and an examination of type and museum-curated material. It is argued that the bispinate type species, E. bispinigerum (Buckman), for which a neotype is designated, is morphologically very close to ammonites from Tethyan and Pacific localities that have been referred in recent literature to the genus Paramicroderoceras Dommergues, Ferretti & Meister. The type species of that genus, namely Microderoceras birchiades Rosenberg, is poorly characterized, and the lectotype designated herein cannot be readily distinguished from Eoderoceras, hence Paramicroderoceras is regarded as a junior synonym.Apart from the type species, which locally dominates a short stratigraphical thickness at its type locality on the Dorset coast, bispinate Eoderoceras are known from only sporadic occurrences in the British Lower Jurassic. However, species interpreted as being the unispinate descendants of Eoderoceras, here accorded generic status as Eteoderoceras gen. nov., are abundant in much of the British Raricostatum Zone and can be divided into a number of different stratigraphically sequential species. The genus Tetraspidoceras Spath is also described as it is thought likely to be a close relative and direct descendant of Eoderoceras that occurs in the British fossil record in the Upper Sinemurian and in the Taylori Subzone of the Jamesoni Zone (Lower Pliensbachian), containing a small number of species. The family Eoderoceratidae, to which these genera belong, is discussed to provide a broader evolutionary context; some important unresolved systematic issues are highlighted.