The 'grooved' Hildoceras ammonite fauna from the Lower Jurassic (Toarcian) of the Apennines. Overview and proposal for a new species

It is historically renown that morphotypes referred to Hildoceras bifrons from the Apennines (Italy) and other Mediterranean localities differ in many ways from the typical species of the Central and North-West Europe. Besides overall smaller size (usually < 40 mm), Mediterranean specimens attain marked whorl embracement, comparable to that of the kin species Hildoceras semipolitum (albeit retaining a less compressed whorl section). Relatively larger (and rare) morphotypes may show slight 'uncoiling' (resembling the typical Hildoceras bifrons), though limitedly to the final part of the adult-stage last whorl. Both the common (small-involute) and rare (larger-slightly uncoiled) morphotypes thus reveal of ambiguous taxonomic attribution either as Hildoceras bifrons or as Hildoceras semipolitum. In front of this ambiguity, the new species Hildoceras appenninicus sp. nov. is proposed for grouping 'bifrons-like' Hildoceras occurring in the Apennines and other Mediterranean localities. In contrast with previous works on this subject, definite ranges for whorl compression indexes relative to whorl diameter and embracement are provided, by this attempting at a more objective distinction of this new species from both Hildoceras bifrons and Hildoceras semipolitum. Finally, along with some biostratigraphic remarks, possible evolutionary and paleobiogeographic implications concerning the arising and spreading of Hildoceras species are outlined.