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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 &lt; 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. lsid: zoobank.org:pub:5C2C159E-61CA-4EE0-996D-D098618B3790

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