Revision of the Middle Jurassic dimorphic ammonite genera Strigoceras/Cadomoceras (Strigoceratidae) and related forms

TLDR: A benefit of this large area of distribution is that strigoceratids may be used to correlate distant locations, but this is partly hampered by the relatively wide range of variability within succeeding chronospecies and remarkably large variation in size of adult macroconchs.

Abstract: We present a modern revision of the dimorphic ammonite pair Strigoceras QUENSTEDT, 1886/Cadomoceras MUNIER-CHALMAS, 1892 from the Middle Jurassic. The stratigraphically oldest hitherto known chronospecies of Strigoceras, S. praenuntium (BUCKMAN) appears in the Middle Aalenian as an extreme rarity. Records of the microconch genus Cadomoceras remain unknown from strata older than Early Bajocian, but otherwise its stratigraphical range corresponds well with that of the macroconchs. The genus probably became extinct in the Early Bathonian (Zigzag Zone). The youngest known Strigoceras is S. callomoni n. sp. described here from the Macrescens Subzone of the Early Bathonian. Another new strigoceratid species, Granulochetoceras oppeliisculptum n. sp., is recorded by a single specimen from the early Garantiana Zone of eastern Spain, representing an interesting phyletic link between Strigoceras QUENSTEDT, 1886 and Granulochetoceras GEYER, 1960. We include a brief review of the latter genus. By including Granulochetoceras in the Strigoceratidae the family would extend to the Late Kimmeridgian of the Late Jurassic. The palaeogeographic distribution of Strigoceras/Cadomoceras is focussed on the western Tethyan Province, but some species also occur in the northern Pacific and its adjacent shelves. A benefit of this large area of distribution is that strigoceratids may be used to correlate distant locations, but this is partly hampered by the relatively wide range of variability within succeeding chronospecies and remarkably large variation in size of adult macroconchs.