Dorsetensia

Dorsetensia is a narrowly coiled discoidal ammonite from the early Middle Jurassic, lower Bajocian,^[1] belonging to the family Sonniniidae of either the superfamily Hammatoceratoidea or Hildoceratoidea. The inner whorls are ribbed or smooth, outer whorl is smooth. The outer rim (venter) is narrow, with a keel running along the middle. The umbilicus, the opening in the middle of the shell exposing inner whorls, is of moderate size with a sharp, sometimes undercut edge.

Dorsetensia is similar in general form to the genus Witchellia, to the point that some have regarded Dorsetensia a subgenus of the latter. Others have regarded both Dorsetensia and Witchellia as subgenera of Soninnia. The prevailing consensus is these are three distinct genera. Dorsetensia and Witchellia are separated on both morphologic and stratigraphic grounds. Witchellia is from the middle Lower Bajocian Laeviuscula subzone, Sowebvyi Zone and has a distinct tabulate venter even in smooth involute forms. Dorsetensia is from the middle Bajocian Humphriesianum Zone and has a narrow fastigate (pitched roof) venter.

The inclusion of alternative superfamilies, Hildoceratoidea and Hammatoceraoidea is because to Soninniidae containing Dorsetensia is included in the Hildoderataceae in the Treatise on Invertebrate Paleontology, (Arkell, 1957) and Donovan, et al. 1981, while it is included in the Hammatoceratoidea in recent classifications, which was separated from the Hildoceratoidea by Schindewolf (1964).

References

• Arkell, et al., 1957. Mesozoic Ammonoidea, Treatise on Invertebrate Paleontology, Part L (Ammonoidea).
• The Bajocian Ammonite Dorsetensia in Skye, Scotland by N. Morton