''Dorsetensia'' is a narrowly coiled discoidal

ammonite Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...

from the early

Middle Jurassic The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174.1 to 163.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively rare, but geological formations co ...

, lower Bajocian, belonging to the family

Sonniniidae Sonniniidae is a diverse family of Middle Jurassic ammonites ranging from those with stout evolute shells to those whose shells are sharply rimmed, oxyconic. The keel, which runs along the middle of the venter, is typically hollow. Sutures vary f ...

of the superfamily

Hildoceratoidea Hildoceratoidea, formerly Hildoceratacaea, is a superfamily of compressed or planulate ammonites, some tending to develop acute outer rims; generally with arcuate or sigmoidal ribs. Aptichus were found in place are double-valved.Arkell ''et al.' ...

. 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 it had been regarded as a subgenus of the latter or as subgenera of '' Soninnia'' along with ''Witchellia''. The prevailing consensus is these are three distinct genera with ''Dorsetensia'' and ''Witchellia'' distinguished by shell morphology and stratigraphic disjunction. ''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.

References

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

Ammonitida genera Sonniniidae Jurassic ammonites Jurassic animals of Asia Bajocian life {{ammonitina-stub