Antarcticeras

Antarcticeras is an extinct genus of enigmatic cephalopod from the Eocene of Antarctica. It contains a single species, A. nordenskjoeldi. It is either considered the last of the "orthocone"-type cephalopods, the only member of its subclass Paracoleoidea & a descendant of the orthoceratids, and a remarkable example of convergent evolution with coleoid cephalopods, or an oegospid squid and a transitional form in the development of the modern squid gladius, of which it is the only preserved example.

It is named after Swedish geologist and Antarctic explorer Otto Nordenskjöld.

Discovery

Antarcticeras is represented by two fossil shell fragments from the Early Eocene La Meseta Formation from Seymour Island, Antarctica. These are straight shells that closely resemble those of the orthocones, although they are thought to have been internal akin to those of certain coleoids such as the cuttlebone of cuttlefish and the "rams horn" shell of spirulids. The shells are weakly mineralized with the presence of soft tissues and were likely only preserved due to a unique depositional environment. Sympatric cephalopod taxa include the coiled nautiloid Euciphoceras.

Taxonomy

Antarcticeras was described in 2017 by Doguzhaeva et al., who recovered it as the only member of a ghost lineage of cephalopods that had diverged from all others during the Paleozoic. Its coleoid-esque internal shell was recovered as an instance of convergent evolution with coleoids.