Annuloceras is an extinct genus of ammonite

Ammonite

Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct subclass within the Molluscan class Cephalopoda which are more closely related to living coleoids Ammonite, as a zoological or paleontological term, refers to any member of the Ammonoidea an extinct...

 cephalopod

Cephalopod

A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...

. Its fossils are found in Lower Barremian

Barremian

The Barremian is an age in the geologic timescale between 130.0 ± 1.5 Ma and 125.0 ± 1.0 Ma). It is a subdivision of the Early Cretaceous epoch...

 sediments from California

California

California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

. The genus is currently placed in the family Aegocrioceratidae.

It is a heteromorph ammonite very similar in form to Macroscaphites, and Shastoceras

Shastoceras

Shastoceras is an extinct genus of ammonite cephalopod found in Lower Aptian sediments in Northern California.It, like its close relative, Annuloceras, is a heteromorph ammonite that strongly resembles Macroscaphites....

, the latter being its close (albeit larger) relative that is also found the same strata in California.