Acanthaecites is an extinct genus of cephalopod belonging to the 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...

 subclass that was first described by Rollier in 1909. Even though some (a minority) of ammonites lived rather sedentary bottom dwelling lives, Acanthaecites was an active, mobile predator that lived a nekton

Nekton

Nekton refers to the aggregate of actively swimming aquatic organisms in a body of water able to move independently of water currents....

ic life style. Acanthaecites both appears and disappears from the fossil record during the Callovian

Callovian

In the geologic timescale, the Callovian is an age or stage in the Middle Jurassic, lasting between 164.7 ± 4.0 Ma and 161.2 ± 4.0 Ma. It is the last stage of the Middle Jurassic, following the Bathonian and preceding the Oxfordian....

stage of the Jurassic Period.