It is known from Lower Ordovician to Upper Devonian deposits[1] both as calcitic shells with a brachiopod-like microstructure[2] and carbonaceous 'linings'.

][5] They may also be related to other conical shells of uncertain affinity including cornulitids, Anticalyptraea, microconchids and trypanoporids.

[6] Their shell microstructure has warranted their comparison with the brachiopods and phoronids,[2][7] and the possible Ediacaran lophophorate Namacalathus.

[8] Tentaculitids have ribbed, cone-shaped shells which range in length from 5 to 20 mm.

Some species septate; their embryonic shell, which is retained, forms a small, sometimes spherical, chamber.