Haplophrentis was a tiny shelled hyolithid which lived in the Cambrian

Cambrian

The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

 Period. Its shell was long and conical, with the open end protected by an operculum

Operculum (gastropod)

The operculum, meaning little lid, is a corneous or calcareous anatomical structure which exists in many groups of sea snails and freshwater snails, and also in a few groups of land snails...

, from which two fleshy arms called helens protruded at the sides. These arms probably acted as stabilisers, or were used for locomotion, rowing the animal across undersea mudflats.

Morphology

Haplophrentis ranged in length between 2–6 cm. It is distinguished from Hyolithes by the presence of a longitudinal septum on the middle of the inner surface of the top of the shell.

Affinity

The hyolithids are thought to belong to the molluscan total-group, although this is uncertain.

Ecology

Specimens of Haplophrentis have been found in the gut of the predator Ottoia

Ottoia

Ottoia is the most abundant priapulid of the Cambrian Burgess Shale formation of British Columbia. It is a stem-group priapulid worm that averaged about 80 mm in length.- Ecology :...

.