Prehistoric protostome classes

The Rostroconchia is a class of extinct molluscs dating from the early Cambrian to the Late Permian. They were initially thought to be bivalves, but were later given their own class. They have a single shell in their larval stage, and the adult typically has a single, pseudo-bivalved shell enclosing the mantle and muscular foot. The anterior part of the shell probably pointed downward and had a gap from which the foot could probably emerge. Rostroconchs probably lived a sedentary semi-infaunal lifestyle. There were probably more than 1,000 species of members of this class.

Dinocaridida is a proposed fossil taxon of basal arthropods, which flourished during the Cambrian period and survived up to Early Devonian. Characterized by a pair of frontal appendages and series of body flaps, the name of Dinocaridids (Greek for deinos "terrible" and Latin for caris "crab") refers to the suggested role of some of these members as the largest marine predators of their time. Dinocaridids are occasionally referred to as the 'AOPK group' by some literatures, as the group composed of Radiodonta (Anomalocaris and relatives), Opabiniidae (Opabinia and relatives), and the "gilled lo

thumb|Yochelcionella cyrano thumb|Pelagiella atlantoides Helcionelloida is an extinct group of ancient molluscs (phylum Mollusca). These are the oldest known conchiferan molluscs, that is, they had a mineralised shell. Some members of this class were mistaken for Monoplacophorans. The class was erected by Peel in 1991.

Megacheira (from Ancient Greek μέγας (mégas), meaning "great", and χείρ (kheír), meaning "hand", also historically great appendage arthropods) is an extinct class of predatory arthropods defined by their possession of spined "great appendages". Their taxonomic position is controversial, with studies generally either considering them stem-group euarthropods, or stem-group chelicerates. The homology of the great appendages to the cephalic appendages of other arthropods is also controversial. Uncontested members of the group were present in marine environments worldwide from the lower Cambrian to

Thylacocephala (from the Greek or ', meaning "pouch", and or ' meaning "head") is an extinct group of mandibulate arthropods, that are generally regarded as a kind of crustacean, though their exact position within this group is uncertain. As a class they have a short research history, having been erected in the early 1980s.

Strophomenata is an extinct class of brachiopods in the subphylum Rhynchonelliformea.