Rhynchonelliformea

One of three subphyla currently recognized in the phylum Brachiopoda. The name is derived from the stratigraphically oldest subclade with extant representatives, the order Rhynchonellida. Rhynchonelliforms constitute over 90% of the more than 4500 named brachiopod genera, most of which (over 95%) are extinct. Rhynchonelliformea includes all taxa formerly referred to the class Articulata, in addition to several groups either newly discovered as fossils, formerly placed in class Inarticulata, or of uncertain taxonomic affiliation.

All rhynchonelliforms have an organic-rich, bivalved calcite shell and can be distinguished from other brachiopods by the following shared, derived characters: the presence of a fibrous secondary shell layer, a pedicle without a coelomic core, a diductor muscle system, and the presence of articulation between the two valves.

Rhynchonelliformea includes 19 orders organized in five classes. Chileata (Chileida and Dictyonellida), Obollellata (Obolellida and Naukatida), Kutorginata (Kutorginida), Stropho-menata (Strophomenida, Productida, Orthotetida, and Billing-sellida), Rhynchonellata (Protorthida, Orthida, Pentamerida, Rhynchonellida, Atrypida, Spiriferida, Spiriferinida, Thecideida, Athyridida, and Terebratulida).

Living rhynchonelliforms are distributed globally, but enjoy their greatest abundance and diversity in temperate latitudes. They live from deep intertidal to abyssal depths, reaching greatest abundance and diversity in the shallow to relatively deep (few hundred meters) subtidal. See also Branchiopoda; Orthida; Pentamerida; Spiriferida; Strophomenida.