The Rhynchonelliformea are one of the three major clades of brachiopods. With 30 living species in 22 genera, thay are by far the most common group today. The Rhynchonelliformea include not only the "articulate" brachiopods, but a number of inarticulate forms as well. Nevertheless, they are commonly treated as coextensive with the traditional Articulata (brachiopods with specialized articular teeth, articulating with sockets that join the dorsal and ventral shells). The gut is blind and without an anus. The Rhynchonelliformea have calcareous shells with two distinct layers: a primary (outer) and secondary inner) layer. The outer layer is lamellar and the inner layer is fibrous.
Extant species are rarely found in tropical waters, but are relatively common in deeper temperate and polar seas. However, this distribution is a relatively recent phenomenon. Paleozoic rhynchonelliforms seemed to have done better in the tropics. Similarly, living rhynchonelliforms are notable for their ability to live at a broad range of water depths. Paleozoic forms seem to have been more specialkized. Kowalewski et al. (2002).
In the currently accepted arrangement, the following groups are regarded as part of the Rhychonelliformea:
Cyrtospirifer from Virtual Fossils (Univ. di Padua)
Kowalewski, M, MG Simões, M Carroll & DL Rodland (2002), Abundant brachiopods on a tropical, upwelling-influenced shelf (Southeast Brazilian Bight, South Atlantic). Palaios 17: 277-286.