Rhynchonelliformea is a major

subphylum In zoological nomenclature, a subphylum is a taxonomic rank below the rank of phylum. The taxonomic rank of " subdivision" in fungi and plant taxonomy is equivalent to "subphylum" in zoological taxonomy. Some plant taxonomists have also used t ...

and clade of

brachiopods Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, wh ...

. It is equivalent to the former

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently ...

Articulata, which was used previously in brachiopod taxonomy. Articulate brachiopods have many anatomical differences relative to "inarticulate" brachiopods of the subphyla

Linguliformea Linguliformea is a subphylum of inarticulate brachiopods. These were the earliest of brachiopods, ranging from the Cambrian into the Holocene. They rapidly diversified during the Cambrian into the Ordovician, but most families became extinct by ...

and

Craniformea Craniata is a class of brachiopods originating in the Cambrian period and still extant today. It is the only class within the subphylum Craniiformea, one of three major subphyla of brachiopods alongside linguliforms and rhynchonelliforms. Cran ...

. Articulates have hard calcium carbonate shells with tongue-and-groove hinge articulations (hence the name) and separate sets of simple opening and closing muscles. The name, Rhynchoelliformea, which replaces Articulata, which is also a class of

crinoids Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which ar ...

, comes from the family Rhynchonellidae, which however is no more representative of articulate brachiopods than any other group such as spirifers or strophomenids. It just happens to be the name chosen, based on an included taxon. The main difference between the Rhynchonelliformea described in the

Treatise A treatise is a formal and systematic written discourse on some subject, generally longer and treating it in greater depth than an essay, and more concerned with investigating or exposing the principles of the subject and its conclusions." Treat ...

Part H, revised 2000/2007, and the Articulata of the Treatise part H, 1965, lies in the groups included, their taxonomic positions and arrangements. The Rhynchonelliformea (Articulata revised) is divided into five classes: Obolellata, Kutorginata, Chileata, Strophomenata, and Rhynchonellata. The Strophomenata and Rhynchonellata are found living today; the Rhynchonellata as the major constituent of modern brachiopod faunas, the Stromphomenata as only a minor contributor. The Obolellata, Kutorginata, and Chileata are all extinct. The Obolellata and Kutorginata are restricted to the Cambrian, the Chileata ranges throughout the extent of the

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...

. In the older classification of the Treatise (1965) the Class Articulata was divided into six orders, the Orthida, Pentamerida, Rhynchonellida, Spiriferida, Terebratulida, and Strophomenida. The Orthida, Pentamerida, Rhynchonellida, Spiriferida, and Terebratulida became combined as the Rhynchonellata. The Strophomenida became the Strophomenata with the addition of the Orthotetida and Billingsellida and separation of the Chileata. The Obolellata and Kutorginata were previously included in the Inarticulata, but have since been recognized as primitive articulates. One of the more significant changes in the new classification is the splitting of the original

Spiriferida Spiriferida is an order of extinct articulate brachiopod fossils which are known for their long hinge-line, which is often the widest part of the shell. In some genera (e.g. '' Mucrospirifer'') it is greatly elongated, giving them a wing-like ap ...

into distinct and separate orders, the Spiriferida as revised, Atrypida,

Athyridida Athyridida is an order of Paleozoic brachiopods included in the Rhynchonellata, which makes up part of the articulate brachiopods. The Athyridida are the Rostrospracea of R.C Moore, 1952,Moore, Lalcker and Fischer, 1952, Invertebrate Fossils, Mc ...

, and Spiriferinida; each with its own derivation and

phylogeny A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spe ...

. Originally these were included as suborders within the Spiriferida which combined brachiopods with spiral (coiled spring-like) brachidia regardless of the orientation or the length of the hinge line or whether the shell was impunctate or punctate. The newer classification recognises the spiral brachidia being a matter of evolutionally convergence. The Athyridida is the Rostropiracea and the Spiriferinida is the Punctospiracea (suborders) of R.C, Moore in Moore, Lalicker, and Fischer, 1952.

References

*Sandra J. Carlson. 2001. Ghosts of the Past, Present, and Future in Brachiopod Systematics. Journal of Paleontology 75(6):1109-1118
BRACHIOPOD TAXONOMY from the original (1965) to the revised (1997-2007)
Treatise on Invertebrate Paleontology Volume H: Brachiopoda
Paleobiology -Rhynchonelliformea
* R.C. Moore. Brachiopoda, Ch 6, Moore, Lalicker, and Fischer 1952. Invertebrate Fossils. {{Taxonbar, from=Q5315 Animal subphyla Brachiopod taxonomy