The taxonomic

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

Rhynchonellida is one of the two main groups of living articulate

brachiopod Brachiopods (), phylum (biology), phylum Brachiopoda, are a phylum of 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 e ...

s, the other being the order

Terebratulida Terebratulids are one of only three living orders of articulate brachiopods, the others being the Rhynchonellida and the Thecideida. Craniida and Lingulida include living brachiopods, but are inarticulates. The name, Terebratula, may be der ...

. They are recognized by their strongly ribbed wedge-shaped or nut-like

shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses Science Biology * Seashell, a hard outer layer of a marine ani ...

s, and the very short

hinge A hinge is a mechanical bearing that connects two solid objects, typically allowing only a limited angle of rotation between them. Two objects connected by an ideal hinge rotate relative to each other about a fixed axis of rotation, with all ...

line. The hinges come to a point, a superficial resemblance to many (

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

ally unrelated)

bivalve Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...

mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...

shells. The loss of the hinge line was an important evolutionary innovation, rhynchonellids being the first truly non-strophic shells with a purely internal articulation (teeth-sockets). Strong radiating ribs are common in this group; and there are generally very strong plications or accordion-like folds on the sulcus (the long middle section) of the shell. This probably helps regulate the flow of water in and out of the shell. All rhynchonellids are biconvex (have a bulbous shell), and have a fold located in the brachial valve. This means that the

commissure A commissure () is the location at which two objects wikt:abut#Verb, abut or are joined. The term is used especially in the fields of anatomy and biology. * The most common usage of the term refers to the brain's commissures, of which there are at ...

, the line between the two valves or shells, is zigzagged, a distinguishing characteristic of this group. The prominent beak of the pedicle valve usually overlaps that of the brachial valve, in order to allow the shell to open and close. There is usually a functional pedicle although the delthyrium may be partially closed. Morphologically, the rhynchonellid has changed little since its appearance during the

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

period. It seems to have evolved from pentamerids, and in turn gave rise to the first atrypids and athyrids, both of which are characterized by the development of a complex spiral brachidium. Although much diminished by the terminal Paleozoic extinction, it experienced a revival during the Early Jurassic period, and became the most abundant of all brachiopods during the

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

Era.

Classification

This classification down to the level of

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

is based on Kazlev and Emig. Extant subgroups * Superfamily Pugnacoidea ** Family Basiliolidae *** Subfamily Acanthobasiliolinae **** '' Acanthobasiliola'' *** Subfamily Basiliolinae **** '' Basiliola'' **** '' Basiliolella'' **** '' Eohemithiris'' **** '' Rhytirhynchia'' *** Subfamily uncertain **** '' Striarina'' * Superfamily Dimerelloidea ** Family Cryptoporidae **** '' Aulites'' **** '' Cryptopora'' * Superfamily Norelloidea ** Family Frieleiidae *** Subfamily Freileiinae **** '' Frieleia'' **** '' Compsothyris'' **** '' Grammetaria'' **** '' Sphenarina'' *** Subfamily Hispanirhynchiinae **** '' Abyssorhynchia'' **** '' Hispanirhynchia'' **** '' Manithyris'' **** '' Parasphenarina'' *** Subfamily Neorhynchiinae **** '' Neorhynchia'' ** Family Tethyrhynchiidae **** '' Tethyrhynchia'' * Superfamily Hemithiridoidea ** Family Hemithyrididae **** '' Hemithiris'' **** '' Pemphixina'' ** Family Notosariidae **** '' Notosaria'' Extinct subgroups * Superfamily Ancistrorhynchoidea * Superfamily Camarotoechioidea * Superfamily Lambdarinoidea * Superfamily Rhynchonelloidea * Superfamily Rhynchoporoidea * Superfamily Rhynchotetradoidea * Superfamily Rhynchotrematoidea * Superfamily Stenoscismatoidea * Superfamily Uncinuloidea * Superfamily Wellerelloidea

References

External links

Phylum Brachiopoda
{{Brachiopod orders Brachiopod orders Extant Ordovician first appearances