Ladogia Nalivkin, 1941 Devonian Western Russia

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

Rhynchonellida is one of the two main groups of living articulate

brachiopod 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, w ...

s, the other being the order Terebratulida. They are recognized by their strongly ribbed wedge-shaped or nut-like shells, 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: all other ...

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

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups ...

ally unrelated)

bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of marine and freshwater Mollusca, molluscs that have laterally compressed bodies enclosed by a shell consisting of two hing ...

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

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, 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, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

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 ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

Era.

Classification

This classification down to the level of

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

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 Rhynchotrematoidea * Superfamily Uncinuloidea * Superfamily Camarotoechioidea * Superfamily Rhynchotetradoidea * Superfamily Lambdarinoidea * Superfamily Wellerelloidea * Superfamily Rhynchoporoidea * Superfamily Stenoscismatoidea

References

External links

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