Ladogia Nalivkin, 1941 Devonian Western Russia

The

taxonomic Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...

order 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 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 derived ...

. 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 ** Thin-shell structure Science Biology * Seashell, a hard o ...

s, and the very short hinge 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 o ...

ally unrelated)

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

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 A commissure () is the location at which two objects 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 five. Such a commi ...

, 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 Pedicle or pedicel may refer to: Human anatomy *Pedicle of vertebral arch, the segment between the transverse process and the vertebral body, and is often used as a radiographic marker and entry point in vertebroplasty and kyphoplasty procedures ...

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. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

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 Cretaceo ...

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