Rhynchonellida
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Rhynchonellida
(′riŋ·kə′nel·əd·ə)
(invertebrate zoology) An order of articulate brachiopods; typical forms are dorsibiconvex, the posterior margin is curved, the dorsal interarea is absent, and the ventral one greatly reduced.
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Rhynchonellida
An extant order of brachiopods that has been an important component of marine benthic communities since the Ordovician. Rhynchonellids possess unequally biconvex valves typically with a fold and sulcus; many species contain strong radial ribs that produce deflections in the commissure, the line of junction between the two valves. Their shells are generally impunctate and also typically lack a hinge line parallel to the hinge axis, resulting in a pointed beak or umbo when viewed in lateral profile. Internally, rhynchonellids possess calcareous processes (crura) that in extant species provide support for the lophophore. Rhynchonellids are sessile, attached, epifaunal suspension feeders. They have a functional pedicle that they use to attach to the substrate. Although rhynchonellids were never diverse compared to other brachiopod orders, they were commonly important members of local communities. They achieved a diversity peak in the Devonian and again in the Jurassic.
Rhynchonellids have shifted their habitat preference in the oceans since their origin in the Middle Ordovician when they originated in shallow low-latitude seas; however, presently they are more common in deep-water habitats from middle and high latitudes and are rare members of benthic communities in low-latitude shallow seas. See also Articulata (Echinodermata); Brachiopoda.
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Rhynchonellida
| Rhynchonellida Temporal range: Ordovician–Recent |
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| Rhynchotrema dentatum, a rhynchonellid brachiopod from the Cincinnatian (Upper Ordovician) of SE Indiana. | |
Scientific classification  |
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| Kingdom: | Animalia |
| Phylum: | Brachiopoda |
| Class: | Rhynchonellata |
| Order: | Rhynchonellida Kuhn, 1949 |
| Superfamilies | |
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The taxonomic order Rhynchonellida is one of the two main groups of living articulate brachiopods, the other being the order Terebratulida. They are recognized by their strongly ribbed wedge-shaped or nut-like shells, and the very short hinge line.
The hinges come to a point, a superficial resemblance to most (phylogenetically unrelated) bivalve mollusk 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).
Rhynchonellids lack any brachidium, and are therefore able to extrude their lophophore out from the shell into the water for more efficient food-gathering. It has been suggested this is why they survived the Devonian, Permian-Triassic, and Jurassic extinctions, while many other lampshells with a brachidium failed to do so.
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 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 Era.
Classification
This classification down to the level of genera is based on Kazlev[1] and Emig[2].
Extant subgroups
- Superfamily Pugnacoidea
- Family Basiliolidae
- Subfamily Acanthobasiliolinae
- Acanthobasiliola
- Subfamily Basiliolinae
- Basiliola
- Basiliolella
- Eohemithiris
- Rhytirhynchia
- Subfamily Uncertain
- Striarina
- Subfamily Acanthobasiliolinae
- Family Basiliolidae
- Superfamily Dimerelloidea
- Family Cryptoporidae
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- Aulites
- Cryptopora
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- Family Cryptoporidae
- Superfamily Norelloidea
- Family Frieleiidae
- Subfamily Freileiinae
- Frieleia
- Compsothyris
- Grammetaria
- Sphenarina
- Subfamily Hispanirhynchiinae
- Abyssorhynchia
- Hispanirhynchia
- Manithyris
- Parasphenarina
- Subfamily Neorhynchiinae
- Neorhynchia
- Subfamily Freileiinae
- Family Tethyrhynchiidae
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- Tethyrhynchia
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- Family Frieleiidae
- Superfamily Hemithiridoidea
- Family Hemithyrididae
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- Hemithiris
- Pemphixina
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- Family Notosariidae
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- Notosaria
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- Family Hemithyrididae
Extinct Subgroups
- Superfamily Ancistrorhynchoidea
- Superfamily Rhynchotrematoidea
- Superfamily Uncinuloidea
- Superfamily Camarotoechioidea
- Superfamily Rhynchotetradoidea
- Superfamily Lambdarinoidea
- Superfamily Wellerelloidea
- Superfamily Rhynchoporoidea
- Superfamily Stenoscismatoidea
References
- ^ M. Alan Kazlev. "Order Rhynchonellida". http://www.palaeos.com/Invertebrates/Lophotrochozoa/Brachiopoda/Rhynchonellida.htm. Retrieved March 10, 2006.
- ^ Christian C. Emig. "Systematics of extant brachiopod taxa". http://paleopolis.rediris.es/BrachNet/CLASS/DOC/Classext.pdf. Retrieved March 10, 2006.
External links
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