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Rhynchonelliformea |
| Brachiopoda | Rhynchonellida |
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This distinctive group of brachiopods - easily recognized by their strongly ribbed wedge-shaped or nut-like shells - first appeared with an evolutionary radiation during the Middle Ordovician and remained prominent throughout much of the Palaeozoic. They were especially common during the Early Carboniferous period. Although much diminished by the terminal Paleozoic extinction they experienced a revival during the Early Jurassic period, and became the most common brachiopods of the Mesozoic Era. The rhynchonellids are one of the three groups of living articulate brachiopods, the other two being the Terebratulida and the very uncommon Thecideida. Today they represented only a fraction of their past glory. This very morphologically conservative group has changed little since their appearance during the Ordovician.
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 and end Permian, and Jurassic extinctions while many other brachiopods with a brachidium failed to do so.
The rhynchonellid shell has a very short hinge line. The hinges come to a point (as in most bivalve mollusk shells), this being the non-strophic condition. 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. All rhynchonellids are biconvex (have a bulbous shell), and have a fold located in the brachial valve. All this means that the commisure, the line between the two valves or shells, is zigzagged, a distinguishing characteristic of this group. Some species are excellent index fossils; others belong to superficially similar genera that can be distinguished only by internal characters. 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. The shell microstructure of most groups is impunctate, but there are a few punctate forms.
The rhynchonellids seem 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.
Classification
des Brachiopoda (Lophophorata) based in turn on Treatise
on Invertebrate Paleontology
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