Rhynchonellida

Rhynchonellida Temporal range: Ordovician–Recent PreЄ Є O S D C P T J K Pg N
Rhynchotrema dentatum, a rhynchonellid brachiopod from the Cincinnatian (Upper Ordovician) of SE Indiana.
Scientific classification
Kingdom:	Animalia
Phylum:	Brachiopoda
Class:	Rhynchonellata
Order:	Rhynchonellida Kuhn, 1949
Subgroups
See text.

File:Ladogia Nalivkin, 1941 Devonian Western Russia.JPG

Ladogia sp., a rhynchonellid brachiopod from the Devonian of western Russia (side view).

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 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

External links

• Phylum Brachiopoda