Trimerellida


Trimerellida is an extinct order of craniate brachiopods, containing the sole superfamily Trimerelloidea and the families Adensuidae, Trimerellidae, and Ussuniidae. Trimerellidae was a widespread family of warm-water brachiopods ranging from the Middle Ordovician to the late Silurian. Adensuidae and Ussuniidae are monogeneric families restricted to the Ordovician of Kazakhstan. Most individuals were free-living, though some clustered into large congregations similar to modern oyster reefs.

Evolution

Trimerellides probably originated from tropical island arcs in the region of Kazakhstania during the "Llandeilo". By the late Sandbian and early Katian stages, many dispersed eastward to nearby regions equivalent to South China and Australia. A few managed to populate the vicinity of Laurentia, possibly through its diminishing proximity to the Australian portion of Gondwana. By the late Katian, trimerellides had also dispersed westward, populating the seas around Baltica, Scotland, and Siberia. Trimerellides were an exclusively tropical group, with most genera endemic to a specific region. There are some exceptions: Eodinobolus and Monomerella were particularly widespread, found at low-latitude ecosystems worldwide. At their acme in the late Katian, trimerellides reached the highest diversity ever seen among craniiform brachiopods, forming a significant component of brachiopod assemblages worldwide.
Trimerellide diversity collapsed during global cooling in the first pulse of the Late Ordovician mass extinction. This fate was shared by several other orders of "inarticulate" brachiopods. Trimerellides are an example of a Lazarus taxon: their fossils are absent from the Hirnantian and the Rhuddanian, with only a few new Silurian genera afterwards. Silurian species were most likely descended from relictual survivors in South China and Australia. Trimerellides managed to recover slightly during the late Wenlock Epoch, experiencing a mild rediversification at the species level. New species emphasized deeper muscle attachments relative to most of their Ordovician counterparts. Silurian trimerellide species may have been too specialized to adapt to rapid changes, resulting in their total extinction in the Ludlow Epoch.

Anatomy

Trimerellides are massive by the standards of early brachiopods. They have fairly smooth and unornamented shells, which were probably aragonitic in composition. The shells are unequally biconvex, in some cases nearly spherical in shape. There is no opening for the pedicle. Trimerellides show some similarities to rhynchonelliform brachiopods, including mixoperipheral shell growth and the development a fixed hinge at the back of the shell. In trimerellides, this hinge is an articulation between a wide plate on the dorsal valve and a socket-like groove on the ventral valve, opposite to the socket-and-teeth articulation of rhynchonelliforms.
Like other craniate brachiopods, the musculature consisted of two pairs of large and vertically-oriented adductor muscles alongside two pairs of horizontally-oriented oblique muscles. The inner pair of oblique muscles extend nearly straight back to the dorsal valve hinge plate. This contrasts with craniids and craniopsids, where the oblique internals splay out and attach besides the posterior adductors. A shelf is usually present near the middle of each valve, in front of the attachments for the anterior adductors. In later trimerellides especially, the anterior adductors sockets are deep vaulted pockets hollowed out from the internal surface of each valve.

Subgroups

From the Treatise on Invertebrate Paleontology Part H, Revised :
  • Family Adensuidae Popov & Rukavishnikova, 1986
  • * Adensu Popov & Rukavishnikova, 1986
  • Family Trimerellidae Davidson & King, 1872
  • * Belubula Percival, 1995
  • * Bowanpodium Percival, 1995
  • * Corystops Percival, 1995
  • * Costitrimella Rong & Li, 1993
  • * Dinobolus Hall, 1871
  • * Eodinobolus Rowell, 1963
  • * Fengzuella Li & Han, 1980
  • * Gasconsia Northrop, 1939
  • * Gyroselenella? Li, 1985 = Selenella
  • * Keteiodoros Strusz et al., 1998
  • * Monomerella Billings, 1871
  • * Ovidiella Nikitin & Popov, 1984
  • * Palaeotrimerella Li & Han, 1980
  • * Paradinobolus Li & Han, 1980
  • * Peritrimerella Liang, 1983
  • * Porcidium Percival, 1995
  • * Rhynobolus Hall, 1871
  • * Sinotrimerella Li & Han, 1980
  • * Trimerella Billings, 1862
  • * Yidurella Zeng, 1987
  • Family Ussuniidae Nikitin & Popov, 1984
  • * Ussunia Nikitin & Popov, 1984