Trimerellida is an Extinction order of craniate , 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 ("Llandeilo" / Darriwilian) to the late Silurian (Ludlow epoch). Adensuidae and Ussuniidae are monogeneric families restricted to the Ordovician of Kazakhstan.
Evolution
Trimerellides probably originated from tropical
Island arc in the region of
Kazakhstania (present-day Kazakhstan) during the "Llandeilo" (late Darriwilian stage). 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 (North America), 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 (eastern Europe), Scotland, and Siberia. Trimerellides were an exclusively tropical group, with most genera
Endemism 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
Craniiforma 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.
Anatomy
Trimerellides are massive by the standards of early brachiopods. They have fairly smooth and unornamented shells, which were probably
Aragonite in composition. The shells are unequally biconvex (both valves convex to different degrees), in some cases nearly spherical in shape.
There is no opening for the pedicle. Trimerellides show some similarities to rhynchonelliform ("articulate") brachiopods, including mixoperipheral shell growth (where the valves converge towards each other) 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 (which close the shell) alongside two pairs of horizontally-oriented oblique muscles (which slide each valve past each other). The inner (internal) pair of oblique muscles extend nearly straight back to the dorsal valve hinge plate. This contrasts with Craniidae and Craniopsidae, 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.
Subgroups
From the
Treatise on Invertebrate Paleontology Part H, Revised (unless stated otherwise):
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Family Adensuidae Popov & Rukavishnikova, 1986
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Adensu Popov & Rukavishnikova, 1986 Upper
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Family Trimerellidae Davidson & King, 1872
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Belubula Percival, 1995 Upper
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Bowanpodium Percival, 1995 Upper
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Corystops Percival, 1995 Upper
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Costitrimella Rong & Li, 1993 Upper
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Dinobolus Hall, 1871 lower-mid
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Eodinobolus Rowell, 1963 Upper
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Fengzuella Li & Han, 1980 Upper
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Gasconsia Northrop, 1939 Upper
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Gyroselenella? Li, 1985 = Selenella Upper
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Keteiodoros Strusz et al., 1998 mid-Silurian
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Monomerella Billings, 1871 Upper
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Ovidiella Nikitin & Popov, 1984 Middle
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Palaeotrimerella Li & Han, 1980 Middle
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Paradinobolus Li & Han, 1980 Upper
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Peritrimerella Liang, 1983 Upper
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Porcidium Percival, 1995 Upper
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Rhynobolus Hall, 1871 mid-Silurian
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Sinotrimerella Li & Han, 1980 Upper
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Trimerella Billings, 1862 Upper
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Yidurella Zeng, 1987 lower
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Family Ussuniidae Nikitin & Popov, 1984
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Ussunia Nikitin & Popov, 1984 Middle
Bibliography
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Modzalevskaya, Tatyana Lvovna (2003). "Silurian and Devonian brachiopods from Severnaya Zemlya" (Russian Arctic). Geodiversitas 25 ( 1). pp. 73–107.
