Oligorhynchiidae

Dorsal view of Oligorhynchia subplana gibbosa, from Cooper (1935).

Belongs within: Rhynchonellida.

The Oligorhynchiidae
Published 11 September 2021

From Oligochiton, we move onto Oligorhynchia. The Oligorhynchiidae are a family of very small brachiopods known from the Middle and Late Ordovician. They were among the earliest representatives of the Rhynchonellida, a major group of brachiopods that survives to the present day. Rhynchonellidan shells are usually characterised by a strong beak associated in life with a well-developed pedicel. In oligorhynchiids, this beak is suberect and the shell as a whole is an elongate subtriangular shape. The valves of the shell are folded into coarse plicae (ridges). At least towards the base of the shells, the major folds are in what is called an inverted arrangement, with a ridge in the dorsal valve matched by a valley in the ventral valve (Schmidt & McLaren 1965). Other structural features defining the group include small plates projecting into the pedicel opening, distinct vertical dental plates and divided hinge plates in the valve articulation, and the usual absence of a median septum or cardinal process inside the shell (Savage 1996).

The oligorhynchiids first arose in the east of what was then the continent of Laurentia (corresponding to modern North America). They subsequently spread across the Iapetus Ocean to the continents of Baltica and Kazakhstan (Jin 1996). The end of the Ordovician saw their replacement by other rhynchonellid families. Nevertheless, their genetic lineage would continue for some time yet as they have been identified as ancestors of later families: the Trigonirhynchiidae and Camarotoechiidae (Jin 1989). The brief oligorhynchiid spark would blossom into later rhynchonellid success.

Systematics of Oligorhynchiidae
<==OligorhynchiidaeAG65
|--Dorytreta Cooper 1956AG65
| `--*D. bella Cooper 1956AG65
|--Sphenotreta Cooper 1956AG65
| `--*S. cuneata Cooper 1956AG65
|--Rhynchotreta Hall 1879AG65
| |--*R. ‘cuneata’ (Dalman 1828) [=Terebratula cuneata non Risso 1826]AG65
| `--R. gansuensis Fu 1983HB93
`--Oligorhynchia Cooper 1935AG65
`--*O. subplana Cooper 1935AG65
|--O. s. subplanaHB93
`--O. s. gibbosa Cooper 1956HB93

*Type species of generic name indicated

References

[AG65] Ager, D. V., R. E. Grant, D. J. McLaren & H. Schmidt. 1965. Rhynchonellida. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt H. Brachiopoda vol. 2 pp. H552–H632. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas Press: Lawrence (Kansas).

[HB93] Harper, D. A. T., C. H. C. Brunton, L. R. M. Cocks, P. Copper, E. N. Doyle, A. L. Jeffrey, E. F. Owen, M. A. Parkes, L. E. Popov & C. D. Prosser. 1993. Brachiopoda. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 427–462. Chapman & Hall: London.

Jin, J. 1989. Late Ordovician–Early Silurian rhynchonellid brachiopods from Anticosti Island, Quebec. Biostratigraphie du Paléozoïque 10: 1–127, 130 pls.

Jin, J. 1996. Ordovician (Llanvirn–Ashgill) rhynchonellid brachiopod biogeography. In: Copper, P., & J. Jin (eds) Brachiopods pp. 123–132. CRC Press.

Savage, N. M. 1996. Classification of Paleozoic rhynchonellid brachiopods. In: P. Copper, & J. Jin (eds) Brachiopods pp. 249–260. CRC Press.

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