Rhipidothyridae

Girtyella indianensis, from here.

Belongs within: Centronellidae.

The Rhipidothyrididae: brachiopods of the Devonian
Published 28 March 2015
Specimen of Rhenorensselaeria, copyright Miguasha National Park.

In the modern world, the brachiopods are an unfamiliar group to most people. To most, they would probably not be readily distinguished from the much more abundant bivalves that they superficially resemble (a resemblance that is literally only skin deep: brachiopods and bivalves are in no way close relatives, and their internal anatomy is fundamentally different). However, this was not always the case. If one was to travel back to some point in the Palaeozoic era, one would find the situation reversed. At this time, it was the brachiopods that dominated the world’s seas, while the bivalves were relegated to a minor supporting role. Their respective fortunes changed around the beginning of the Mesozoic, though whether that was because changing conditions favoured the bivalves, or whether the bivalves simply got a head start in recovering from the Rocks Fall, Everyone Dies clusterf*** that was the end-Permian extinction event, I couldn’t tell you.

The fossil shown at the top of this post is one of these Palaeozoic brachiopods, a member of the family Rhipidothyrididae. Rhipidothyridids were among the earliest families of the order Terebratulida, which includes the majority of surviving brachiopods but in the Palaeozoic was just one group among many. Half a dozen genera from the Devonian period have been assigned the Rhipidothyrididae (Lee 2006). They often occur in mass assemblages, with a low diversity of other fossils (Boucot & Wilson 2004). That these assemblages represent their habits in life is indicated by the fact that the individual brachiopods in them are usually articulated; because the shells lacked a toothed hinge, the valves would soon become disassociated if transported after death.

The relationships of the rhipidothyridids are somewhat uncertain. A significant feature used in terebratulid classification is the morphology of the loop, a calcified ring at the base of the shell that provides part of the support for the lophophore in life. In some terebratulids, the loop is long and provides most of the lophophore support; in others, the loop is much shorter and lophophore support is partially taken over by free spicules embedded in the lophophore itself. However, because the loop is a quite delicate structure, its study in fossil taxa requires careful sectioning of specimens, with due consideration of the possibility of post-mortem damage. To date, this has not yet been done for the rhipidothyridids, so their loop morphology remains unknown.

Systematics of Rhipidothyridae
Rhipidothyridae [Rhipidothyrididae]HB93
| i. s.: AdreniaXY86
| CydimiaXY86
| Lingshanella Xu & Yao 1986XY86
| |--L. changlingensis Xu & Yao 1986XY86
| `--L. convexa Xu & Yao 1986XY86
|--GirtyellinaeM-WS65
| |--Harttella Bell 1929M-WS65
| | `--*H. parva Bell 1929M-WS65
| `--Girtyella Weller 1914HB93, M-WS65
| |--*G. indianensis (Girty 1908) [=Harttina indianensis]M-WS65
| `--G. intermedia Weller 1911HB93
`--Globothyridinae [Globothyrinae]M-WS65
|--Prorensselaeria Raymond 1923B75, M-WS65
| `--*P. nylanderi Raymond 1923M-WS65
`--+--Globothyris Cloud 1942B75, M-WS65
| `--*G. callida (Clarke 1907) [=Rensselaeria callida]M-WS65
`--Rhipidothyridinae [Rhipidothyrinae]M-WS65
|--Rhipidothyris Cooper & Williams 1935B75, M-WS65
| `--*R. plicata Cooper & Williams 1935M-WS65
`--Septothyris Cooper & Williams 1935M-WS65
`--*S. septata Cooper & Williams 1935M-WS65

*Type species of generic name indicated

References

[B75] Boucot, A. J. 1975. Evolution and Extinction Rate Controls. Elsevier Scientific Publishing Company.

Boucot, A. J. & R. A. Wilson. 1994. Origin and early radiation of terebratuloid brachiopods: thoughts provoked by Prorensselaeria and Nanothyris. Journal of Paleontology 68 (5): 1002–1025.

[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.

Lee, D. E. 2006. Stringocephaloidea. In: Kaesler, R. L. (ed.) Treatise on Invertebrate Paleontology pt H. Brachiopoda (Revised) vol. 5. Rhynchonelliformea (part) pp. 1994–2018.

[M-WS65] Muir-Wood, H. M., F. G. Stehli, G. F. Elliott & K. Hatai. 1965. Terebratulida. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt H. Brachiopoda vol. 2 pp. H728–H857. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas Press: Lawrence (Kansas).

[XY86] Xu H.-K. & Yao Z.-G. 1986. The Lower Devonian and its brachiopods from Lingshan, Guangxi. Acta Palaeontologica Sinica 25 (2): 169–179.

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