Mussel of the Month

Elliptoideus is another one of those monotypic genera that may require a little scrutiny. As we have discussed previously, monotypic genera have their time and place. But, on the other hand, there are some such taxa that are just hold-overs from the pre-cladistic era.

For much of the 19th century, there were basically only two genera of mussels that concerned most people: Unio (for species with hinge teeth) and Anodonta (for those without). Those genera are still with us, though over the last century, their constituent species have been whittled down by the removal species to their own genera (as well as tribes, subfamilies, and families).

The earliest placement of Unio sloatianus Lea, 1840 into a different genus was accomplished by Conrad (1853): Plectomerus sloatianus. The genus Plectomerus is still in use, but only for P. dombeyanus, rather than the whole slate of original species that have since been removed to Amblema, Megalonaias, and Elliptoideus. The important point is that Conrad’s system of genera was largely ignored, and U. sloatianus remained in Unio – even after it was classified in the "section" Elliptio Rafinesque, 1819 by Simpson (1900).*

Ortmann (1912) raised Elliptio to a genus, and from that Frierson (1927) split out E. sloatianus into its own subgenus, Elliptoideus, because unlike other species of Elliptio, our Mussel of the Month broods in all four demibranchs. Elliptoideus was raised to a full-on genus by Modell (1942), then he sank it back into Plectomerus (Modell, 1964). Haas (1969a) returned Elliptoideus to a subgenus of Elliptio, though Haas (1969b) classified it as a subgenus of Nephronaias. Heart & Guckert (1970) reinstated Elliptoideus has a genus, and that rank as generally stuck ever since.** Given all the taxonomic flux described in this paragraph, it should be emphasized that these changes were not based so much on new data (that largely ended with Ortmann). Rather, these taxonomists were using classification to reflect their own views of how the data could/should be organized.

In the cladistic era, molecular phylogenetic analyses have recovered E. sloatianus in various positions (e.g., Serb et al., 2003; Campbell et al., 2005), and thus the sister lineage remains unclear. Elliptoideus is apparently a good use of a monotypic genus – at least until we get this stuff sorted out.

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* Despite the obvious phylogenetic distance between true Unio and Elliptio even on anatomic grounds, the legitimate placement of Unio wasn’t correctly sorted until Graf (2002).

** Oesch (1984) inexplicably synonymized the species E. sloatianus with Plectomerus dombeyanus —the species, not just the genera — and this was followed by Howells et al. (1996), and then we were done with that.