The classification of the genus Plethobasus has been pretty consistent over the last hundred years. Many Nearctic freshwater mussel genera of the 20th century have been reimagined and revised since Y2K — for example, Pleurobema, Pleuronaia, Parvaspina, Elliptio, and Eurynia (all Pleurobemini, like Plethobasus). But, the Plethobasus of today is basically the same genus we have recognized since at least the 1970s.
It took a little jostling to get the three species of Plethobasus to settle into their current classification. Simpson (1900), Frierson (1927), and others recognized Plethobasus as a subgenus of Pleurobema based on limited observations of females brooding their larvae only in the outer demibranchs, but Ortmann (1912) considered the genera distinct. He considered Plethobasus cyphyus (which he called P. aesopus) to represent the intersection in a Venn diagram of multiple genera: Pleurobema (ectobranchus), Pustulosa (bumpy shells), and Fusconaia (reddish body color and conglutinate shape).
Ortmann & Walker (1922) considered this month’s mussel, Plethobasus cicatricosus, to be a synonym of the more widespread (but still uncommon) Plethobasus cyphyus, Mussel of the Month back in December 2013. Haas (1930) selected a neotype (the specimen pictured here) from the Senckenberg Museum in Frankfurt, Germany to replace the lost type specimen of Thomas Say, and that apparently cemented the validity of this month's mussel, Plethobasus cicatricosus.*
However, the taxonomic stability of Plethobasus is less a story about consensus based on multiple lines of evidence and more about the challenge of testing the phylogeny of a genus of rare (and perhaps extinct) species. For starters, only a single species in the genus, Plethobasus cyphyus, has been included in any molecular phylogenetic analyses. Many of the analyses reported on the Plethobasus Cladomics page use the same sequences published by Campbell et al. (2005) to resolve the position of Plethobasus cyphyus among the Tribe Pleurobemini, and the results have been somewhat consistent: Plethobasus is often in a clade with Elliptio and/or Hemistena, and it is generally distinct from Fusconaia or the multiple clades of Pleurobema (i.e., Pleurobema and Sintoxia). The monophyly of the genus Plethobasus has yet to be tested. Since the other two species of Plethobasus, P. cicatricosus and P. cooperianus, are both critically endangered (according to the IUCN Red List) and specimens are rarely (if ever) encountered, we may have to wait for some new types of data and analyses to get the full picture of Plethobasus.
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* Morrison (1969) argued that the long-unused Obovaria pachostea Rafinesque, 1820 was the senior synonym for P. cicatricosus, but he was mostly alone in that. |
