The genus Alasminota represents a shard of the shattered taxonomic bowls that contained the pre-cladistic concepts of Lasmigona and Alasmidonta. If you aren’t deep in the weeds of the cladistic and taxonomic literature, you might not have even noticed that Lasmigona and Alasmidonta weren’t as good as they have been since the late 20th century (e.g., Williams et al., 1993).
The traditional distinction between these genera has been based on Lasmigona having more robust hinge teeth, double-looped umbo sculpture, and the inner demibranchs of the ctenidia typically not connected to their visceral mass posterior to the foot. Species of Alasmidonta, on the other hand, have weaker hinge teeth (if any), unlooped umbo sculpture, and the inner lamellae of their inner demibranchs separate the infra- and supra-branchial chambers of the mantle cavity more completely (Ortmann, 1912, 1914).
It turns out that the old constructions of Lasmigona and Alasmidonta are just not monophyletic. That is, the morphological characters that malacologists used to recognize those genera don’t diagnose natural evolutionary groups — at least as evidenced by their independent genetic traits. These data are summarized (with lots of refernces) on the relevant Cladomics pages on this web site: Lasmigona, Alasmidonta, and the subtribe Alasmidontina.
If Lasmigona complanata, L. compressa, L. holstonia, and the other species of Lasmigona sensu lato represented a natural genus, then they should be recovered as monophyletic in phylogenetic analyses. They should be consistently found to form single clade, exclusive of species in other genera. But, they aren’t (at least when taxon sampling has been sufficient for a rigorous test). The same should be true of Alasmidonta marginalis, A. viridis, and A. heterdon, but it isn’t. Each of those six species represents a subset of their respective genera that shares a more recent common ancestor with mussels of other genera than with other species of the genus to which they have been traditionally assigned.
It also turns out that pre-cladistic freshwater mussel taxonomists like Ortmann (1914), Ortmann & Walker (1922), and Clark (1981, 1985) recognized these taxonomically distinct entities as subgenera and gave them names. So it is relatively straightforward (since it requires absolutely formal taxonomic work) to simply elevate those recognized subgenera to genera.
That is what we have done in the MUSSEL Project Database. Prolasmidonta heterodon and Pressodonta viridis has been separated from the rest of Alasmidonta, and Lasmigona is distinct from Platynaias and Alasminota holstonia, this month’s Mussel of the Month.
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