Mussel of the Month

The genus Trapezoideus has taxonomically exploded over the last decade. Back in when we made our global checklist (Graf & Cummings, 2007), Trapezoideus was composed of 2 (maybe 4) species. The principle species was the widespread Trapezoideus exolescens. Back in December 2012, T. exolescens was the Mussel of the Month. Then Bolotov et al. (2017) found that at least two species comprised T. exolescens sensu lato, precipitating the first round of revisions. The name-sake of the species, Unio exolescens Gould 1843, was recovered in a clade with Lamellidens, whereas the type species of Trapezoideus was found to share a more recent common ancestor with Contradens. The type species, Unio foliaceus Gould, 1843, had been regarded as a junior synonym (Subba Rao, 1989). With T. exolescens escaping to a new genus (Trapezidens), T. foliaceus has been bumped to seniority.

Our previous selection of T. exolescens as Mussel of Month is slightly more complicated. We had used the type specimen of Unio siamensis Lea 1866, which was considered a junior synonym (Subba Rao, 1989). But additional splitting has moved U. siamensis under Contradens misellus. As things currently stand, Trapezoideus exolescens sensu lato has been split into nine species in five genera divided among three tribes, including T. foliaceus (Bolotov et al., 2020).

The Splitting of Trapezoideus exolescens senu lato
Trapezoideus foliaceus (Contradentini)
Contradens misellus (Contradentini)
Contradens pallegoixi (Contradentini)
Contradens comptus (Contradentini)
Yaukthwa dalliana (Contradentini)
Yaukthwa peguensis (Contradentini)
Yaukthwa zayleymanensis (Contradentini)
Indonaia subclathrata (Indochinellini)
Trapezidens exolescens (Lamellidentini)