Mussel of the Month

MNHN. Senegal
(type of Iridina splendida Chenu, 1858).

The unique hinge morphology of Pleiodon makes it an interesting genus. Members of the genus Pleiodon (as well as a couple species of Mutela) have what is referred to as "pseudotaxodont" hinge dentition, for its convergent similarity to the taxodont teeth seen in marine protobranch bivalves. The purpose of hinge teeth in a bivalve is to help the valves articulate and to reduce twisting at the ligament. Typical freshwater mussels have schizodont hinge teeth composed of anterior pseudocardinal teeth and posterior laterals. Both the anterior and posterior teeth, although differing in length, can be described as interlocking ridges running in the anterior-posterior direction. Among some species, especially thick-shelled North American amblemines, the anterior pseudocardinals may be more peg-like than lamellar. Among the species of the Iridinidae generally, however, the hinge is usually edentulous, lacking hinge teeth. In the pseudotaxodont teeth of Pleiodon species, the parallel ridges run in the dorsal-ventral direction on the hinge and are more numerous.

The value of this conspicuous, diagnostic characteristic is that it can be easily recognized in the fossil record — as opposed to the diagnostic characters of other genera that are in the soft-parts and typically not preserved. The freshwater mussel family Iridinidae is currently restricted to the Afrotropics, but, because species referrable to Pleiodon are so distinctive as fossils, it is hypothesized that the paleo-distribution of the family might be much wider. Parodiz and Morris & Williamson have reported Pleiodon from Cretaceous strata in both South and North America. This fits well with the fossil record of other etherioidean families and further research would be valuable for understanding the evolution of the Unionoida.

Classification:

Phylum Mollusca
Class Bivalvia
Subclass Palaeoheterodonta
Order Unionoida

Superfamily ETHERIOIDEA Deshayes, 1830

Family IRIDINIDAE Swainson, 1840
Subfamily IRIDININAE s.s.

Genus Pleiodon Conrad, 1834

Species Pleiodon ovata (Swainson, 1823)