Redescription of the genus Modiomytilus Griffin, 1990 (Bivalvia: Mytilidae) from Southern Patagonia with remarks on the paleobiogeography of the genus

Abstract

Modiomytilus Griffin, 1990, was based on composite molds collected from Cenozoic rocks in southern Patagonia. The type species comes from the early Miocene Centinela Formation, exposed along the southern shore of Lake Argentino. An additional species is known from the Eocene Rı´o Turbio Formation, which outcrops only at the southwestern corner of Santa Cruz Province. New material preserved as original shell was collected recently in beds referred to the Centinela Formation exposed in the same area as the Rı´o Turbio Formation, about 150 km south of Lake Argentino. Enough detail is preserved in collected the shells to warrant an emended diagnosis and a new and more complete description of the genus, as they provide a set of characters not recorded in the currently known material of any of the species described. The shells clearly belong to the type species, i.e., Modiomytilus argentinensis Griffin, 1990. These characters, among which are included the ligament, adductor muscle scar, pallial line, and byssal retractor muscle scar allow a reinterpretation of this genus and confirm its validity. At the same time these new morphological characters may be potentially useful for further phylogenetic analyses and a new interpretation of the currently available material, a prerequisite for understanding the palaeobiogeographic history of this genus.