The Bipolar Bivalve Kolymonectes in South America and the Diversity of Propeamussiidae in Mesozoic Times

DAMBORENEA, S.E.

Bivalves: An Eon of Evolution. Paleobiological Studies Honoring Norman D. Newell

University of Calgary Press

Lugar: Caldary; Año: 1998; p. 143 - 155

The family Propeamussiidae is a well-characterized group of pectinoideans represented in Recent seas by several genera. One genus of internally ribbed Propeamussiidae (which should be referred to Parvamussium rather than to Propeamussium) is known from the Jurassic worldwide, but the family diversity in Mesozoic times may have been greater than previously thought if internally non-ribbed forms such as Kolymonectes are included, as proposed here. Abundant and well-preserved material from Pliensbachian beds in the southern Andes reveals new morphological data for the genus Kolymonectes, until now of uncertain affinities. Kolymonectes is here referred to the Propeamussiidae on account of the absence of  a ctenolium and the inferred presence of an extensive outer prismatic calcite layer on the right valve. Kolymonectes had a bipolar distribution: formerly known from the Late Triassic to Early Jurassic of the northern polar regions, and now reported from Sinemurian and Pliensbachian beds in southern South America, it did not extend to low palaeolatitudes. According to up-dated palaeogeographic reconstructions, the latitudinal range of this genus was apparently greater in the southern Hemisphere. Argentine species (Kolymonectes weaveri n. sp. and K. sp.) only occur in fine-grained, mostly light-coloured, off-shore shales and are rare in black shales. They seem to have preferred well-oxygenated, open sea conditions. As a result of the Mesozoic marine revolution, propeamussiids in general, having limited antipredatory and competitive capabilities, seem to have been relegated to safe places in the deep sea where predators are less efficient and competition is less intense.