The bipolar bivalve Kolymonectes and the diversity of Propeamussiidae in Mesozoic times

DAMBORENEA, S.E.

Drumheller, Canada

Simposio; The Fifth Canadian Paleontology Conference and International Symposium on the Paleobiology and Evolution of the Bivalvia; 1995

The family Propeamussiidae is a well characterized group of pectinaceans represented in Recent seas by several genera. One genus of internally ribbed Propeamusiidae is also known from the Jurassic worldwide, but the family diversity in Mesozoic times may have been greater than previously thought if non-internally ribbed forms such as Kolymonectes are included with it, as proposed here. Very abundant and well preserved material from Pliensbachian beds in the southern Andes revealed a set of interesting shell features which considerably add to the knowledge of the genus Kolymonectes, up to now of uncertain affinities. This genus is here referred to the Propeamussiidae on account of the absence of a ctenolium and the inferred presence of an extensive outer prismatic calcitic layer on the right valve. Although no proper study of the shell microstructure could be performed so far, the different ornament on both valves and the presence of a distal apron on the right valve point to the existence of a thin prismatic calcitic layer over most of the right valve disc, extending beyond it along the marginal region of the valve. Kolymonectes had a bipolar distribution: formerly known from the Late Triassic to Early Jurassic of the northern polar regions, now reported from Pliensbachian beds in southern South America, it did not reach low palaeolatitudes. Its southernmost reach in the Northern Hemisphere is used to define the boundary of the Arctic Subprovince of the Boreal Province. According to updated palaeogeographical reconstructions, the latitudinal range of this genus was apparently greater in the Southern Hemisphere. Argentinian species only occur in very fine-grained, mostly light-colored, off-shore shales and are rare in black shales. They seem to have preferred well oxygenated, open sea conditions as revealed by statistical studies of shell size in different populations of the Neuquén Basin.