Evolutionary conservatism of color pattern in freshwater bivalves

Abstract

The phylum Bivalvia is one of the most successful at survival groups of animals with diverse color patterns on their shells, and they are occasionally preserved in the fossil record as residual color patterns. However, the fossil record of the residual color patterns in freshwater bivalves could be traced only to the Miocene, greatly limiting color pattern evolution knowledge. We present the color patterns of the Cretaceous freshwater bivalves (hereinafter the Kitadani Freshwater Bivalves) from Japan, which is the oldest and the second fossil record of freshwater molluscan color patterns. The Kitadani Freshwater Bivalves consists of two types of color patterns: stripes along the growth lines and radial rays tapered toward the umbo, which resemble that of the colored bands of extant freshwater bivalves. This resemblance of the color patterns between the Kitadani Freshwater Bivalves and the extant species indicates that the color patterns of the freshwater bivalves have remained unchanged for approximately 120 million years, representing an extreme evolutionary conservatism. To explain this evolutionary conservatism, we advocate two conceivable factors: one is promoted by monotonous habitats typical of freshwater ecosystems, and the predation pressure by visual predators against such pressure for camouflaging in freshwater sediments.