Case study of the convergent evolution in the color patterns in the freshwater bivalves.

The class Bivalvia (phylum Mollusca) 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 belonging to three extinct families of the order Trigoniida (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 represent the convergent evolution between Trigoniida and Unionida. To explain this convergent evolution, we advocate three conceivable factors: the phylogenetic constraints, monotonous habitats typical of freshwater ecosystems, and the predation pressure by visual predators in freshwater sediments.