Fukui Prefectural Dinosaur Museum, 51-11 Terao, Muroko, Katsuyama, Fukui, 911-8601, Japan.
Institute of Dinosaur Research, Fukui Prefectural University, Katsuyama, Japan.
Sci Rep. 2022 Jul 13;12(1):10885. doi: 10.1038/s41598-022-14469-3.
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.
双壳纲（软体动物门）是动物中最成功的生存群体之一，其贝壳上具有多种多样的颜色图案，并且它们偶尔会作为残余颜色图案在化石记录中保存下来。然而，淡水双壳类动物的残余颜色图案的化石记录只能追溯到中新世，这极大地限制了对颜色图案进化的了解。我们展示了来自日本的已灭绝的 Trigoniida 目三个科的白垩纪淡水双壳类动物（以下简称北谷淡水双壳类动物）的颜色图案，这是最古老的也是第二个淡水软体动物颜色图案的化石记录。北谷淡水双壳类动物有两种类型的颜色图案：沿生长线的条纹和向喙部逐渐变细的放射状射线，类似于现存淡水双壳类动物的有色带。北谷淡水双壳类动物和现存物种的颜色图案之间的相似性表明，淡水双壳类动物的颜色图案代表了 Trigoniida 和 Unionida 之间的趋同进化。为了解释这种趋同进化，我们主张三个可以想象的因素：系统发育的限制、淡水生态系统特有的单调生境，以及视觉捕食者在淡水沉积物中的捕食压力。
