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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.

No abstract

We propose a new stratigraphic unit, the Nagasaki Kitaura Formation, for the Upper Cretaceous sedimentary rocks on the coast of Kitaura, Nagasaki City, Nagasaki Peninsula, northwestern Kyushu, Japan, to replace the previously named Kitaura Formation. The Nagasaki Kitaura Formation is divided into two lithostratigraphic units that are in fault contact: the Akasakino-hana Sandstone and Mudstone Member (ASMM), the lower, thin unit (>13 m thick), and the Zatobama Gravelly Sandstone and Mudstone Member (ZGSMM), the upper, thicker unit (>140 m). The ASMM consists of shallow marine deposits yielding ammonoid (Polyptychoceras obatai and cf. Phylloceras sp.) and bivalve fossils including an inoceramid (Platyceramus japonicus), and an incomplete femur of a hadrosauroid dinosaur was unearthed from the fluvial ZGSMM. Based on the biostratigraphic range of Platyceramus japonicus (late Santonian? to early Campanian), Polyptychoceras obatai (late Santonian), and the 206 Pb/ 238 U dates of the detrital zircon from the ASMM (the youngest concordant date = 83.6±5.0 Ma; the weighted mean age of the youngest date cluster = 85.74±0.75 Ma; 95% confidence level), the Nagasaki Kitaura Formation is no older than the late Santonian and potentially extends to the Campanian in age. The main body of the formation, the ZGSMM, is correlated with the lower part of the Mitsuse Formation (middle Campanian) on the western Nagasaki Peninsula. The stratigraphy of the Nagasaki Kitaura Formation can be assessed in association with the depositional environments of the lower half of the Upper Cretaceous Himenoura Group in western Kyushu.

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.