Preservation of larval bivalve shells in a radiolarian chert in the Late Triassic (Early Norian) interval of the Malampaya Sound Group, Calamian Island, western Philippines

Silicified thin larval bivalve shells occur in Upper Triassic radiolarian chert (Early Norian) in the Liminangcong Formation of the Malampaya Sound Group, part of a Late Jurassic to Early Cretaceous subduction-related accretionary complex in the North Palawan Block, western Philippines. The bivalve-bearing radiolarian chert, which we term "bivalve chert," is considered to be a deep-sea sediment that accumulated in an open-ocean realm of the Panthalassa Ocean. Radiolarian and conodont biostratigraphies indicate an early Norian deposition of the thin-shelled bivalves. The bivalve morphology, thin shells (less than 30 μm thick), smooth ornamentation, and small size (less than 1.3. mm long), suggests they represent larval shells. Although their taxonomic identity cannot be established with certainty, halobiid bivalves that lived in Norian times apparently had such a planktonic larval mode of life.