Exceptionally preserved embryos reveal maternal care in freshwater bivalves since the Cretaceous

As an adaptive reproductive strategy to their habitat, the majority of freshwater bivalve lineages incubate their larvae in the adults' gills. The Unionida (pearly mussels) with up to 1000 living species worldwide, are widely accepted as key components of modern freshwater ecosystems. Furthermore, they are unique amongst other freshwater bivalves because their larvae, after finishing maternal incubation, must also parasitize fish to complete their embryonic development and dispersal. Here, we report fossil evidence of a functional freshwater bivalve reproductive system in Margaritifera valdensis, a unionoid from the iconic Lower Cretaceous Iguanodon locality on the Isle of Wight, southern England. We document four interconnected bioelements of their gill anatomy: gill supports, interlamellar junctions within the gill demibranchs, mineral concretions, and fossilized gill soft tissue. Moreover, diverse developmental stages of brooded embryos and larvae are identified. These data reveal larval incubation within modified gills and evidence a calcium source for their shell formation. These highlight a key evolutionary innovation that facilitated the Mesozoic diversification of unionoid bivalves in calcium-deficient freshwater habitats. Our findings provide the first fossil evidence that by the Early Cretaceous this successful brooding adaptation in freshwater unionoid bivalves developed, providing a significant clue to understanding the evolution and general role of bivalves' gill anatomy in their function for reproduction.