 Bivalve fossils are formed when the sediment in which the shells are buried hardens into rock . Often , the impression made by the valves remains as the fossil rather than the valves . During the Early Ordovician , a great increase in the diversity of bivalve species occurred , and the <unk> , heterodont , and <unk> dentitions evolved . By the early Silurian , the gills were becoming adapted for filter feeding , and during the Devonian and Carboniferous periods , siphons first appeared , which , with the newly developed muscular foot , allowed the animals to bury themselves deep in the sediment . 
