SILURIAN MEGALODONT BIVALVES OF THE CANADIAN ARCTIC AND AUSTRALIA - PALEOECOLOGY AND EVOLUTIONARY SIGNIFICANCE

The oldest occurrence (Early Silurian, Llandovery) megalodont bivalves is reported from the Canadian Arctic. Megalodonts, although occurring through most of the Silurian, are abundant in Ludlow strata in three main areas: Australia, the Arctic, and the Michigan Basin. They occupi ed several different depositional environments, but they tended to be most common in shallow-water, protected back-barrier, lime mud-rich se ttings. Many of the Silurian megalodonts were reclined, mud-resting, e pifaunal suspension feeders. Because of their abnormally large size, t hick shell, probable high skeleton-to-body ratio, and epifaunal, tropi cal reef-associated habitat, many probably contained photosynthetic en dosymbionts. Some megalodont-bearing calcimuds were likely influenced by microbial communities, which may have increased substrate bearing s trength and support for the robust megalodonts. Megalodonts represent the earliest record of algal symbiosis in Bivalvia, a characteristic w hich is believed to have occurred in other groups of Phanerozoic shell y fauna, particularly the bivalves. The Permian alatoconchiids, Triass ic megalodonts, and Cretaceous rudists represent important occurrences of bivalve endosymbiosis which may have had an origin in the megalodo nts.