The first specimen of the giant marine snakePterosphenusLucas is recorded from a carbonate platform environment in the Late Eocene (Jackson Age) of the Crystal River Formation of Alachua County, Florida. The great enlargement of the marrow cavities and concomitant thinning of the vertebral walls are interpreted as specializations to a completely aquatic and fully marine habitus. The Florida specimen indicates thatPterosphenuswas not limited to estuarine and low salinity environments as previously proposed.