The Joggins Fossil Cliffs site was selected as a UNESCO World Heritage site because of its unparalleled preservation of Pennsylvanian terrestrial organisms in their environmental context. Despite an abundance of research over the past 150 years, significant questions remain regarding the Joggins paleoenvironment, including the degree of marine influence and whether the gymnospermous Cordaites trees may represent the earliest mangroves. Sedimentologic and paleontologic data from interbedded limestone beds indicate open marine conditions in the oldest part of the section, with a waning marine influence up section. Limestone beds, which occur primarily at the base of cycles interbedded with coal and floodplain deposits, are 15–100 cm thick and contain ostracodes, bivalves, and echinoderm fragments. Independent lines of evidence to support a diminishing marine influence in fluvial and coastal deposits with time include: (1) the presence of punctate brachiopods, echinoderm fragments, and ostracodes infilled with framboidal pyrite in older limestone beds; (2) antithetic abundances between ostracodes and freshwater bivalves; and (3) an overall coarsening upward in the sequence. These results indicate that Joggins was, at least in the oldest portion of the formation, closer to the open ocean than previously surmised.