Extant estuarine and freshwater animals show a variety of adaptations to marine life, which could reflect transitional stages in a gradual evolution from freshwater to the sea. Our aim was to identify the temporal and spatial environment associated with pig-nosed turtles Carettochelys insculpta coastal nesting in the Kikori Region, Papua New Guinea (PNG). We also related the use of coastal areas with size within and among different populations of C.insculpta and species of the superfamily Trionychoidea. Throughout its range, C.insculpta nests during the drier months when suitable sandbanks are exposed. In PNG, rainfall in the drier season dilutes salinities and C.insculpta nests in coastal sandbanks. In Australia, high salinities prevail in the river mouths during the nesting season and no coastal use is observed. Trends toward a larger body size in coastal areas suggest that size is an important factor to explore coastal environments. It is unlikely that female C.insculpta with less than 50cm (curve carapace length) would be able to cope with the Kikori coastal environment. Expanding this trend to its superfamily Trionychoidea, only species larger than 37cm (leathery carapace length) explore coastal environments. As the Australian coast is not suitable for nesting, the selection for larger body sizes was probably relieved. Of course, the reverse could be true, but our study provides an example of the caution required when placing evolutionary interpretations on life-history traits whose manifestation is studied only within a restricted portion of a species range.