Transition from swimming to walking preserved in tetrapod trackways from the Late Carboniferous of Bjørnøya, Svalbard

The Late Carboniferous was a crucial interval for the establishment of terrestrial ecosystems. A dramatic change in tetrapod distribution and ecology is coupled with an ongoing transition from amphibian to amniote domination. Presented here is a new set of tetrapod footprints from a single slab discovered on the island of Bjørnøya in the Norwegian High Arctic. A three-dimensional photogrammetric model was constructed to allow analysis of the trackway, and palaeoenvironmental observations were taken to provide context to the ichnological determinations. The slab appears to preserve the transition from swimming to walking. Statistical tests provide indication that there is a significant change in locomotive behaviour (swimming to walking) present on the slab (p = 0.0026, n = 15). This coincides with a change in the preservation style and an apparent change in the depth of water traversed by the tracemaker. Two trackways can be assigned to the ichnogenus Limnopus Marsh, 1894 (temnospondyl amphibians). They consist of 36 and 24 prints, respectively, and have widths and lengths of 151 mm x 2149 mm and 166 mm x 1226 mm. Two other trackways represent the traces of indeterminate tetrapods. Palaeoenvironmental analysis suggests that the trackways had lain in a fluvial floodplain setting in a palaeo-river valley system, in agreement with regional-scale analyses. Locomotion analysis suggests that on moving from submerged walking and swimming to terrestrial walking, this large Late Carboniferous temnospondyl increased its pace angulation and lengthened its stride. At ~30°N, these tracks may be the farthest north Limnopus trackways yet found in terms of palaeolatitude. They are the first Carboniferous tetrapod traces discovered from Svalbard and are probably among the oldest examples of Limnopus yet found.