Nothosaur tracks from the Middle Triassic of Spain suggest a global distribution for distinctive swim track assemblages

The seas of the Mesozoic were remarkable for predatory marine reptiles, but the track record of these vertebrates is extremely scarce and morphologically distinct. In this paper, Middle Triassic seabed tracks made by the paddles of nothosaurs (Reptilia, Sauropterygia) from the Valdelcubo locality, Riba area, Guadalajara, Spain, are reported. The trackway occurs at the base of Cuesta del Castillo Sandstones Formation. The palaeoenvironment was a shallow near-coastal marine habitat. The tracks from Valdelcubo are broad, elliptical, with the claw marks locally differentiated, with double row imprints. The left tracks of the trackway are better recognized than the right ones. This record is interpreted as a trackway of a buoyant and punting tetrapod, using its forefeet only. The features of these tracks and trackway have a strong affinity with the ichnogenus Dikoposichnus. Nevertheless, some tracks from Valdelcubo are associated swim traces and claw marks, being tentatively classified as aff. Dikoposichnus. This finding adds new data to the little-known type of locomotion of nothosaurs. Additionally, this is the first Triassic record of subaqueously-registered paddle tracks attributed to sauropterygians outside China and enhances the information about its palaeobiogeographical distribution, suggesting a global distribution for distinctive swim track assemblages.