Ichnological evidence of semi-aquatic locomotion in early turtles from eastern Iberia during the Carnian Humid Episode (Late Triassic)

Abstract

Some of the earliest European records of fossil turtle footprints (Late Triassic, Middle Carnian, ~227–237Ma) are interpreted from 46 footprints from three outcrops, Domeño, Quesa and Cortes de Pallas, located in the Iberian Range (eastern Spain). The samples were obtained from Upper Triassic rocks in Keuper Facies. They are characterized in the studied area by two well-defined evaporitic sequences, separated by a detrital stratigraphic interval, constituting the Manuel Sandstones Formation in which the studied fossil footprints were recorded. These fluvial deposits are correlatable with the Carnian Humid Episode. The footprints are tridactyl and tetradactyl, mainly digitigrade, and elongated scratch marks are common. The digit traces present triangular and curved claws. The morphology of these marks suggests they were produced by the autopodium of a tetrapod animal while swimming near the bottom, and they are consistent with the anatomical features of a turtle's manus. Other footprints were produced by turtles walking in wet medium- to coarse-grained sand in a subaerial environment. A short trackway is identified, sharing similarities with both the ichnogenera Emydhipus and Chelonipus. The observed tracks support a freshwater semi-aquatic habit for some stem turtles during the early Late Triassic. These are remarkably ancient turtle tracks, close to the origin of the group, as indicated by skeletal fossils; they confirm that the earliest forms were narrowly related to aquatic (fluvial) environments.