Tetrapod ichnotaxonomy in eolian paleoenvironments (Coconino and De Chelly formations, Arizona) and late Cisuralian (Permian) sauropsid radiation

The tetrapod footprint record of Permian eolian environments has long been underestimated because of overall poor preservation and its apparent monospecifity. The best known and most abundant Cisuralian record of tetrapod footprints is from the Coconino and De Chelly formations of Arizona, which, however, thus far encompassed only the ichnogenera Chelichnus and Dromopus. We revised the locomotion and taphonomy of these footprints and propose a new model, basing it on: 1) trackways changing direction, 2) trackways heading in different directions on the same surface, 3) trackways in situ, and 4) laboratory experiments with common wall lizards, Podarcis muralis. In all cases, the Chelichnus-like appearance of footprints is due to digit tip sliding on inclined depositional surfaces, masking the original footprint shape and orientation. Also, the trackway pattern and body position are largely influenced by the angle of inclination (dip) of the substrate being walked on. Based on an anatomy-consistent ichnotaxonomy, Chelichnus and Laoporus are here considered nomina dubia, and the footprints from the Coconino and De Chelly formations are revised and assigned to: parareptiles/captorhinomorph eureptiles (Erpetopus, Varanopus curvidactylus), bolosaurid parareptiles/ diapsid eureptiles (cf. Dromopus), varanopid synapsid (cf. Tambachichnium) and reptiliomorph amphibians (Amphisauropus, Ichniotherium sphaerodactylum). The ichnoassociation is dominated by parareptile/captorhinomorph tracks, similarly to all the late Cisuralian marginal marine, floodplain, alluvial fan and ephemeral lacustrine tetrapod ichnoassociations of North America, Europe and North Africa. A review of all the available data including the new results suggests a facies-crossing transition between an early-Cisuralian amphibian- and synapsid-dominated ichnofauna (Dromopus track biochron) and a late Cisuralian parareptile/captorhinomorph-dominated ichnofauna (Erpetopus track biochron) at low latitudes of Pangea