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Triassic chirotheriid footprints from the Swiss Alps: ichnotaxonomy and depositional environment (Cantons Wallis & Glarus)

June 30 , 2016

Autochthonous Triassic sediments of the Vieux Emosson Formation near Lac d’Emosson, southwestern Switzerland, have yielded assemblages with abundant archosaur footprints that are assigned to chirotheriids based on pentadactyl pes and manus imprints with characteristic digit proportions. Tridactyl footprints formerly considered as those of dinosaurs are identified as incomplete extramorphological variants of chirotheriids. Recently discovered new sites, including a surface with about 1500 imprints, permit re-evaluation of ichnotaxonomy and modes of preservation. Most common are oval to circular impressions arranged in an “hourglass-like” shape, corresponding to pes-manus couples. Sediment displacement rims indicate the presence of true tracks rather than undertracks. A few well-preserved footprints with distinct digit traces allow closer assignments. Several chirotheriid ichnotaxa are present with Chirotherium barthii, ?Chirotherium sickleri, Isochirotherium herculis, Chirotheriidae cf. Isochirotherium isp. and indeterminate forms. This corresponds with characteristic assemblages from the Buntsandstein of the Germanic Basin. In the study area, the Vieux Emosson Formation is an up to 10 m thick fining-upward sequence with conglomerates, rippled sandstones, siltstones and mudstones and occasionally carbonate nodules. Sedimentological features such as high relief erosion, immature sediments, erosionally truncated metre-scale fining-upward sequences, palaeosols and unidirectional palaeocurrents clearly prove a fluvial depositional environment with sediment transport towards the northwest and the Germanic Basin. This contrasts with former assumptions of a coastal marine environment and a south-facing transport towards the Tethys. The footprints occur in the coarser lower portion of the sequence that is interpreted as a shallow braided river. From Obersand in the eastern Swiss Alps, a surface in dolomitic limestone (Röti Dolomite) is re-examined. The footprints are identified as Chirotherium barthii and were impressed in a carbonate tidal flat environment. Biostratigraphically, the occurrence of characteristic Buntsandstein assemblages with Chirotherium barthii supports an Anisian age of both locations.

Hendrik Klein , Michael C. Wizevich, Basil Thüring, Daniel Marty, Silvan Thüring, Peter Falkingham & Christian A. Meyer (2016)
Triassic chirotheriid footprints from the Swiss Alps: ichnotaxonomy and depositional environment (Cantons Wallis & Glarus)
Swiss Journal of Palaeontology (advance online publication)
DOI: 10.1007/s13358-016-0119-0



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