Ichnofacies associations of the Bletterbach section

I already introduced the fossil site of the Bletterbach gorge, recognized by geologists for the first time in 1951. However after some preliminary studies and some recuperated fossil imprints a systematic research began only in 1973, resulting in the discovery of a large number of tracks and trackways.

These discoveries were followed by a systematic stratigraphic and paleoenvironmental reconstruction of the site, and led to the revision of its chronological attribution (from the Middle to the Late Permian, 260-251 Ma) and its ichnological content. Recognized the significance of the Bletterbach gorge, research continued and the site became some years ago a geology park with marked itineraries for visits and local documentation centres. In June 2009 it was inserted as "Geotop" along the Dolomites in the list of World Heritage sites of the UNESCO.

The Bletterbach gorge is one of only five major outcrops of tracksites worldwide dated to the Late Permian and from these possesses the most diverse assemblage of ichnofossils.
The facies association occurs in sandstones deposited in the alluvial plain and channels of a river system that regularly flooded the former semi-desertic landscape. The ichnofauna assemblage is extraordinary rich with 9 ichnospecies belonging to 8 ichnogenera.
The presumed terrestrial track makers range from gigantic pareiasaurs, medium-sized to large herbivorous anapsid reptiles, small and relatively primitive diapsids and to large synapsids. In some cases, the foot-prints are exceptionally well preserved, showing details of the skin and imprints of the single claws.
Pelycosaurs, one of the dominant reptilian groups during the Permian, are present by the imprints of large caseids, reaching an estimated length of 2 meters. An important peculiarity of the site is the discovery of tracks attributed to gorgonopsians, the only known example of imprints of this group recorded in central Europe. Other therapsida imprints, only 5cm small and with the classic 4 to 5 forward facing digits, were left probably by cynodonts.

Fig.1. Pachypes dolomiticus (LEONARDI et al. 1975), amongst the largest Paleozoic tetrapod footprints found in the Bletterbach, has been referred to pareiasaurs by means of comparison with the skeletal features of several Palaeozoic reptilian groups (digits show upward).

Fig.2. Morphofamily Chiroteriidae (ABEL, 1935) - imprint of a large archosaur and the oldest known examples of this kind of tracks (digits show to the right).

Fig.3. Rhynchosauroides in the foreground - three distinct ichnospecies are recognized in the Bletterbach of this track, all of them attributed to small, lizard-like creatures (lepidosauromorpha).

Fig.4. Imprint of a large synapsid (gorgonopsia?) with five digits, even the marks of the claws are recognizable.

Fig.5. The mid-nineteenth century fanciful view of the trackmakers: a labyrinthodont amphibian (centre) leaves a Chirotherium trackway watched by dicynodonts (left) and rhynchosaurs (right).
(B.W. Hawkins archive,The NaturalHistory Museum, London), found in BOWDEN et al. 2010

Fig.6. A more realistic reconstruction - Tetrapod Footprints from Bletterbach and causer: a) Pachypes dolomiticus; b) Rhynchosauroides pallini; c) Ichniotherium accordii; d) Dycinodotipus isp.; e) Chirotheriidae (From PIERO et al. 2010).

Bibliography:

AVANZINI, M. & TOMASINI, R. (2004): Giornate di Paleontologia 2004 Bolzano 21-23 Maggio 2004 Guida all´escursione: la gola del Bletternach. Studi Trentini di Scienze Naturali - Acta Geologica Supplemento al v.79 (2002):1-34
BOWDEN, A.J.; TRESISE, G.R. & SIMKISS, W. (2010): Chirotherium, the Liverpool footprint hunters and their interpretation of the Middle Trias environment. In: MOODY, R. T. J., BUFFETAUT, E., NAISH, D. & MARTILL, D. M. (eds) Dinosaurs and Other Extinct Saurians: A Historical Perspective. Geological Society, London, Special Publications, 343, 209 - 228
LEONARDI, G. (2008): Vertebrate ichnology in Italy. Studi Trent. Sci. Nat., Acta Geol., 83 (2008): 213-221
PIERO, G.; AVANZINI, M.; BREDA, A.; KUSTATSCHER, E.; PRETO, N.; ROGHI, G.; FURIN, S.; MASSARI, F. PICOTTI, V. & STEFANI, M. (2010): Dolomites 7th international Triassic Field Workshop Pan-European correlation of the Triassic. Field trip to the world heritage site of the Tethyan Triassic September 5-10, 2010 Dolomites, Southern Alps, Italy: 122
VALENTINI, M.; CONTI, M.A. & NICOSIA, U. (2008): Linking tetrapod tracks to the biodynamics, paleobiogeography, and paleobiology of their trackmakers: Pachypes dolomiticus Leonardi et al., 1975, a case study. Studi Trent. Sci. Nat., Acta Geol., 83: 237-246