A DINOSAUR TRACKSITE IN AN EARLY LIASSIC TIDAL FLAT IN NORTHERN ITALY- PALEOENVIRONMENTAL RECONSTRUCTION FROM SEDIMENTOLOGY AND GEOCHEMISTRY

Lateral and vertical sedimentological, ichnological, and geochemical c haracteristics of a tracksite at Lavini di Marco (Southern Alps, North ern Italy) allowed for the reconstruction of a semi-arid, coastal, lif e-sustaining environment for sauropods and theropods that lived in the western margin of tropical Pangaea at the Triassic-Jurassic transitio n. Isotopic analyses reveal that there was little influence of meteori c water in the diagenetic transformation of trampled sediments. Conseq uently, freshwater lenses were probably ephemeral, but could still sus tain some vegetation and support animal life. The environment was domi nated by the presence of marine waters in sediment pores. Intense evap oration caused partial dolomitization in the morphologically ''low lyi ng'' parts of the site by modified marine waters. Dolomite precipitati on favored the preservation, via early cementation, of dinosaur footpr ints by carbonates and, possibly, other soluble minerals. Early cement ation in semi-arid and arid settings could play a major role in the fo ssilization of dinosaur footprints on carbonate tidal flats. Dinosaur footprints provide unequivocal proof of subaerial exposure of the plat form, which has important consequences for both cycle-and sequence str atigraphic studies. Commonly, bed surfaces in Alpine outcrops are scar ce and paleontologic studies have had to rely upon vertical outcrop fa ces. In this case, the recognition of dinoturbation structures becomes very important. At Lavini we had the opportunity to study the same fo otprints both on the bedding surface and in bed cross-section. Here we provide detailed descriptions of dinoturbation structures that should aid in the recognition of subaerial exposure surfaces in Mesozoic tid al flats. The preserved footprints at Lavini indicate that fossilizati on of ichnofaunas is possible in a variety of subenvironments within a tidal flat, in a general inter-supratidal setting characterized by al ternating periods of seawater influx and dryness.