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 characteristics of a tracksite at Lavini di Marco (Southern Alps, Northern Italy) allowed for the reconstruction of a semi-arid, coastal, life-sustaining environment for sauropods and theropods that lived in the western margin of tropical Pangaea at the Triassic-Jurassic transition. Isotopic analyses reveal that there was little influence of meteoric water in the diagenetic transformation of trampled sediments. Consequently, freshwater lenses were probably ephemeral, but could still sustain some vegetation and support animal life. The environment was dominated by the presence of marine waters in sediment pores. Intense evaporation caused partial dolomitization in the morphologically "low lying" parts of the site by modified marine waters. Dolomite precipitation favored the preservation, via early cementation, of dinosaur footprints by carbonates and, possibly, other soluble minerals. Early cementation in semi-arid and arid settings could play a major role in the fossilization of dinosaur footprints on carbonate tidal flats. Dinosaur footprints provide unequivocal proof of subaerial exposure of the platform, which has important consequences for both cycle-and sequence stratigraphic studies. Commonly, bed surfaces in Alpine outcrops are scarce and paleontologic studies have had to rely upon vertical outcrop faces. In this case, the recognition of dinoturbation structures becomes very important. At Lavini we had the opportunity to study the same footprints 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 tidal flats. The preserved footprints at Lavini indicate that fossilization of ichnofaunas is possible in a variety of subenvironments within a tidal flat, in a general inter-supratidal setting characterized by alternating periods of seawater influx and dryness.