LOWER JURASSIC (HETTANGIAN–SINEMURIAN) DINOSAUR TRACK MEGASITES, SOUTHERN ALPS, NORTHERN ITALY

Two Early Jurassic dinosaur track megasites and one isolated track site are reported in the Trento Platform of the Italian Southern Alps. Their trace fossils occur in carbonate units once thought to be totally marine in origin, and constitute the two most extensive Early Jurassic dinosaur track sites currently known in Europe. The older of these sites spans multiple horizons within a 7 m interval in the middle part of the Lower Hettangian Monte Zugna Formation of the Calcari Grigi Group. The second track site occurs primarily in two horizons within a 1 m interval near the top of the Monte Zugna Fm. and is Early Sinemurian in age. The youngest track site, apparently isolated, is preserved on the top of the Loppio Oolite, which is Sinemurian in age. The tracks attest to the presence of many dinosaur taxa. The most abundant are theropods; other taxa represented are early sauropods and several unidentified bipedal dinosaurs, including small ornithischians. Swim tracks of bipedal dinosaurs and some nondinosaurian reptiles have also been found. The lower Monte Zugna Fm. track megasite is well exposed at the Lavini di Marco locality (1.5 km2). A conservative estimate based on the currently studied area indicates that there may be more than 500,000 in situ footprints per square kilometer. Other, less spectacular track sites in the same stratigraphic interval occur along a 50 km, east-west transect (Chizzola, Pizzo di Levico, Becco di Filadonna). The track-bearing surface of the upper Monte Zugna Fm. occurs in a 5 km2 area of Monte Pasubio (Dente italiano). Other tracks occur on the same horizon about 15 km to the northwest in the Becco di Filadonna Massif. The Loppio Oolite track site is apparently confined to the Monti Lessini area. However, it constitutes a brackish water limestone layer that covers the entire Trento Platform and that preserves scattered vertebrate body fossils (crocodyliforms and fishes). The older track megasites evidently formed in an arid, upper intertidal to supratidal environment. Microbial mat growth on the ancient tidal flats and early dolomitization initiated preservation of the footprints. At the two younger track sites, the trampled layers indicate formation in different environments, with more humid climates and rare supratidal conditions. The differences in faunal densities and diversities of the three track megasites seem to be related to these differences in climate and environment