Multifacted Examination of Carbonate Platform Deposits with Dinosaur Footprints: Upper Jurassic of Istria, Croatia

Mezga et al. (2007) described sauropod dinosaur footprints from Upper Jurassic limestones of the Adriatic-Dinaric Carbonate Platform (ADCP) in a quarry near Kirmenjak village in western Istria, Croatia. With 23 trackways and almost 1000 individual footprints found in one quarry horizon, this site represents the largest and oldest evidence for dinosaur presence on the ADCP. Multifaceted examination of the quarry exposures (locally referred to as stylolitic limestones and known as architectural stone “Pietra d'Istria” or “Kirmenjak”) reveals details about marginal marine environments in which the sauropods left their footprints. Biostratigraphic analysis indicates Late Tithonian age (Cvetko Tesovic et al., 2008). The track-bearing horizon is in intertidal fenestral mudstones that form the top of a shallowing-upward succession capped with thin peloidal packstone/grainstone and overlain by subtidal mudstone. The footprints are shallow (<2 cm) and up to 52 cm long oval (pes) and horseshoe (manus) prints without clear digit impressions. Calculated dinosaur hip height is 1.5–3 m. The trackways are narrow-gauge type and frequently overlap. The pace and stride lengths indicate slow walking in a herd. The footprints are assigned to the ichnogenus Brontopodus. Their formation and preservation was favored by short-duration exposure of muddy sediment and its rapid burial underneath more mud. Fortuitous absence of substantial pressure dissolution (stylolites) along the bedding plane with the footprints further aided their preservation. Stable isotope composition of carbonate sediment from the track-bearing horizon is not substantially different from that of an adjacent area without footprints and from the overlying mudstone. Isotopic analysis supports petrographic observations that the conditions on the carbonate tidal flat during formation of dinosaur footprints were not unique. Documented variations in isotopic compositions reflect minor differences in depositional and diagenetic history of the Kirmenjak succession, which faces an uncertain future in this active quarry.