A dinosaur track assemblage from the Upper Hettangian (Lower Jurassic) marginal-marine deposits of Zapniów, Holy Cross Mountains, Poland

Abstract

Upper Hettangian (Lower Jurassic) marginal-marine Przysucha Ore-bearing Formation exposed at Zapniów mine and clay-pit (northwestern part of the Holy Cross Mountains area, central Poland) revealed an interesting assemblage of dinosaur tracks. Although mostly yielding poorly preserved and isolated tracks probably left in shallow water conditions, the site nevertheless indicates a diversity of early saurischian (theropods and sauropodomorphs) and ornithischian (thyreophorans) dinosaur trackmakers. This new assemblage is partly consistent with the Lower-Middle Hettangian ichnorecord from the same region. The tracks are preserved in sediments of a barrier-foreshore origin within a barrier/lagoonal depositional system, belonging to the highstand systems tract, located below a marked erosional surface (sequence boundary), which is associated with a substantial fall in sea level at the end of Hettangian times. Here we described all previously and newly collected or observed in the field dinosaur tracks from both surface and underground exposures at Zapniów. Four ichnospecies: Grallator isp., Anchisauripus isp., cf. Tetrasauropus isp., and cf. Anomoepus isp. were identified. The theropod and ornithischian tracks show distinct similarities to those described from the richest in this region and most famous Gliniany Las dinosaur track assemblage, in age approximately coeval to Zapniów. Two sizes of theropod tracks (small and medium) indicate the presence of two different size classes or species of predators in this area. The described cf. Tetrasauropus isp. from Zapniów is the first unquestioned evidence of basal sauropodomorphs in the Upper Hettangian of the Holy Cross Mountains and first record of this ichnotaxa in the Lower Jurassic of Poland. Additionally, two theropod trackways (Anchisauripus isp.) show evidence for trotting. The new finds suggest similarities between marginal-marine environments (delta-plain and foreshore-barrier/lagoon lithofacies) association of dinosaurs containing low-browsing thyreophorans accompanied by small or juvenile sauropodomorphs and small to medium sized theropods. Presence of the ornithischian footprints suggests their prominent role as a major component in Middle-Upper Hettangian dinosaur faunas in marginal-marine environments dominating in the region