Storm origin of bone-bearing beds in the Lower Devonian placoderm sandstone from podłazie Hill (Holy Cross Mountains, central Poland)

Abstract

The placoderm sandstone (Emsian, Holy Cross Mountains) exposed in the abandoned quarry at Podłazie Hill was revisited and excavated during fieldwork conducted in 2011-2013. Bone-bearing breccias were identified for the first time at this site and subjected to taphonomic analysis. Vertebrate remains are dominated by heterostracans, while true placoderms compose less than 20% of the total vertebrate assemblage. The high degree of fragmentation of the bones and low degree of abrasion indicate that the remains were reworked and transported before final burial. This is consistent with the mixed character of the bone accumulations, which comprise both open-shelf forms (acanthodians, chondrichthyans) as well as those related to marginal-marine environments (placoderms and sarcopterygians). The bone-bearing succession has been subdivided into five depositional facies attributed to a coastal lagoon influenced by stormy, possibly tidal conditions. The occurrence of the invertebrate trace fossil Ilmenichnus sp. accompanied by Lockeia and Monomorphichnus supports this interpretation.