Analysis of marrow cavity fillings as a tool to recognise diverse taphonomic histories of fossil reptile bones: Implications for the genesis of the Lower Muschelkalk marine bone-bearing bed (Middle Triassic, Żyglin, S Poland)
Abstract The bone-bearing bed from Żyglin (S Poland), which likely represents the oldest Lower Muschelkalk accumulation of reptile remains in the SE part of the Germanic Basin, has been investigated in order to determine its genesis. The basic methods were supported by petrographic analysis of the marrow cavity and large inter-trabecular pore space fillings of fossil bones, which was used to check its usefulness to identify the environments where the bones were initially deposited and to decipher taphonomic histories of the remains. The bone-bearing bed is a composite deposit, which consists of three distinct layers (from bottom to top): micritic limestone (mudstone, bioturbated autochthonous mud), crinoidal limestone (grainstone to packstone, calcirudite), and shell-rich limestone (packstone to wackstone, calcirudite). The crinoidal limestone layer, the main bone-bearing bed, is recognised as the proximal tempestite deposited in the mid-ramp zone. The petrographic analysis of the fillings reveals the prevalence of minute ostracod carapaces, accompanied by other grains, embedded together with micrite to microspar. Such compositions suggest that these sediments may have been inserted into the bone pore spaces in lagoons and tidal-flat ponds. Features of some bones record their early diagenetic burial and lithification before the final redeposition. The isopachous spar, blocky spar and weathered pyrite document changes in the chemical composition of fluids that flowed into the bone interiors. The burrow found in the marrow cavity of specimen IGUAM-ZOV-7 provides evidence that some remains were inhabited by minute or juvenile invertebrates. All recognised features indicate that both invertebrate and vertebrate bioclasts, included in the tempestite, were initially deposited in various settings of the carbonate ramp and in the end redeposited as a result of a heavy storm or a hurricane. The examined bone-bearing bed represents time-averaged assemblage, which originated due to hydraulic concentration of the vertebrate bioclasts. The petrographic analysis of the fillings is a valuable tool to identify complex taphonomic pathways of vertebrate bioclasts.