Petrographic analysis of trace fossil Desmograpton Fuchs, 1895 from the Miocene of Northern Apennines (Italy): a methodology to detect hypichnial features and implications on the graphoglyptid ethology

Abstract The trace fossil Desmograpton Fuchs, 1895, as many other graphoglyptids, has been considered to be a typical pre-turbidite, three-dimensional burrow system formed in mud and preserved as a casting at sole of turbidites. This classic interpretation was mostly based on hypichnial preservation, for the lack of direct evidences (e.g. modern seafloor pictures) for pre-depositional or post-depositional origin. Due to the accidental series of processes invoked to explain the perfect hypichnial preservation of this delicate trace fossil, several doubts and questions have been raised. To try to explain this aspect, many Desmograpton dertonensis Sacco, 1888 and Desmograpton ichthyforme Macsotay, 1967 specimens have been analysed at sand/clay interface of thin-bedded calcarenites in the Miocene marl deposits of Verghereto, Northern Apennines (Italy). Petrographic analysis of textures and grains of the burrow-filling sediments points out that most of the grains of burrows differs from those of overlying sand, being either more or less selected differing in composition and locally also in grain size. Burrows are often characterised by rims enriched in minerals as glauconite, phosphates, pyrite or quartz. Axial cuts of the strings show that rims were involved in diagenetic compaction, squeezing and fluid migration modifying the original shape of the trace fossil. Oblique-oriented filling or compaction produced asymmetry in tunnels and in original mud. Analysis of microstructures provides doubts about the classic filling model in pre-depositional condition. This petrographic approach, performed for the first time on graphoglyptids of central Italy, allows to summarize the complex interaction of several taphonomic processes that led to the preservation of Desmograpton; this study can helps to better understand also the origin and ethology of many other graphoglyptids that need further detailed investigations.