Macaronichnus ‘co-occurrence’ in offshore transition settings: discussing the role of tidal versus fluid muds influence

Macaronichnus is a key trace fossil in palaeoceanographic, palaeoclimatic, and petroleum exploration research. Small ichnosubspecies such as Macaronichnus segregatis segregatis, M. s. lineiformins, M. s. maeandriformis and M. s. spiriformis, typically occur in wave-dominated foreshore sands where large M. s. degiberti was never found. The latter shows a wide environmental distribution, occurring in sandy deposits of tidal channels, tidal bar sand sandridges, tidal-flat sand sheets, shorefaces, bioturbated sandy shelf, shelf storm-sheets, shelf sand ridges, and upper slopes. Small M. segregatis and large M. s. degiberti have not been observed to date due the ecological segregation of the tracemakers. An abundant record of large M. s. degiberti in a Tortonian (Late Miocene) mixed carbonate-siliciclastic unit from the Betic Cordillera (southern Spain) has been studied. Occurrence of M. s. degiberti is the result of the interaction of tidal and waves, storm influenced environment determining high-energy conditions and associated palaeoenvironmental parameters as shifting substrates, organic matter availability, and oxygenated pore and bottom-waters. Locally, associated to M. s. degiberti appear small, sinuous traces infilled by light material that were originally assigned to M. s. maeandriformis, and very rare M. s. spiriformis. However, the absence of the typical rim of Macaronichnus avoid a conclusive assignment. The coexistence of both small traces (?M. s. maeandriformis) and large M. s. degiberti is identified in the deposits underlying mudstone layers, revealing the importance of mud deposition during tidal slack water intervals or linked to fluid mud events favouring the co-occurrence of the trace makers of both Macaronichnus ichnosubespecies. This fact would have significant palaeobiological and palaeoecological implications, and could be the first record of both ichnosubspecies in the same intervals.