Sclerobionts on biogenic substrates from the Monte Leon Formation (lower Miocene) in Santa Cruz Province, Argentina: Taphonomic and paleoenvironmental considerations

Study of hard-substrate communities in a paleoenvironmental and taphonomic context contributes towards increasing knowledge on the depositional environments and on the history of the substrate colonized before final burial. In this work we study traces produced by sclerobionts on biogenic substrates from two shell concentrations in the Monte Leon Formation (lower Miocene), exposed at Cabeza de Leon, southern Patagonia, Argentina. The aims of this study are to describe the bioerosion traces and to infer the taphonomic history of the hard substrates (mollusk shells) based on ichnodiversity and sclerobiont distribution together with other preservational features. Fifteen ichnotaxa were identified at different taxonomic ranks (one ichnofamily, three ichnogenera and eleven ichnospecies). Traces produced by bryozoans were highly predominant followed by those produced by sponges, (?)foraminifera and polychaetes. The biogenic substrates on which the sclerobionts settled were gastropod and bivalve shells showing different taphonomic signatures. Shells with high fragmentation and surface-alteration are dominant, suggesting that they were reworked by wave action before deposition and final burial. On the other hand, well preserved shells reflect less reworking and shorter transport distance. The mixture of these elements could be the result of short-term high-energy events such as storms. Yet, these taphonomic signatures were modified later by biologic factors, mainly bryozoan bioerosion in a quiet and shallow environment with a low sedimentation rate and abundant nutrient availability. Bioerosion valuation in conjunction with other taphonomic attributes allowed identifying the concentrations as of mixed origin (biogenic-sedimentologic) and including elements that underwent different taphonomic histories. The results contribute to improve the knowledge about marine hard substrate communities in middle to high latitudes of the southern hemisphere. In addition, they highlight the importance of conducting studies on these communities-in conjunction with other taphonomic data-when elucidating the history of colonized substrates.