Distinctive morphological features of Skolithos linearis from the Eocene of the Aínsa-Jaca Basin (South-Central Pyrenees)

A sandstone bed with abundant Skolithos linearis burrows is described from the Late Eocene of Aguilar, Aínsa-Jaca Basin, (South-Central Pyrenees). Ichnogenus Skolithos is a simple structure consisting of vertical and unbranched cilindrical burrows. It's considered as a dwelling structure (i.e. Domichnia) and as a facies-crossing trace fossil. Although annelids have been proposed as the most likely tracemakers, phoronids and crustaceans have been not discarded. Despite being common in all Phanerozoic, these traces are not-so-common during the Eocene. The presence of abundant Skolithos indicates a shallow marine paleoenvironment. The detailed study of the ichnofossils in this outcrop shows some special and uniform characteristics, such as having a large apertural diameter and their multidirectional obliquity. These features are analyzed and discussed.