Helical crustacean burrows: <em>Gyrolithes</em> ichnofabrics from the Pliocene of Lepe (Huelva, SW Spain)

Abstract

Two ichnofabrics characterized by abundant vertical and helical burrows (ichnogenus Gyrolithes) are described from the Pliocene siliciclastic facies of the southwestern sector of the Guadalquivir Basin (Lepe, Huelva, SW Spain). These ichnofabrics, associated with shallow and marginal marine environments, characterize two consecutive and concordant stratigraphic units: (1) the lower one is dominated by G. nodosus (together with other pellet-lined ichnotaxa), occurs in fine- to medium-grained, massive sands and silty sands, and is characterized by moderate to high bioturbation; (2) the upper ichnofabric is dominated by G. variabilis (and other unlined ichnofossils), occurs in sandy silts, and is characterized by low to high bioturbation. The transition of these two ichnofabrics clearly reflects the ability of an infaunal community to assimilate environmental changes over time. Additionally, new observations at the type locality of G. nodosus, the description of a new locality for G. variabilis and review of existing literature on this ichnogenus have provided the bases for emending the diagnoses of both ichnospecies, to propose a neotype for G. nodosus and to suggest a new type locality for G. variabilis. According to the main architectural features of Gyrolithes specimens studied herein and by comparison with modern analogues, 'thalassinidean' shrimps are proposed as their most likely tracemakers. Although it is known that these kinds of crustaceans exhibit a great variability in regards to their burrowing behaviors, further study is needed in order to more fully understand the purpose of these helical bioturbation structures.