Taphonomy of decapod-bearing concretions and their associated trace fossils from the Agrio Formation (Lower Cretaceous, Neuquén Basin), with paleobiological implications for axiid shrimps

LAZO, DARÍO GUSTAVO; ANDRADA, ALEJANDRA MARIEL; BRESSAN, GRACIELA SUSANA

Revista de la Asociación Geológica Argentina

Asociación Geológica Argentina

Lugar: Ciudad Autónoma de Buenos Aires; Año: 2020 vol. 77 p. 366 - 383

1853-7138

A total of 22 concretions containing 37 decapod crustacean specimens belonging to the family Axiidae, from the Hauterivian (Lower Cretaceous) of the Agrio Formation in the Neuquén Basin, were studied. The decapods were assigned to Protaxius sp., which likely had a fossorial life habit. In this work we attempt to determine, through taphonomic analysis, if the specimens fossilized within their burrows or outside them in order to interpret paleobiological aspects of the studied taxon. Several taphonomic criteria of the specimens and concretions, known from the literature, were applied and their utility discussed, including anatomical disposition, completeness of specimens, orientation and alignment, dispersion of exoskeletal elements in the concretion, preservation of both chelae, sedimentary fabric of the concretion, position within the concretion, and shape of the concretions. The relative importance of these criteria was discussed especially in those concretions with conflicting evidence pointing to either fossilization within or outside burrow systems. In the studied case, the specimens preserved in eight concretions were interpreted as fossilized within their burrows while two represent preservation outside burrow systems, on the sediment-water interface. The remaining concretions were difficult to interpret and taphonomic information was insufficient to conclude; however, the completeness and articulation of the specimens suggest preservation within burrow systems. The preservation of several specimens within the same burrow suggests a gregarious behavior, spending most of their lives in their galleries, and molting in special chambers without external disturbances.