In most oxygen-deficient ancient successions, Chondrites and Zoophycos are typical and recurrent trace fossils that are the last to disappear during deoxygenation events. Here, we report an unusual case of an oxygen-deficient ichnofauna lacking Chondrites and showing scarce Zoophycos, from the Upper Jurassic-Lower Cretaceous Vaca Muerta Formation of Argentina. One outcrop and cores from nine wells were analysed. The succession comprises fine-grained, mixed carbonate-siliciclastic deposits accumu-lated in marginal-marine, basin, drift, slope, and outer ramp environments. It displays abundant biodeformational structures mostly producing irregular-laminated and mas-sive fabrics in fine to medium mudstone. However, discrete trace fossils can be found within interbedded coarser-grained intervals, namely (in order of decreased ichnogen-era abundance), Teichichnus, Alcyonidiopsis, Coprulus, Phycosiphon, Planolites, Lockeia, Thalassinoides, Palaeophycus, Nereites, Crininicaminus, Zoophycos, Diplocraterion, and?Skolithos. Considering all ichnotaxa, the Vaca Muerta Formation may be seen as host-ing a relatively diverse ichnofauna, yet, this is misleading because most occurrences are of low diversity. This atypical ichnofauna may be explained by either a lack of strong sea-sonality and existence of background food-rich environments, or by salinity variations, precluding the specialized feeding style of the Chondrites and Zoophycos producers. The dominance of feeding trace fossils with spreite (Teichichnus) and the low ichnodiversity suggest that the studied ichnofauna belongs to the Zoophycos Ichnofacies.

Fil: Rodriguez, Maximiliano Nicolás. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigación en Paleobiología y Geología; Argentina

Fil: Parada, Martín Nazareno. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigación en Paleobiología y Geología; Argentina

Fil: Minisini, Daniel. Shell Technology Center Houston; Estados Unidos. Rice University; Estados Unidos

Fil: Desjardins, Patricio R.. Shell Exploration And Production Company; Estados Unidos

Fil: Pereira, Egberto. Universidade do Estado de Rio do Janeiro; Brasil

Fil: González Tomassini, Federico. Phoenix Global Resources; Argentina

Fil: Mángano, M. Gabriela. University of Saskatchewan; Canadá

Fil: Paz, Maximiliano. University of Saskatchewan; Canadá

Fil: Buatois, Luis A.. University of Saskatchewan; Canadá