Ophiomorpha irregulaire and associated trace fossils from the Upper Cretaceous of Patagonia, Argentina: Palaeogeographical and ethological significance

There are two contrasting opinions on the stratigraphic and palaeogeographic distribution of the relatively rare trace fossil Ophiomorpha irregulaire: 1) the trace is mostly restricted to the Upper Cretaceous of the Western Interior Seaway, or 2) it has a post-Palaeozoic record and a worldwide geographic distribution. In this study we document the finding of O. irregulaire in Upper Cretaceous turbidites of the Alta Vista Formation, southern Patagonia, providing additional evidence on the morphology and composition of its pelletal masses, the ethology of its producer and the palaeogeographical and palaeoenvironmental distribution of the trace fossil. The burrow walls are lined with dark mud and their elongate, spiky pellets are formed by a sand core that is also lined with mud. The association with typical fodinichnia, such as Halopoa, Spirophyton, and Zoophycos and the common presence of O. irregulaire in sediments with abundant plant detritus, suggest that the producer was a deposit feeder. Crosscutting relationships and occurrences at several levels within thick-bedded turbidites, suggest a relatively deep tier emplacement of the trace. The finding of the trace in the Southern Hemisphere supports its envisaged worldwide palaeogeographic distribution.