Late Ordovician massive-bedded Thalassinoides ichnofacies along the palaeoequator of Laurentia

Massive-bedded Thalassinoides ichnofacies occurs pervasively in widely separated areas on the vast Upper Ordovician carbonate platforms along the palaeoequator of Laurentia, from the Bّrglum River Formation in North Greenland, through the Red River Formation and Bighorn Dolomite of the Williston Basin (Manitoba, Canada to Wyoming, USA), to the Ely Springs Dolomite and equivalent strata of the eastern Great Basin (Utah and Nevada, USA). The Late Ordovician Thalassinoides is characterised by complex, three-dimensional, anastomosing or multi-level galleries that are consistent in size, pattern and density, with preserved depth of 1 m or greater. In addition to the similar Thalassinoides burrows, the ichnofacies across the palaeocontinent also contains typical taxa of the Red River biota, such as the soccer-ball sized receptaculitid Fisherites, large-shelled, diverse and abundant nautiloids, the distinctively planispiral gastropod Maclurina, as well as diverse tabulate and rugose corals. The consistent characteristics of the Red River biofacies and Thalassinoides ichnofacies suggest a palaeogeographically homogeneous and temporally stable depositional environment along the palaeoequatorial belt of Laurentia during the pre-Hirnatian Late Ordovician. The Thalassinoides ichnofacies marked the first phase of mega-scale invasion of burrowers into relatively deep-water platform and shelf environments by successively creating a well-oxygenated deep burrow system for protection and deposit feeding. Such substrate tiering expansion constituted an important aspect of the Great Ordovician Biodiversification Event.