ARTHROPOD PRODUCED BURROWS FROM THE EARLY TRIASSIC OF CANADA: GULLIVER'S FOOTPRINTS?

Paleoecological studies of the Early Triassic are paramount to the understanding of post-extinction recovery dynamics. Trace fossil analysis has proven useful to paleoecological investigations in areas comprising northwest Pangea; in part because lithofacies, and possibly ocean chemistry, were prohibitive to body fossil preservation. Research on Triassic strata from the Western Canadian Sedimentary Basin and the Sverdrup Basin has yielded numerous instances of ichnofossil assemblages dominated by forms interpreted to be produced by arthropods. While the trace makers themselves are rarely preserved or identified, ethologies such as feeding, dwelling, crawling and resting are readily recognizable in the rock record. In many cases, arthropod produced traces represent the largest and most robust forms within a given assemblage.

Previous reports of Early Triassic ichnofossil assemblages have focused on an apparent reduction in overall size and diversity with respect to those of the Late Paleozoic. Indeed, in many Permian – Triassic boundary sections, simple, diminutive traces are often the only ichnofossils present. However, numerous examples of arthropod produced ichnofossils that exceed the size and diversity of the nearest preceding Permian assemblages exist within Lower Triassic strata in western and arctic Canada. For example, Spongeliomorpha and Thalassinoides in lower Griesbachian strata are comparable in size to those of the underlying Middle Permian in the subsurface of western Alberta. In the Canadian Arctic, Dienerian age Kouphichnium, Spongeliomorpha, and Cruziana are larger than comparable ichnofossils in the Permian. Furthermore, in southwestern Alberta, examples of Spathian age Thalassinoides burrows exist that are in excess of 15 cm average burrow diameter. Considering the apparent dwarfism exhibited in many taxa of this period, arthropods may well have been virtual giants of the Early Triassic world.