EARLIEST TRACES OF LIFE ON LAND: ARTHROPOD TRACKWAYS AND OTHER ICHNA IN EARLY CAMBRIAN (NEVADELLA ZONE) INTERTIDAL TO SUPRATIDAL DEPOSITS, MACKENZIE MOUNTAINS, NORTHWEST TERRITORIES, CANADA

Intertidal to supratidal deposits of the lower part of the Lower Cambrian Sekwi Formation in the Mackenzie Mountains, Northwest Territories, Canada, contain several distinct arthropod trackways and trampled grounds. These are associated with unique ichna that likely represent arthropods, annelids, and other soft-bodied organisms preserved in sediment bound by microbial mats. The ichna are contained within beige and red, inclined, heterolithic strata composed of fine-grained sandstone and siltstone with parallel, flaser, and ripple laminations draped by mudstone. Many of the upper parts of these strata are modified by tepee structures and minor to major sets of desiccation cracks. Within the tepee structures are relatively undisturbed beds with small-scale shrinkage cracks several millimeters in length and tenths of millimeters in width.

The trackways are composed of what appear to be sets of 8 simple circular to slightly tapered tracks. The trackways have more or less a constant width from 3 to 5 mm with incompletely preserved lengths of about 80 mm. Several trackways cross over desiccation cracks, demonstrating that they were produced while the substratum was subaerially exposed. In a few specimens, numbers of co-occurring tracks produce a sort of trampled ground where individual trackways are difficult to determine. We hypothesize that the tracks represent an organism with uniramous appendages similar to those of lobopods. These organisms spent much of their time in intertidal to supratidal deposits grazing the microbially bound mats. These mats also preserved the locomotion trails of probable annelids that produced Cochlichnus, and circular to elliptical, bilaterally symmetrical impressions of unknown origin that may be related to slug-like molluscs.