LATE CAMBRIAN ARTHROPOD TRACKWAYS IN SUBAERIALLY EXPOSED ENVIRONMENTS

Little is know about the early Paleozoic terrestrial colonization of Earth by animals. Trace fossils provide our most important insights into this interval because, unlike body fossils, they reveal behavior and are directly linked to specific environments. For example, trace fossils of the ichnogenus Protichnites from Lower Ordovician-?Upper Cambrian aeolian deposits in Ontario show that the first known organisms to move out of the ocean were amphibious arthropods. Continued study of fossilized traces from marginal marine settings of this era is thus important for expanding our understanding of this marine-terrestrial transition. Upper Cambrian strata of Quebec, New York, and Wisconsin were examined in this study because they have a wide range of ephemerally exposed marginal marine environments and abundant unstudied occurrences of Protichnites.

Extensive work was done in the quartz sandstones of the Upper Cambrian Elk Mound Group in central Wisconsin, where tracks are associated with trough cross-bedding, oscillation ripples, low-angle lateral accretion surfaces, channel-levee complexes and related features - suggesting intertidal to shallow subtidal conditions. In situ trackways were analyzed in detail, and morphologic data about the individual prints, medial impressions, and lateral track sets were collected. Additionally, the taphonomy of each trace and its association with other traces and microbial structures was noted. Sedimentologic features of each trackway surface as well as that of bounding layers were examined to infer trackway depositional environment(s). In particular, four of the studied surfaces are characterized by either raindrop imprints, adhesion structures, or high ripple-index ripples, suggesting subaerial exposure occurred at or near the time the tracks were made. Cross-cutting relationships of these trackways and sedimentary structures are being studied to test the hypothesis that these organisms ventured out of the water.