The First Animals on Land: Al Curran(t) State of Knowledge

Aeolian foresets of the Paibian-Stage 10 Gunter and Lamotte Sandstones of Missouri, and the Furongian Keeseville Member of the Potsdam Group of New York have arthropod trackways (Protichnites, Diplichnites) that were produced subaerially, by euthycarcinoid-like animals crawling on coastal dune slip faces. These new fossils, together with trackways on aeolian foresets in the Furongian-Floian Potsdam Group (Nepean Formation) of Ontario, represent some of the first evidence of land-going animals. However, in order to get to these dunes, animals must first have crawled across other intermittently wet coastal landforms, such as tide- and sand-flats and beaches. It is in these settings where they first withstood the vicissitudes of crossing the subaqueous-subaerial divide – learning to withstand loss of buoyancy and aqueous respiration, and the hazards of increased desiccation, ultraviolet radiation and temperature variation.

In earliest Late Cambrian (Paibian) intertidal-supratidal lithofacies of the Elk Mound Group of Wisconsin, a low diversity community of large euthycarcinoid-like arthropods, soft-bodied or weakly sclerotized molluscs, and ?annelids? crawled across exposed sand flats during and between episodes of subaerial exposure. Although the arthropods are represented by both body and trace fossils, most of this community is only known from its trackways (Protichnites, Diplichnites, Climactichnites, Aulichnites, Helminthoidichnites) or body impressions (Musculopodus). The trackways link the timing of animal activity to specific depositional events, including individual episodes of subaerial exposure. Two rain-dropped, polygonally-cracked bed surfaces have many individual trackways that cross-cut and are cross-cut by raindrops – indicating animal movement during or between rain showers. Four surfaces bear adhesion structures and sand shadows indicating animal movement close to the time of subaerial exposure. Six surfaces have tracks on and between polygonally-cracked tidal levees and supratidal lithofacies that are riddled with low amplitude wind ripples. In WI, NY, and QUE, 23 other trackway surfaces are sandwiched between closely spaced scyphomedusae stranding horizons.