RETHINKING THE ICHNOLOGY OF THE LOWER PERMIAN COCONINO SANDSTONE, GRAND CANYON, ARIZONA

The lower Permian (Leonardian) Coconino Sandstone in the Grand Canyon contains one of the most significant trace fossil (ichnological) records known from a Permian eolianite. Research nearly a century ago, particularly by Smithsonian paleontologist Charles W. Gilmore, based on extensive Coconino collections made along the Hermit Trail, established much of the ichnotaxonomy within which Permian eolianite ichnology was interpreted for about 70 years. Gilmore recognized a plethora of ichnotaxa from the Coconino Sandstone, which indicated a great diversity of invertebrate and vertebrate trace makers in the Coconino desert. However, in the 1990s, German ichnologist Hartmut Haubold and collaborators presented a totally different view, that, in particular, recognized little more than one tetrapod ichnogenus in the Coconino (Chelichnus) and attributed it to synapsid trackmakers. This lumping suggested that only the relatively specialized Permian synapsids could live in the harsh desert environments represented by the Coconino Sandstone. However, new discoveries and a recent critical re-examination of Coconino ichnology has found a middle ground in which the Coconino tetrapod tracks are understood to represent more than one ichnogenus, made by diverse trackmakers, but not as diverse as the early studies indicated. This new conclusion is largely based on recognition that the extramorphology of Coconino tracks mostly reflects the slope of the substrate upon which the animals walked: up-, down- or side-slope walking produces very different looking tracks by the same trackmaker. Thus, the deep reaches of the Coconino desert are now seen to have been inhabited by small parareptiles/captorhinomorph eureptiles, bolosaurid parareptiles/diapsid eureptiles, varanopid synapsids and reptiliomorph amphibians. Coconino invertebrate traces are dominated by arthropod walking traces that represent the Octopodichnus ichnofacies of eolian depositional systems. The so-called Entradichnus ichnofacies is based on a depauperate ichnoassemblage of the Scoyenia ichnofacies, representatives of which are found in the interdunal facies of the Coconino Sandstone. The new understanding of Coconino Sandstone ichnology provides a template with which to re-evaluate the ichnology of all ancient eolian depositional systems.