STRATIGRAPHIC DISTRIBUTION OF TETRAPOD AND INVERTEBRATE ICHNOFOSSILS IN THE PERMIAN COCONINO SANDSTONE OF GRAND CANYON NATIONAL PARK AND ADJACENT AREAS, NORTHERN ARIZONA

The Coconino Sandstone has yielded the largest assemblage of ichnofaunas of the Late Paleozoic eolian Chelichnus ichnofacies. Lull first described tetrapod tracks from the Coconino of Grand Canyon National Park (GRCA). Gilmore described the majority of specimens from GRCA based on specimens collected by three expedition of the Smithsonian Institution during1924, 1926 and 1927. Later museum collections from the Coconino were made north of Ash Fork and Seligman, south of GRCA. In the last decade, we have investigated new in situ tetrapod specimens from GRCA. Tetrapod and invertebrate ichnofossils are non-randomly distributed in the Coconino Sandstone. The vast majority are restricted to the lower half of the unit. The Hermit basin at GRCA has yielded the largest number of ichnological specimens in stratigraphic context. At the Hermit Trail, the Coconino is about 106 m thick. The basal 6 m is barren, and tracks are rare until an interval 39.5-45.7 m above the base of the formation that yields over 90% of all tetrapod tracks and seemingly all invertebrate trace fossils. The upper half of the formation is essentially barren other than rare, poorly-preserved specimens of Chelichnus. South of GRCA, tracks in the Coconino are also restricted to an approximately 5-6 m-thick interval in the medial Coconino. McKee and later workers demonstrated a subtle sedimentologic change through the Coconino section. The lower Coconino is characterized by coarser grain size and steeper, shorter bedding planes. The upper Coconino has a finer grain size and the majority of the known wind ripples. These features apparently represent a drying trend through Coconino time. Clearly water, possibly as dew, was necessary to preserve the tracks. We hypothesize that an ideal taphonomic setting for preserving tracks that included fine grain size and the presence of water may have only been present for a short interval during medial Coconino time.