WHEN DID AMNIOTES FIRST ADAPT TO EOLIAN HABITATS?: EVIDENCE FROM TRACKWAYS IN THE MID-PENNSYLVANIAN MANAKACHA FORMATION OF GRAND CANYON, ARIZONA

The skeletal record of early amniotes, including reptiles and synapsids, is poor. Much of what we know about the habitats and ecology of Pennsylvanian amniotes comes from trace fossils. Reptiles, specifically, are archtypical inhabitants of eolian dunefields today, but it is not known when their Carboniferous or Permian ancestors first became adapted to such settings. Eolian sandstones rarely contain body fossils, but they commonly contain trace fossils, so the ichnofossil record offers our best opportunity for identifying the early adaptation to eolian dunefields by amniotes.

The earliest reported occurrence of tetrapod tracks in eolianite is in the Pennsylvanian-Permian Weber Sandstone of northeastern Utah. The age of the Weber Sandstone tracks is poorly constrained; they are probably Late Pennsylvanian or Early Permian, which is quite late in the history of amniotes.

Here we report the discovery of two amniote trackways on one eolianite bedding plane, within the Manakacha Formation in Grand Canyon National Park. These tracks are early Middle Pennsylvanian in age, about 313 Ma, which is only a million years younger than the earliest undisputed amniote body fossils globally. The Manakacha tracks occur in two boulders adjacent to the Bright Angel Trail. The trackway-bearing bedding planes are part and counterpart surfaces that split apart when the rock fell off the adjacent cliff face. One of the trackways is Chelichnus-like, while the other is defined only by aligned sets of claw marks. They are the first tetrapod tracks reported from the Manakacha Formation, and the oldest in the Grand Canyon region. These trackways represent the earliest known occurrence of dunefield-dwelling amniotes (either basal reptiles or basal synapsids), thereby extending the known utilization of eolian settings by amniotes by at least eight million years, into the Atokan/Moscovian Age of the Pennsylvanian.