Steve Rowland (Geoscience) recently published "Early Adaptation to Eolian Sand Dunes by Basal Amniotes is Documented in Two Pennsylvanian Grand Canyon Trackways" in PLOS One. The study confirmed the oldest vertebrate tracks in the Grand Canyon. The two sets of fossilized tracks are approximately 313 million years old and are lying in view on the park's Bright Angel Trail. The tracks are among the oldest tracks on Earth of shelled-egg-laying animals, such as reptiles, and the earliest evidence of vertebrate animals walking in sand dunes.
The original story "Tiny Footprints, Big Discovery: Reptile Tracks Oldest Ever Found in Grand Canyon" was written after the tracks were discovered and Rowland presented the findings at the Society of Vertebrate Paleontology's annual meeting, but before being published in PLOS One.