Fig 5. A composite stratigraphic column of the Green River Formation showing the relative position of the Fossil Butte Member and stratigraphic levels where various bat fossils have been found. The Fossil Butte Member is the thinnest of the members in the Green River Formation, but was deposited when the lake was deepest, providing conditions that were optimal for exceptional preservation. Five units in the Fossil Butte Member (Sandwich Beds, 18-inch Layer, Minifish Bed, Gastropod Beds, and Upper Split-fish Bed) are actively quarried on private and stateowned land by companies collecting the abundant fossil fish to sell. Among the fish, one bat is found on average every two years. To date, all bats have come from the 18-inch Layer and Sandwich Beds. Bat specimens are coded by color: Icaronycteris gunnelli in red, I. index in orange, Onychonycteris finneyi in blue, and unidentified or indeterminate are black. More than 10 bat specimens are held in private collections (not shown here), but none are confirmed lower in the section than the two specimens of I. gunnelli. Left two columns modified after Buchheim et al, 2011 [40]. https://doi.org/10.1371/journal.pone.0283505.g005