Report of a Diverse Vertebrate Body Fossil Assemblage in the Maroon Formation (Carboniferous–Permian), Eagle County, Colorado, U.S.A.

The Carboniferous–Permian (C–P) Maroon Formation of western Colorado preserves a paleotropical record of continuous terrestrial sedimentation during Earth's penultimate icehouse-hothouse state change: the "Late Paleozoic Ice Age." In the last twenty years, reports of vertebrate fossils, including tetrapod trackways, have suggested a diverse terrestrial vertebrate fauna adapted to dryland conditions comparable to the famous Permian Tambach vertebrate assemblage of Germany, but body fossils in the formation have remained elusive. Here, we describe the first vertebrate body fossils from the Maroon Formation and discuss them in the contexts of the known ichnoassemblage and C–P environmental change. The vertebrate fossils occur within a sequence of thin carbonate beds in the lower portion of the formation and represent a largely aquatic assemblage. We identify the remains of xenacanth and hybodont chondrichthyans, including the hybodont Hamiltonichthys, actinopterygians, including a platysomid and a pygopterid closely comparable to Progyrolepis, the lungfish Sagenodus, and an amphibamiform temnospondyl (among other rare and isolated tetrapod elements). The assemblage provides biostratigraphic data that may correlate the lower portion of the Maroon Formation to the late Carboniferous and the stratigraphically higher red siltstones to the early Permian (Wolfcampian), and therefore a continuous record of deposition across the C–P boundary. The new assemblage preserves a snapshot of one of the latest known Carboniferous wetland faunas in western Colorado prior to Pangean climate-drying during the peak Late Paleozoic Ice Age.