VERTEBRATE FOSSIL PRESERVATION IN BLUE PALEOSOLS FROM THE PETRIFIED FOREST NATIONAL PARK, ARIZONA, AND ITS IMPLICATIONS FOR VERTEBRATE BIOSTRATIGRAPHY AND PRESERVATION IN THE COLORADO PLATEAU

The Chinle Formation in Petrified Forest National Park (PEFO) produces one of the most complete Late Triassic terrestrial faunal assemblages in the world. Phytosaurs, aetosaurs, and metoposaurs are the most common components of this assemblage and are known from most fossil sites in PEFO, but rare components of the Chinle faunal assemblage, including dinosaurs and small-bodied amniotes, appear to be restricted to distinctive blue-colored horizons observed at only a few sites in PEFO. A similar facies bias also seems to occur in other fossiliferous units in pre-Cretaceous sequences of the Colorado Plateau. In the Chinle Formation, the blue sites represent paleosols formed in fine-grained sediments deposited in abandoned channels, and the blue color is interpreted as a weathered feature of hydromorphically reduced iron. These drab paleosols contrast markedly with red-colored floodplain sediments and paleosols that characterize most fossil localities in the Chinle Formation. The coincidence of rare taxa and the blue-colored paleosols in the Chinle Formation suggests that the stratigraphic occurrences of these taxa are determined more by preservation than by evolution; this may be the case in other pre-Cretaceous sites in the Colorado Plateau as well. Indeed, the stratigraphic distribution of the sites and the preservation of the rare taxa they contain may result in part from fluvial sequence tracts. In PEFO, as well as in all deposits of all ages, taxa that are facies-biased do not have reliable ranges and thus are unsuitable index taxa for biostratigraphy.