USE OF MICROVERTEBRATE FOSSILS IN DETERMINING DEPOSITIONAL ENVIRONMENT OF THE SONSELA MEMBER, CHINLE FORMATION, ARIZONA

The excellent exposures of Upper Triassic Chinle Formation in and around Petrified Forest National Park have resulted in the Chinle Formation becoming one of the most well-studied geologic locales in the southwestern United States. Here, previous studies on the sedimentology have suggested perennial fluvial deposition, with a shift into a more arid ephemeral depositional environment nearer the end of the Triassic. Microvertebrate remains in the Chinle Formation are often used as index fossils and paleoenvironmental indicators. The Chinle Formation is home to at least 18 holotype macrovertebrate specimens ranging from aetosaurs/phytosaurs to dinosauromorphs and other archosaurs, in addition to many microvertebrate species ranging from jawed fish to amphibians. Microvertebrate-rich sediments collected from a Typothorax quarry within the Sonsela Member of the Chinle Formation, located northeast of Holbrook, Arizona, were examined to identify and catalogue any microvertebrate fauna. This data was used to aid with tentative reconstructions of the paleoecology. As many of the remains recovered consisted of teeth, a new dental morphology-based cataloguing system was used. Four dominant morphotypes were identified (Morphotypes A through D), with Morphotype A (circular basal shape with no serrations) and Morphotype C (compressed basal shape, long anterior/posterior axis, no serrations) being the most common. These morphotypes are characteristic of known terrestrial and aquatic taxa commonly found within the Chinle Formation. These results are consistent with a perennial stream deposit, which conforms with existing paleoenvironmental and depositional interpretations of the Sonsela Member.