NEW, WESTERNMOST OCCURRENCE OF LARGE-BODIED TEMNOSPONDYLS FROM THE UPPER TRIASSIC CHINLE FORMATION OF SOUTHERN NEVADA

The Upper Triassic Chinle Formation of the American Southwest has long been known as a prolific fossiliferous unit, producing numerous terrestrial fossil assemblages across the southern Colorado Plateau. The Chinle fossil record for the Basin and Range is virtually unknown. In 2015, local hiker Harold Larson came upon a large vertebra from Spring Mountain Ranch State Park at the eastern base of the Spring Mountains of Clark County, Nevada, and brought it to the attention of JWB. A follow-up field outing was able to place the specimen in stratigraphic context and additional specimens were found and salvaged. The stratigraphy of the Chinle Formation in the Basin and Range is not as well defined as it is in Colorado Plateau sections. The fossils were found approximately 2 meters above the top of the Shinarump Member, and we interpret this unit as the Cameron Member following interpretations of similar facies in southwestern Utah. The Shinarump Member here is buff tan, with well rounded, multiple-colored chert pebbles, with large lateral accretion structures. Large tree impressions are found toward the top of the member. The overlying Cameron Member consists of purplish-mottled sandy mudstones with interspersed dark brown, course, lenticular, channel sandstones. There is common agatized petrified wood throughout the member. This unit eventually gives way upsection into redder sediments mapped as undifferentiated Moenave/Kayenta equivalent rocks. The vertebrate specimens were found as float atop the mottled purple beds, here interpreted to be inter-channel, poorly drained floodplain deposits. The fossils are found mostly with thin veneers of carbonate concretion surrounding them. A string of 4-5 associated vertebrae and textured bone fragments were found in close proximity. The vertebrae consist only of the centra, which are concave on one side, and convex on the other, consistent with presacral centra of metoposaurids. The larger of the vertebrae measures 7 cm tall, 8.5 cm wide, and 3.2 cm thick. The ornamentation and shape of the textured elements and the centra are consistent with temnospondyl amphibians, and the size suggests a large individual of metoposaur-grade, comparable to Anaschisma (=Koskinonodon). This is the westernmost occurrence of temnospondyl amphibians from the Late Triassic of North America.