Collapse of the Late Triassic megamonsoon in western equatorial Pangea, present-day American Southwest

A juvenile Koskinonodon perfectus (Temnospondyli, Metoposauridae) from the Upper Triassic of Arizona and its implications for the taxonomy of North American metoposaurids

“…The occurrence of a juvenile K. perfectus in younger sediments than previously documented for the taxon is important for improving our understanding of the Adamanian-Revueltian faunal turnover, thought to correlate with a climatic shift to an increasingly arid regime with more pronounced seasonality (Dubiel et al, 1991;Parker and Martz, 2011;Nordt et al, 2015). Material of K. perfectus is common in Adamanian strata (Blue Mesa Member, lower Sonsela Member) of the park and rare in Revueltian strata (upper Sonsela Member, Petrified Forest Member), while material of A. gregorii demonstrates the inverse distribution pattern (Hunt and Lucas, 1993), which is thought to reflect a climate-associated transition from large metoposaurids to small ones (Parker and Martz, 2011;Atchley et al, 2013).…”

“…Given the pronounced seasonality that characterized the equatorial regions of the Upper Triassic, it would not be unexpected for aquatic taxa, particularly desiccationprone amphibians, to acquire adaptations that would allow them to tolerate more arid conditions during the dry season (Dubiel et al, 1991;Martz and Parker, 2010;Atchley et al, 2013;Nordt et al, 2015). For example, workers have postulated that M. diagnosticus, which features more robust forelimbs and musculature relative to Dutuitosaurus ouazzoui (two taxa with well-described articulated skeletons), may have been adapted for burrowing into lake sediments during the dry season .…”

Eogenetic diagenesis of Chinle sandstones, Petrified Forest National Park (Arizona, USA): A record of Late Triassic climate change

“…(A) Mean annual precipitation ( MAP ) derived from the chemical index of alteration minus potassium ( CIA –K) transfer function (Nordt et al ., ). (B) Mean annual temperatures ( MAT ) from the NAK (salinization) and δ 18 O PDB transfer functions (Nordt et al ., ).…”

“…The varying extent of feldspar weathering, kaolinite and smectite neoformation, and pseudomatrix development indicates differences in the intensity of meteoric eogenetic diagenesis. The evolution of meteoric diagenetic features can be compared and correlated to other climate studies of western equatorial Pangea that document an anomalously wet climate in the earlier Chinle that gradually transitioned to a drier climate during later Chinle deposition (Robinson, ; Parrish & Peterson, ; Sims & Ruffell, ; Dubiel et al ., ; Parrish, ; Dubiel, ; Frances, ; Retallack et al ., ; Tanner, ; Prochnow et al ., ; Tanner & Lucas, ; Dubiel & Hasiotis, ; Trendell et al ., ; Atchley et al ., ; Nordt et al ., ). These studies document the climate evolution using a wide range of indicators, including megascopic pedological features, associated geochemical signatures, the distribution of aquatic and semi‐aquatic tetrapods (phytosaurs and metasaurs), abundance of water loving plants (i.e.…”

“…(A) Location of Petrified Forest National Park ( PEFO ) in north‐eastern Arizona, and the eight outcrops comprising the composite section of the Chinle Formation used in this study. Outcrop numbers are in order from oldest to youngest: 1 – Hunt Valley; 2 – Lower Tepees; 3 – Upper Tepees; 4 – Blue Mesa Overlook; 5 – Mountain Lion Mesa; 6 – Citadel; 7 – Lacey Point; 8 – Chinde Mesa (modified from Nordt et al ., ). (B) Chinle Formation stratigraphy including lithostratigraphic, geochronological and sequence stratigraphic framework.…”