A Late Triassic Faunal Transition and Associated Events

Figure 1. Chinle Formation stratigraphy in southern Petrified Forest National Park with approximate ages. (Adapted from a figure by Jeff Martz)

Recent revisions in the stratigraphy of the Chinle Formation along with more precise documentation of vertebrate fossil occurrences have revealed a distinct faunal transition, which coincides with several other biologic and geologic events. The Chinle Formation is exposed throughout northern Arizona, southern Utah, southwestern Colorado, and northwestern New Mexico and represents river deposits from the Late Triassic.  Although the event was first identified in Petrified Forest National Park (PEFO), it can be identified throughout the Chinle. It has not yet been identified outside of the southwestern United States. Lithologically, the transition is represented by a layer of bright red, petrified "mulch", known as the "persistent red silcrete zone" (PRSZ). This layer occurs only a few meters above the base of the Jim Camp Wash beds in the Sonsela Member of the Chinle Formation. It is most easily observed in the southern half of PEFO, where the Sonsela Member is widely exposed.

Figure 2. The persistent red silcrete zone in the Jim Camp Wash beds near the Painted Desert Museum of Petrified Forest National Park. (Photo by Susan Drymala)

The transition divides two distinct biostratigraphic units (biozones), defined by phytosaur taxa, with 'Leptosuchus' only occurring below the turnover level (defining the Adamanian biozone) and Pseudopalatus only occurring above (defining the Revueltian biozone). As large, robust, river dwelling animals, phytosaurs were easily preserved in the fossil record, especially their squamosals (large, knob-shaped bones at the back of the skull, useful in identifying species). Aetosaurs also show a distinct turnover at the Adamanian-Revueltian boundary and are represented by abundant fossils (primarily their armor plates - a.k.a. osteoderms - which are also useful in species identification). Although there are many species of aetosaurs present throughout the Chinle Formation Calyptosuchus and Typothorax can be used to identify the Adamanian and Revueltian, respectively. The boundary can be identified through patterns in other vertebrates as well, but phytosaurs and aetosaurs are the most densely sampled, especially at stratigraphic intervals directly above and below the transition zone, suggesting that the turnover represents a real event rather than an affect of sampling bias.

Figure 3. Phytosaur (left) and aetosaur (right) taxa representative of the two biozones. (Adapted from an image by Jeff Martz)

Although the Adamanian-Revueltian was first observed in vertebrate fossils, several other transitions have now been identified. Lithology and isotope records suggest a steady drying of the climate over this period, which may be due to the slow movement of the North American continent northward, away from the equatorial zone. The Revueltian shows an increase in well-developed paleosols and pedogenic carbonate nodules, compared to the Adamanian. This coincides with the appearance of large beds of unionid bivalves (known today as river mussels) which are known to prefer relatively alkaline waters. These beds only appear above the PRSZ of the Jim Camp Wash beds. A turnover in flora has also been identified, represented by the transition from Pollen Zone II to Pollen Zone III. Current absolute ages (U-Pb ages from detrital zircons) for Chinle strata place the persistent red silcrete zone and the Adamanian- Revueltian boundary between about 218 (lower Rainbow Forest Bed) and 214 Ma (middle Jim Camp wash beds). A cause for the event has not yet been identified, but some possibilities exist. Interestingly, the Manicouagan bolide impact in Quebec, Canada occurred at 215.5 Ma, although it currently cannot be directly tied to the Adamanian-Revueltian turnover.

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Martz, J. W. & W. G. Parker. 2010. "Revised lithostratigraphy of the Sonsela Member (Chinle Formation, Upper Triassic) in the southern part of Petrified Forest National Park, Arizona." PLoS ONE 5 (2)

Parker, W. G. & J. W. Martz. 2011. "The Late Triassic (Norian) Adamanian-Revueltian tetrapod faunal transition in the Chinle Formation of Petrified Forest National Park, Arizona." Earth and Environmental Science Transactions of the Royal Society of Edinburgh 101: 231-260

Ramezani, J., S. A. Bowring, M. S. Pringle, F. D. Winslow, & E. T. Rasbury. 2005. "The Manicouagan impact melt rock: a proposed standard for the intercalibration of U–Pb and 40Ar/39Ar isotopic systems." Geochimica et Cosmochimica Acta 69

Ramezani, J., G. D. Hoke, D. E. Fastovsky, S. A. Bowring, F. Therrien, S. I. Dworkin, S. C. Atchley, & L. C. Nordt. 2011. "High-precision U-Pb zircon geochronology of the Late Triassic Chinle Formation, Petrified Forest National Park (Arizona, USA): Temporal constraints on the early evolution of dinosaurs." Geological Society of America Bulletin