Abstract

Three localities in the Chinle Formation (Late Triassic), Petrified Forest National Park (PEFO), Arizona, provide insights into the paleoenvironments frequented by primitive North American theropods. At the Dinosaur Wash locality, an undetermined theropod is preserved in paleosols that indicate a transition from wet to dry conditions on the floodplain. Coelophysis bauri remains from the Dinosaur Hill locality are preserved in a filled channel scour. The paleosols at this locality contain carbonate nodules intimately associated with Fe and Mn oxides, indicative of alternating alkaline/acidic conditions around roots in response to a semi-arid climate with strong seasonal precipitation. At the Dinosaur Hollow locality, Chindesaurus bryansmalli is preserved in a setting similar to those encountered at Dinosaur Hill. The paleosols exhibit vertic features, such as pseudoanticlines, and are indicative of water-deficit periods during the year.

The most complete theropod remains at PEFO are predominantly preserved in distinctive blue-colored paleosol horizons showing depletion in iron and aluminum, and exhibiting features such as localized Fe concentrations and mottling. These are interpreted as A-horizons of redoximorphic paleosols developed in wet areas of the floodplain where the degree of water saturation fluctuated.

Preservational conditions of PEFO theropod localities indicate they are time-averaged attritional mortality assemblages representative of contemporaneous organisms. Comparison of these localities to the well-known Ghost Ranch, New Mexico, Coelophysis quarry reinforces the idea that the quarry represents unique preservational conditions.

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