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Taphonomy and Depositional Setting of the Placerias Quarry (Chinle Formation: Late Triassic, Arizona)

¹ Dallas Museum of Natural History, P. O. Box 150349, Dallas, TX 75315
² Museum of Paleontology, University of California, Berkeley, CA 94720

The Placerias Quarry of northeastern Arizona is one of the most diverse Triassic vertebrate fossil localities in the world. Several long-standing hypotheses are tested about depositional environment, paleoecology, and thanatocoenotic influences. Sedimentological features indicate a low-energy depositional environment. Bones are associated mostly with mudstones and a layer that contains numerous carbonate nodules. Thin-sections and field characteristics reflect that this nodule zone is pedogenic in origin. Taphonomic data show that the bones were not transported into the site by fluvial processes. The frequency of modification features is minimal and there is little evidence of postmortem alteration of the bones from prolonged exposure, trampling, or predation.

Element survivorship curves were generated for the five most abundant taxa in the quarry. Curves for left and right elements tracked each other closely, but no pattern of preservation of cranial and postcranial elements was evident either within or among taxa. No clear signal was provided by either size or hydrodynamic potential of the elements. Because the same elements were not preserved preferentially among the best represented taxa, there is no systematic bias of preservation or removal by predators, though it cannot be eliminated, and taphonomic evidence provides no reason to suspect stream transport as a preservational bias. It is proposed as a general guide for assessing the relative completeness of preservation of an assemblage that the convexity of the element survivorship curve is proportional to the representation of the original death assemblage in the fossil assemblage. Thus, it provides increasing confidence in the assessment of the numbers of individuals actually represented.

Sedimentologic and taphonomic data indicate that the Placerias Quarry was a site of bone accumulation in a soil with a high water table. The causes of death for the animals remain largely unknown, but recently proposed climatic models for the Chinle Formation are consistent with the hypothesis that seasonal droughts may have caused mortality events like this one.

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