Chemical and petrographic analysis of bone and sediment from the Coelophysis quarry, Ghost Ranch, New Mexico

An abstract describing the possible causes of death of the Coelophysis at the Ghost Ranch quarry through the analysis of sediment samples.

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The Coelophysis quarry was discovered in the Upper Triassic Chinle Formation at Ghost Ranch in 1947 by Edwin H. Colbert, then of the American Museum of Natural History. It has since become well known for its abundant, essentially monospecific fossil assemblage which includes a remarkable number of articulated skeletons and partial skeletons. The bones occur in a siltstone sequence of channel and overbank deposits, approximately 40m below the Chinle-Entrada contact. The stratigraphy and taphonomy of the site indicate that the Coelophysis remains were transported and oriented by currents prior to burial in flood-borne sediments. Field work has not substantiated the actual cause of death of the quarry dinosaurs, although the facies suggests drowning as a possibility.

Ongoing petrographic and geochemical analysis of bone sediment from the quarry was undertaken in hope of learning more about the Coelophysis mass death as well as details of logical diagenesis. Petrography shows that the quarry bone is enclosed carbonate pebble - and mudclast-bearing siltstones, with little evidence of remnant volcanic ash. It is therefore unlikely that the dinosaurs expired due to volcanic eruption. The bone is preserved as apatite, and individual bones maintain structural integrity, with little or no distortion of osteon microstructure. Exterior edges of bones are smooth and unbroken, suggesting (along with the degree of articulation at the site) that the Coelophysis remains were buried soon after death. Unexpanded pressure cracks developed in some bone after burial, despite early cementation of the siltstones by calcite, hematitic clay, and quartz.

Three techniques have been used to analyze the elemental composition of quarry sediment and bone: electron microprobe (point analysis), SEM (X-ray maps) and neutron activation (bulk analysis), together yielding information on more than 50 different elements. These data indicate the Coelophysis bone is typical fossil bone fluorapatite, rich in fluorine (2.8-4.5 Wt.%), with traces of sulfur, sodium, and, in some cases, barium. Calcium is relatively depleted near pressure cracks. Bulk analysis (for 46 elements) of 108 sediment samples from the quarry and other Chinle strata at Ghost Ranch and the Petrified Forest, Arizona, show significant compositional variations. Most can be attributed to facies or color differences, However, some of the quarry strata have an arsenic content more than four times greater (14ppm) than the average for all Ghost Ranch samples, and five times greater than the average Petrified Forest samples. Further analysis will be necessary to determine whether the demise of the quarry dinosaurs is related to these high local levels of arsenic.