Technical_article

Geology and taphonomy of the Coelophysis Quarry, Upper Triassic Chinle Formation, Ghost Ranch, New Mexico

A detailed taphonomic study that concludes that currents transported and concentrated the Coelophysis skeletons in the Ghost Ranch bonebed.

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Abstract

The Coelophysis dinosaur quarry at Ghost Ranch, near Abiquiu, New Mexico, is unique among Triassic fossil sites for its yield of numerous complete and partial skeletons of a single species of theropod dinosaur (Coelophysis bauri). Since its discovery in 1947 by E.H. Colbert in the red siltstone beds of the Upper Triassic Chinle Formation, the quarry has yet to yield the remains of at least 1,000 individuals from approximately 30 cubic meters of excavated material. The main bone-bearing strata are abandoned channel deposits that are part of a siltstone overbank sequence. The Coelophysis remains found at the quarry are remarkably whole and well preserved, though they range in degree of articulation from complete skeletons to isolated limbs and bones. Skeletons, partial skeletons, and bones are crudely aligned and show little evidence of predator or scavanger disturbance or surface weathering. Geologic and taphonomic evidence suggests that the dinosaurs preserved in the Ghost Ranch quarry were transported to the site as carcasses by fluvial currents. The carcasses blocked a small channel and were subsequently buried by silts. Petrographic study and neutron activation analysis reveal no evidence of volcanic ash, paleopathologic osteology, or unusual chemistry in the quarry bone and sediments. The virtual monospecificity, taphonomy, and ecology of the assemblage suggest that the dinosaurs perished due to a regional environmental crisis, such as drought.