How did all of the Coelophysis at Ghost Ranch die?

Paleontologists don’t know what killed all the Coelophysis in the Ghost Ranch bonebed. Various suggestions include a flash flood, disease or poison swamp gas.

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Paleontologists are not sure how the Coelophysis in the Ghost Ranch bonebed died. There is no real evidence on the fossils themselves of the cause of death. However, the fact that hundreds of complete skeletons are jammed together in one rock layer suggests a fast and catastrophic death of the dinosaurs. Furthermore, there is no evidence of weathering or scavenging of the fossil bones, so they must have been buried rapidly after death.

Some paleontologists suggested that the Coelophysis at Ghost Ranch may have been poisoned or died from disease. However, no data support these suggestions. Indeed, in all cases, fossil evidence of poisoning or disease is almost impossible to find.

Perhaps the best suggestion is that a flash flood killed all the Coelophysis, swept their bodies into a depression on the Late Triassic flood plain and buried them rapidly. Evidence supporting this idea include: the presence of a pond immediately beneath the bonebed (and thus a depression in the flood plain), the fact that all the bodies are jumbled together as if they were simply swept in by water, and the lack of evidence of scavenging, suggesting quick burial.

So, at present, the best inference is that a flash flood killed all the Coelophysis at Ghost Ranch.