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Forensic paleontologists use blowfly ‘cocoons’ to solve a 200-year-old cold case

Tom Gordon was digging a trench for a waterline in his Carson City backyard when he discovered fossil bones. “We need a paleontologist,” said his daughter Brittaney, a law student who was living at home with her parents due to the COVID-19 epidemic. Brittaney contacted paleontologist Steve Rowland at the Las Vegas Natural History Museum, and a full-scale paleontological excavation resulted. The paleontologists excavated a nearly complete bison skeleton, along with portions of three additional bison and a pronghorn antelope. Some of the bones have distinct cut marks, indicating that the animals were butchered and probably skinned, presumably by members of the Washoe Tribe. Radiocarbon dating indicates that the bones are approximately 200 years old, according to Rowland, which is much younger than he expected. “We think Tom’s backyard was a Washoe hunting camp two hundred years ago,” Rowland reported.

Artist’s reconstruction of the skinning and butchering of the GOR-1 bison. Note position of left foreleg.

Along with the bones, the researchers dug up thousands of blow fly ‘cocoons’ (technically called ‘puparia’) that were closely associated with the nearly complete bison skeleton. Blow flies lay their eggs on the bodies of dead animals. To explore the significance of the blow fly puparia, entomologist and blow fly specialist Terry Whitworth of Washington State University was recruited to join the research effort. Whitworth identified the species of blow fly as Phormia Regina, the black blow fly. Because of its importance in determining the length of time since the death of a human accident and murder victims, the life cycle characteristics and temperature tolerance of black blow flies have been well studied. This information was put to use by Rowland and his team to determine the season of death of the Carson City bison. They concluded that the animal died in the spring. Following the infestation of blow flies, the butchered and skinned carcass was apparently buried in sand by a flood of the Carson River and its tributaries, thus preventing the carcass from being dismembered by carnivores such as wolves and coyotes. A research paper describing the results of the study has been published in the journal PALAIOS.

Photograph of two puparia. The anterior end of the upper puparium has broken off due to the emergence of the adult fly.

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