A Jurassic graveyard
In the remote badlands of Xinjiang, the Shishugou Formation holds a series of grim tableaus from the Middle to Late Jurassic, roughly 164 to 159 million years ago. These are not typical fossil sites with scattered bones. Instead, they are "dinosaur death traps"—ancient, muddy pits that captured a cross-section of an entire ecosystem. Evidence shows a climate with intense seasonal droughts, where large sauropods congregated near dwindling water sources. Their immense weight, trampling the wet, volcanic ash-laden soil, created deep, inescapable mud pits.
These pits, some one to two meters deep, became graves for smaller vertebrates. The primary victims were small theropods, creatures under three meters long that became hopelessly mired in the thick mud. The vertical stacking of articulated skeletons shows that animals were trapped successively, their struggles perhaps attracting others who then met the same fate. Paleontologists have found numerous individuals preserved together, showing community structure of community structure and environmental pressures during the Jurassic. The arrangement of the skeletons, many found with their hindlimbs stuck deep in the mud, shows a slow death.
The fossils here are exceptionally well-preserved, suggesting rapid burial within days or months. This has allowed for detailed study of many species, including the primitive crested tyrannosauroid Guanlong wucaii and the small ceratosaur Limusaurus inextricabilis. In one pit, an adult and a juvenile Guanlong were found together, suggesting the adult may have been trapped while attempting to get to the already-mired younger animal.
A diverse and deadly ecosystem
The Junggar Basin death traps provide a complete record of the local fauna. The sites are dominated by the small theropod Limusaurus, with at least 15 individuals found in the pits. The basin also contained much larger animals. The giant sauropod Mamenchisaurus sinocanadorum, discovered here in 1987, possessed one of the longest necks of any known animal, measuring approximately 15.1 meters (49.5 feet). Its vertebrae were mostly hollow—up to 77% air—to reduce weight. These giants likely created the deadly mud pits that trapped their smaller contemporaries.
Other dinosaurs from the formation include the large carnosaur Sinraptor, discovered by a Sino-Canadian expedition in 1987, and various smaller sauropods like Bellusaurus. The ecosystem also supported early mammals, turtles, and crocodylomorphs, whose remains are sometimes found within the same assemblages.
The taphonomy of the sites—the study of how the organisms were fossilized—reveals details. Some bones show evidence of scavenging, such as separated body parts. Others have spherical boreholes, likely made by necrophagous insects like dermestid beetles feeding on the desiccated carcasses before burial. This indicates the bodies were exposed on the surface for at least several weeks in a semi-arid climate before being covered by sediment. The combination of drought, mud traps, and a diverse community of dinosaurs makes the Junggar Basin an informative record of life during the Jurassic period.