What Created Dinosaur Death Pits?

Despite how fun it sounds, walking with dinosaurs probably wasn’t the best idea.

One must look no further than China’s Shishugou formation to see why. Within three separate bonebeds, the remains of over a dozen small dinosaurs from the Late Jurassic period were discovered buried together. Most specimens represent the small theropod Limusaurus, though fossils of crocodiles, mammals, turtles, and even the early Tyrannosaurid Guanlong have been discovered too! The discovery of these bonebeds led paleontologists to question the events leading to the animal’s demise.

Let’s examine the possibilities. Bonebeds typically result from catastrophic environmental events, such as flash floods, volcanic eruptions, mudslides, and elongated drought periods. While these natural causes were responsible for dinosaur bonebeds in places like Canada and the United States, the evidence at Shishugou points towards another factor being involved.

Bonebeds resulting from catastrophes are typically messy, to say the least. The force of such events leads to bones becoming disarticulated, incomplete and scattered across large areas. The dinosaurs at Shishugou display no such qualities, with complete and articulated skeletons representing most specimens. Instead of laying side-by-side, the remains are stacked on top of each other. What’s more interesting is that the bonebeds are confined to small pits that range between 1-2 meters deep and wide, far smaller than typical dinosaur bonebeds. With such well-articulated and confined skeletons, the question remains: what could have caused their doom?

Could it have been quicksand? After all, these natural traps are confined to just a few meters in most cases. While the narrative is fun, it seems unlikely. The composition of quicksand doesn’t match the sediments found within the pits. So, while the notion of dinosaurs falling victim to a Hollywood trap is entertaining, it doesn’t quite line up with the science.

Could the composition of the pits provide an answer? Analysis has revealed that most pit sediments were volcanic stones deposited following volcanic eruptions. This came as little surprise to paleontologists, as volcanism in China during the Mesozoic epoch was common (Jiang et. Al). Despite this, the vertical stacking of skeletons means that a volcanic eruption wouldn’t have caused these dinosaurs to die from a catastrophe on par with Pompeii. While volcanic eruptions certainly posed a threat, the dinosaurs at Shishuguo weren’t victims of a lava flow. 

This doesn’t mean that the volcanic sediments were harmless, however. After an eruption, the volcanic debris and ash would have settled on the surface and may have formed a deadly quicksand like-substance. If small animals were to step on this molten mixture, they would become trapped and potentially die from either suffocation or exhaustion. Talk about unlucky…

Two questions remain: Why were the pits so concentrated? And why were only small animals victim to these pits? A closer look at the pit dimensions may provide a clue. The pits are almost the exact shape and size as the footprints of the largest animals living in the region: the sauropods. This matching has led to the emergence of an interesting theory: what if a sauropod caused the death pits?

If this was the case, it would explain why the surrounding areas are absent of fossils. In the years following an eruption, sediment would amass over the volcanic quicksand, forming a crust. If a small dinosaur were to pass over the crust, the crust could withstand it. But when a 20-tonne sauropod like Mamenchisaurus decided to go for a stroll, its immense body weight would have crushed through the thin layer of sediment and exposed the dangerous pits. While the surrounding areas would have been safe, walking in sauropod footprints would have proved fatal.

If the sauropods were walking in volcanic quicksand, then why didn’t they get stuck too? Simply put, their immense body weight gave them enough strength to pull themselves up without getting stuck. This is likely why the pits are absent of any large dinosaurs, as they were strong enough to pull themselves out without trouble. Being big certainly had its advantages!

For small dinosaurs, getting out wasn’t as easy. The majority of dinosaurs were small theropods, animals that were bipedal walkers. Their reliance on their hindlegs may have been what doomed them; without the ability to use their forelimbs for support and to crawl out, they were helpless. This could be the reason why theropods were more vulnerable than ornithischians like the similarly sized Yinlong, though their presence may just reflect local populations more than survival proficiency.

The presence of these pits does not paint a good picture for life next to sauropod dinosaurs. In Switzerland, a turtle fossil was discovered that may have been crushed by a meandering sauropod. Talk about a bad way to go! 

The lesson is that if you want a pet dinosaur, stay away from sauropods. Because even if they don’t crush you initially, walking in their steps may have been just as dangerous.

Works Cited:

I do not take credit for any images found in this article.

Header image courtesy of Michael Skrepnick, found here

Dinosaur stuck in mud courtesy of Zhao Chuang, found here

Fossil pit courtesy of National Geographic, found here

Sauropod trackway courtesy of Bob Nicholls, found at his twitter here

Eberth, D. A., et al. “Dinosaur Death Pits from the Jurassic of China.” Palaios, vol. 25, no. 2, 2010, pp. 112–125., https://doi.org/10.2110/palo.2009.p09-028r.

Jiang, Baoyu, et al. “Early Cretaceous Volcanism and Its Impact on Fossil Preservation in Western Liaoning, Ne China.” Palaeogeography, Palaeoclimatology, Palaeoecology, vol. 302, no. 3-4, 2011, pp. 255–269., https://doi.org/10.1016/j.palaeo.2011.01.016.

Püntener, Christian, et al. “Under the Feet of Sauropods: a Trampled Coastal Marine Turtle from the Late Jurassic of Switzerland?” 2019, https://doi.org/10.31233/osf.io/2atnq.

Wings, Oliver, et al. “New Sauropod Material from the Late Jurassic Part of the Shishugou Formation (Junggar Basin, Xinjiang, NW China).” Neues Jahrbuch Für Geologie Und Paläontologie – Abhandlungen, vol. 262, no. 2, 2011, pp. 129–150., https://doi.org/10.1127/0077-7749/2011/0183.