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Sauropod stomps snare smaller saurians, suggest scientists

The Late Jurassic "death traps" of the Shishugou formation, notable for having yielded the likes of Limusaurus and Guanlong, as well as a National Geographic documentary, are pretty much unique. Hundreds of dinosaur skeletons have been mired in them, most notably small theropods like the aforementioned taxa. When together, Limusaurus (the most common theropod there by far) tend to be found towards the bottom. while Guanlong is near the top (represented in the figure below).

The pits themselves indicate that a redbed crust on top collapsed, revealing mud below that managed to trap small animals. How did these death pits come to be?

Eberth et al. (2010) believe they have the answer. The pits are newly interpreted as none other than sauropod footprints, albeit virtually unrecognizable sauropod footprints. This conclusion was based on several observations: the omnipresence and regularity of the pits, preservation of two or more pit assemblages that can be interpreted as linear pathways, and the existence of large sauropods in the area such as Mamenchisaurus (Eberth et al., 2010).

Something like this would happen. A sauropod, ambling nonchalantly across the landscape as sauropods are wont to do, would stray onto the death-pit field, composed of a dry reddish crust masking the semisolid mud layers underneath. As it walked there, it would crunch through the crust into the mud; however, its great size and strength prevented it from getting stuck, and it would pull its feet out, proceeding across the flat with "a loud squelching noise similar to that made by a hippopotamus when lowering its foot into the mud on the banks of the Limpopo River" (Dahl, 1988).

Small theropods where not so lucky. Stumbling into the death pits, they were quickly trapped and sucked down to their doom. The pits show a definite bias towards animals less than 3 meters long and 1 meter tall; no sauropods are preserved anywhere. The large number of Limusaurus is likely due to their having existed in large flocks. Finally, the stratification seems to be based on the fact that Limusaurus would get caught first, and the scavenging Guanlong would get caught last. The pits themselves are a succession in time of different victims (Eberth et al., 2010).

Whatever the cause of the death traps of Shishugou, we remain thankful that they should have preserved all those otherwise hard-to-find little theropods. Props to Eberth et al. for bolstering the hypothesis that sauropods were the driving force for the evolution of flight in... nah, just kidding. It's still a cool thought.

All images from Eberth et al. (2010).

References

Eberth, D. A.; Xing, X.; and Clark, J. M. (2010) Dinosaur death pits from the Jurassic of China. Palaios, 2010, v. 25, pp. 112-125

Dahl, R. (1988) Matilda. Puffin Books.

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