For palaeontologists, the Cleveland-Lloyd Dinosaur Quarry in the US state of Utah has long been a site of broad abundance and deep mystery.
The quarry is crammed with Jurassic period fossils – in far greater quantity than those found at any other site so far discovered. If that wasn’t weird enough, the fossil debris is dominated by the massive predator species, Allosaurus.
Ever since the density of the fossil bed was first realized in 1927, scientists have been trying to work out why such an unusually large collection of dinosaurs – including more predators than should be feasible – died there.
Theories have included mass fatality events, perhaps caused by poisoned water or drought, or maybe a build-up over time caused by sucking, sticky mud.
Now two new and rigorous analytical approaches by a team led by Joseph Peterson, of the University of Wisconsin-Oshkosh, in Wisconsin, US, may at last have uncovered the answers.
Peterson and colleagues first subjected sediments in the quarry to X-ray fluorescence, and then minutely examined the abrasion patterns found on bone fragments scattered among the larger fossils.
The first approach yielded higher levels of barite and sulfide minerals, compared to the surrounding environment, and the second revealed that the bones had been damaged while being deposited on the site.
The appropriate conclusion, write the scientists in the journal PeerJ, is that there was no mass fatality event – and no over-population of Allosaurus.
Instead, it is likely that most, if not all, of the dinosaurs died at different times some little distance from the quarry and were then washed into their final resting place by periodic flooding.
In Jurassic times, the team writes, the quarry was “an ephemeral pond”.
The finding neatly solves two other mysteries associated with the fossil site. The first is the lack of fish, turtle or crocodile fossils, which would be expected if the water body were permanent.
The second is the lack of gnaw marks on the dinosaur bones themselves. For some reason, the corpses weren’t attractive to carrion eaters.
The idea that the corpses, already rotting, were essentially washed into place, forming layer after putrid layer, suggests that the water, when present, was deeply foul.
The team confirmed potentially lethal levels of heavy metals in the old pond. In the past, these have been suggested as the cause of the dino deaths. Peterson and his colleague, however, suggest they were in fact a consequence of high levels of decomposition.
The Cleveland-Lloyd Dinosaur Quarry, it seems, was never a nice place to be.