Dinosaur Bone Bed Indicates Underwater Graveyard

Multiple specimens of Edmontosaurus give evidence of catastrophic water burial after transport by an underwater debris flow.

Picture this: a crowd of large dinosaurs drowns suddenly. Their carcasses are carried by a debris flow of in a muddy matrix for miles. The debris flow lands in deep water, sorting the bones by weight. And here’s the most amazing part: the site where this was analyzed appears to be characteristic of many dinosaur deposits.

Sound like a widespread flood happened? Can a paper with such anti-consensus conclusions be published these days? Yes. The paper is open-access at the following link, so readers can learn all about an amazing dinosaur dig site.

Snyder, McClain, Wood and Chadwick, “Over 13,000 elements from a single bonebed help elucidate disarticulation and transport of an Edmontosaurus thanatocoenosis.” PLoS One May 21, 2020, DOI:10.1371/journal.pone.0233182.

This peer-reviewed paper is the result of a 20-year analysis of dinosaur bones in the Hanson Ranch in eastern Wyoming, where some dinosaur bones can be found sticking out of the ground. The authors dug up, measured and catalogued 13,000 bones of a single species called Edmontosaurus, a type of hadrosaur standing 10 feet tall as an adult and weighing up to 3 tons. This genus with two species was named for a geologic formation in central Alberta called the Edmonton Formation (after the city of Edmonton to the north). More recently however the Edmonton Formation has been renamed the Horseshoe Canyon Formation (after a steep canyon further south in Alberta). As for the city of Edmonton, Edmontosaurus bones were only discovered there in 1989, but now a major study has been published on a bone bed there as well. How did they end up together, disarticulated and sorted by size, with heaviest bones at the bottom? And why are there so few predator or scavenger tooth marks or scratches on them?

In our opinion, the best explanation for the transport and deposition of the bones and the containing sediment is that they were carried together as part of a seismically activated subaqueous debris flow possibly originating from a terrestrial source an unknown distance to the west. Because the bones exhibit very little transport-induced abrasion, the flow must have been matrix-supported.

Did the big beasts die of old age? Did the bonebed form over millions of years?

The pristine condition of these bones and the uniformity across all quarries suggests that these specimens originated from a single catastrophic event, as opposed to an attritional assemblage over time. They were probably not exposed for more than a short period of time and thus exposed to minimal pre-depositional weathering.

Edmontosaurs for scale. E. annectens (right) is the species found at Hansen Ranch. Credit: Arthur Weasley, Matt Martyniuk, Wiki Commons.

Is this an isolated case? No; the authors identify six other hadrosaur bonebeds that give evidence of transport by debris flows. Dinosaurs were hanging out doing dinosaur things, when sudden destruction came upon them out of nowhere.

There are a number of other dinosaur bonebeds which share a suite of characteristics (including stratigraphy, sedimentology, condition of bones, and orientation), with the HR Bonebed. These include Upper Cretaceous, monodominant hadrosaur bonebeds containing primarily disarticulated elements or completely or almost completely disarticulated elements. A similar biotic or abiotic mechanism may be at work in all these examples….  The uniformity of pristine bone preservation indicates limited exposure. However, extremely limited articulation in the bonebed suggests that almost complete disarticulation prior to transport and burial occurred. The presence of concreted bones in the deposit is consistent with the burial of bones having biogenic fluids still associated and occurring shortly after disarticulation.

The authors identify other bonebeds as far apart as Utah, Canada, Alaska, and Russia having similar characteristics. Do these dinosaur graveyards fit the consensus view that they perished because of an asteroid impact in the Yucatan?

Because these features are present at numerous horizons within the Lance [Formation], the seismic disturbance could not be directly attributable to a single event such as, for instance, an asteroid impact, and the explanation must be sought in other tectonic processes such as mountain building, perhaps associated with the Laramide orogeny. The transport direction based on paleocurrents in the sandstones indicates the source area (and possibly the paleoslope) was from the west and northwest.

Those are not from the south, where the Yucatan is. The rate of mountain building is a function of the forces involved. Consider this evidence that suggests the mountains were pushed up rapidly at about the same time:

Rapid deposition of the sandstone is attested to by beds displaying seismic disruptions that are uniform throughout the bed, but do not extend into the underlying or overlying sediments (Fig 2), indicating that dewatering had not occurred prior to the seismic event.

What kind of force would produce debris flow deposits in numerous widespread locations concurrent with seismic events and mountain building processes powerful and sudden enough to bury whole herds of dinosaurs?

A single subaqueous debris flow then moved the disarticulated elements far enough to permit vertical sorting of elements before coming to rest. The primarily silt and clay matrix must have been viscous enough and transport brief enough to provide protection from abrasion, but liquid enough for sorting and grading to have taken place. This sub-aqueous debris flow transported bones and the surrounding sediment into deeper water. According to Moore, a large majority of other Upper Cretaceous dinosaur bonebeds are recognized as being deposited in a catastrophic setting, and this bonebed appears to follow that trend.

Read the paper; see the diagrams; and think. The kind of catastrophe that could do this is left as an exercise.