How paleontologists interpret the fossil record could be set to change, as new research demonstrates that floodwaters alter the disposition of bones in ways that run counter to decades of understanding.
During flood events, dinosaur and mammal bones are transported from their original locations by raging waters and buried elsewhere before becoming fossils. Researchers at the University of Minnesota Twin Cities reported their discovery of flood-based fossil transport in a recent paper published in Paleobiology.
An Unpredictable Environment
Decades ago, scientists conducted foundational research that is still used to understand how water flow affects bone transport prior to fossilization. Unfortunately, that work was limited in scope, focusing solely on typical river-flow conditions and not accounting for periodic disruptions. Despite a lack of technical understanding of how flooding affects bone placement, researchers have repeatedly invoked flooding to explain animal burials.
The new work focuses on the types of singular flooding events that can greatly alter the disposition of materials in sediment. To better understand this, they tested how bones move under the unsteady flow dynamics common in natural sheet floods.
“Paleontologists try to piece together the stories of how fossil sites actually came to be, sort of CSI style,” said lead author Michael Chiappone, a University of Minnesota Ph.D. candidate. “So we asked ourselves: ‘Are fossil organisms preserved in the places where they died? Or are we finding them after they’ve been moved some distance after death by scavengers or water flow?’”
Reproducing Fossil Movement
St. Anthony Falls Laboratory at the University of Minnesota, specializing in fluid mechanics, provided the researchers with an ideal venue to begin testing real-world flooding conditions. With 24 facilities, St. Anthony’s Falls has been at the heart of many practical studies in civil engineering, geology, and ecology since opening in 1938.
The team recreated river features, including ripples, dunes, and bars, over which they reproduced the chaotic waves found in flood events. The experiments enabled researchers to determine how size, shape, and density affect the sorting of bones in water flow.
“Our experiments were conducted in the ‘Main Channel’ flume at the University of Minnesota St. Anthony Falls Laboratory, which is an 80-meter-long, 3-meter-wide flume that channels water from the Mississippi River outside through the building,” Chiappone told The Debrief, describing the experiments. “For our purposes, we ran on a flattened bed of sand so that we could run multiple floods that were reproducible each time with different flow rates (more or less powerful floods).”
Fossils in Context
“Our main takeaway is that those established methods don’t always work outside of the context of the original experiments, and that we should be careful in applying them to a really broad range of conditions and organisms that we find in the real-world fossil record,” Chiappone explained.
The underlying finding was that normal seasonal flooding has little impact: it moves only the smallest bones far from their original locations, and requires exceptionally strong floods to significantly affect their position. In tests, some of the heaviest pieces, such as articulated skulls, hardly moved at all, while hip bones and other varieties were more likely to travel, with some even exiting the experimental area if flooding was strong enough.
Paleontologists presently use what is known as the Voorhies Groups, a four-tier classification system developed by Michael Voorhies in his 1969 study, to determine how flowing water moves bones. The University of Minnesota research challenges these long-accepted standards, finding in its experiments that bones don’t always behave according to the Vorhies Groups’ expectations.
A Complex Natural World
The new work indicates that paleontologists should consider a broader range of variables when evaluating fossil discoveries.
“The results from our experiments will help us better interpret how bones were sorted, accumulated, and buried at paleontological sites we excavate,” said co-author Peter Makovicky, a professor and paleontologist in the University of Minnesota Department of Earth and Environmental Sciences. “This provides the basic information we work back from to reconstruct extinct animals and the environments they lived in.”
The team is already working on a follow-up involving the largest bones from creatures such as elephants, whales, and bison.
“In light of the limits of using the classic Voorhies Groups we saw in this study, we are currently taking a broader look at bone transport potentials for a wider suite of water flow regimes and organisms from a physics perspective in hopes of being able to predict how easily fossil bones might have been transported relative to each other before they were buried,” Chiappone concluded.
“From there, we want to be able to apply these methods at real fossil sites to determine more about what kinds of flows can move and bury bones,” he added, “and what kinds of biases that imparts on the fossil record we use to understand ancient ecosystems.”
The paper, “When the Levee Breaks: Experimentally Testing Dinosaur and Mammal Bone Transport in Unsteady Flows,” appeared in Paleobiology on January 12, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.