Enlarge / Full-color reconstruction of the life of Edmontosaurus.
We rarely have time to write about every great science story we come across. So this year we’re running a special Twelve Days of Christmas series of posts again, highlighting one science story that fell through the cracks in 2022, every day from December 25th to January 5th. Today: Why dinosaur ‘mummies’ may not be as rare as scientists believed.
Under certain conditions, dinosaur fossils can include exceptionally well-preserved skin, an event long thought to be rare. But the authors of an October paper published in the journal PLoS ONE suggest that these dinosaur “mummies” may be more common than previously thought, based on their analysis of a mummified duck-billed hadrosaur with well-preserved skin that shows unusual telltale signs of cleaning in the form of bite marks.
In this case, the term “mummy” refers to fossils with well-preserved skin and sometimes other soft tissues. As we’ve previously reported, most fossils are bones, shells, teeth, and other forms of “hard” tissue, but sometimes rare fossils are found that preserve soft tissue like skin, muscles, organs, or even the occasional eyeball. This can tell scientists a lot about aspects of the biology, ecology and evolution of such ancient organisms that skeletons alone cannot convey.
For example, last year researchers created an extremely detailed 3D model of a 365-million-year-old ammonite fossil from the Jurassic period by combining advanced imaging techniques, revealing internal muscles that had never been seen before. Another team of British researchers conducted experiments involving the observation of rotting carcasses of dead sea perch to learn more about how (and why) the soft tissues of internal organs can be selectively preserved in fossils.
In the case of dinosaur mummies, there is an ongoing debate about what appears to be a central contradiction. Dinosaur mummies discovered so far show signs of two different mummification processes. One is quick burial, where the body is quickly covered and advanced decomposition is greatly delayed and the remains are protected from scavenging. The other common route is desiccation, which requires the body to remain exposed to the landscape for a period of time before burial.
The specimen in question is a partial skeleton of Edmontosaurus, a duck-billed hadrosaur found in the Hell Creek Formation of southwestern North Dakota and now part of the North Dakota State Fossil Collection. Named “Dakota,” this mummified dinosaur showed evidence of rapid burial and desiccation. The remains have been examined with various tools and techniques since 2008. The PLoS ONE paper’s authors also performed a CT scan of the mummy, along with a grain-size analysis of the surrounding sediments in which the fossil was found.
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There was evidence of multiple lacerations and lacerations to the forelimb and tail, as well as holes and abrasions on the arm, arm bones and skin in an arc-shaped pattern very similar to the shape of a crocodile’s teeth. There were also longer V-shaped notches on the tail that could have been made by a larger carnivore, such as a young Tyrannosaurus rex.
Zoom in / Proposed soft tissue preservation pathway based on the specimen examined.
Becky Barnes/PloS ONE
The authors conclude that there is likely more than one pathway for dinosaur mummification, resolving the debate in a way that does not “require an extremely unlikely convergence of events.” In short, dinosaur remains may be mummified more often than previously thought.
In Dakota’s case, the bulging appearance of the skin over the underlying bones has been seen in other dinosaur mummies and is also well documented in modern forensic research. The authors believe Dakota was “mummified” through a process called “desiccation and deflation,” involving incomplete scavenging in which animal carcasses are emptied as scavengers and decomposers target the internal tissues, leaving skin and bones behind. According to Forbes’ David Bressan, here’s what likely happened to Dakota:
After the animal’s death, its body was probably scavenged by a group of crocodiles, opening the carcass in its abdomen and colonized by flies and beetles, cleaning the bones and skin from the rotting flesh. Such incomplete cleansing would expose the interior of the dermal tissue, after which the outer layers would slowly dry out. The underlying bones would prevent the empty hull from shrinking too much, preserving the finer details of the scaly skin. Finally, the now mummified remains were buried under mud, perhaps by a sudden flash flood, and the circulating fluids deposited minerals, replacing the remaining soft tissue and preserving a cast in the rock.
“Dakota not only taught us that durable soft tissues such as skin can be preserved on partially scavenged carcasses, but these soft tissues can also provide a unique source of information about the other animals that interacted with the carcass after death,” said co-author Clint Boyd, a paleontologist at the North Dakota Geological Survey.
DOI: PLoS ONE, 2022. 10.1371/journal.pone.0275240 (About DOI).
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