The term “mummy” is often used to describe dinosaur fossils with fossilized skin. It is usually suggested that such fossils only form in exceptional circumstances and that a carcass must be protected from scavenging and decomposition by rapid burial and/or desiccation in order for the skin to fossilize.
In a new study, researchers at the University of Tennessee-Knoxville combined fossils with observations of modern animal carcasses to propose a new explanation for how such “mummies” might form.
The researchers examined one Edmontosaurus named Dakota—after the site in North Dakota where the fossil was found—which preserves large patches of desiccated and apparently deflated skin on its limbs and tail. They found bite marks on the dinosaur’s skin. These are the first examples of unhealed damage by carnivores on the skin of fossil dinosaurs, and in addition, this is evidence that the carcass of the dinosaur was not protected by scavengers, but nevertheless became a mummy.
Modern scavengers and decomposers target the soft internal tissues by biting or tearing apart an animal’s carcass. This creates openings in the body wall through which fluids and gases can escape. Also, invertebrates and microbes use these openings to access the internal tissues, initially accelerating decomposition.
But removal of internal soft tissues and drainage of fluids and gases associated with decomposition allows external soft tissue, such as skin and other skin tissues, to dry out quickly. This process facilitates the long-term persistence of skin and other durable soft tissues until final burial and fossilization.
Based on the available geologic evidence, the authors suggest that the fossilization of the Dakotas occurred as follows:
After the animal died, its body was probably scavenged by a pack of crocodiles, opening the carcass in its belly and colonized by flies and beetles, cleaning the bones and skin from the rotting flesh. Such an incomplete scan would have exposed the inner skin tissue, after which the outer layers would slowly dry out. The underlying bones will prevent the empty hull from shrinking too much, preserving the fine details of the scaly skin. Finally, the mummified remains were buried under mud, perhaps from a flash flood, and the circulating fluids deposited minerals, replacing the remaining soft tissue and preserving a mold in the rock.
This process, which the authors call “drying and deflation,” is common with modern corpses and explains how dinosaur mummies can form under relatively ordinary conditions. The authors point out that there are likely multiple pathways through which a dinosaur mummy could develop. Understanding these mechanisms will guide how paleontologists collect and interpret such rare and informative fossils.
The paper “Biostratinomic alterations of an Edmontosaurus “mummy” reveals a pathway to soft tissue preservation without invoking “exceptional conditions” is published in PLoS One (2022). Material provided by PLoS.