Life reconstruction of Edmontosaurus.
The term “mummy” is often used to describe dinosaur fossils with fossilized skin. It is commonly suggested that such fossils only form under exceptional circumstances and that a carcass must be shielded from scavenging and decomposition by rapid burial and/or desiccation in order for skin to become fossilized.
In a new study, researchers of the University of Tennessee–Knoxville combined fossil evidence with observations on modern animal carcasses to propose a new explanation for how such “mummies” might form.
The researchers examined an Edmontosaurus named Dakota - after the site in North Dakota where the fossil was found - which preserves large patches of desiccated and seemingly deflated skin on the limbs and tail. They identified bite marks upon the dinosaur’s skin. These are the first examples of unhealed carnivore damage on fossil dinosaur skin, and furthermore, this is evidence that the dinosaur carcass was not protected from scavengers, yet it became a mummy nonetheless.
Dakota's leg showing finer details, like scaly skin and hoofed digits.
Modern scavengers and decomposers target the soft internal tissues, biting or ripping apart an animal carcasses. This creates openings in the body wall through which fluids and gasses can escape. Also invertebrates and microbes use those openings to access the internal tissues, at first speeding up decomposition.
But the removal of internal soft tissues and drainage of fluids and gasses associated with decomposition allows the outer soft tissue, like skin and other dermal tissues, to quickly desiccate. This process facilitates longer-term persistence of the skin and other resistant soft tissues until eventual burial and fossilization.
Based on the available geological evidence, the authors propose Dakota's fossilization happened as follows:
After the animal died, his body was likely scavenged by a bunch of crocodiles, opening the carcass at its belly, and colonized by flies and beetles, cleaning bones and skin from the rotting flesh. Such incomplete scavenging would have exposed the dermal tissue insides, after which the outer layers became slowly desiccated. 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 beneath mud, maybe by a sudden flash flood, and circulating fluids deposited minerals, replacing the remaining soft tissue and preserving a cast in the rock.
Proposed soft tissue preservational pathway based on the studied specimen.
This process, which the authors call “desiccation and deflation,” is common with modern carcasses and explains how dinosaur mummies might form under relatively ordinary circumstances. The authors stress that there are likely numerous pathways by which a dinosaur mummy might develop. Understanding these mechanisms will guide how paleontologists collect and interpret such rare and informative fossils.
The paper "Biostratinomic alterations of an Edmontosaurus "mummy" reveal a pathway for soft tissue preservation without invoking "exceptional conditions" is published in PLoS One (2022). Material provided by PLoS.
