In the early 1900s, fossil collectors found two skeletons of the duck-billed dinosaur Edmontosaurus annectens in east-central Wyoming, both covered in large patches of scaly skin.
Further studies identified impressions of spikes and hooves, as well as the upper bill of one of the dinosaurs. These extraordinary finds prompted researchers to dub the specimens ‘mummies’.
Typically, soft tissues like skin only preserve in a handful of environments and under a specific set of conditions, such as in a lagoon or on a seabed starved of oxygen and covered in fine-grained sediments.
What sets Wyoming’s duck-billed dinosaur ‘mummies’ apart is the fact they were discovered in coarse, oxygenated river deposits - a type of environment that very rarely preserves soft tissues.
The mystery surrounding these dinosaurs has long puzzled palaeontologists, until now.
As part of a study published today in Science, the University of Chicago’s Paul Sereno - along with a team of other palaeontologists - found evidence to suggest the soft tissues of Wyoming’s duck-billed dinosaur ‘mummies’ are preserved as delicate, ~1mm-thick clay moulds.
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According to Sereno and colleagues, these clay moulds formed during the early part of the decay process as microorganisms broke down the soft tissues, allowing sediment to rapidly infill and preserve the three-dimensional morphology of fleshy parts like skin, spikes and hooves.
This particular style of preservation is incredibly rare and has only previously been documented in anoxic (no oxygen) marine settings.
In order to solve this more than 100-year-old mystery surrounding Wyoming’s duck-billed dinosaur ‘mummies’, Sereno and colleagues played detectives and used historical documents to relocate the site where the original specimens were found. Here, they discovered even more ‘mummies’: another adult E.annectens and a juvenile of the same species.
The juvenile is the first subadult dinosaur ‘mummy’ ever found and the first large-bodied dinosaur with a fully preserved fleshy outline, including a neck and trunk. The adult, on the other hand, preserves the earliest known, wedge-shaped hooves in the fossil record.
These ‘mummies’ also reveal far more complex soft tissues than previous restorations have suggested, including rows of spikes similar to those seen on the necks and backs of modern lizards.
The fact this style of preservation has been identified in specimens found in coarse, oxygenated river deposits is incredibly important as it broadens the range of known depositional settings in which soft tissues can be preserved.
Now researchers know what to look for, and the specific conditions needed to create such clay moulds, the hunt is on to discover more dinosaur ‘mummies’.
Top image: Wyoming’s duck-billed dinosaur ‘mummies’. Credit: University of Chicago Fossil Lab
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