Duck-billed dinosaur “mummies” preserve fleshy hide and hooves in thin layers of clay

New paleontological findings offer insights into Wyoming’s “dinosaur mummies,” revealing that the stunningly preserved skin, spikes, and hooves of duck-billed dinosaurs are not fossilized flesh at all, but delicate clay molds formed by microbes as the creatures decayed, researchers report. Soft-tissue preservation in fossils usually occurs in fine-grained, oxygen-poor environments such as lagoons or seabeds, which enable the fossilization of delicate structures like feathers and skin. However, the so-called “dinosaur mummies” of duck-billed dinosaurs (Edmontosaurus annectens), which were discovered in the early 1900s in eastern Wyoming with what appeared to be fossilized skin texture and fleshy body parts, were found in coarse, oxygenated river deposits, making the nature of their remarkable preservation a long-standing mystery. Using historical documents, Paul Sereno and colleagues approximated the location where these original specimens were discovered more than a century ago and report on new findings from the site, including newly discovered juvenile and adult Edmontosaurus mummies. Notably, the late juvenile represents the first subadult dinosaur mummy and the first large-bodied dinosaur with a fully preserved fleshy outline, including a neck and trunk crest. The adult is the first hadrosaurid to retain the full tail spike row and the earliest known hooves in any tetrapod, marking the first reptile with hooved feet. According to the authors, the midline crest and tail spikes of E. annectens reveal a far more complex hide than historical restorations have suggested. Comparisons to living reptiles suggest functional and morphological similarities to some modern squamates. Sereno et al. also show that these features are preserved as thin (<1 millimeter) clay layers bounded by sandstone, rather than as true fossilized soft tissue. Detailed analyses using optical, CT, and electron microscopy, and X-ray spectroscopy revealed no evidence of original organics within the clay or surrounding matrix. The uniform clay layer likely formed as a surface template over the decaying carcass, aided by biofilms, rather than replicating original tissue. This clay templating mechanism preserves external integumental forms in three dimensions, even in coarse, oxygenated fluvial deposits, expanding the range of depositional settings in which soft-tissue features can fossilize.