In a groundbreaking paper published in the esteemed journal Science, a team of researchers from the University of Chicago has unveiled the astonishing transformation of Edmontosaurus annectens, a duck-billed dinosaur, into remarkably detailed "mummies" approximately 66 million years ago. These ancient remains, dating from the very end of the Cretaceous Period, offer an unparalleled glimpse into the external anatomy of a large dinosaur, preserving intricate features of skin, scales, and even hooves through a newly identified process dubbed "clay templating." This unique taphonomic phenomenon involves the outer soft tissues of the deceased animal being preserved not as original organic material, but as a delicate clay coating, less than 1/100th of an inch thick, that settled over the skeleton after burial, effectively forming a three-dimensional mask.
A Window into the Late Cretaceous: The World of Edmontosaurus
The discovery fundamentally reshapes our understanding of one of the most iconic dinosaurs of the Late Cretaceous. Edmontosaurus annectens was a large, herbivorous hadrosaur, or "duck-billed" dinosaur, common across western North America during the Maastrichtian age, the final epoch before the cataclysmic asteroid impact that ended the age of dinosaurs. These animals were dominant herbivores, often moving in large herds, and known for their complex dental batteries designed for grinding tough plant matter. While skeletal remains of Edmontosaurus are relatively abundant, soft tissue preservation – particularly of the entire external surface – has remained exceedingly rare. Prior reconstructions of hadrosaurs, often based on skeletal morphology and sparse skin impressions, left much to speculation regarding their full integumentary details, including crests, defensive structures, and the precise nature of their feet. The new findings from the University of Chicago team, led by senior author Paul Sereno, PhD, Professor of Organismal Biology and Anatomy at UChicago, provide the first comprehensive, fleshed-out view of a large dinosaur with a high degree of confidence, pushing the boundaries of paleoart and scientific reconstruction.
The "Mummy Zone": A Unique Preservation Hotspot in Wyoming
The extraordinary fossils originate from a specific geological locale in east-central Wyoming, which the team has aptly termed a "mummy zone." This region, characterized by stacked river sands from the Late Cretaceous, proved to be an exceptionally fertile ground for this rare form of preservation. The historical significance of this area is not new; several classic dinosaur mummies, though less understood at the time, were first uncovered here in the early 1900s. Through meticulous detective work, utilizing old field photographs and geological mapping, Sereno and his colleagues painstakingly retraced these historical locations, pinpointing a compact area within these rock layers conducive to "clay templating."
The team’s modern expeditions in this "mummy zone" led to the excavation of two new Edmontosaurus mummies: a younger individual and a somewhat older one. Crucially, both specimens preserved large, continuous patches of the external skin surface, providing complementary pieces of a jigsaw puzzle needed to construct a complete and accurate profile of the animal. This collaborative effort, extending from field discovery to advanced laboratory analysis, underscores the multidisciplinary nature of contemporary paleontological research.
Clay Templating: A Novel Preservation Mechanism
Unlike the deliberately mummified remains of ancient humans, these dinosaur "mummies" bear no original organic material. Instead, the preserved skin, spikes, and hooves exist as an incredibly thin clay coating that formed around the carcass shortly after burial. Professor Sereno emphasizes the delicate nature of this preservation, describing it as "a mask, a template, a clay layer so thin you could blow it away." The scientific process behind this "clay templating" is as fascinating as the fossils themselves.
The researchers propose a specific sequence of events for this rare preservation style. Following the dinosaur’s death, its body likely underwent a period of drying in the sun, leading to a degree of desiccation. Subsequently, the carcass was rapidly buried, most probably during sudden flash floods, which are common in riverine environments. Crucially, a microbial film developed on the outer surface of the carcass. This biofilm, through electrostatic forces, then attracted fine clay particles suspended in the surrounding wet sediment. This interaction resulted in the formation of a wafer-thin clay template that faithfully replicated the animal’s outer shape in three dimensions. Over extended geological timescales, the original soft tissues decayed away, leaving behind the clay film and the skeleton, which subsequently fossilized. This mechanism represents a new paradigm in taphonomy, offering insights into how such ephemeral structures can be preserved under very specific environmental conditions.
To unravel the intricacies of this extraordinary fossilization, the team employed a sophisticated suite of imaging and analytical approaches. These included high-resolution hospital and micro-CT scans, thin sections for microscopic analysis, X-ray spectroscopy to determine elemental composition, clay mineral analyses to identify the specific types of clays involved, and a detailed study of the rock layers where the fossils were found. Every line of evidence converged, supporting the proposed "clay templating" sequence.
From Fossil to Form: Reconstructing a Living Dinosaur
The journey from delicate clay impression to a fully fleshed-out digital reconstruction was an arduous yet rewarding endeavor. Fossil Lab manager Tyler Keillor, a co-author on the study, spearheaded the painstaking preparation process. Revealing a boundary as fragile as a paper-thin clay layer required immense patience and skill to uncover the crucial surfaces without causing irreparable damage.
Following preparation, another key group, led by postdoctoral scholar Evan Saitta, utilized advanced 3D surface imaging and CT scans. They meticulously traced the preserved soft anatomy, analyzed the sediments both inside and outside the mummy, and even matched the dinosaur’s hooves back to fossil footprints from the same time period. This multi-modal data was then handed to digital artists who, working in close collaboration with the scientists, created lifelike reconstructions. These digital models vividly portray how the Edmontosaurus would have appeared and moved, walking across the soft mudflats of the Late Cretaceous, offering a dynamic view of this ancient giant.
Anatomical Revelations: Crests, Spikes, Scales, and Thin Skin
The two newly described mummies complemented each other, allowing the researchers to assemble an unprecedented complete, fleshy outline of Edmontosaurus annectens. "For the first time, we could see the whole profile rather than scattered patches," Sereno remarked, highlighting the synergistic nature of the finds.
Among the most striking discoveries was a continuous feature running along the center of the dinosaur’s back. This began as a fleshy crest over the neck and torso, a feature long speculated but never definitively proven for Edmontosaurus. Over the hips, this crest seamlessly transitioned into a single row of prominent spikes along the tail, with each spike precisely aligned above a vertebra and fitting neatly with its neighbors. Such structures likely served purposes related to display, species recognition, or perhaps even defense against predators like Tyrannosaurus rex, which coexisted with Edmontosaurus.
The team also meticulously documented the dinosaur’s scale patterns. Contrary to some popular depictions, Edmontosaurus was not uniformly covered in large, reptilian scales. The largest polygonal scales were found along the lower body and tail, but surprisingly, the majority of the animal was covered in very small, pebble-like scales, measuring only 1-4 millimeters across. This is remarkably tiny for a dinosaur that could reach lengths exceeding 40 feet. Furthermore, fine wrinkles preserved over the ribcage indicated that the skin of this duckbill was relatively thin, suggesting a more supple and perhaps less armored integument than previously imagined for some large dinosaurs.
The Hoofed Dinosaur: A Revolutionary Discovery
Perhaps the most unexpected and significant discovery came from the larger mummy’s hind feet: Edmontosaurus had hooves. The tips of each of its three hind toes were enclosed in a wedge-shaped hoof with a flat underside, bearing a striking resemblance to the hooves of modern horses.
To confirm the appearance and function of these feet in life, the researchers combined CT scans of the mummified feet with 3D images of the best-preserved duckbill footprints from the same time period. By carefully aligning the bone structures with the soft-tissue impressions in the footprints, they produced a detailed and accurate reconstruction of the hind foot. Unlike the forefoot, which appeared to make contact with the ground primarily through its hooves, the hind feet also featured a fleshy heel pad located behind the hooves, providing additional support and cushioning.
Professor Sereno highlighted the revolutionary nature of this finding, enumerating several "firsts" preserved in these duck-billed mummies: "the earliest hooves documented in a land vertebrate, the first confirmed hooved reptile, and the first hooved four-legged animal with different forelimb and hindlimb posture." This discovery profoundly alters our understanding of dinosaur locomotion and evolutionary adaptations, suggesting that hooves, once thought to be primarily a mammalian innovation for cursorial locomotion, evolved convergently in other vertebrate lineages. The presence of hooves implies adaptations for navigating varied terrain, potentially including soft, muddy substrates, consistent with their riverine habitat.
A New Toolkit for Dinosaur Soft-Tissue Research and Future Implications
Beyond the striking anatomical revelations, this research provides a practical and robust framework for future studies of dinosaur soft tissue. The authors have outlined new preparation techniques specifically tailored for such delicate clay-templated fossils, established a clear set of terms for describing soft structures and scale types, presented a step-by-step imaging pathway from fossil specimen to fleshed-out digital model, and provided a detailed recipe for how a dinosaur mummy can form under natural conditions.
This comprehensive approach offers more than a series of isolated findings; it proposes a general model for dinosaur mummification based on "clay templating." This model can now be tested on other fossils that exhibit similar styles of preservation, potentially unlocking secrets from countless other ancient creatures. The implications for paleontological methodology are significant, offering new avenues for interpreting seemingly unremarkable fossil impressions.
The team also outlined crucial next steps for the field. These include focused searches for additional specimens with this kind of preservation in the same Wyoming rock layers and in other regions globally. Furthermore, the accurate external body outlines derived from this study will enable more precise biomechanical analyses, shedding light on how Edmontosaurus moved and interacted with its environment. Complementary analyses designed to understand the precise geochemical and environmental conditions when and where "clay templating" is most likely to occur will also be vital.
Professor Sereno, reflecting on the magnitude of the work, concluded, "This may be the single best paper I’ve released. From field to lab to 3D reconstructions along with a suite of useful terms defined, it’s a tour de force, and it tells a coherent story about how these remarkable fossils come to be and what we can learn from them." This sentiment underscores not only the scientific rigor but also the broader impact of the discovery, which promises to captivate the public imagination and inspire new generations of paleontologists. The timing of the publication, "just in time for Halloween," as Sereno noted, adds a touch of accessible wonder to a truly monumental scientific achievement, ensuring that the true face of Edmontosaurus will be remembered for years to come.