Leonardo Dinosaur Mummy

On July 20, 2000, a remarkable discovery came to light in the Judith River Formation in Montana. It dated back 77 million years, to the late Cretaceous Period. Discovered by staff and volunteers from the Judith River Foundation (JRF), the find was a subadult hadrosaur, a Brachylophosaurus canadensis. This was another of the so-called duck-billed dinosaurs, a plant eater about seven meters in length. Excavation would reveal that the skeleton was nearly complete and fully articulated. That means the bones remain after fossilization largely in the same relation to each other as they did in the animal's life.

Such articulated finds rightly gain media coverage and frequently command pride of place in museum displays, but they are not what paleontologists normally find in the field. Even partial or disarticulated specimens are rare. Isolated bones are the norm —a sampling of bones from a carcass that was largely devoured, torn apart, and scattered by predators and scavengers before final burial—or isolated bones that turn up here and there, having been carried along prehistoric rivers or by flood washes, leaving puzzle pieces that today are collected piecemeal. The find of even a single articulated limb can be a valuable discovery. More than half of an articulated dinosaur—either half!— is truly an occasion for celebration.

Excavation of the Judith River Formation discovery was carried out the following year, beginning in May 2001. Overburden above the skeleton was so extensive and resistant as to require a bulldozer, drilling, and blasting. When only a meter of sediment remained above the specimen, the team took up hand tools and began working their way down, employing pneumatic tools where the sandstone grew tough and concreted. As the find was gradually exposed, project leader and JRF paleontologist Nate L. Murphy and his team found that their Brachylophosaurus appeared to be not only articulated but almost complete, with only a section of its tail missing. Much more striking than this degree of completeness, however, was the fact that diggers kept uncovering what appeared to be skin impressions.

Ordinarily a dig team separates a fossil dinosaur skeleton into manageable portions for transport out of the field. Many bones may be individually isolated before being protected by plaster jackets and carried away. Smaller bones may be removed in groups for convenience. The separation of elements creates no special problems because the original orientations are all recorded carefully before anything is taken out of the ground. In the case of encountering fossil skin, however, one does not want to just chop through it in order to isolate separate pieces of the skeleton beneath. Fossil skin impressions are so precious that one wants to protect every square inch of them and sacrifice as little as possible. As they dug downward, Murphy's team kept hitting skin impressions here, there, and everywhere on their Brachylophosaurus. To preserve whatever traces might remain, they decided the whole beast had to be taken out of the ground in a single block. Only an unconnected tail sequence was removed separately.

To surround the large block in a protective field jacket required approximately a ton of plaster. To give strength to the package, it was surrounded by a sturdy steel framework of heavy square tubing. Altogether, the single mega-jacket weighed 6.5 tons. As we will discover later, Tyler's fossil gave us an even larger plaster of Paris bill!

As has become the custom ever since a Tyrannosaurus rex fossil was named Sue, the striking new find was soon given a name: Leonardo. The name was derived from that of two lovers who had carved their names in the rock face above the excavation site many years ago. Who said that paleontologists are not romantics? The fossil's soft-tissue preservation ratcheted up interest dramatically as lab preparation proceeded at the Judith River Institute. Leonardo appeared to be covered to a substantial degree with fossil skin. Murphy reported that it was present over an astonishing 90 percent of the preserved body, lacking only on part of the right forelimb and the right side of the skull. This degree of preservation indicated the presence of a feature that could not be detected from bones alone: a vertical comb or frill of skin that rose along the spine from the skull to the middle of the body. Whether the fossil skin represented any preservation of original biomolecules, or whether it might simply be impressions made by the original skin preserved as mold-and-cast and preserved as a trace fossil, the investigators have refrained from concluding either way. They have adopted the term "integument trace" to refer without commitment to what the popular press would call "fossil skin" or "skin impressions." Having seen many images of the fossil and spoken to Murphy at length, I can confidently state that Leonardo is a remarkable find. The key to all such finds is that the data be used in conjunction with other fossil finds, both old and new, to help fill gaps in our knowledge.

As reconstructed by Murphy and his colleagues, the body of their subadult Brachylophosaurus floated down a river headfirst until it hit a sandbar with its neck and right shoulder. Somehow the bulk of the animal's flesh appears to have been preserved in the fossil in the area where it initially impacted the sandbar. Most of the rest of the body appears to have lost its flesh before fos-silization, with the integument trace ending up tightly wrapped against the skeleton like the skin of a desiccated mummy. In the shoulder area, however, the fossil appears to reflect the geometry of the musculature of the living hadrosaur. The investigative team interprets the sedimentology and the preservation as indicating rapid deposition that covered over the carcass before it could be scavenged. What exactly accounts for the elimination of the internal flesh, and yet the preservation of the skin, in what was clearly a wet environment, which would ordinarily promote rapid and thorough decay, is not yet clear.

During preparation of Leonardo, a hole was accidentally made through the skin in the belly area. This hole revealed a darker mass within the area enclosed by the animal's ribs: a "lignaceous mass" of what appear to be gastric tract contents in the form of numerous chewed-up and partly digested plant fragments. These are reported as only 1-5 millimeters in size, presumably reflecting effective mastication by the hadrosaur's powerful tooth batteries. Because the mass of plant material appeared to be completely contained within continuous soft-tissue traces, the Leonardo investigation team concluded that it truly represents the gastric contents of the dinosaur, rather than the remains of plants that might have randomly collected in the corpse after the animal died. The Sternberg hadrosaur mummies had not offered the same kind of evidence in the stomach cavity, since their body cavities appeared to have been incompletely preserved. The investigative team for Leonardo concluded from their examination that the stomach cavity of the new mummy was, quite remarkably, still intact. This is a surprising state of preservation because under ordinary conditions an animal's stomach acids and other digestive juices begin acting to break down the animal's own gastric tissues very soon after it dies.The depositional conditions that would account for the apparently excellent condition of the digestive tract in Leonardo remain to be identified and explained.

A second hole was also inadvertently opened in the Leonardo fossil during preparation, this one in the pelvic area. The second opening revealed additional lignaceous material within the body, presumably from farther down the digestive tract. As one would anticipate with more fully digested plant material, the material here is more homogenous, and the fragments are smaller and much less identifiable. Its compressed aspect is consistent with the state of meal remains more completely processed by an animal's system.

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