The excavation of the tail. The pickaxe gouges above the fossil attest to the density of the sandstone

Painstaikingly removing the matrix, one grain of sand at a time.

Close up of the preserved Dinosaur Skin

A DINOSAUR TAIL

 
A Dinosaur Tail With bone quality this nice, I wonder if we'll find preserved skin on this tail? With that thought in mind, we were very careful to remove and plaster-jacket the surrounding matrix along with the bones of the tail.
 
 
Rob Sula
 
When people ask me what I did over the summer, my answer for well over a decade has been the same: "I was digging dinosaurs!" I am the Senior Field Supervisor for Paleo Prospectors, a company that takes paleontology enthusiasts on tour of late Cretaceous fossil sites in Montana, Wyoming, Nebraska and North and South Dakota. I then incorporate what I've learned in the field into the paleontology classes that I teach at the College of DuPage in Glen Ellyn, Illinois.


As amazing and relatively rare as dinosaur bones are, if you go to the right places and know how to look for them, the fossils are abundant. Our team focuses primarily on the Hell Creek formation, which ranges from 65.5 to 67.2 million years from top to bottom. The Hell Creek formation is significant because it is one of the best geologic representations of the Cretaceous-Tertiary (K-T) boundary. In other words, it's perhaps the best place in the world to see the final days of that remarkable group of animals known as dinosaurs.



Dinosaurs are by no means the only group of animals found in the Hell Creek. A diverse fauna of at least 61 taxa of vertebrates that include fish, turtles, crocodilians, lizards, amphibians, pterosaurs, birds and mammals represent the formation. That being said, the headliners of this act are, of course, the dinosaurs. And not just any dinosaurs mind you—the Hell Creek hosts some of the world's most well-known dinosaurs including Triceratops, Edmontosaurus, Ankylosaurus, Pachycephalosaurus, and perhaps the most famous of all the dinosaurs, Tyrannosaurus rex.
Triceratops and Edmontosaurus are by far the most common dinosaur remains found in the Hell Creek. This makes sense as 66 million years ago there were herds of these animals that may have numbered in the thousands. Furthermore, these were both large and sturdy animals, in life probably weighing in the range of 4 to 12 tons. Therefore, the bones of these dinosaurs were robust enough to survive the ravages of time and erosion. As common as the fossils from these two dinosaurs are, their complete skeletons are rare. When we find one, the bones are often spread over large areas or stacked on top of one another in a complicated mess. These finds are described as associated. That is, the bones are from the same animal, but they are not in life position.



True articulation (the animal's bones laying neatly together as they were in life) is very rare in the Hell Creek. This is due to the turbulent nature of the environment that these animals lived in. 66 million years ago eastern Montana and the western extremes of the Dakotas were a series of river deltas flowing down eastward from the Rockies and feeding into the Western Interior Seaway that effectively bisected North America. Think of southern Mississippi today: lush, dynamic, and crisscrossed by river systems.



These rivers not only attracted the dinosaurs (providing them with fresh water and lush vegetation) but their ensuing floods and storm surges buried the animals rapidly enough to give them a chance to eventually become fossils. Unlike the gentle silting of lake bed deposits, which can often preserve articulated specimens, the more violent deposition of flood plains like the Hell Creek tend to scatter skeletons about. As a result what you typically find there are what appears to be isolated bones. The bones may in reality be part of associated skeletons, but the massive proportions and unpredictable patterns of these sites often make excavations impractical.



That's why our team was so excited when we found a perfectly articulated hadrosaur tail in 1999. With the exception of a few associated aspects of the pelvis, the tail was all that was left of the animal. There was not enough of the animal preserved to positively identify it to a species. Based on the length of the tail (23 feet), we project the overall body length of this hadrosaur to be over 40 feet. Based on that immense size, there was some speculation that this specimen is an Anatotitan ("large duck"). Some dispute the validity of this species believing all Anatotitans to be large examples of Edmontosaurus. This specimen may indeed be an exceptionally large Edmontosaurus but without anterior elements of the skeleton, we cannot be certain.



Every excavation is different and each one always presents its own unique engineering challenges. This hadrosaur tail was no exception. For starters, the tail was protruding out of a cliff facing over 100 feet to the ground. Secondly, the matrix that entombed the fossil was a sandstone that was harder than concrete. And finally the tail went directly into the cliff. It would have been very convenient if the tail had been at ground level, oriented parallel to a nice, soft clay or mudstone butte and (as long as we're fantasizing) next to a road. Naturally, we had no such luck. We essentially had to dig a 25-foot long tunnel directly into an extremely dense cliff in order to excavate this specimen. That all being said, we weren't complaining.



Any kind of articulation is a cause for celebration in the Hell Creek, and the bone quality of this specimen was exceptional.



One of my paleontological mantras is to never do preparation in the field. The purpose of an excavation is to collect data, document the site and safely remove and transport the specimen to the lab. Once delivered, the fossil can then be safely prepared in a controlled environment. As events unfolded, this turned out to be a good policy.



The excavation was challenging but satisfying. Once we plaster-jacketed the bones and surrounding matrix, we used ropes to raise them up to the top of the cliff. This was no small task considering the fact that some of these jackets weighed hundreds of pounds. Once safely to the top, we plastered 2x4s to the jackets and carried them out stretcher style over almost a mile of rough terrain to the jeeps.



Once the excavation was completed, my job was done and I made my way back to Chicagoland. A few weeks later I received a very odd message on my answering machine: "SKIN! THERE'S SKIN ALL OVER THIS TAIL!" It was my partner, the owner of Paleo Prospectors, Steve Nicklas. He had been preparing the tail in his lab in Georgia. What Steve had discovered was profound. I got in my car and began to drive south.



Soft tissue preservation like skin is the rarest of the rare in dinosaur paleontology. Of the few existing examples, most specimens of dinosaur skin are relatively small. Here was a 23-foot long tail and over 70% of it was covered in skin! At the time of its discovery, from the standpoint of square footage, this was the third largest amount of skin ever found on a dinosaur. Another thing worth mentioning is that most specimens of preserved dinosaur skin are "skin impressions." That is, a trace fossil formed when a dinosaur pressed its skin into some matrix (usually sand) and left a molded negative fossil copy of the skin (think of footprints.) The skin on our fossil is not an impression. The skin itself was mineralized and preserved which can clearly be seen in cross-section.



The skin itself superficially looks like hexagonal scales ranging in size from 1/8 inch to almost 1/2 inch in diameter. These "scales" are more accurately epidermal tubercles not dramatically different from our own skin when looked at under magnification. These tubercles provided the dinosaur armor-like strength as well as a reasonable amount of flexibility.



The quality of the preservation on this fossil is remarkable. Different locations on the tail have different kinds of tubercles. Some are thick and almost pebble-like while others are delicate and reminiscent of snake skin. There is even what appears to be keloidal scar tissue from and a serious injury that the hadrosaur had suffered and survived.



Preparation of the skin was a long, arduous process. The skin layer was so delicate that typical preparation methods such as pneumatic air scribes and even air abrasion would have blown right through it. Instead, once we got close to the skin layer, we had to remove the sandstone with scalpels and nylon brushes often one grain at a time.



I'm often asked how dinosaur skin can survive 66 million years. It's a legitimate question. Indeed, mineralization of even sturdy anatomical features like bones and teeth are somewhat miraculous; the idea that something as fragile as skin can be preserved seems impossible. But there it is, mineralized skin along with ossified tendons and bone.



The circumstances by which such soft tissue preservation can occur are extraordinary. First, as with an articulated specimen, there must be little or no scavenging prior to burial. The animal needs to be essentially intact when it is buried. Secondly, once it is buried, the environment must either be very arid or one that lacks enough oxygen to support decomposing organisms. Examples of such an environment would include a desert, the bed of a swamp, or the bottom of an ocean. None of these conditions matches what we believe Hell Creek looked like 66 million years ago. Considering the Hell Creek's river delta environment, the following is one theory on how this animal was preserved in such a pristine state.



The western extreme of late Cretaceous South Dakota was experiencing a significant drought. Whether it was due to this drought or to some other unknown reason, our hadrosaur died in a dried-up riverbed. The animals that would typically scavenge its carcass had migrated to find water or had also succumbed to the drought. In any case, our deceased dinosaur lay in an undisturbed state baking in the sun. The drought created a micro-arid climate in which microorganisms could not thrive. This resulted in only partial decomposition of our hadrosaur. Aspects of its body that were moisture-rich like eyes, muscle and internal organs decayed, while bone, tendon and even skin remained mostly undisturbed. As the animal desiccated, it lost volume and appeared to deflate. More importantly, the skin and connective tissues hardened. Now fully desiccated, the hadrosaur had a minimum of microbial activity within it and could be accurately described as mummified.



Then something in the environment dramatically changed. The hadrosaur was very rapidly covered in sand. Whether the drought ended abruptly and the hadrosaur was buried in a flash flood or the carcass was buried by some other means is unclear. What is certain is that this dried and hardened hadrosaur was buried rapidly in a low oxygen



environment resulting in almost no further decomposition. Over the next several million years this unlikely combination of events resulted in the mineralization of not only the dinosaur's bones but also its connective tissue and skin.



Paleontologists are always looking for more information. This amazing fossil can teach us things about hadrosaurs that bones alone could never do. We can actually see how the ossified tendons supported the weight of the tail confirming that hadrosaurs did indeed carry their tails off the ground. We do not have to speculate about what hadrosaur skin looks like and as a result we have a much better idea what the living animal looked like. Specimens like this can even shed some light on that most mysterious aspect of paleontology . . . the behavior of extinct animals. Is the scar a healed wound from a large carnivore bite? If so, what does this say about Tyrannosaurus rex being a scavenger?



Some people ask me why I go digging for dinosaurs for two months every summer. I tell them that it's a rescue mission. If we had discovered that hadrosaur a few decades or perhaps only a few years earlier, we may have found more of the animal's body. As things turned out, the majority of the hadrosaur's body (which was most likely also covered by skin) weathered out of the cliff facing, crashed down to the bottom of the valley and eventually eroded into dust.



I wonder what we'll rescue next summer?