By Jon Tennant
Hadrosaur bones, courtesy U.S. Bureau of Land Management. Fossils, as we typically think of them, tell us about the death of an animal. The teeth, bones, shells, fragmented pseudopods and other weird and wonderful bits of carcass all only ever reflect one thing: a permanent geological limbo. These types of fossil are known as body fossils. The other major group of fossils, that are generally less common, less researched, less known about, but arguably more important for guiding our understanding of the history of life on Earth, are trace fossils. The study of trace fossils is called ichnology, and the fossils don’t represent death; they represent life, behavior, activity. Often trace fossils are actually found in or on body fossils---anything from boring holes from bivalves on other bivalves, to bite marks on bones detailing a poor creature's last painful seconds as a living animal. Or, in this case, an ancient injury preserved on a dinosaur's skin. Dinosaur skin is one of the rarest treasures the paleontological record can reveal to us. We now have a pretty good idea about the texture and nature of dinosaur skin, thanks to a couple of exceptional "dinosaur mummies" and the occasional fragments of skin preserved not as a mold of skin impressed on the surrounding sediment but of the actual fleshy flesh. But even rarer than these spectacular glimpses are bits of dinosaur skin with trace fossils on them. In fact the history of the study of traces on dinosaur skin only goes back to 2008, when it was first noticed that a ceratopsian dinosaur, Psittacosaurus, had fossilized skin that showed signs of substantial trauma; that is, getting nommed on by a meat-eating dinosaur during predation. The study, however, missed half the story; there was no demonstration that the dinosaur was alive at the time of trauma, so it could have been a scavenging trace.
A patch of fossil skin showing a 1.3-inch long oblong area of healed skin trauma. Now evidence of trauma followed by healing has emerged, published in Cretaceous Research. This fossilized skin of the skull of hadrosaur Edmontosaurus annectens is more convincing evidence for a predator attack which the prey dinosaur survived. What the trace fossil reveals to us, then, is the story of a failed predation attempt on a dinosaur! Given the time and place (latest Cretaceous, Hell Creek Formation [formations are a type of geological unit], USA), the only likely candidate, based on the spacing of the tooth-induced traumas, would be the notorious Tyrannosaurus rex! Maybe big old T. rex wasn’t such a badass hunting machine after all if it sometimes couldn’t even take down a wimpy little hadrosaur. The scale-like structures you see on dinosaur skin are known as called tubercles, and resemble the polygonal desiccation cracks that you might see on a dried up mud flat (because we all investigate sedimentary structures when we’re out, right..?) On one particular specimen though, this normal pattern has been disrupted by penetration marks and replaced with tissue where the skin has healed itself over time before fossilisation. Woah. There are two macro-structural indicators that this is a healed flesh wound. Firstly, there are a whole bunch of wrinkles radiating out from the amorphous center of impact. Secondly, the surrounding tubercles to the impact scar are smaller than the others, and arranged in a more chaotic manner, reflecting the odd way in which the skin healed itself. Both of these are characteristic features of healing in modern reptile skin, which is known to heal at a slower rate than mammals' skin. As such, these structures are strongly suggestive of a failed predation attempt, in which the lucky hadrosaur has managed to scuttle away to live another day. This study is actually a pretty cool one in illustrating both the scientific and communicative value of trace fossils. If this were just plain old boring (cough) dinosaur skin, we’d be able to tell, well, about dinosaur skin. Instead, due to the traces of dinosaur behavior present, we’re able to conjure up the image of a T. rex and Edmontosaurus having a playground scrap, and then for some reason (speculate away!), the Edmontosaurus manages to leave with just a slight scratch and go back to spending all day eating---and poor T. rex is forced to either go hungry or vegetarian.
Jon Tennant is currently undertaking a PhD in vertebrate macroevolution in London. He co-hosts a podcast series called Palaeocast and tweets as @protohedgehog. This post originally appeared on his blog Green Tea and Velociraptors.