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This famous Edmontosaurus skeleton was found with intricate traces of skin over much of its body. Image in Osborn, 1916, from Wikipedia.

Last week, I wrote about attempts by paleontologist Phil Bell and colleagues to extract biological secrets from fossilized traces of dinosaur skin. Among the questions the study might help answer is why so many hadrosaurs are found with remnants of their soft tissue intact. Specimens from almost every dinosaur subgroup have been found with some kind of soft tissue preservation, yet, out of all these, the shovel-beaked hadrosaurs of the Late Cretaceous are found with skin impressions and casts most often. Why?

Yale University graduate student Matt Davis has taken a stab at the mystery in an in-press Acta Paleontologica Polonica paper. Previously researchers have proposed that the abundance of hadrosaur skin remnants is attributable to large hadrosaur populations (the more hadrosaurs there were, the more likely their skin might be preserved), the habits of the dinosaurs (perhaps they lived in environments where fine-resolution fossilization was more likely) or some internal factor that made their skin more resilient after burial. to examine these ideas, Davis compiled a database of dinosaur skin traces to see if there was any pattern consistent with these ideas.

According to Davis, the large collection of hadrosaur skin fossils isn’t attributable to their population sizes or to death in a particular kind of environment. The horned ceratopsid dinosaurs–namely Triceratops–were even more numerous on the latest Cretaceous landscape, yet we don’t have as many skin fossils from them. And hadrosaur skin impressions have been found in several different kinds of rock, meaning that the intricate fossilization occurred in multiple types of settings and not just sandy river channels. While Davis doesn’t speculate about what made hadrosaurs so different, he proposes that their skin might have been thicker or otherwise more resistant than that of other dinosaurs. A sturdy hide might have offered the dinosaurs protection from injury in life and survived into the fossil record after death.

Still, I have to wonder if there was something about the behavior or ecology of hadrosaurs that drew them to environments where there was a greater chance of rapid burial (regardless of whether the sediment was sandy, silty or muddy). And the trouble with ceratopsids is that they have historically been head-hunted. Is it possible that we’ve missed a number of ceratopsid skin traces because paleontologists have often collected skulls rather than whole skeletons? The few ceratopsid skin fossils found so far indicate that they, too, had thick hides ornamented with large, scale-like structures. Were such tough-looking dinosaur hides really weaker than they appear, or is something else at play? Hadrosaurs may very well have had extra-sturdy skin, but the trick is testing whether that characteristic really accounts for the many hadrosaur skin patches resting in museum collections.

Reference:

Davis, M. 2012. Census of dinosaur skin reveals lithology may not be the most important factor in increased preservation of hadrosaurid skin. Acta Paleontologica Polonica http://dx.doi.org/10.4202/app.2012.0077

Osborn, H. 1916. Integument of the iguanodon dinosaur Trachodon. Memoirs of the American Museum of Natural History. 1, 2: 33-54

Sternberg, C.M. 1925. Integument of Chasmosaurus belli. The Canadian Field Naturalist. XXXIX, 5: 108-110

Dinosaur reconstructions often begin and end with bones. Dinosaur muscles and organs usually don’t survive the processes that turn bodies into fossils, with casts of the intestinal tract–called cololites–and other soft tissue clues being rarities. Restoration of those squishy bits relies on comparison with modern animals, muscle scars on bones and other lines of evidence. Yet paleontologists have found a great deal of dinosaur skin impressions, especially from the shovel-beaked hadrosaurs of the Cretaceous. We probably know more about the actual external appearance of hadrosaurs such as Edmontosaurus and Saurolophus than almost any other dinosaurs.

Hadrosaurs found with skin impressions are often called “mummies.” This isn’t quite right. Natural mummies–human and otherwise–preserve the organism’s actual skin due to any number of environmental conditions, from arid heat to extreme cold or preservation in a bog. What we know of hadrosaur skin isn’t the original organic material that made up the dinosaur’s flesh, but rock that has made a mold or cast of the dinosaur’s pebbly outer coating. Terminology aside, though, paleontologists have found enough dinosaur skin impressions that the fossils can be used to detect different ornamentation patterns and may even help distinguish one species from another. Earlier this year, paleontologist Phil Bell demonstrated that two Saurolophus species exhibited different patterns on their bumpy skins–an additional kind of ornamentation aside from their prominent head crests.

But how do skin impressions became preserved? And why are such traces so often found with hadrosaurs but not other dinosaurs? Is it because hadrosaurs frequented environments where such preservation was more likely, or are we just missing similar impressions associated with other fossils? There’s much about dinosaur skin impressions that we don’t yet understand. In the video above, Bell gives us a preview of new research on a recently collected hadrosaur that has skin traces, in the hope that some high-tech analysis will help him better understand how such fossils form.

Hadrosaurs have often been called “duck-billed dinosaurs.” You don’t have to look at their skulls for very long to see this analogy is wide of the mark. Not only did hadrosaurs such as Edmontosaurus have shovel-shaped, grooved beaks, but their jaws were lined with rows of cropping, crushing teeth. These dinosaurs didn’t dabble in Cretaceous swamps – they grazed the prehistoric plains. And, up until recently, it was thought that these huge herbivores possessed an evolutionary innovation that made them the dinosaurian equivalent to cows.

In general, dinosaur jaws and teeth were for cutting, plucking, and tearing. Dinosaurs didn’t chew their food, but instead ripped or clipped their morsels, which were then swallowed whole. (Strange as it may seem, this style of eating might have had a role to play in why sauropods were able to maintain such large body sizes.) But hardosaurs were thought to be different.

The idea I encountered as a kid was that when hadrosaurs such as Edmontosaurus opened their jaws, the tooth-bearing bones of their upper jaws – the maxillae – swung inwards. Then, when the lower jaws came back up, the lower teeth met the upper teeth and ground the plant food across the tooth surfaces. This wasn’t chewing like mammalian herbivores do it, but it was an evolutionary alternative that allowed hadrosaurs to better break down their food before swallowing. You can see a visualization of this hypothesis in action in this YouTube video.

But this model of hadrosaur chewing required a great deal of flexibility in the skull to create a complex chewing motion. As a video uploaded by the Canadian Museum of Nature – posted above – shows, hadrosaur jaw movements were probably a great deal simpler. The key to the puzzle is the interlocking group of small bones at the back of the skull. When the virtual Edmontosaurus drops its lower jaw, the movement compresses some of these bones at the back of the skull, which moves the upper tooth rows slightly inward. But the lower jaw doesn’t just drop – a joint at the back of the mandible also allows the lower jaw to extend forward. When the jaws close, the lower jaw moves back in a diagonal motion, and the contact of the upper and lower teeth gently push the maxilla slightly outwards. There is still a lot of movement in the skull, but it’s not quite as dramatic as the swingin’ maxilla version. And this goes to show just how much we still have to learn about dinosaurs. Even though we know more about Edmontosaurus and its kin than ever before, the basics of dinosaur biology remain rich grounds for investigation and debate.

[Hat-tip to Thomas Holtz for sharing this video on Twitter.]

A reconstruction of the Edmontosaurus skull LACM 23502, with a beak based on a natural mold. From Morris, 1970.

I’ve never liked the term “duck-billed dinosaur.” I know it’s part of the accepted dinosaur lexicon, just like “raptor” is, but every time I hear the phrase I think of a sluggish, swamp-bound Edmontosaurus dabbling in the water for soft water plants and algae. Paleontologists tossed out this imagery decades ago—hadrosaurs were terrestrial creatures with jaws specially adapted to grinding down tough vegetation.

I admit that the skull of Edmontosaurus looks superficially duck-like. Much like a mallard’s, the Late Cretaceous hadrosaur’s mouth is long, low and generally bill-shaped. The resemblance between these very, very distant relatives helped inspire images of wading hadrosaurs. But most Edmontosaurus skulls you see in museums present only the bony framework of the skull. The tough keratinous beak that tipped the skull typically decayed during the fossilization process, but in 1970, paleontologist William Morris described a rare Edmontosaurus skull with a beak trace.

You can see the specimen on display at the Natural History Museum of Los Angeles today. Designated LACM 23502, this Edmontosaurus skull was collected by Harley Garbani near Montana’s Ft. Peck Reservoir. Other Edmontosaurus have been found here, but this fossil included a natural mold of the dinosaur’s beak. (While the beak itself was not preserved, the mold showed what the internal surface looked like. In life, the actual beak sat on top of the fossilized mold.) The structure was not shaped just like a duck’s bill. On the bottom jaw, the beak surface curved slightly upward, and the upper half of the beak created a vertical, fluted surface that hung over the tip of the lower jaw. Maybe the term isn’t the most apt—and I’m open to suggestions—but Edmontosaurus seemed to be a shovel-beaked dinosaur rather than a duck-billed one.

At the time Morris described the skull, though, hadrosaurs were still thought to be semi-aquatic dinosaurs. Morris believed that the bill traces he described supported this idea and imagined that ridges on the interior part of the mold helped the dinosaurs strain plants and small invertebrates from the water. “A filtering device would be very important in assuring that these large animals could ingest large amounts of concentrated food relatively free of water in a manner similar to that of the dabbler ducks,” Morris wrote, which made the term “duck-bill” seem all the more apt for these dinosaurs.

Despite Morris’ insistence that hadrosaurs nourished themselves by slurping plant-heavy Cretaceous soup, though, we now know that Edmontosaurus and kin were terrestrial animals capable of breaking down tougher plant materials. Exactly how the beak of Edmontosaurus contributed to feeding is not entirely clear—perhaps the beak cropped vegetation that was broken down by the rows of small teeth lining the jaws. One thing is for sure, though. The duck-bills weren’t really so duck-like after all.

Reference:

Morris, William J. (1970). “Hadrosaurian dinosaur bills — morphology and function“. Contributions in Science (Los Angeles County Museum of Natural History) 193: 1–14.

Bones from the foot of a hadrosaur attributed to Edmontosaurus annectens. Modified from Zheng et al., 2011.

Sometimes, hadrosaurs can be a real pain. Even though they are some of the most abundant dinosaurs among Late Cretaceous fossil sites, and are therefore an excellent resource for investigating the biology of dinosaurs, the fact is that there are far more isolated bits and pieces of them than complete skeletons. Properly identifying and cataloging these single bones can be difficult—you need a comprehensive knowledge of dinosaur anatomy to know what a lonely bone once belonged to. Now students Rachel Zheng and Gy-Su Kim from southern California’s Webb Schools and paleontologist Andy Farke have taken a step towards offering their colleagues a way to recognize isolated bones from hadrosaurine dinosaurs.

Zheng, Farke, and Kim have just published a hadrosaur atlas in PalArch’s Journal of Vertebrate Palaeontology. Their aim was to fill in a gap in the literature. Even though lots of hadrosaurs have previously been described, seemingly no one had published a detailed, illustrated guide to the hadrosaur foot. To remedy this, the researchers decided to compose a detailed description of the well-preserved foot of a specimen tentatively attributed to the common Late Cretaceous hadrosaur Edmontosaurus annectens. With this atlas to the hadrosaur foot, they propose, other researchers and collections managers may be better able to properly identify hadrosaur foot bones, especially if those researchers don’t already have a reference collection to make comparisons with.

Frustratingly, the precise identity of the dinosaur used to create the atlas is uncertain. Hadrosaurs are notoriously difficult to identify without their skulls, and the specimen in question was missing one. Nevertheless, a combination of anatomical and geological detail allow Zheng, Farke and Kim to hypothesize that the dinosaur in their atlas is an Edmontosaurus annectens. Along with the foot and other bones, part of the right hip (the ischium) of the dinosaur was found. The distal tip of this hip bone is narrow, and this feature identifies the dinosaur as belonging to the hadrosaurine lineage of hadrosaurs. (The other major hadrosaur lineage—the ornately-crested lambeosaurines—had a flared ischium tip.) Since Edmontosaurus annectens is the only hadrosaurine dinosaur known from the Hell Creek strata where this specimen was uncovered, the identification is the most reasonable one on the basis of the material at hand.

The bulk of the paper consists of labeled color photographs of the hadrosaur’s foot from different angle. This is not the super-sexy kind of research that’s going to end up in Nature or Science. That’s a good thing. Some of the biggest gaps in our understanding about dinosaurs involve relatively simple things. There is a definite need for detailed descriptions and comprehensive atlases that will allow other researchers to easily compare and identify different dinosaurs. I love paleobiology and wondering about the lives of dinosaurs as much as anybody else, but in order to generate hypotheses we need a solid foundation of descriptive analysis. I certainly hope that other researchers take the time to go through their own collections, identify well-preserved specimens, and create similar guides so that various mystery bits scattered through museums can be better identified and cataloged.

References:

Zheng, R.; Farke, A.; Kim, G. (2011). A Photographic Atlas of the Pes from a Hadrosaurine Hadrosaurid Dinosaur PalArch’s Journal of Vertebrate Palaeontology, 8 (7), 1-12