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Soft tissue traces of the ankylosaur Tarchia. Black asterisks denote large osteoderms, scale impressions are pointed out by an arrowhead and small ossicles are identified by the arrow. From Arbour et al., 2012.

Ankylosaurs can be frustrating dinosaurs. In life, armor covered the bodies of these dinosaurs from snout to tail, but those bony adornments often fell out of place between the death and ultimate burial of the ankylosaurs. Reconstructing an ankylosaur, therefore, requires that paleontologists not only figure out the articulations of the bones but also the arrangement of the armor. Every now and then, though, researchers discover one of these dinosaurs with some armor still in place. According to an in-press Acta Palaeontologica Polonica paper, ankylosaur expert Victoria Arbour and colleagues have just identified one such specimen from the Late Cretaceous of Mongolia.

The dinosaur in question is most likely a specimen of Tarchia–an ankylosaur that could grow to about 26 feet long and, like many of its close relatives, carried a tail club. Rather than being a brand new find, though, this Tarchia was originally discovered in 1971 during the Polish-Mongolian Palaeontological Expedition and was sent to the Geological Museum in Oslo, Norway in 1998. Now, after over three decades, the dinosaur gets its time in the scientific spotlight.

What makes this Tarchia so significant isn’t the completeness of the skeleton. Only the left side of the back half of the body, including most of the tail, is preserved. What’s special is that parts of the dinosaur’s armor are still in place, including triangle-shaped bits of armor along the dinosaur’s slender tail and impressions of the tough sheaths that covered some of the armor in life. Indeed, the bony armor of dinosaurs was not exposed to the outside but was covered in a hard keratinous coating–horns, claws, plates and spikes were all covered in this, often making weapons sharper and ornaments more expansive.

While such soft tissue fossils are relatively rare, Arbour and her co-authors follow what paleontologist Phil Bell recently suggested on the basis of hadrosaur skin impressions–that preserved soft tissue impressions such as these might eventually be useful in distinguishing between different genera or species of dinosaur. In fact, this may be particularly important in cases like this exceptional ankylosaur. While the specimen is most similar to other specimens of Tarchia, it also differs in some minute tail characteristics. Are the differences the result of growth or individual variation, or could they be signs of a previously-unrecognized species? Detailed comparisons of skin impressions, in addition to skeletal differences, may help paleontologists winnow down the possibilities. We just need a better collection of ankylosaur soft tissue traces first.

Reference:

Arbour, V.M., Lech-Hernes, N.L., Guldberg, T.E., Hurum, J.H., and Currie P.J. (2012). An ankylosaurid dinosaur from Mongolia with in situ armour and keratinous scale impressions Acta Palaeontologica Polonica DOI: 10.4202/app.2011.0081

A gliding Stegosaurus. From the Ogden Standard-Examiner, 1920.

Stegosaurus is undoubtedly one of the most perplexing dinosaurs. What was all that iconic armor for? (And how did amorous stegosaurs get around that complication?) Paleontologists have been investigating and debating the function of Stegosaurus ornamentation for decades, but without much consensus. The dinosaur’s spectacular plates were certainly prominent visual signals, but could they also have been used for regulating body temperature? Or might there be some evolutionary impetus we’re not thinking of?

Of course, a few ideas have been tossed in the scientific wastebasket. Despite what 19th and early 20th century paleontologists thought, Stegosaurus plates were not protective armor. And, contrary to numerous restorations I saw as a child, Stegosaurus could not waggle or flap its plates around. But the weirdest idea of all was forwarded by paleontology enthusiast and writer W.H. Ballou in 1920. Stegosaurus plates were not armor, heat regulators, or flashy ornaments, Ballou wrote, but were wings that allowed the dinosaur to glide.

Ballou’s article appeared in the Utah’s Ogden Standard-Examiner. And, fortunately for fans of bizarre fossil ideas, a large illustration of flying Stegosaurus graces the piece. One stegosaur crouches to take off, another perches on a rock, and a third buzzes a prehistoric human. (Ballou pointed out in the article that humans originated after dinosaurs, but apparently the artist decided to take some historical license.) This ungainly and aerodynamically-challenged dinosaur, the paper said, was the “Father of All the Birds.” “Crude aeroplane or glider as the Stegosaur was, the principle of all flight was there in the parallel rows of flaps upon his back,” Ballou wrote, concluding, “Certainly he was the factory in which the first bird was built.”

There wasn’t any scientific evidence behind this. While Ballou mentioned the recent discovery of the lovely Stegosaurus skeleton now on display at Smithsonian’s National Museum of Natural History as the inspiration for the idea, the wild notion seems to have been entirely his. The vision of swooping stegosaurs isn’t attributed to any paleontological authority.

But Ballou did draw from a few references that offer a clue to his bizarre vision of gliding stegosaurs. Ballou pointed out that Stegosaurus was an ornithischian, or “bird-hipped” dinosaur. If Stegosaurus was bird-hipped, he reasoned, it must have been close to avian ancestry. Yet Ballou was confused by terminology. Despite having generally bird-like hips, the ornithischian dinosaurs—the hadrosaurs, ceratopsids, ankylosaurs, stegosaurs and others—were nowhere near the bird lineage. Their hip shape was a red herring, a case of superficial convergence. Ironically, the hips of birds were modified from an earlier “lizard-hipped” saurischian form. Ballou wasn’t the only one to be fooled by ornithischian hips—from the 1870s to the 1960s, some paleontologists thought that birds evolved from an ornithischian root—but he certainly ran with his mistaken assumption as far as he could possibly go.

Ballou wasn’t the only one taken with the dramatic idea. In a comment thread about the strange article at Dave Hone’s Archosaur Musings, paleontologist Mike Taylor points out that science fiction writer Edgar Rice Burroughs later imagined a flying stegosaur in one of his novels. In Burroughs’ world, Stegosaurus was a formidable aerial attacker that used its fearsome, thagomizer-tipped tail as a rudder, and it’s certainly possible that the ludicrous image was inspired by Ballou’s article. Sadly, though, Stegosaurus was less aerodynamic than a brick, so we shouldn’t expect any paleo documentary scenes of angry stegosaurs dive-bombing Allosaurus.

[Hat-tip to Dave Hone, by way of John Hutchinson and Jeff Martz.]

A reconstructed skeleton of Rapetosaurus on display at the Field Museum of Natural History in Chicago. Image by Lisa Andres, from Wikipedia.

Everyone knows the sauropod body plan: thin at one end, much thicker in the middle, and then thin again at the far end. Yet simply calling these dinosaurs “long necks” or focusing on their often enormous size doesn’t do justice to the diversity of forms within this group. Different sauropods had vacuum-shaped heads, whiplash tails, long bony spines jutting out of their necks, tail clubs and, among other things, armor. Regarding this latter feature, some sauropods within the titanosaur subgroup had bones embedded within their skin—called osteoderms—that would seem to have strengthened their hides against attack. According to a new Nature Communications report by paleontologist Kristina Curry Rogers and colleagues, however, an inside look at two such osteoderms yielded new evidence that these bones might have had a different function.

The pair of osteoderms that are the focus of the new study were found in association with two different specimens of Rapetosaurus, a titanosaur estimated to have reached an adult length of about 50 feet. These dinosaurs lived sometime between 70 million and 65 million years ago on what is now the island of Madagascar. One piece of armor was found next to the tail vertebrae of a juvenile individual. As seen in osteoderms of other animals, the bone had a dense outer layer surrounding spongy bone inside.

When the paleontologists used CT-scanning technology to look inside a larger, approximately 22-inch-long osteoderm found near the hips of an adult Rapetosaurus, however, they found something unusual. The inside of the osteoderm was mostly hollow. What’s more, the thickness of the outer layer of bone varied around the internal cavity, and the microscopic bone structure inside the osteoderm showed signs that bone was actually being resorbed by the body.

Maybe the osteoderms in the adult animals were not actually armor at all. A mostly hollow, relatively thin-walled bone is not exactly the sort of structure that is going to protect a sauropod from attack, especially since Curry Rogers and co-authors suggest that sauropods like Rapetosaurus were probably not fully covered in osteoderms, anyway. Instead, the paleontologists take the bone resorption within the larger osteoderm as a clue that these bones might have been mineral reservoirs for when times got tough or when egg-laying dinosaurs required extra calcium to give their a hard shell. While small Rapetosaurus might have had relatively solid osteoderms, adult individuals may have drawn upon the calcium and phosphorous in these bones to meet the demands of growing, reproducing, or living in an arid environment poor in such minerals. These dinosaur decorations may have had little to do with attack or defense.

References:

Curry Rogers, K., D’Emic, M., Rogers, R., Vickaryous, M., & Cagan, A. (2011). Sauropod dinosaur osteoderms from the Late Cretaceous of Madagascar Nature Communications, 2 DOI: 10.1038/ncomms1578

A restoration of the ankylosaur Sauropelta by John Conway. From Wikipedia.

Many dinosaurs were adorned with spikes, horns and plates, but it was the ankylosaurs that took armor to the extreme. These dinosaurs were covered in bony armor from snout to tail-tip, yet, as a new study suggests, there may have been more to some of these structures than just attack and defense.

As reviewed by paleontologists Shoji Hayashi, Kenneth Carpenter, Torsten Scheyer, Mahito Watabe and Daisuke Suzuki in the journal Acta Palaeontologica Polonica, the ankylosaurs can be subdivided into three smaller groups. There was the Polacanthidae (a group with large shoulder spikes and a “shield” over the hips), the Nodosauridae (forms with narrow heads and lacking tail-clubs) and the Ankylosauridae (the classic type with heavy armor over the body and tail-clubs). (There is some debate as to whether the Polacanthidae should be thought of as a distinct group, but since the authors separate it from the others I will follow their lead here.) Members of each group can be distinguished from each other on the basis of features which can be seen with the naked eye, but they are also different at the microscopic level. The arrangement of collagen fibers—one of the chief components of bone—differs in each group, as does the thickness of the bone composing the armor.

The differences in the bony armor of each kind of ankylosaur may help paleontologists determine to which group a specimen belongs based upon fragmentary material, but they may also indicate the different ways in which ankylosaurs used their armor. When the scientists looked at pieces of armor (including spikes and clubs) from several different dinosaurs across the three groups, they found that some of what might be thought to be weaponry was not well suited to the task. The outer layer of bone in the spikes of the polacanthids, for example, was relatively thin, especially in comparison to similar structures from the skeletons of the nodosaurids. This may mean that while the large spikes on the nodosaurids were sturdy enough to be used as weapons, the more fragile spikes of the polacanthids may have played a role primarily in display or regulating body temperature instead.

Additionally, the partial ankylosaurid tail club the researchers examined still showed signs of bone growth even though it appeared to have come from an adult animal. Combined with other recent findings, such as a possible lack of tail clubs among some juvenile ankylosaurids, this may mean that this structure developed later in life and was not initially used as a weapon. Perhaps, the authors hypothesize, developing tail clubs were used by juveniles and young adults for display, but it was not until later that the clubs could also be used for defense. Whatever they were doing, this study confirms that scientists are still learning much about dinosaurs by looking inside their bones.

Hayashi, S. (2010). Function and evolution of ankylosaur dermal armor Acta Palaeontologica Polonica DOI: 10.4202/app.2009.0103

Nodosaurus textilis, courtesy of WikiCommons

Even though museums all over the world are filled with dinosaur skeletons, it is very rare for paleontologists to find a complete, articulated specimen. Scraps and fragments of dinosaur bone are far more common, and often only the hardest parts of the skeleton become fosslized. In the case of the armored dinosaurs, the ankylosaurus, the plates, spikes, and bony knobs called tubercles are much more common than complete skeletons. A new study published in the Journal of Vertebrate Paleontology confirms that it is possible to use some of these tell-tale fossils to identify these dinosaurs.

While fossil hunting in the San Juan Basin of New Mexico in 1919, the paleontologist Charles Gilmore discovered a few bony scutes, fossils similar to the armor of other ankylosaurs. When the specimens were re-examined over 80 years later, they were thought to represent a new genus and species, Glyptodontopelta mimus, but could a new kind of dinosaur be established on the basis of just a few bits of armor? In the new study, paleontologist Michael Burns compared the Glyptodontopelta material to new armor fossils found from the same area to determine if they could be used to tell the difference between different ankylosaurs.

Burns’ analysis showed that both Gilmore’s fossils and the new ones did, in fact, belong to Glyptodontopelta, and that this dinosaur was a nodosaurid. (Nodosaurids were a kind of armored dinosaur that, unlike many ankylosaurids, lacked a bony tail club.) More importantly, however, by looking at the details of the texture of the armor (the pits, pores, and furrows that mark each specimen), Burns was able to reliably differentiate between genera and, in some cases, species. Since the armor of ankylosaurs is common in many Cretaceous deposits, the comparison of the armor fossils has the potential to detect the presence of new genera or even species of armored dinosaurs that are otherwise unknown.

Still, as Burns points out in the paper, paleontologists do not yet have a good idea how the armor grew on any one individual over time, and it may be possible to mistake the armor of a young individual (or variations in the armor of among individuals) for a new kind of dinosaur. This was not the case with Glyptodontopelta, however, as various remains matched each other to the exclusion of other types of ankylosaur known from more complete skeletons. Great care must be taken in such comparisons, but at least in this case, the existence of a new genus of nodosaurid was confirmed.