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The dinosaur William Parks described as Dyoplosaurus, showing where the bones would have fit on the actual animal. From Arbour et al., 2009.

If I started this Dinosaur Alphabet series just a few years ago, I wouldn’t have included Dyoplosaurus. Up until 2009, the dinosaur was hiding within another genus of heavily-armored ankylosaur. But after decades of discovery and debate, Dyoplosaurus is back, and the Cretaceous club-tail has its own role to play in wider discussions about the tempo and mode of dinosaur evolution.

Canadian paleontologist William Parks named the ankylosaur in 1924. Just a few field seasons earlier, in 1920, a University of Toronto crew found the partial skeleton of an armored dinosaur in the Late Cretaceous rock along the Red Deer River in Alberta. “The anterior part of the skeleton had been long exposed and had suffered in consequence,” Parks later wrote, but the team was still able to collect part of the skull, some tooth fragments, ribs and, best of all, the articulated hip and tail. Some of the armor remained in place, and the preservation was delicate enough to include skin impressions and the long ossified tendons that helped support the ankylosaur’s tail. If only the front half had remained intact!

This partial skeleton wasn’t the first ankylosaur to be found in the Late Cretaceous of North America. But, Parks wrote in his report, the animal’s tail club was “distinctly different from any previously described and, as far as I am aware, from any that have been collected.” Based on this slender oval of bone and other features, Parks distinguished the skeleton as Dyoplosaurus acutosquameus. And while the front half of the animal was almost entirely missing, the detail of the back half gave paleontologists a detailed look at how the armor, bones and tendons of ankylosaurids were arranged.

Then researchers sunk Dyoplosaurus. In 1971, in a huge revision of the ankylosaurs, paleontologist Walter Coombs proposed that Dyoplosaurus was not so unique as Parks had proposed. A jaw fragment found with the original Dyoplosaurus specimen was virtually identical to part of a jaw referred to the more famous armored dinosaur Euoplocephalus, Coombs wrote, and therefore Parks’ dinosaur should be considered a Euoplocephalus.

Since this other ankylosaur was named on the basis of even more fragmentary material, the addition of the “Dyoplosaurus” specimen gave paleontologists a new reference for what the hips, tail, and armor of Euoplocephalus looked like. More than that, the find extended the range of Euoplocephalus through Alberta’s Late Cretaceous rock. The “Dyoplosaurus” material was found in a roughly 76-million-year-old park of the Dinosaur Park Formation, and bones referred to Euoplocephalus had also been found in the geologically younger Horseshoe Canyon Formation. Altogether, Euoplocephalus seemed to persist for almost ten million years–quite a feat given that many neighboring genera and species of dinosaur came and went during the same span of time.

As paleontologists found additional ankylosaurs and compared previously discovered material, though, it became apparent that Euoplocephalus had become an osteological umbrella that was hiding more than one dinosaur genus. Indeed, since the original Euoplocephalus material consisted of a partial skull and a half ring or neck armor, it was difficult for paleontologists to compare and accurately refer specimens when there was a lack of overlapping material. As researchers investigated more complete material that was undeniably Euoplocephalus, it became apparent that other specimens from a wide range of time and displaying a wide range of variation had been incorrectly assigned to this dinosaur. Among the incorrectly lumped dinosaurs was Dyoplosaurus.

Ankylosaur expert Victoria Arbour and her colleagues resurrected Parks’ ankylosaur in 2009. While the anatomy of the animal’s skull fragment wasn’t easily distinguishable from the original Euoplocephalus fossils, details of the hips and vertebrae, especially in the tail, distinguished Dyoplosaurus from all other ankylosaurs. From the hips back, Dyoplosaurus was a distinct dinosaur.

Despite what Parks had written, though, Arbour and her coauthors cautioned that the tail club of Dyoplosaurus was not an easy-to-spot difference. As far as paleontologists know now, ankylosaurid dinosaurs were not born with tail clubs. The osteoderms that formed the bludgeon grew later in life, and, since Parks’ Dyoplosaurus specimen was relatively small compared to Euoplocephalus specimens, it’s possible that the dinosaur’s tail club had not finished growing. When comparing dinosaurs, it’s always important to  keep the animal’s stage of development in mind–features that may seem to characterize a new species may only indicate immaturity.

Other ankylosaurs are probably hiding within Euoplocephalus. Properly identifying and categorizing them will take years. Studies of hadrosaurs, ceratopsians, tyrannosaurs and other dinosaurs have shown that Late Cretaceous dinosaurs on the western subcontinent of Laramidia–isolated from their eastern cousins by the vanished Western Interior Seaway–that the genera and species differed along the latitudes. Rather than finding the same dinosaurs from Alberta to Utah, paleontologists have found distinct assemblages of dinosaurs that belie isolated evolutionary pockets. And analyses of Canada’s Late Cretaceous species have tracked turnover patterns among the dinosaurs, timing the pulse of evolution and extinction. Splitting out Dyoplosaurus is one more step towards understanding what North America’s dinosaurs can tell us about how evolution works.

Want to know more about other unsung dinosaurs? Check out previous entries in the Dinosaur Alphabet.

References:

Arbour, V. Burns, M. Sissons, R. 2009. A redescription of the ankylosaurid dinosaur Dyoplosaurus acutosquameus Parks, 1924 (Ornithischia: Ankylosauria) and a revision of the genus. Journal of Vertebrate Paleontology 29, 4: 1117–1135. doi:10.1671/039.029.0405

Parks, W. 1924. Dyoplosaurus acutosquameus, a new genus and species of armored dinosaur; and notes on a skeleton of Prosaurolophus maximus. University of Toronto Studies Geological Series 18: 1–35.

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