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In this restoration by Emiliano Troco, a Sauroniops feeds on a juvenile Spinosaurus. (And yes, all the dinosaurs in this image are fluffy.) Image courtesy Andrea Cau.

Earlier this year, paleontologists Andrea Cau, Fabio Dalla Vecchia and Matteo Fabbri described a strange, 95-million-year-old skull scrap from an unknown dinosaur. Acquired by a commercial collector from Morocco’s Kem Kem beds and later donated to Italy’s Museo Paleontologico di Montevarchi, the bone showed signs that it belonged to a carcharodontosaurid–massive cousins of the familiar Allosaurus. There was something odd about the fossil. The bone was a frontal–situated at the top of the skull just above and in front of the dinosaur’s eye opening–but, unlike the same bone in related species like Carcharodontosaurus, a small dome protruded from the middle of the specimen. No caracharodontosaurid has been found with a dome before.

The skull fragment of Sauroniops, showing where it would have fit on a carcharodontosaur skull (human skull for scale). Image courtesy Andrea Cau.

While a single piece of skull isn’t much to go on, Cau and colleagues nevertheless were able draw on the dome and other subtle features to determine that the frontal didn’t belong to any previously known dinosaur. Still, at the conclusion of their brief Acta Palaeontologica Polonica report, the scientists cautioned against naming a new species from an isolated skull bone. “Although the combination of features present in [the frontal] is unique and should support the institution of a new species,” Cau and coauthors concluded, “pending more complete specimens we feel it would be inappropriate to erect a new taxon.”

Cau, Dalla Vecchia and Fabbri quickly changed their minds. While the rest of the dinosaur remains unknown, after reanalyzing the frontal the paleontologists decided that it was truly unique enough to merit establishing a new name. The subtly-domed dinosaur is now known as Sauroniops pachytholus–the genus name a tribute to the demonic Sauron of the Lord of the Rings series, and the species name for the thick dome on the dinosaur’s head.

I emailed Cau to ask why he changed his mind about the dinosaur so quickly. During the year between the time the two papers were submitted, Cau replied, several papers were published showing that carcharodontosaurids–such as the high-spined Acrocanthosaurus from North America–had frontal bones that were so distinct that they could be use to tell one theropod genus from another. That inspired Cau to take another look at the domed specimen from Morocco.

Ultimately, Cau wrote, “the collected data showed that the unique morphology of our specimen was as diagnostic as those available from the type specimens of other African carcharodontosaurids (e.g., the holotypes of Eocarcharia [a single postorbital bone], Carcharodontosaurus iguidensis [a single maxilla], Veterupristisaurus [a single caudal vertebra]).” If all these dinosaurs were based on isolated bones, Cau explained, “then there are no real objections for erecting Sauroniops even from a single frontal.”

Frustratingly, though, the limited material means that we only have the barest outline of what Sauroniops was like in life. The size of the frontal, compared to the bone in other carcharodontosaurs, indicates that the dinosaur probably exceeded thirty feet in length. The carnivore was probably just as big as the better-known Carcharodontosaurus, which it lived alongside, but such estimates always await the test of more fossils.

And then there’s the dome. Why did such a large theropod have a prominent bump on its head? In other theropod lineages, such as the abelisaurids, bumps, knobs and horns are common forms of ornamentation. Perhaps the same was true for Sauroniops–thanks to Acrocanthosaurus and the sail-backed Concavenator, we know that carcharodontosaurs showed off with visual signals. Then again, Cau and coauthors speculate that the dome might have been a sexual signal or might have even been used in head-butting behavior. I think the last hypothesis is unlikely, especially since we don’t know what the microstructure of the dome looks like and there’s no evidence of pathology, but it’s still a distant possibility.

So Sauroniops has a name and a family. Like its cousins Kelmayisaurus and Shaochilong, though, we don’t know very much about this dinosaur’s appearance or biology. The lone frontal is a tantalizing glimpse at a dinosaur that paleontologists will have to hunt down in the deserts of Morocco. With some luck, and a lot of persistence, we may eventually become better acquainted with the dome-skulled dinosaur.

For more on this discovery, see Cau’s blog post at Theropoda.

References:

Cau, A., Dalla Vecchia, F., Fabbri, M. 2012. Evidence of a new carcharodontosaurid from the Upper Cretaceous of Morocco. Acta Palaeontologica Polonica 57, 3. 661-665

Cau, A., Dalla Vecchia, F., Fabbri, M. 2012. A thick-skulled theropod (Dinosauria, Saurischia) from the Upper Cretaceous of Morocco with implications for carcharodontosaurid cranial evolution. Cretaceous Research, in press. DOI: 10.1016/j.cretres.2012.09.002

A headless Haplocanthosaurus, laid out at the Utah Field House of Natural History. Photo by the author.

The Morrison Formation is one of the most wonderful slices of prehistoric time found anywhere in the world. Parts of this Late Jurassic record pop up all over the American west, from Montana to Texas, and the sequence harbors wonderful bonebeds such as those at Dinosaur National Monument, Utah, and Bone Cabin Quarry, Wyoming. Yet, while the upper part of the Morrison has yielded splendid specimens of famous dinosaurs such as Apatosaurus, Stegosaurus, Allosaurus and more, the lower part of the formation contains a gaggle of puzzling dinosaurs. Haplocanthosaurus is one of these enigmas.

When discussing any geologic formation, it’s easy to talk about it as if it’s just a narrow slice of time. Yet distinct formations can record many millions of years of evolution and extinction. The Morrison Formation, for one, records about 10 million years of Jurassic history, from about 156 to 146 million years ago. And the dinosaurs paleontologists find near the top are not the same as the ones they found lower down in the formation.

Haplocanthosaurus, one of the long-necked sauropods, was part of the lower Morrison fauna. The 50-foot herbivore trod the Jurassic landscape around 155 million years ago and lived alongside the equally unfamiliar forerunners of famous dinosaurs. The stegosaur Hesperosaurus, the slender Allosaurus “jimmadseni” and hefty Eobrontosaurus also lived during this earlier portion of Morrison time.

Despite the fact that the dinosaur was named in 1903, however, paleontologists are still confounded by Haplocanthosaurus. The mid-sized sauropod appears to have been a close relative of the extremely common, blunt-headed dinosaur Camarasaurus. Frustratingly, though, Haplocanthosaurus is extremely rare, and no one has found the dinosaur’s skull just yet. With a skull, the dinosaur’s relationships and biology would come into sharper focus, but no such luck.

Haplocanthosaurus is a symbol of how much we still have to learn about even long-known dinosaurs. The lower part of the Morrison Formation, in particular, seems to be filled with strange dinosaurs that may offer clues about how the exceptionally rich fauna of the later Morrison–filled with sauropods and knife-toothed predators–evolved. Were Hesperosaurus, Eobrontosaurus, Allosaurus “jimmadseni” and Haplocanthosaurus ancestral to any of the later forms? Or did they fall away as new species migrated into the same habitats from elsewhere? The depths of the Morrison Formation still hold Jurassic mysteries worth investigating.

A pair of Stegoceras on display at the Royal Tyrrell Museum, Alberta, Canada. Photo by Sebastian Bergmann, image from Wikipedia.

The history of pachycephalosaurs is mostly a story of domes. Even though some skeletons have been uncovered over the years, the most commonly-found part of these bipedal Cretaceous herbivores is the thickened, decorated skull. As a result, much of what we know about these dinosaurs comes from skull fragments, and this can sometimes seed confusion about which fossils represent new species and which are individuals of already-known dinosaurs.

Take the partial pachycephalosaur skull UCMP 130051, for example. In 1990, paleontologist Mark Goodwin described the skull–discovered in the Judith River Formation of Montana–as an adult of the previously-known dinosaur Stegoceras. The skull was large for a Stegoceras, and lacked the array of nodes commonly seen on the back shelf of the skull but was otherwise matched the anatomy of the common pachycephalosaur. But when paleontologist Robert Sullivan wrote a review of known Stegoceras material in 2003, he thought that UCMP 130051 was distinct enough that it belonged to a new kind of pachycephalosaur he named Hanssuesia sternbergi.

Now the story of UCMP 130051 has taken another turn. In the latest issue of the Journal of Vertebrate Paleontology, Ryan Schott and David Evans argue that the skull is really an adult Stegoceras after all. After reconstructing a Stegoceras growth series with juvenile and subadult specimens, Schott and Evans found that UCMP 130051 more closely resembled younger Stegoceras than other skulls Sullivan attributed to Hanssuesia. UCMP 130051 was just a bit bigger and lacked the nodes on the back of the skull that characterized younger individuals–the rest of the anatomy was “indistinguishable” from Stegoceras.

Exactly why UCMP 130051 was missing the set of bumps seen on younger Stegoceras fits into a wider debate about how much dinosaurs changed as they grew up. The “Toroceratops” controversy is the most prominent example, perhaps matched by the longer debate over “Nanotyrannus“, but pachycephalosaurs also form a facet of discussion. In 2009, Jack Horner and Mark Goodwin proposed that the dome-headed dinosaurs Dracorex and Stygimoloch were really just younger individuals of the contemporary dinosaur Pachycephalosaurus. This proposal required drastic changes to the dinosaur’s skull during its life, including forming a dome, growing long skull spikes, and then resorbing those spikes. The transformation must have been spectacular.

While not quite as drastic as in the transition from the spiky “Stygimoloch” form to adult Pachycephalosaurus, Schott and Evans found that Stegoceras probably went through similar changes. In their study, which focused on the ornamented squamosal bones at the back of the skull, younger individuals had prominent nodes that varied in size and shape. In UCMP 130051, though, those bumps were missing, indicating that they were resorbed when Stegoceras reached adulthood. And while they are tentative about this identification, Schott and Evans point out that some Stegoceras specimens–including UCMP 130051–appear to have resorption pits on the surface of the bone; an indicator that their skull ornaments were changing shape as they dinosaurs reached skeletal maturity. Stegoceras didn’t undergo the same back-and-forth horn growth suggested for Pachycephalosaurus, but the change in those little skull nodes hint that the dinosaur went through a more subdued change as it reached full size.

But the new study by Schott and Evans isn’t just about how young Stegoceras changed into adults. By reconstructing the dinosaur’s growth series, the paleontologists also discovered clues that may help paleontologists parse the ever-growing number of dinosaur species, as well as what all that crazy headgear was for. While young Stegoceras showed a high degree of variation in the shape and number of ornaments on their squamosal bones, for example, the dinosaur’s retained the same general “ornamental pattern” throughout their lives. This means that isolated squamosal bones can be useful in identifying pachycephalosaurs known only from partial skulls (and there are quite a few of them).

Of course, one of the biggest mysteries about pachycephalosaurs is why they had domes and spikes in the first place. Depending on who you ask, the ornaments were used to help the dinosaurs recognize members of their own kind, as sexual signals, as weapons or some combination of these. Schott and Evans prefer a mosaic approach to the problem. The fact that even the youngest Stegoceras specimens had recognizable, diagnostic ornaments on their squamosal bones, the researchers argue, indicates that these bumpy adornments probably acted as species recognition signals. They don’t seem to have any role in defense, and the fact that dinosaurs grew these signals before sexual maturity means that they probably weren’t advertisements for mates. If this is true, though, the question is why adult specimens would lose the display structures so late in life.

Then there’s the dome. Young Stegoceras, Schott and Evans point out, were relatively flat-headed. Thick domes developed as the dinosaurs grew up, and previous studies of Stegoceras skulls hinted that the rounded structures were capable of taking quite a shock. (Some pachycephalosaur fossils may even preserve damage from bouts gone awry.) Paleontologists are not agreed on this point, but it is possible that these dinosaurs really did butt heads. This idea, combined with the fact that domes grew as the dinosaurs approached reproductive and skeletal maturity, might mean that domes were sexual signals, and possibly even used in competitions to garner mates. Frustratingly, though, testing these ideas is extremely difficult. We can’t observe the animals themselves, and can only approach these aspects of their lives indirectly through the detail of fossilized bone. We know more about pachycephalosaurs than ever before, but the evolution of their bizarre features remains contentious.

Reference:

Schott, R., Evans, D. (2012). Squamosal ontogeny and variation in the pachycephalosaurian dinosaur Stegoceras validum Lambe, 1902, from the Dinosaur Park Formation, Alberta. Journal of Vertebrate Paleontology, 32 (4), 903-913 DOI: 10.1080/02724634.2012.679878

The skull of a mounted Tarbosaurus. Photo by Jordi Payà, from Wikipedia.

Last Friday, the United States government captured a tyrannosaur. The scene was more Law & Order than Jurassic Park. The million-dollar Tarbosaurus skeleton was seized in an ongoing legal dispute about the origins of the dinosaur and how it was imported to the United States. To date, the evidence suggests that the giant Cretaceous predator was illegally collected from Mongolia (a country with strict heritage laws), smuggled to England and then imported to the United States under false pretenses, all before a private buyer bid more than a million dollars for the skeleton at auction. (For full details on the ongoing controversy, see my previous posts on the story.) Now that the dinosaur has been rescued from the private dinosaur market, I can only hope that the skeleton is swiftly returned to the people of Mongolia.

But there’s one aspect of the dispute that I haven’t said anything about. Heritage Auctions, press releases and news reports have been calling the illicit dinosaur a Tyrannosaurus bataar, while I have been referring to the dinosaur as Tarbosaurus. Depending on who you ask, either name might be correct. Embedded in this tale of black market fossils is a scientific argument over whether this dinosaur species was a “tyrant lizard” or an “alarming lizard.”

Paleontologist Victoria Arbour recently wrote an excellent summary of this issue on her blog. In general appearance, North America’s Tyrannosaurus rex and Mongolia’s Tarbosaurus bataar were very similar animals. They were both huge tyrannosaurs with short arms and deep skulls. Unless you really know your dinosaurs, it’s easy to confuse the two. But there are a few significant differences between Tyrannosaurus rex and Tarbosaurus bataar.

Line drawings of Tarbosaurus (left) and Tyrannosaurus (right) showing the differences in their skulls. Not only is the skull of Tarbosaurus more slender from front to back, but the lacrimal (in light grey) has more of a domed shape. From Hurum and Sabath, 2003.

In 2003, paleontologists Jørn Hurum and Karol Sabath counted the ways the two dinosaur species differed [PDF]. The most obvious is in the top-down profiles of the tyrannosaur skulls. The skull of Tyrannosaurus rex looks much more heavily built and flares out abruptly at the back, while the skull of Tarbosaurus bataar is narrower and doesn’t have the same degree of expansion at the rear of the skull. A more subtle difference is the shape of the lacrimal bone, which made up the front part of the eye socket and was also part of the dinosaur’s skull ornamentation. In Tyrannosaurus rex, the top portion of the lacrimal has a concave shape, but in Tarbosaurus bataar the same portion of bone is domed. And as Arbour mentioned in her post, the arms of Tarbosaurus bataar are proportionally shorter compared to the rest of the body than in Tyrannosaurus rex—so there are three quick ways to tell the dinosaurs apart.

As Arbour noted, the two dinosaurs definitely belong to different species. As it stands now, the two appear to be each other’s closest relatives. The question is whether they should be two species in the same genus—Tyrannosaurus, which was established first and has priority—or whether each species belongs in its own genus. That decision is influenced as much by a paleontologist’s view of how prehistoric animals should be lumped or split into different taxa as anything else. Some prefer to call the Mongolian form Tyrannosaurus bataar, and others view the tyrannosaur as a very different animal rightly called Tarbosaurus bataar. As you might guess, my vote is for Tarbosaurus.

Like Arbour, I suspect that Heritage Auctions advertised the dinosaur as a Tyrannosaurus to get more attention. Tyrannosaurus is the essence of prehistoric ferocity, and putting a Tyrannosaurus up for sale—rather than a Tarbosaurus—will undoubtedly gain more attention every time. In fact, we know that celebrity has a lot to do with why the legal dispute over the auctioned specimen erupted in the first place. There were other Mongolian dinosaur specimens for sale on auction day, such as a rare ankylosaur skull, but almost no one paid any attention to these specimens. The nearly complete Tarbosaurus was a vacuum for media attention, and it was the most powerful symbol of the rampant fossil smuggling problem. But this isn’t necessarily bad. Perhaps, in time, one outcome of this high-profile case will be to keep other, less charismatic dinosaurs from winding up in the homes of affluent private collectors.

Reference:

Hurum, J.H. and Sabath, K. 2003. Giant theropod dinosaurs from Asia and North America: Skulls of Tarbosaurus bataar and Tyrannosaurus rex compared. Acta Palaeontologica Polonica 48 (2): 161–190.

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.]