November 26, 2012
Stegosaurus was a weird dinosaur. We’ve known that for well over a century, but, as Darren Naish has often pointed out, Stegosaurus was strange even compared to its Jurassic relatives. The dinosaur’s arrangement of broad, alternating plates is a departure from the arrangements of smaller plates, back spikes and accessory spines seen on many other stegosaurs, including the perplexingly well-armed Gigantspinosaurus sichuanensis.
Ornamented with a double row of short, narrow plates along its back, the roughly 160-million-year-old Gigantspinosaurus generally resembled other stegosaurs from Late Jurassic Asia, such as Tuojiangosaurus. But, as you might be able to guess from the dinosaur’s name, the feature that immediately sets Gigantspinosaurus apart from similar species is a enormous hooked spine that jutted out from behind the shoulder blade. These striking spikes were found close to their life position on the first skeleton of this dinosaur to be found–erroneously attributed to Tuojiangosaurus, before being redescribed as Gigantspinosaurus in 1992–although their exact orientation isn’t entirely clear. Did the shoulder spikes curve straight backward, or were they tiled slightly upwards? And, more significantly, how did such prominent ornaments evolve? No one knows.
As yet, we know relatively little about the natural history of Gigantspinosaurus. The dinosaur has a name, and skin impressions have helped researchers restore what the stegosaur looked like, but many aspects of the spiky herbivore’s biology remain mysterious. In the grand scheme of stegosaur evolution, though, the ornamentation of Gigantspinosaurus has sometimes been taken as evidence that similar forms had shoulder spikes. In addition to paired spikes along its tail, the Late Jurassic stegosaur Kentrosaurus possessed an extra pair of spikes along its side. These were originally placed over the hips, but, due to the discovery of Gigantspinosaurus, some researchers have argued that the spikes truly belong at the shoulders.
Frustratingly, paleontologists have yet to find a Kentrosaurus skeleton with side spikes in place. But the discovery of Gigantspinosaurus doesn’t necessarily mean that its cousin Kentrosaurus had the same arrangement. Among stegosaurs, the two genera were relatively distantly related, and it’s entirely possible that more than one side spike arrangement evolved. As paleontologist Heinrich Mallison has argued, the hips of Kentrosaurus seem to possess areas where the spikes could have articulated, and this arrangement would be consistent with the dinosaur’s ornamentation pattern–small plates at the front give way to spikes along the stegosaur’s back and tail. Indeed, the side spikes on Kentrosaurus more closely resemble the same structures along the dinosaur’s back and tail and the shoulder spike of Gigantspinosaurus. If Kentrosaurus had plates up front and serially homologous spikes along the back, then why shouldn’t the hip spikes remain a reasonable hypothesis? Together, Gigantspinosaurus and Kentrosaurus might represent different alternatives in the stegosaur armory.
August 22, 2012
When I think of theropod tracks, the mental image that immediately pops up is of three-toed depressions with conspicuous indentations where the dinosaur’s claws dug into the substrate. After all, theropod means “beast foot,” and many theropod tracks seem to fit the name. But not all theropod dinosaurs balanced on three toes. The deinonychosaurs–the group of sickle-clawed dinosaurs that included the more slender troodontids and the bulky hypercarnivorous dromaeosaurids–ambled through the Mesozoic on two toes, with their curved switchblade claw held off the ground. Over the past two decades, paleontologists have found these two-toed tracks at multiple sites around the world. Most, however, have been found in China, and researchers have just issued a profile of one of the richest deinonychosaur tracksites anywhere.
The in-press Acta Palaeontologica Polonica study, written by Lida Xing and colleagues, describes an Early Cretaceous tracksite at the Liujiaxia Dinosaur National Geopark in Gansu Province, China. The site preserves 71 deinonychosaur tracks. These footprints are unique enough that the paleontologists gave them a new name–Dromaeosauripus yongjingensis. It may seem strange to name tracks, especially since we don’t know exactly what species of dinosaur made them, but trace fossil experts name particular types of tracks to keep track of the different kinds of animals that left the impressions. If a track type is unique and consistent from print to print, then it often gets its own moniker.
Of course, tracks record fossil behavior. At the new site, at least two trackways show that individual dinosaurs turned while walking. Unlike other sites, though, there isn’t any evidence for social raptors here. Based upon the spacing of the tracks and other details, the paleontologists Xing and co-authors hypothesize that the deinonychosaur trackways were made by individual animals that walked along the same surface at different times.
But what kind of deinonychosaur made the tracks? This is the Cinderella Syndrome–fitting the right trace to the proper tracemaker. Unless a dinosaur literally dies in its tracks, paleontologists can only outline the general kind of dinosaur who left the footprints. In this case, the choice is between a troodontid and a dromaeosaurid.
Footprint size isn’t especially helpful. Most troodontids were relatively small, while dromaeosaurids could grow to 20 feet long or more. The footprints at the site were made by dinosaurs ranging from two to 10 feet long, and this means that the larger animals were close to the known upper limit for troodontid size. Frustratingly, size alone is too ambiguous to distinguish between a troodontid or dromaeosaurid trackmaker.
To narrow down the possibilities, Xing and collaborators turned to another clue. The toe lengths of troodontids and dromaeosaurids are slightly different. While the fourth (or outermost) toe of troodontids is slightly shorter than the neighboring third toe, the two supporting toes in dromaeosaurids are typically just about equal in length. The tracks at the site more closely match the dromaeosaurid pattern. Thanks to this tracksite, we can imagine feathery dinosaurs akin to Deinonychus strutting across Cretaceous China.
Xing, L., Li, D., Harris, J.D., Bell, P.R., Azuma, Y., Fujita, M., Lee, Y.-N., and Currie, P.J. (2012). A new Dromaeosauripus (Dinosauria: Theropoda) ichnospecies from the Lower Cretaceous Hekou Group, Gansu Province, China Acta Palaeontologica Polonica DOI: 10.4202/app.2011.0115
April 5, 2012
Science is awesome. I know this because paleontologists have just announced the discovery of a giant, feather-covered tyrannosaur.
The freshly described dinosaur—dubbed Yutyrannus huali by Xu Xing and co-authors—stretched about 30 feet long as an adult. Thanks to the fine preservation of three skeletons that represent this roughly 125-million-year-old carnivore, we know that much of this dinosaur’s body was covered in fine, wispy feathers. These were not flight feathers or down that you might see on a modern bird, but simpler structures best described as dino-fuzz. This makes Yutyrannus the largest creature with observed plumage ever to have lived.
I have been waiting for Yutyrannus or something like it for a long time. The dinosaur is a beautiful confirmation of an evolutionary hypothesis made years ago. In 2004, Xu and collaborators described a much smaller tyrant: Dilong paradoxus, which lived only about five million years before Yutyrannus, was a small coelurosaur with a coat of simple fuzz. And Dilong appears to have been an archaic tyrannosauroid, a dinosaur near the base of the family that contained later tyrants such as Gorgosaurus and Teratophoneus. If a tyrannosauroid had feathers, and almost every other lineage closely related to the tyrannosauroids had feathers, then even Tyrannosaurus rex might have been at least partly coated in plumage.
Giant tyrannosaurs with feathers was a respectable idea, but there was no direct evidence. In North America, at least, tyrannosaurs were not entombed in the kind of environments with the high-fidelity preservation potential for feathers to make it into the fossil record. And, while they have frustratingly never been published, rumored specimens of tyrannosaur skin have hinted that adult animals had naked hides. Maybe tyrannosaur chicks were fluffy while adults, no longer needing an insulating coat, lost their feathers.
Not everyone has been on board with the idea of fluffy tyrannosaurs. The humor website Cracked.com listed an illustration of a feather-covered Tyrannosaurus one as of “17 Images That Will Ruin Your Childhood,” and the same image posted at BuzzFeed attracted more than a few negative responses. (“Dear god no!” wailed on commenter.) The smooth-skinned monsters of the Jurassic Park franchise remained the canonical pop culture image of everything a Tyrannosaurus should be.
I was ecstatic when news of Yutyrannus reached by inbox. Killjoy that I am, I loved the idea that the dinosaur made it all the more likely that other big tyrannosaurs were at least partly covered in filamentous protofeathers. I have no sympathy for immature attachment to traditional visions of scaly, drab tyrannosaurs. And, despite all the cries of “Ow! My childhood!” in reaction to feathered dinosaurs, Tyrannosaurus and kin would have been just as fearsome as ever. As tyrannosaur expert Thomas Holtz pointed out in a National Geographic news item, feathers “might make [Tyrannosaurus] a little more amusing, but only until the point right before it tears you to shreds.”
The extent of feathers on Yutyrannus and other tyrannosaurs isn’t entirely clear. Although I think Brian Choo’s illustrations of Yutyrannus are fantastic, and a full coat of fuzz is a fair hypothesis, patches of feathers were only found in a few places among the three specimens: the tail, hip, foot, neck and arm. That’s enough to hypothesize that much of the dinosaur was covered in feathers, but there’s always the possibility that non-avian dinosaurs had feathers on some parts of their bodies and not on others. Any restoration opting for either pattern is a hypothesis based on the available evidence.
Still, the discovery of any feathers at all means that we might find out what color Yutyrannus was. Microscopic studies of dinosaur feathers have helped establish the palettes of small feathered dinosaurs such as Anchiornis, Archaeopteryx and Microraptor. Now there’s the possibility of unlocking tyrannosaur colors, too. Was Yutyrannus mostly covered in dark plumage, like the other dinosaurs studied so far? Or did the tyrannosaur have a different color scheme? I guess we’ll have to wait and see—according to an interview with Xu on the Nature podcast, this research is already underway.
In spite of my overwhelming excitement about all this, though, there are two wrinkles in the story. The first is that there is a slight possibility that Yutyrannus may not actually be a tyrannosaur. As paleontologist Darren Naish points out at Tetrapod Zoology, Yutyrannus shows some subtle similarities to carcharodontosaurids, a subgroup of large predatory dinosaurs more closely related to Allosaurus. Exactly where Yutyrannus fits in the dinosaur family tree awaits confirmation by way of future analyses.
Should Yutyrannus turn out to be something other than a tyrannosauroid, that would immediately make the predator that much more important. At first, it seemed that only coelurosaurs—the group containing tyrannosauroids and sundry other theropod lineages, including birds—had feathers. Then paleontologists discovered feather-like structures on two very distantly related dinosaurs—the small ceratopsian Psittacosaurus and the diminutive, bipedal herbivore Tianyulong. (Following that, the carcharodontosaurid Concavenator supposedly showed evidence of bristles on its arms, but this evidence has been disputed.)
The spread of feathers and feather-like structures among dinosaurs might mean that secondary body coverings evolved at least twice on two different sides of the dinosaur family tree. Or it might indicate that simplified integument was a common trait shared among dinosaurs—a very old feature that was retained in some groups and lost in others. And here’s where Yutyrannus comes in. If Yutyrannus is not a coelurosaur but a carcharodontosaurid or something else, then it adds another feathery point in the dinosaur family tree and suggests that a wider array of dinosaurs had feather-like body coverings.
Yutyrannus isn’t even the only dinosaur that may shake things up. A smaller, earlier theropod called Juravenator was preserved with traces of dinofuzz, and there have been rumors that this dinosaur might turn out to be something other than a coelurosaur. Much remains to be established and tested, but the emerging picture is that several dinosaur lineages—very distantly related to birds—had secondary body coverings of one sort or another. It wouldn’t be altogether surprising if Yutyrannus turned out to be additional evidence of this trend. For now, though, the primary hypothesis is that Yutyrannus was an archaic form of tyrant dinosaur.
According to comments Xu made during a Nature podcast interview, the behavior of Yutyrannus may have made the predator even sexier still. The paper mentions three Yutyrannus individuals of different sizes, all found together. Other bonebeds of multiple tyrannosaurs have been used to propose that tyrant dinosaurs were highly coordinated pack hunters, and Xu follows suits with this discovery. Since the three predators were found together in the same quarry, and a sauropod skeleton has also turned up at the site, Xu says that the Yutyrannus were members of a pack that attacked the even bigger sauropod. For some unknown reason, all died together.
I’m not convinced that this was the case. Bonebeds are tricky things—there are many reasons why multiple skeletons may come to rest in the same place. The animals could have been forced into a relatively small area by flooding or storms, they could have died elsewhere and all been washed into the same place, or the site could have been some sort of predator trap. Very careful analysis of the geology and taphonomy of such sites is required to figure out why all those bodies wound up in the same place, and we shouldn’t take the association of skeletons at face value when trying to reconstruct dinosaur behavior. Could tyrannosaurs have hunted in groups? Certainly. But solid evidence for rapacious packs of big tyrannosaurs has yet to be found.
Alone or in coordinated social groups, though, Yutyrannus must have been a fantastic sight. Discoveries like this beautifully underscore just how wonderful dinosaurs really were. If previous discoveries hadn’t led us to expect the existence of this fuzzy dinosaurian hypercarnivore, I sincerely doubt that we could have imagined such a creature.
Xu, X., Norell, M., Kuang, X., Wang, X., Zhao, Q., & Jia, C. (2004). Basal tyrannosauroids from China and evidence for protofeathers in tyrannosauroids Nature, 431 (7009), 680-684 DOI: 10.1038/nature02855
Xu, X., Wang, K., Zhang, K., Ma, Q., Xing, L., Sullivan, C., Hu, D., Cheng, S., & Wang, S. (2012). A gigantic feathered dinosaur from the Lower Cretaceous of China Nature, 484 (7392), 92-95 DOI: 10.1038/nature10906
January 4, 2012
Even before we knew what they really were, dinosaurs inspired our imagination. Unidentifiable bones and tracks formed the basis of legend–they were the evidence of great battles, fearsome monsters and times when the world was new and hostile to human existence. Indeed, contrary to what John Noble Wilford wrote in The Riddle of the Dinosaur, fossilized bones were not just ignored or ground up for “dragon-bone medicine” in the centuries prior to the scientific discovery of dinosaurs. People have puzzled over dinosaurian fossils for centuries. Some of that folklore still persists today.
In a paper recently published in Ichnos, researchers Lida Xing, Adrienne Mayor, Yu Chen, Jerald Harris and Michael Burns focus on one particular source of dinosaur-inspired myths–trackways found in China. Just as dinosaur tracks in New England generated tales about primeval monsters, huge turkeys and ostrich-like birds, the tracks in China motivated the creation of different stories to explain just what left such imposing footprints.
According to the new study, Chinese folklore about dinosaur tracks can be divided into four categories–mythical birds, mammals, plants, and gods or heroes. In the case of three-toed theropod tracks discovered in Chabu, Inner Mongolia, for example, the footprints had been known to local farmers since the 1950s and were believed to be footprints of a “divine bird.” As explained by Xing and co-authors, “The herders believed that the tracks represented beautiful wishes for human happiness left by the sacred bird Shen Niao.” This is a common theme across sites where theropod tracks are found. Three-toed dinosaur footprints have often been interpreted as the steps of birds, and other sites in Heibei, Yunnan, Guizhou and Liaoning provinces have been attributed to other mythical birds, such as the golden and heavenly chickens.
Not all the dinosaur tracks are associated with supernatural avians. The fossil footprints of a sauropod dinosaur near Zigong City have traditionally been cast as the footprints of a rhinoceros–”The tradition of counting the footprints to pray for good fortune is popular,” the authors note–and hadrosaur tracks at Qijang County may have been interpreted as impressions of lotus flowers on stone. The size of the impressions and the fact that they were made on stone were often taken to mean that some supernatural agency was involved. What else could leave such detailed markings on rock?
One such powerful figure, according to myths about footprints found in Changdu County, Tibet, was the Mountain Deity. During the construction of a highway through the area in 1999, construction crews found several large footprints. Local villagers believed that all the noise had disturbed a god who dwelt in the mountains, and when the deity fled, it left the footprints in stone. Though not everyone agrees. Others think that the footprints represent King Gesar, a warrior featured in an epic poem about Tibet’s history. In reality, the tracks are the fore- and hindfoot impressions of a sauropod dinosaur. The shape of the tracks and their arrangement roughly resemble a large human footprint, and so the legendary explanation was born. Indeed, not all myths about dinosaur remains are ancient. In places where people don’t know about dinosaurs or paleontology, fantastic stories are still employed to explain the origin of fossils.
The nature of tracksites themselves may explain why they often find their way into folklore. Fossilized bone is often fragile and visible on the surface for a short time before eroding away. Exposed tracks, on the other hand, often remain in place for generations before fully succumbing to the wear of wind and water. The persistence of the tracks may allow them to become more readily established in cultural tradition–the stone footprints are visible for years and act as evidence of the stories.
And these legends have practical applications for paleontologists. By using rumors of “dragon bones” and stories about stone footprints, researchers can use local folklore to locate previously-unknown fossil localities. Folklore may tell tales too fantastic to believe, but they may be based on very real traces of prehistoric life.
Xing, L., Mayor, A., Chen, Y., Harris, J., & Burns, M. (2011). The Folklore of Dinosaur Trackways in China: Impact on Paleontology Ichnos, 18 (4), 213-220 DOI: 10.1080/10420940.2011.634038
November 22, 2011
In life, Microraptor gui must have been an elegant dinosaur. This small, sickle-clawed dromaeosaurid was covered in plumage, including long feathers along its arms and legs. We know this thanks to the exquisite preservation of multiple Microraptor specimens found in the roughly 120-million-year-old strata of northeastern China. But feathers aren’t the only delicate dinosaur features that remained intact during the process of death, burial and fossilization. In at least one Microraptor specimen, paleontologists have found scraps of the dinosaur’s last meal.
Attendees to the 71st annual Society of Vertebrate Paleontology meeting in Las Vegas, Nevada earlier this month got a preview of the specimen during one of the conference’s poster sessions. Now the full paper describing the fossil, written by Jingmai O’Connor, Zhonghe Zhou and Xing Xu of Beijing’s Institute of Vertebrate Paleontology and Paleoanthropology, has been published in PNAS. There are a few notable details of the feathery dinosaur.
The skeleton of this Microraptor, like others, is arched into the classic dinosaur death pose with the head arched back and the tail angled upwards. Whether the trigger for this posture turns out to be death throes, a result of immersion, or something else, the posture may be a clue to how the dinosaurs died or were rapidly buried. This Microraptor is also of interest because the dinosaur’s skull appears to be more complete and less crushed than some of the other specimens published so far, though the authors note that this specimen is relatively poorly preserved and therefore difficult to study. As for feathers, only a few tufts were preserved along the dinosaur’s head, neck and back. But the focus in the new paper isn’t on the dinosaur’s skeleton or outside appearance. The study is about what was inside the dinosaur’s body cavity when it died. There, hidden beneath the ribs, are parts of the wing and feet of a Cretaceous bird.
Exactly what genus of bird Microraptor consumed is impossible to say at the moment. Even so, anatomical characteristics of the bird feet allowed O’Connor and colleagues to classify the unfortunate avian as an enantiornithine, a form of archaic and now extinct bird. The position of this bird’s remains within the dinosaur is as good an indication as any that the feathered, non-avian dinosaur Microraptor at least sometimes consumed its distant avian cousins. But what happened just before the Microraptor swallowed the bird?
According to O’Connor and co-authors, the position of the bird bones within the Microraptor indicate predation rather than scavenging. The fact that the feet of the bird are closer to the front end of the dinosaur indicate that the prey was swallowed head first. The paleontologists cite this hypothesis as evidence that Microraptor was an arboreal dinosaur. Since the avian prey had anatomical specializations for life in the trees, and Microraptor supposedly caught the bird while the prey was still alive, then Microraptor must have been a skilled climber if not a regular tree-dweller.
Strangely, however, the paleontologists did not explore other scenarios for what might have happened in the moments before the Microraptor consumed the bird. Scavenging is briefly mentioned and dismissed as a possibility, but otherwise the idea that Microraptor scrambled up trees to catch birds is taken as the primary hypothesis. We know the facts—that a Microraptor swallowed a bird—but there is more than one pathway to that point.
Let’s assume that Microraptor truly did capture a live bird. But there is no indication whether the prey was caught on the ground or in the trees. In fact, as I sit here writing this, my cat Teddy is sitting in front of the window watching chickadees forage on the ground on my front lawn. Anatomically, the birds in my yard are specialized for life in the trees, but they do spend a considerable amount of time on the ground, and birds are often caught by cats and other terrestrial predators when the birds come down from their perches. Perhaps early birds also foraged on the ground, and when doing so they would have been vulnerable to attack by dinosaurs such as Microraptor.
Furthermore, there is nothing that tells us whether the bird was alive or dead when the dinosaur consumed it. Perhaps the bird died, fell to the ground, and the Microraptor was the recipient of a relatively fresh, free meal. All we know is that the bird was probably intact when the dinosaur ate it, but we can’t tell whether the bird was alive or recently deceased at the time.
We don’t know exactly what happened to the little bird, and therefore the association between the dinosaur and its prey can’t be cited as supporting either a ground- or tree-dwelling lifestyle for Microraptor. Nevertheless, the discovery that Microraptor ate birds adds one more piece to our understanding of this peculiar dinosaur, and I, for one, am a little tickled by the description of an avian dinosaur within a feathered non-avian dinosaur just prior to Thanksgiving. Turducken, anyone?
O’Connor, J., Zhou, Z., & Xu, X. (2011). Additional specimen of Microraptor provides unique evidence of dinosaurs preying on birds Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1117727108