October 27, 2010
Since the description of the fuzzy-feathered dinosaur Sinosauropteryx in 1996, paleontologists have been inundated with a still-flowing flood of fossil evidence confirming that birds are living dinosaurs. More than that, many of the characteristics we once thought were unique to birds—from air-sacs to infestations of peculiar microorganisms—were common among dinosaurs, too, and every year it seems that dinosaurs become just a little more bird-like. This does not mean that we now understand everything we need to know about the origin of birds, however. With so many unique fossils changing our understanding at such a rapid rate, the exact details of when the first birds evolved and which lineage of feathered dinosaurs they originated from are still unclear.
Our changing understanding of bird origins is addressed in the Chinese Science Bulletin by paleontologists Xu Xing, Ma Qing Yu and Hu Dong Yu. The key to this evolutionary pattern is Archaeopteryx, a 150-million-year-old feathered dinosaur traditionally regarded as the earliest known bird. This sets the origin of birds in the Late Jurassic, but many of the feathered coelurosaurs—the larger group of theropod dinosaurs which birds are nested in—known so far lived after Archaeopteryx. The earlier, Jurassic dinosaurs that would have been ancestral to both birds and the other feathered dinosaurs have been notoriously difficult to find, but better sampling of Jurassic-age strata have provided more context for the origin of birds and feathered dinosaurs.
In their review, the authors list the recent discovery of many Jurassic and Early Cretaceous coelurosaurs, from the early tyrannosaur Proceratosaurus to the strange, tiny dinosaur Epidexipteryx. Together these specimens help flesh out the pattern of early coelurosaur evolution by allowing scientists to determine which traits are archaic and which are later specializations, and this may shake up the traditional picture of bird origins.
Parsing the evolutionary relationships of birds requires a fair amount of esoteric scientific terms. Even though the deinonychosaurs—a group made up of troodontids such as Saurornithoides and dromaeosaurids such as Velociraptor—have typically been taken as the closest relatives of the first birds, the new paper proposes that they are a bit further removed from bird origins. The breakdown would look something like this. Archaeopteryx, placed in the context of all the feathered dinosaurs we now know of, would group with the deinonychosaurs, whereas all definitive early birds would be more closely related to Epidexipteryx and oviraptorosaurs such as Citipati and Incisivosaurus. (See the evolutionary tree on the right above.)
This new arrangement has yet to be fully tested and analyzed—it is a provisional hypothesis which will rest on further discoveries—but if correct it raises the sticky question of what we call a bird. If we keep Archaeopteryx as a bird in this arrangement, then all the deinonychosaurs, the oviraptorosaurs, and Epidexipteryx would be birds, too. Then again, we could strip Archaeopteryx of its long-held title of “earliest known bird” and give that title to Jeholornis, thus keeping the more traditional image of what a bird is. Admittedly, the latter option makes more sense to me than extending the “bird” designation to such a wide group of feathered dinosaurs, but no doubt what is or is not an early bird will be something that paleontologists will be grappling with for some time to come. Frustrating, perhaps, but it is also wonderful that we have so many well-preserved fossils that the distinction between bird and non-avian dinosaur has become so difficult to figure out!
Xu, X., Ma, Q., & Hu, D. (2010). Pre-Archaeopteryx coelurosaurian dinosaurs and their implications for understanding avian origins Chinese Science Bulletin DOI: 10.1007/s11434-010-4150-z
September 2, 2010
It has been a good year for horned dinosaurs. The recent description of Mojoceratops, the discovery of a ceratopsian in Europe, and the long-awaited publication of the New Perspectives on Horned Dinosaurs volume have all given paleontologists reason to celebrate, and a new study led by Xu Xing reports on another significant discovery: the first ceratopsid dinosaur from China.
Understanding the significance of the newly-described dinosaur requires a little background information. As with tyrannosaurs, horned dinosaurs are organized via a nested tree of different groups, with each term marking a different degree of specificity. Within this scheme the Ceratopsia is the most inclusive group—containing everything from early forms such as Yinlong all the way to Triceratops—whereas the Ceratopsidae is subset of this larger group which includes the stereotypical quadrupedal herbivores with gnarly frills and horns on their faces. Ceratopsians which sit near the base of the horned dinosaur family tree have been found in China before, but the new Chinese Science Bulletin paper is the first notice of one of the specialized ceratopsids from the area.
Named Sinoceratops zhuchengensis, the new dinosaur is represented by the top portion of a skull and a partial frill discovered in the approximately 75-million-year-old rock of Shandong, China. From what is known of it, Sinoceratops appears to have been a centrosaurine ceratopsid, or, in other words, belonged to the subset of ceratopsid dinosaurs with large nasal horns and small frills which includes Styracosaurus and Pachyrhinosaurus. Given that the scientists placed it at the base of the centrosaurine family tree, Sinoceratops does not share some of the specializations seen among other members of the group, but its archaic traits may indicate what the ancestors of the first centrosaurines were like. Provided further discoveries support the hypotheses put forward in the new paper, Sinoceratops may act as a transitional form which could help explain how the centrosaurine dinosaurs evolved and may even indicate that ceratopsid dinosaurs evolved in Asia before dispersing to North America.
Still, as the new paper points out, among the most important aspects of the discovery of Sinoceratops is that it shows that ceratopsids were present in China. As is well known among paleontologists, there is a remarkable correspondence between the Late Cretaceous dinosaurs of North America and Asia—tyrannosaurus, hadrosaurs, ankylosaurs, pachycephalosaurs, and others—but until now ceratopsids seemed to have stuck to North America. Now we know otherwise, although the fact that it took so long to find the bones of a ceratopsid in Asia may be indicative of barriers—in terms of geography or environment—that prevented this group of horned dinosaurs from proliferating in Asia as they did in North America.
XU Xing, WANG KeBai, ZHAO XiJin & LI DunJing (2010). First ceratopsid dinosaur from China and its biogeographical
implications Chinese Science Bulletin, 55 (16), 1631-1635 DOI: 10.1007/s11434-009-3614-5
June 3, 2010
Last month a group of nine students from Montana State University, Dawson Community College and Rocky Mountain College left for China to study dinosaur eggs, and they have been chronicling their experiences on the new blog MSU China Paleontology Expedition. About two weeks into their six-week stay, most of the posts so far cover “culture shock” (both on the part of the Montana students and the Chinese citizens they encounter), though some of the more recent posts have begun to share the scientific work being undertaken by the team. Among things the students have been studying: the sediment encasing some of the eggs to figure out what kind of environment they were preserved in, and I look forward to reading more updates from the students during the rest of their stay in China.
April 22, 2010
Around 160 million years ago, an enormous sauropod dinosaur trudged across an ancient marsh in what is now Xinjiang, China. It was not easy going. The eruption of a nearby volcano coated the area in a layer of ash which formed a thin surface over a morass of mud and volcanic debris, and as it walked it left deep holes which quickly filled in with sticky muck. As some of the smaller dinosaurs that lived in the area would find out, these in-filled footprints would soon become dinosaur death traps.
As reported by paleontologists David Eberth, Xu Xing and James Clark in the journal Palaios, the sauropod-made pits created circumstances in which small theropod dinosaurs would be more likely to be preserved. Once filled in with mud, it would have been hard to see the large holes, and when a small dinosaur stepped into them they would have had a very difficult time getting out. As we can tell from the layers of fossils preserved within the footprints, many of the dinosaurs became permanently stuck and died, piling one on top of the other (although, in some cases, trapped dinosaurs might have been able to gain better footing on the remains of previously trapped individuals and push themselves out).
These dinosaur death traps have been especially significant because at least two previously-unknown species of dinosaur have been recovered from them. Guanlong, an early cousin of Tyrannosaurus, and the bizarre theropod Limusaurus were both described from the pile of skeletons contained within the footprints. Chances are that there are other yet-undescribed dinosaurs contained within similar assemblages in the area, too, showing how something as simple as a mud-filled footprint can tell us much about the life of the past.
EBERTH, D., XING, X., & CLARK, J. (2010). DINOSAUR DEATH PITS FROM THE JURASSIC OF CHINA PALAIOS, 25 (2), 112-125 DOI: 10.2110/palo.2009.p09-028r
March 19, 2010
Between 84 million and 75 million years ago, near the end of the Cretaceous, part of the land now known as the Gobi Desert was host to a variety of raptors. There were two species of Velociraptor, a similar predator named Tsaagan mangas, a tiny feathered dinosaur called Mahakala omnogovae, and, as just announced in the journal Zootaxa, a previously unknown type represented by an exquisitely-preserved specimen. It is called Linheraptor exquisitus.
As described by paleontologists Xing Xu, Jonah Choiniere, Michael Pittman, Qingwei Tan, Dong Xiao, Zhiquan Li, Lin Tan, James Clark, Mark Norell, David Hone and Corwin Sullivan, Linheraptor was a relatively small predatory dinosaur most closely related to Tsaagan. Outside of some small differences in the skull, such as the size and placement of small holes (called fenestrae) towards the front of the skull, the two appear to represent a group of unique dromaeosaurs which, while close cousins of their neighbor Velociraptor, lacked some of the specialized characteristics which distinguish their more famous relative.
Further research on Linheraptor has been planned, but I find it especially interesting that the famous Djadokhta Formation (home of the Flaming Cliffs) has yielded another predatory dinosaur. What could it have been eating, and how did it avoid competition with the other raptors in the area? Famous specimens such as the “fighting dinosaurs” have confirmed that some of the raptors fed on Protoceratops, and the numerous kinds of small mammals which lived in the area were probably prey, but the general scheme of “who ate whom” is still incompletely known. The preservation in the Djadokhta Formation is so good, however, that scientists have been able to get a well-defined look into this part of Earth’s history, and with any luck further discoveries will tell us more about the ecology of the area during the time of Linheraptor.
David Hone, one of the authors of the new Linheraptor paper, has more about the find at his blog Archosaur Musings.
XING XU, JONAH CHOINIERE, MICHAEL PITTMAN, QINGWEI TAN, DONG XIAO,, & ZHIQUAN LI, LIN TAN, JAMES M. CLARK, MARK A. NORELL, DAVID W. E. HONE, CORWIN SULLIVAN (2010). A new dromaeosaurid (Dinosauria: Theropoda) from the Upper Cretaceous Wulansuhai Formation of Inner Mongolia, China Zootaxa, 1-9