August 31, 2012
Earlier this week, I got into a snit over the blinkered assertion that feathery dinosaurs are lame. I argued the opposite point–as I wrote at the time “Feathered dinosaurs are awesome. Deal with it.” How fortunate that a new paper this week offers proof of fuzzy dinosaur superiority. The evidence comes in the form of gut contents found within predatory dinosaurs that stalked Cretaceous China around 125 million years ago.
The carnivores in question are a pair of Sinocalliopteryx. These dinosaurs were close cousins of the much earlier Compsognathus, albeit quite a bit larger. While Compsognathus was turkey-size, about three feet long, Sinocalliopteryx grew to be about eight feet long. And this big predator was fluffy. The original description of the dinosaur mentioned the vestiges of simplified dinofuzz around the body of Sinocalliopteryx, and this makes sense given the dinosaur’s relationships. While considerably bigger than its close relatives, Sinocalliopteryx was a compsognathid–a group of theropod dinosaurs that also includes fuzzy forms such as Sinosauropteryx and Juravenator. Big or small, the compsognathids were hunters wrapped in wispy plumage.
And the initial description of Sinocalliopteryx mentioned something else. The skeleton that formed the basis of the original paper contained the leg of an unidentified dromaeosaurid dinosaur in its gut contents. Even though dromaeosaurids have long been cherished as sickle-clawed uber-predators, Sinocalliopteryx had clearly eaten the drumstick of one of the smaller feathered predators. Since then, paleontologists have identified a second Sinocalliopteryx with gut contents, and the two dinosaurs form the basis of a new PLoS One study by University of Alberta paleontologist Lida Xing and colleagues.
Looking back at the first Sinocalliopteryx, Xing and colleagues identified the victim as Sinosauropteryx. The second Sinocalliopteryx specimen had a different menu before it perished–its stomach contains the remains of two Confuciusornis, an archaic bird, and bones from an unidentified ornithischian dinosaur. But these gut contents invoke an aggravating mystery. Did these Sinocalliopteryx hunt their dinosaurian prey, or did they scavenge their meals?
This isn’t the first time paleontologists have puzzled over the meaning of predatory dinosaur gut contents. Earlier this year, Dave Hone and collaborators investigated a pterosaur bone found inside a Velociraptor, and last year Jingmai O’Connor and colleagues described a Microraptor with the remains of a bird in its gut (just to pick two examples of many). Frustratingly, though, it’s difficult to say how the dinosaurs obtained the meat. In the case of the Velociraptor, the researchers could not rule out hunting even though scavenging seemed the more likely option. Likewise, even though O’Connor and co-authors suggested their Microraptor hunted birds in the trees, the non-avian dinosaur could have just as easily scavenged a dead bird that fell to the forest floor. Gut contents tell us about what dinosaurs consumed, but they almost never provide direct evidence of how carnivores obtained flesh and bone to eat.
In the case of Sinocalliopteryx, the PLoS One study concludes that the dinosaur may have been skilled at catching live avian prey. The fact that one Sinocalliopteryx fed on two Confuciusornis in quick succession could mean that the large dinosaur was adept at nabbing early birds. “[T]he evidence of bird predation in Sinocalliopteryx,” Xing and colleagues conclude, “suggests that it was a highly capable stealth hunter.” Then again, the same researchers also note that their scenario “is speculative.” While it may seem improbable, the Sinocalliopteryx in question could have scavenged one or both of those birds, as well as the non-avian dinosaur remains in its stomach. We just don’t know. Like many predators, Sinocalliopteryx most likely hunted live prey and took advantage of carrion. Frustratingly, these fossil gut contents can’t tell us what happened in each case. Sinocalliopteryx may very well have been a skilled bird-slayer. Or perhaps not. The fact is that we don’t know for sure.
Perplexing feeding habits aside, there’s something else about the gut contents of Sinocalliopteryx that can give us a closer look at the dinosaur’s biology. In the dinosaur that ate the two birds and the ornithischian, the bone of the ornithischian dinosaur was corroded by stomach acid. The more delicate bird bones, by contrast, had not been so damaged. This means that the Sinocalliopteryx ate the ornithischian first, followed by one bird and, later, another. More than that, the acid damage indicates that at least some dinosaurs had highly-acidic foreguts where bone was broken down–comparable, but not exactly like, the stomachs of crocodilians and perhaps some bone-eating birds like the bearded vulture.
All of which is to say that Sinocalliopteryx is a great example of a fluffy dinosaur you wouldn’t want to mess with. Even if we can’t discern the backstory of each meaty morsel, the variety of prey in the Sinocalliopteryx stomachs shows that this dinosaur wasn’t a picky eater and may have even been a quick hunter that specializing in snapping up other feathery dinosaurs. For our fuzzy mammalian predecessors, hiding the Cretaceous forests, this would have been one scary dinosaur.
Xing L, Bell PR, Persons WS IV, Ji S, Miyashita T, et al. (2012) Abdominal Contents from Two Large Early Cretaceous Compsognathids (Dinosauria: Theropoda) Demonstrate Feeding on Confuciusornithids and Dromaeosaurids. PLoS ONE 7(8): e44012. doi:10.1371/journal.pone.0044012
March 9, 2012
Microraptor was an exquisitely feathered dinosaur. The small, sickle-clawed predator, which lived about 120 million years ago, was covered in well-developed plumage, including long feathers on its arms and legs. But we now know that Microraptor was not only beautiful in an anatomical structure sense. A detailed new study has painted this dinosaur in a glossy black sheen.
The range of the dinosaur palette has been one of the most mysterious aspects of dinosaur biology. For most species, we just don’t know—bones and teeth can’t tell us anything about skin color. But feathered dinosaurs contain evidence of their hues within their feathers. Microscopic organelles called melanosomes are the key. In fossil creatures—just as in living ones—the size, shape, density and distribution of these tiny, pigment-filled blobs created different colors. By studying the characteristics of melanosomes in feathered dinosaurs and comparing the patterns with those that create the colors of modern birds, paleontologists can reconstruct dinosaur feather colors.
Several dinosaurs have already received a color treatment. After establishing that fossil melanosomes are faithful indicators of prehistoric color in ancient birds, paleontologist Jakob Vinther and colleagues restored the full-body coloring of the feathered, non-avian dinosaur Anchiornis. This small dinosaur looked something like a magpie with a bright red splash of feathers on top of its head. Earlier this year, Vinther, Ryan Carney and co-authors determined that the famous feather used to name the earliest known bird—Archaeopteryx—was black. And a different team of researchers, led by paleontologist Fucheng Zhang, hypothesized that the fuzzy Sinosauropteryx had a candy-cane tail ringed in white and rusty red. Paper by paper, dinosaurs are being colored in.
In the case of Microraptor, the dinosaur did not turn out quite like any of the restorations that artists had previously composed. Many Microraptor illustrations envisioned the dinosaur in shades of brown, white and blue. But when Vinther, Quanguo Li and collaborators studied the melanosomes sampled from 26 different locations on a Microraptor specimen designated BMNHC PH881, they didn’t find those colors. Microraptor feathers were iridescent blue-black. In appearance, Vinther said via email, Microraptor would have looked similar to “grackles or a magpie, or indeed a crow.”
Black was apparently quite fashionable among feathered dinosaurs. Anchiornis, while overall more colorful, was also predominantly black, and the lone Archaeopteryx feather was also black. Why black was so common for dinosaurs with complex, specialized feathers isn’t clear. Vinther pointed out that the small sample size might be creating this pattern, especially since other, unpublished specimens show different colors. Then again, black and other dark shades might have had something to do with where the animals lived. Citing a phenomenon called Gloger’s rule, Vinther explained that mammals and birds that live in hot, humid environments near the equator have more of the pigment melanin, and therefore appear darker, than those living closer to the poles, though “sample size needs to be increased to make any generalizations like these,” he cautioned.
Vinther is confident that further studies will increase the number of dinosaurs for comparison. “The material is clearly there,” he said. It is only a matter of time before paleontologists can start to understand how color varied between individuals, and possibly even between the sexes. For the moment, though, the handful of dinosaurs that have been restored in color have shown that intricate avian traits existed far back in the past. “We were speculating about how deep iridescent colors might be and we were pretty excited when we realized that Microraptor indeed is iridescent,” Vinther said, and this discovery can tell us something about how feathers and even behaviors evolved among early birds and their dinosaurian kin.
“We can see that the paravian clade,” the group that contains birds and non-avian dinosaurs more closely related to birds than dinosaurs , “has complex feather morphologies and exhibit colors and color patterns for display and even iridescence like in modern birds, so these features are ancient and indeed suggest that at least the derived theropod dinosaurs were more similar in ecology and behavior to birds,” Vinther said. And, as research continues on feathered dinosaurs more distantly related to birds, Vinther suspects that many characteristics of modern birds will be pulled “deep down” the dinosaurian tree. The more we learn about feathered dinosaurs, the further back we can draw traits seen among birds today.
And there are still things to learn about the anatomy of feathered dinosaur plumage. While the iridescent hues of Microraptor are the major finding of the new paper, the study also pointed out that specimen BMNHC PH881 had a specialized set of paired feathers at the end of the tail. Similar feathers had been noted in other Microraptor specimens before, but this fossil had an especially lovely set. The structures are “simply too small and the feathers too spaced to create any lift,” Vinther said, so it’s unlikely that they aided the dinosaur in gliding or flying. Instead, citing the assessment of co-author Julia Clarke, Vinther said that the feathers might have been a display structure. Combined with the flashy feathers, these structures might be another clue that display and visual communication were very important factors in the early evolution of feather anatomy and color.
For most of my life, I was told that we would never know what colors dinosaurs were. Now, amazingly, there is a way to restore the appearances of some dinosaurs with a fidelity never thought possible. But I had to wonder if paleo-artists have felt any aggravation about such discoveries. As new studies establish feather colors for dinosaurs, the realistic palettes for those dinosaurs are constrained. I asked Vinther if he has received any irritated comments from artists about his work. He replied that, to the contrary, his research has been greeted with excitement. And while defining dinosaur colors “might take some of the imagination from the artists,” Vinther said, “I think that their fascination with these beasts gives them a desire to make them more scientifically correct.” The colorfully restored dinosaurs seem to be a hit. “I am struck by awe when I google-image Anchiornis and see forty plus versions of Anchiornis by various artists all over the world and even tattoos of it,” Vinther said. With any luck, the new glossy Microraptor will be just as popular.
Carney, R., Vinther, J., Shawkey, M., D’Alba, L., & Ackermann, J. (2012). New evidence on the colour and nature of the isolated Archaeopteryx feather Nature Communications, 3 DOI: 10.1038/ncomms1642
Li, Q., Gao, K., Vinther, J., Shawkey, M., Clarke, J., D’Alba, L., Meng, Q., Briggs, D., & Prum, R. (2010). Plumage Color Patterns of an Extinct Dinosaur Science, 327 (5971), 1369-1372 DOI: 10.1126/science.1186290
Li, Q., Gao, K., Meng, Q., Clarke, J., Shawkey, M., D’Alba, L., Pei, R., Ellison, M., Norell, M., & Vinther, J. (2012). Reconstruction of Microraptor and the Evolution of Iridescent Plumage Science, 335 (6073), 1215-1219 DOI: 10.1126/science.1213780
Zhang, F., Kearns, S., Orr, P., Benton, M., Zhou, Z., Johnson, D., Xu, X., & Wang, X. (2010). Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds Nature, 463 (7284), 1075-1078 DOI: 10.1038/nature08740
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
November 3, 2011
“The fossil record is incredible when it preserves things,” paleontologist Jack Horner said during his talk about dinosaurs and evolution the other night, “but it’s not a complete record.” Many of the sessions and posters I have seen at the annual Society of Vertebrate Paleontology meeting so far are a testament to that truth, either in a positive or negative sense.
In one of the most talked-about presentations delivered so far, McMaster University masters student Ben Novak brought up some substantial obstacles that he and his co-authors have discovered to the hypothesis that remnants of dinosaur soft tissues and proteins have been found in the fossil record. The evidence for long-lived Tyrannosaurus goo may not be as good as previously thought, Novak explained, and the record of proposed dinosaur soft tissue remnants accumulated so far should be reexamined. The fossil record may not be as kind to us with dinosaur remnants as we would like.
Then again, there were notices of some exquisite finds which will provide researchers with a way to better understand dinosaur lives. A poster created by paleontologists Jingmai O’Connor, Zhou Zhonghe and Xu Xing from Beijing’s Institute of Vertebrate Paleontology and Paleoanthropology presented fossil evidence for a Cretaceous turducken. Inside the gut contents of the non-avian, feathered dinosaur Microraptor were the partial remains of a prehistoric bird, and the fact that the bird probably lived in the trees may provide some supporting evidence for the notion that Microraptor may have also been an arboreal animal. Like anything presented at the conference, these findings will be further researched, scrutinized and hopefully published, but such preliminary announcements illustrate the difficulties and the wonders of the fossil record.
But not all the cool announcements are exclusive to SVP. Significant new discoveries pop up regularly in journals, and one that caught my eye is the first description of a Protoceratops nest by University of Rhode Island paleontologist David Fastovsky and colleagues in the Journal of Paleontology. This discovery has been a long time coming.
During the 1920s, American Museum of Natural History expeditions to Mongolia brought back, among other things, dinosaur eggs that they attributed to the horned dinosaur Protoceratops. The researchers were so confident in this assignment that the remains of a small theropod dinosaur found in the same deposits as the supposed Protoceratops eggs was named Oviraptor: “egg thief.” Restorations of Protoceratops parents guarding their nests from Oviraptor hungry from an omelet proliferated through dinosaur books. But reexamination of those eggs during the 1990s showed that paleontologists had the story wrong. Developing dinosaurs preserved inside some eggs were actually oviraptorid dinosaurs—the “egg thief” was more likely a parent! Good thing for us Oviraptor can’t sure for defamation of character.
How Protoceratops nested once again became a mystery, as paleontologists continued to amass more evidence of oviraptorid nests. The closest thing to a Protoceratops nest was an aggregation of small, juvenile dinosaurs found in China and attributable to an evolutionary cousin known as Psittacosaurus. But the new paper by Fastovsky and colleagues documents a rare discovery than can give us some insight into how Protoceratops reproduced and grew up.
The nest in question was found in the roughly 84- to 75-million-year-old strata of the Upper Cretaceous Djadokhta Formation in central Asia. Rather than being a nest full of eggs, though, this Protoceratops nest is packed with baby dinosaurs. Fastovsky and co-authors count as many as 15 juvenile animals inside the nest, but these were not newborns. The degree of skeletal development among the little dinosaurs and a lack of eggshells within the nest indicates that they had already been in the nest for some time. Sadly, these little dinosaurs were buried alive, probably by a sandstorm.
What this discovery indicates about parental care in Protoceratops is uncertain. No adult dinosaur was found in association with the babies. Perhaps the adult continued to care for the little dinosaurs while they remained in the nest, or perhaps they left the nest and the baby dinosaurs remained together in the nest area. With any luck, future discoveries will provide more insight into these points. Nevertheless, the new find adds to the growing body of evidence that many dinosaurs stuck together as juveniles. Their tragedy is a boon for paleontologists hoping to understand dinosaur lives.
Fastovsky, D., Weishampel, D., Watabe, M., Barsbold, R., Tsogtbaatar, K., & Narmandakh, P. (2011). A Nest of Protoceratops andrewsi (Dinosauria, Ornithischia) Journal of Paleontology, 85 (6), 1035-1041 DOI: 10.1666/11-008.1
October 1, 2010
Ever since the announcement of an exquisitely-preserved specimen of the feathered dinosaur Microraptor gui in 2003, paleontologists have been debating how it might have flown and what relevance it might have to the origin of birds. How did it hold its legs? Could it really fly, or just glide? Is is representative of a stage in the origin of flight, or does it represent a different way of taking to the air? Answers to these questions depend on who you ask. Earlier this year a pair of papers appeared in the journal PNAS hypothesizing that the dinosaur held its hindlimbs out to the side—like a crocodile—to create a second set of wings behind the first.
According to a new commentary published in the same journal by American Museum of Natural History paleontologist Stephen Brusatte and colleague Jason Brougham, however, the authors of the recent Microraptor study made some significant errors. It all comes down to a few bits of esoteric anatomy about where the head of the femur (thighbone) articulates with the hip. In the original research paper published by David Alexander and colleagues, the scientists asserted that the hip of Microraptor—as well as all dromaeosaurid dinosaurs (roughly, “raptors” and their kin)—lacked two features of the pelvis called the supracetabular crest and antitrochanter, which normally constrain the flexibility of the hip socket. Without these features, Microraptor could have splayed its legs out to the side to glide.
Not so fast, say Brusatte and Brougham. Dromaeosaurid dinosaurs have supracetabular crests which are reduced in size, but their antitrochanters are actually enlarged in size, and these features would have prevented Microraptor from splaying its legs out in the manner that Alexander and co-authors proposed. This would have made the posture favored by Alexander and colleagues “anatomically implausible,” says Brusatte, adding, “if the femur was held completely lateral to the body, then it would have been dislocated out of its socket.” It does not matter whether the posture hypothesized by the other team of scientists would have made Microraptor a better glider. It simply could not have held its limbs in that position, Brusatte argues, “so flying a model with this posture tells us nothing about how the living animal could actually fly.”
The reason for this difference between the scientists may be a result of the preservation of the dinosaur. The fact that the Microraptor hips Alexander and co-authors used were crushed flat means that they may have mistakenly thought the constraining features were absent. “Even though the [known Microraptor] fossils are crushed,” Brusatte says, “it is still clear that they possessed supracetabular crests and antitrochanters.” Furthermore, Microraptor was closely-related to the recently described dinosaur Hesperonychus, which was preserved with an uncrushed pelvis. In this dinosaur the constraining features are present, Brusatte observes. Although a peculiarity of its hip socket might have given the legs of Hesperonychus a little more flexibility, “there is no way that Hesperonychus could have splayed its legs completely laterally,” says Brusatte.
It is noteworthy that the researchers who published the first PNAS paper have been long-time critics of the well-supported hypothesis that birds evolved from feathered dinosaurs. Their preference for a crocodile-like posture for the hindlimbs of Microraptor is more consistent with their previously stated idea that the first birds evolved from a yet-unidentified lineage of archosaurs.
Naturally, Alexander and his co-authors disagree with the criticisms of Brusatte and Brougham. They state that the hip specimen on which this entire argument hinges truly lacks the constraining features, and they suggest that other small dromaeosaurid dinosaurs lacked them as well. Frustratingly, however, the hip in question has not been extensively described in the accessible peer-reviewed literature. Paleontologist David Burnham featured it in his 2007 thesis and the image has been reproduced in a print-on-demand version of that thesis, but it has yet to be presented to the paleontological community through a detailed analysis published in a peer-reviewed journal. This step would have been essential for building a rigorous case for a sprawl-legged Microraptor, but it was not done in the PNAS study by Alexander, Burnham and their peers.
In the larger context of the origin of flight, though, it is unclear how significant Microraptor might be in investigating how the first birds evolved. Early birds already existed by the time Microraptor lived 120 million years ago, and it is possible that it was simply part of an array of small feathered dinosaurs that independently evolved the ability to glide. “It is unclear whether the gliding capabilities of Microraptor were an odd feature of this dinosaur only, or whether dromaeosaurids more broadly were capable of gliding,” Brusatte says. How significant Microraptor is to the question of how the first birds evolved is something which will require further evidence, but as Brusatte summarizes, understanding the paleobiology of Microraptor will help place the evolution of its close relatives in context:
It is important to study Microraptor, but there are over 40 dromaeosaurids and troodontids—the closet relatives to birds—and these vary greatly in their size, feathery integument, and presumed lifestyle. It is no more fair to say that Microraptor is the key to understanding the origins of avian flight than to say that Deinonychus is. In order to argue that Microraptor‘s gliding ability was a precursor to the origin of flight, it must be demonstrated that the its gliding ability was retained by the immediate ancestors of birds. That is not certain, or even likely, based on current theropod phylogenies.
Alexander DE, Gong E, Martin LD, Burnham DA, & Falk AR (2010). Model tests of gliding with different hindwing configurations in the four-winged dromaeosaurid Microraptor gui. Proceedings of the National Academy of Sciences of the United States of America, 107 (7), 2972-6 PMID: 20133792
Alexander, D., Gong, E., Martin, L., Burnham, D., & Falk, A. (2010). Reply to Brougham and Brusatte: Overall anatomy confirms posture and flight model offers insight into the evolution of bird flight Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1007798107
Brougham J, & Brusatte SL (2010). Distorted Microraptor specimen is not ideal for understanding the origin of avian flight. Proceedings of the National Academy of Sciences of the United States of America PMID: 20864633
Ruben, J. (2010). Paleobiology and the origins of avian flight Proceedings of the National Academy of Sciences, 107 (7), 2733-2734 DOI: 10.1073/pnas.0915099107