November 28, 2012
How did feathered dinosaurs take to the air? Paleontologists have been investigating and debating this essential aspect of avian evolution for over a century. Indeed, there have been almost as many ideas as they have been experts, envisioning scenarios of dinosaurs gliding through trees, theropods trapping insects with their feathery wings and even aquatic Iguanodon flapping primitive flippers as flight precursors (I didn’t say that all the ideas were good ones). The biomechanical abilities of bird ancestors and their natural history has always been at the center of the debate, and a new Current Biology paper adds more fuel to the long-running discussion.
At present, hypotheses for the origin of avian flight typically fall into one of two categories. Either bird ancestors accrued the adaptations necessary for flight on the ground and, through evolutionary happenstance, were eventually able to take off, or small tree-dwelling dinosaurs used their feathery coats to glide between trees and, eventually, flapped their way into a flying lifestyle. There are variations on both themes, but feathers and the characteristic avian flight stroke are at the core of any such scenario. In the case of the new paper, Yale University paleontologist Nicholas Longrich and colleagues draw from the plumage of early bird Archaeopteryx and the troodontid Anchiornis to examine how feathers changed as dinosaurs started to fly.
In modern flying birds, Longrich and coauthors point out, the wing arrangement typically consists of “long, asymmetrical flight feathers overlain by short covert feathers.” This pattern creates a stable airfoil but also lets the flight feathers separate a little during the upstroke of a wing beat, therefore reducing drag. When the paleontologists examined the fossilized wings of Archaeopteryx and Anchiornis, they found different feather arrangements that would have constrained the flight abilities of the Jurassic dinosaurs.
Both prehistoric creatures had long covert feathers layered on top of the flight feathers. Anchiornis, in particular, appeared to have an archaic wing form characterized by layers of short, symmetrical flight feathers and similarly shaped coverts. Archaeopteryx showed more specialization between the flight feathers and the coverts but still did not have a wing just like that of a modern bird. As a result, Longrich and collaborators hypothesize, both arrangements would have stabilized the wing at the cost of increased drag at low speeds, making it especially difficult for Anchiornis and Archaeopteryx to take off. As an alternative, the researchers suggest that these dinosaurs might have been parachuters who jumped into the air from trees, which might hint that “powered flight was preceded by arboreal parachuting and gliding.”
The trick is determining whether Anchiornis and Archaeopteryx actually represent the form of bird ancestors, or whether the dinosaurs, like Microraptor, were independent experiments in flight evolution. At the Society of Vertebrate Paleontology conference in Raleigh, North Carolina last month, flight expert Michael Habib quipped that all that was needed to make dromaeosaurs aerially competent was the addition of feathers. If Habib is right, and I think he is, then there could have been multiple evolutionary experiments in flying, gliding, wing-assisted-incline-running and other such activities. There’s no reason to think that flight evolved only once in a neat, clean march of ever-increasing aerodynamic perfection. Evolution is messy, and who knows how many ultimately failed variations there were among flight-capable dinosaurs?
The three-step Anchiornis-Archaeopteryx-modern bird scenario of wing evolution fits our expectations of what a stepwise evolutionary pattern would look like, but, as the authors of the new paper point out, shifting evolutionary trees currently confound our ability to know what represents the ancestral bird condition and what characterized a more distant branch of the feathered dinosaur family tree. We need more feathery fossils to further investigate and test this hypothesis, as well as additional biomechanical and paleoecological information to determine whether such dinosaurs really took off from trees. We must take great care in distinguishing between what an organism could do and what it actually did, and with so much up in the air, the debate on the origin of flight will undoubtedly continue for decades to come.
Longrich, N., Vinther, J., Meng, Q., Li, Q., Russell, A. 2012. Primitive wing feather arrangement in Archaeopteryx lithographica and Anchiornis huxleyi. Current Biology DOI: 10.1016/j.cub.2012.09.052
November 22, 2012
Tonight, at dinner tables all around the country, families are going to dine on dinosaur. If you dissect your holiday theropod just right, the ancient nature of the tasty avian is strikingly evident–right down to the wishbone. But what kind of dinosaur is a turkey, anyway?
Birds are dinosaurs. That’s a fact. But birds are really just one kind of dinosaur. Indeed, we call Triceratops, Euoplocephalus, Futalognkosaurus, Allosaurus and their ilk non-avian dinosaurs because these lineages fell outside the bird subgroup at greater or lesser distances. Birds are a distinct form of dinosaur, nested within a great group of fuzzy and feathery forms.
Let’s start from the bottom up. The dinosaur family tree is divided into two major branches–the ornithischians (the ceratopsids, hadrosaurs, stegosaurs and their relatives) and the saurischians. The saurischian side is made up of the long-necked, big-bodied sauropodomorphs and the bipedal, often-carnivorous theropods. The theropod subset is further subdivided into various groups, one of the major ones being the coelurosaurs. This subset includes the the famous tyrannosaurs, ostrich-like ornithimomosaurs, odd-looking oviraptorosaurs, sickle-clawed deinonychosaurs and birds, among a few others. Every lineage within this group contained at least one representative with feathers, and many of these dinosaurs were quite bird-like both anatomically and behaviorally.
Now here’s where things get tricky. For decades, numerous anatomical characteristics seemed to link the earliest birds, represented by Archaeopteryx, with deinonychosaurs similar to Velociraptor and Troodon. But some paleontologists have questioned this hypothesis. Last year, a controversial Nature paper suggested that the resemblance was because Archaeopteryx wasn’t actually a bird but a non-avian dinosaur more closely related to Deinonychus, while the first birds evolved from feathered dinosaurs akin to Oviraptor or the enigmatic Epidexipteryx. Rather than being deadly hypercarnivores, these alternative candidates for avian ancestry were oddball omnivores that often sported flashy tail feathers.
Not everyone agrees with the new proposal. For now, Archaeopteryx is still widely regarded to be at the base of the bird family tree, recently branched off from a deinonychosaur ancestor. Nevertheless, the argument underscores the point that many traits thought to be exclusively avian evolved much earlier in dinosaurian history than we previously expected. The more dinosaurs we find, the smaller the difference between the earliest avian dinosaurs and their non-avian ancestors. I know the pudgy kid in Jurassic Park called Velociraptor as “six foot turkey” as a put-down, but the comment isn’t too far of the mark. When you pick at the bird on your plate tonight, you’re devouring the dressed remains of a distant Deinonychus cousin.
October 25, 2012
Another week, another feathery dinosaur. Since the discovery of the fluffy Sinosauropteryx in 1996, paleontologists have discovered direct evidence of fuzz, feather-like bristles and complex plumage on over two dozen dinosaur genera. I love it, and I’m especially excited about a discovery announced today. In the latest issue of Science, University of Calgary paleontologist Darla Zelenitsky adds another enfluffled species to the dinosaurian ranks. Even better, the specimens raise hopes that many more dinosaurs might be preserved with their feathery coats intact.
Zelenitsky’s downy dinosaurs are not newly discovered species. Ornithomimus edmontonicus was initially described by famed bone hunter C.H. Sternberg in 1933, and it is one of the characteristic Late Cretaceous species found in Alberta, Canada’s fossil-rich Horseshoe Canyon Formation. In Sternberg’s time, these dinosaurs were thought to be scaly, but recent finds of so many feathery dinosaurs has raised the likeliehood that the “ostrich mimic” dinosaur was at least coated in some sort of dinofuzz.
The prediction of fluffy Ornithomimus came from the spread of feathers on the coelurosaur family tree. The Coelurosauria is a major dinosaur group that encompasses tyrannosaurs, compsognathids, ornithomimosaurs, alvarezsaurs, oviraptorosaurs, deinonychosaurs and birds. To date, evidence of feathers has been found in every coelurosaur lineage except one–the ornithomimosaurs. The spread of feathers hinted that some sort of plumage was present in the common ancestor of all coelurosaurs and therefore should have been inherited by the ornithomimosaurs, but, until now, no one had found direct evidence.
A trio of Ornithomimus skeletons have finally confirmed what paleontologists expected. Zelenitsky enthusiastically explained the details to me by phone earlier this week. In 1995, when Zelenitsky was a graduate student, paleontologists uncovered an articulated Ornithomimus with weird marks on its forearms. No one knew what they were. But in 2008 and 2009 a juvenile and an adult Ornithomimus turned up with preserved tufts of filamentous feathers. “When we found these specimens,” Zelenitsky said, “we made the link to the 1995 dinosaur.” All those strange marks on the arms of the previously discovered Ornithomimus, Zelenitsky and colleagues argue, are traces of longer, shafted feathers.
Even though paleontologists expected feathery Ornithomimus, the discovery was still a surprise. “I was in disbelief,” Zelenitsky said. “They’re the first feathered dinosaurs from the Americas, and the first ornithomimosaurs with feathers, as well. It was shocking to say the least.”
But there’s more to the find than simply adding another species of fluffy dinosaurs to the list. The fact that the adult and juvenile animals had different kinds of plumage adds new evidence that coelurosaurs changed their fluffy coats as they aged. “The one juvenile was completely covered in filamentous type feathers,” Zelenitsky said. What the adults looked like comes from the two other specimens. One adult skeleton, lacking forearms, preserves fuzzy feathers, and “the second adult had markings on the forearm.” Together, the specimens indicate that adult Ornithomimus were mostly covered in fuzz but developed more complex arm feathers by adulthood.
Sex is probably behind the plumage change. “We infer that because these wing feathers are not showing up until later in life, they were used for reproductive purposes,” Zelenitsky said. Perhaps adult Ornithomimus used flashy arm feathers to strut their stuff in front of potential mates. Then again, based upon the resting and brooding postures of other theropod dinosaurs, adult Ornithomimus could have used their proto-wings to cover their nests. We don’t know for sure, but the developmental change appears to be another example of dinosaurs undergoing significant changes as they approach sexual maturity. This discovery, and others like it, will undoubtedly play into the ongoing discussion about the role of sexual selection in dinosaur biology and evolution.
Best of all, the new study indicates that paleontologists may soon find more feathered dinosaurs in unexpected places. The Ornithomimus skeletons were found in prehistoric river deposits composed of sandstone. Since almost all feathered non-avian dinosaurs have been found in fine-grained sediment–such as those around Liaoning, China–paleontologists thought that coarser-grained sandstone deposits were too rough to record such fine details. Now we know better. “That’s the really exciting part of it,” Zelenitsky says. If traces of dinosaur feathers can be preserved in sandstone, the twist opens up the possibility that paleontologists might find fluff and feathers with a greater array of dinosaurs–including the tyrannosaurs, deinonychosaurs, therizinosaurs and other coelurosaurs of North America. The trick is recognizing the traces before they’re destroyed during excavation and preparation. Rock saws and airscribes can all too easily obliterate the delicate fossils. A word to researchers–keep your excavation tools sharp, and your eyes sharper.
Zelenitsky, D., Therrien, F., Erickson, G., DeBuhr, C., Kobayashi, Y., Eberth, D., Hadfield, F. 2012. Feathered non-avian dinosaurs from North American provide insight into wing origins. Science. 338, 510-514
October 11, 2012
How dinosaurs took to the air is one of the longest-running debates in paleontology. Ever since the first skeleton of Archaeopteryx was discovered in 1861, researchers have wondered what the archaic bird might tell us about how flight evolved and how the feathery creature connected its reptilian ancestors with modern birds. Even now, when we know that birds are a feathered dinosaur lineage, the origins of flight remain a contentious issue constrained by the available fossil evidence and our ability to reconstruct how prehistoric creatures moved.
Before paleontologists confirmed that birds are dinosaurs, though, various researchers came up with speculative schemes to explain how birds originated. Naturalist William Beebe, for one, proposed that bird ancestors started off as parachuting reptiles that benefited from expanded scales (his conception of protofeathers). Other scientists came up with their own ideas, imagining everything from seagoing protobirds to gliding reptiles.
When ornithologist Colin Pennycuick wrote his paper “Mechanical Constraints on the Evolution of Flight” in 1986, however, paleontologists were warming to the idea that Archaeopteryx spanned the evolutionary space between living birds and dinosaurs like Deinonychus. This narrowed down the list of early flight scenarios to hotly debated “ground up” or “trees down” hypotheses for the origin of flight, and raised the possibility that feathers evolved among non-avian dinosaurs first. Within these debates, Pennycuick put forward his own idiosyncratic proposal.
Pennycuick believed that birds took to the air by way of the trees. Bird ancestors progressively shrunk in size over time, he believed, and started gliding before they could actually fly. He couldn’t envision that birds evolved from a running, leaping ancestor, as other researchers suggested. For Pennycuick, flight was a gradual extension of gliding.
But what did the ancestor of Archaeopteryx look like? Pennycuick assumed that feathers and flight were closely tied together–something that is not true at all and had already been pointed out by paleontologist John Ostrom in his work on bird origins. Feathers are important for display and insulation and were only later co-opted for flight. All the same, Pennycuick needed a gliding–but featherless–ancestor for Archaeopteryx to make his idea work. So he conjured something really weird.
Pennycuick was puzzled by the clawed fingers of Archaeopteryx. Why would a bird have differentiated fingers? Rather than look at the fingers as just a holdover from dinosaurian ancestry, Pennycuick assumed that they had some kind of flight function. The fingers of Archaeopteryx, he proposed, “could have supported a small, batlike hand-wing.” Such a structure would have been inherited from the featherless ancestor of Archaeopteryx, he proposed, “constituting the main wing area in the stage before feathers were developed.”
Where the feathers of Archaeopteryx came from, Pennycuick couldn’t say. He mused on the need for feathers in the transition from gliding to flight, but he didn’t offer an explanation for how feathers evolved. He only mentioned that “The development of down feathers as thermal insulation is a separate process that may or may not have preceded the development of flight feathers.”
The fuzzy dinosaur Sinosauropteryx proved Pennycuick wrong a decade later. Paleontologists like Ostrom and artists such as Gregory S. Paul had long suspected that feathers were a widespread trait among bird-like theropod dinosaurs, and a flood of exceptional fossils has shown that feathers and their precursors have a deep, deep history. Dinofuzz, or structurally similar body coverings, might even go back to the root of the Dinosauria. How evolutionary forces molded those adornments, however, and what drove the evolution of flight feathers, remain as aggravatingly contentious as ever.
[Hat-tip to paleontologist Victoria Arbour for bringing this paper to my attention]
Pennycuick, C. 1986. Mechanical Constraints on the Evolution of Flight. Memoirs of the California Academy of Sciences. 8, 83-98
September 6, 2012
Anyone who regularly reads this blog knows that there’s a very easy way to make me annoyed–all you have to do is start whining about how dinosaurs are less cool since paleontologists discovered that many non-avian species sported tufts and coats of fluff, fuzz, bristles and feathers. My reaction is usually along the lines of “Brian SMASH!” Even though I understand that some people find scaly, monstrous dinosaurs aesthetically appealing, I have no patience for the callow assertion that science has somehow ruined dinosaurs through the addition of plumage.
Cartoonist Randall Munroe summed up my feelings–albeit in a more concise and positive way–this week at XKCD. Restoring dinosaurs with protofuzz and feathers isn’t just about giving Tyrannosaurus, Velociraptor and company a new look. Dinosaur feathers, and feather-like structures, are allowing paleontologists to think of dinosaurs in new ways. In particular, Munroe cites a PLoS One study about how feathers may have played into the predatory behavior of sickle-clawed dromaeosaurs such as Deinonychus. According to paleontologist Denver Fowler and co-authors, Deinonychus may have used its famous “killing claw” to pin down small prey just like modern hawks and eagles do. More than that, the avian raptors flap to help stabilize themselves while immobilizing their prey, and Deinonychus–almost certainly a feathered dinosaur–may have done the same.
We can’t know for sure whether Deinonychus killed prey like a big, grounded version of a hawk. But it’s possible. Either way, though, studies like these show that prehistoric dinosaur feathers are allowing paleontologists to look to modern birds to generate new hypotheses and tease out previously-unknown aspects of dinosaur lives. As I’ve mentioned before, feathers are the key to figuring out dinosaur colors. How wonderful is that? Again, Munroe says it better than I can: “The past keeps getting cooler!”
Post script: Munroe isn’t the only cartoonist to take on dinosaurs this week. FoxTrot’s Bill Amend had a few suggestions for the Smithsonian National Museum of Natural History’s dinosaur hall renovation. Paleontology curator Matt Carrano responded to the idea of installing a “Tourist Chompsognathus” at our Around the Mall blog.