December 14, 2012
There’s always something new to learn about dinosaurs. Whether it’s the description of a previously-unknown species or a twist in what we thought we knew about their lives, our understanding of the evolution, biology, and extinction is shifting on a near-daily basis. Even now, paleontologists are pushing new dinosaurs to publication and debating the natural history of these wonderful animals, but the end of the year is as good a time as any to take a brief look back at what we learned in 2012.
For one thing, there was an exceptional amount of dino-hype this year. A retracted paper that mused on the nature of hypothetical space dinosaurs, a credulous report on an amateur scientist who said he had evidence that all dinosaurs were aquatic, and overblown nonsense about dinosaurs farting themselves into extinction all hit the headlines. (And the less said about the Ancient Aliens dinosaur episode, the better.) Dinosaurs are amazing enough without such sensationalist dreck, or, for that matter, being transformed into abominable human-raptor hybrids by Hollywood.
Not all the dinosaurs to wander into the media spotlight were atrocious, though. The glossy book Dinosaur Art collected some of the best prehistoric illustrations ever created, and the recently-released All Yesterdays presented dinosaurs in unfamiliar scenes as a way to push artists to break from severely-constrained traditions. Dinosaurs were probably much more unusual than we have ever imagined.
Indeed, new discoveries this year extended the range of fluff and feathers among dinosaurs and raised the question of whether “enfluffledness” was an ancient, common dinosaur trait. Paleontologists confirmed that the ostrich-like Ornithomimus–long suspected to have plumage–sported different arrangements of feathers as it aged. New insight on the 30-foot-long carnivore Yutyrannus affirmed that even big tyrannosaurs were covered in dinofuzz. And while both Ornithomimus and Yutyrannus belonged to the feathery subset of the dinosaur family tree that includes birds, the discovery of fluff on a much more distantly related theropod–Sciurumimus–hints that feathers were a much older, more widespread dinosaur feature than previously expected. Paired with previous finds, Sciurumimus suggests that protofeathers either evolved multiple times in dinosaurian history, or that the simple structures are a common inheritance at the base of the dinosaur family tree that was later lost in some groups and modified in others.
While some traditionalists might prefer scaly dinosaurs over fuzzy ones, feathers and their antecedents are important clues that can help paleontologists explore other aspects of paleobiology. This year, for example, researchers reconstructed dark, iridescent plumage on Microraptor on the basis of fossil feathers, and, as display structures, feathery decorations will undoubtedly have a role to play in the ongoing debate about how sexual selection influenced dinosaur forms. Feathers can also be frustrating–a new look at the plumage of Anchiornis and Archaeopteryx will undoubtedly alter our expectations of how aerially capable these bird-like dinosaurs were and how they might have escaped predatory dinosaurs that dined on the prehistoric fowl. Such lines of inquiry are where the past and present meet–after all, birds are modern dinosaurs.
Feathers aren’t the only dinosaur body coverings we know about. Skin impressions, such as those found with the ankylosaur Tarchia, have also helped paleontologists discern what dinosaurs actually looked like. Pebbly patterns in Saurolophus skin can even be used to differentiate species, although paleontologists are still puzzled as to why hadrosaurs seem to be found with fossil skin traces more often than other varieties of dinosaur.
And, speaking of ornamentation, a damaged Pachycephalosaurus skull dome might provide evidence that these dinosaurs really did butt heads. How the adornments of such dinosaurs changed as they aged, though, is still a point of controversy. One of this year’s papers threw support to the idea that Torosaurus really is a distinct dinosaur, rather than a mature Triceratops, but that debate is far from over.
Other studies provided new insights into how some dinosaurs slept, the evolutionary pattern of dinosaur succession, what dinosaur diversity was like at the end of the Cretaceous, and how dinosaurs grew up, but, of course, how dinosaurs fed is a favorite place that lies at the intersection of science and imagination. A poster at the annual Society of Vertebrate Paleontology meeting deconstructed how Tyrannosaurus rex–suggested to have the most powerful bite of any terrestrial animal ever–tore the heads off of deceased Triceratops. The herbivorous Diplodocus, by contrast, munched soft plants and stripped branches of vegetation rather than gnawing on tree bark, and the tiny, omnivorous Fruitadens probably mixed insects with its Jurassic salads. Studying dinosaur leftovers also explained why paleontologists didn’t find more of the mysterious Deinocheirus, which thus far has been identified by only one incomplete fossil–the long-armed ornithomimosaur was eaten by a Tarbosaurus.
We also met a slew of new dinosaurs this year, including the many-horned Xenoceratops, the archaic coelurosaur Bicentenaria, the sail-backed Ichthyovenator, the stubby-armed Eoabelisaurus, and the early tyrannosaur Juratyrant. This is just a short list of species I wrote about–a few that add to the ever-increasing list.
To properly study dinosaurs and learn their secrets, though, we must protect them. One of the most important dinosaur stories this year wasn’t about science, but about theft. An illicit Tarbosaurus skeleton – pieced together from multiple specimens smuggled out of Mongolia–has brought wide attention to the fossil black market, as well as the poachers and commercial dealers who fuel it. The fate of this dinosaur remains to be resolved, but I’m hopeful that the dinosaur will be returned home and will set a precedent for more vigorously going after fossil thieves and their accomplices.
Out of all the 2012 dinosaur stories, though, I’m especially excited about Nyasasaurus. The creature’s skeleton is as yet too fragmentary to know whether it was true dinosaur or the closest relative to the Dinosauria as a whole, but, at approximately 243 million years old, this creature extends the range of dinosaurs back in time at least 10 million years. That’s another vast swath of time for paleontologists to examine as they search for where dinosaurs came from, and those discoveries will help us better understand the opening chapters in the dinosaurian saga. That’s the wonderful thing about paleontology–new discoveries open new questions, and those mysteries keep us going back into the rock record.
And with that, I must say goodbye to Dinosaur Tracking. On Tuesday I’m starting my new gig at National Geographic’s Phenomena. I’ve had a blast during my time here at Smithsonian, and I bid all my editors a fond farewell as I and my favorite dinosaurs head off to our new home.
Editor’s Note: Best wishes to Brian on his future travels and we all thank him for his hard work over the past 4 (!) years, writing every day about something new on dinosaurs. It’s not nearly as easy as he makes it look. – BW
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
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
December 30, 2011
Another year, another fantastic spate of dinosaur discoveries. Even as 2011 draws to a close, the findings keep rolling in—from the way Deinonychus used its killer cutlery to the first record of sauropod dinosaurs from Antarctica and sexual selection among dinosaurs. There has been such a glut of interesting papers that it would be impossible to mention every bit of dinosauriana from this year, but here is a partial listing of some of the stories that caught my eye.
Everyone knows that there are lots of unknown dinosaur species left to be discovered. What has become increasingly contentious is the question of how many species can be counted among what has already been collected. This year saw a continuation of the 2010 “Toroceratops” debate with a paper on the enigmatic Nedoceratops by Andrew Farke early in the year, followed by a response to his paper by John Scannella and Jack Horner this month. Likewise, paleontologists suggested that the hadrosaur Anatotitan and the tyrannosaur Raptorex were really just growth stages of other known dinosaurs (the latter being similar to Tarbosaurus, a juvenile of which was also described this year).
How did dinosaurs perceive their world? Two significant papers approached this question—one focused on smell (see the video above), and the other vision. As with studies of dinosaur growth, though, investigations of dinosaur senses can be controversial. Last week’s issue of Science included a comment and reply about the idea that the bony rings preserved in the eyes of some dinosaurs might be used to reconstruct the time of day when the animals were most active.
This year marked the 150th anniversary of the discovery of Archaeopteryx. But 2011 has been full of ups and downs for the Urvogel. Even though an 11th specimen of the feathered dinosaur was announced, a controversial paper proposed that the creature was not an early bird but rather a non-avian dinosaur more distantly related to the first birds. Exactly what Archaeopteryx is and what that interpretation means for our understanding of bird evolution will continue to be debated.
New dinosaurs are named just about every week, but two in particular caught my eye: Brontomerus, a sauropod whose name translates to “thunder thighs,” and Teratophoneus, a short-snouted tyrannosaur. (I just realized that both were found in Utah, though, so perhaps I have a bias for my adoptive state!)
That is just a smattering of findings from 2011. Shout out your favorite 2011 dinosaur discoveries in the comments. And, if you want to see how 2011 compares to previous years, see my lists from 2010 and 2009.
October 26, 2011
Since the time the English anatomist Richard Owen described Archaeopteryx as the “by-fossil-remains-oldest-known feathered Vertebrate” in 1863, the curious creature has been widely regarded as the earliest known bird. Lately, though, the status of the iconic animal has been up for debate. Earlier this summer, one team of paleontologists proposed that Archaeopteryx was not a bird but actually a feather-covered, non-avian dinosaur more closely related to genera like Microraptor and Troodon. Now a different team of paleontologists has published a paper in Biology Letters that says Archaeopteryx was an early bird after all.
The ongoing back and forth over Archaeopteryx reminds me of the old Looney Tunes bit where Bugs Bunny and Daffy Duck keep going back and forth over which hunting season it is. “Duck season.” “Wabbit season!” “Duck season” “WABBIT SEASON!” In the same way, the argument over Archaeopteryx could seemingly go on indefinitely. The reasons why have everything to do with how both science and evolution work.
The study of prehistoric life, like any other science, is not restricted to the slow and steady accumulation of facts. Facts are most certainly acquired through studies in the field and lab alike, but to tell us anything significant about dinosaurs, these facts must be understood according to theories and hypotheses. An exasperated Charles Darwin conveyed this truth eloquently in an 1861 letter he wrote to colleague Henry Fawcett:
About thirty years ago there was much talk that geologists ought only to observe and not theorise; and I well remember some one saying that at this rate a man might as well go into a gravel-pit and count the pebbles and describe the colours. How odd it is that anyone should not see that all observation must be for or against some view of it is to be of any service!
Facts, theories and hypotheses are all necessary and interacting parts of the scientific process. As new discoveries are made and ideas are tested, the context by which we understand what dinosaurs were and how they lived changes. This is to be expected—there are always more questions and mysteries about dinosaurs than readily available answers. In the case of Archaeopteryx, we know this feather-covered dinosaur lived on a group of roughly 150-million-year-old islands that would eventually become southeastern Germany. Whether or not Archaeopteryx belonged to that successful lineage of feathered dinosaurs called birds, though, is something that depends on other feathered dinosaur discoveries and the techniques used to test ideas about relationships among animals.
Teasing out relationships among prehistoric animals is a comparative science. The key is finding traits that are shared in some organisms due to common ancestry but are absent in others. This can be a tricky process. Due to a shared way of life, for example, unrelated organisms may have developed superficially similar traits through a phenomenon called convergent evolution. Paleontologists must carefully choose the traits being compared, and the discovery of additional dinosaurs adds more grist to the comparative mill.
Archaeopteryx is actually a perfect example of how new discoveries can change our perception of relationships. When the first skeleton was discovered in 1861, nothing quite like it had been found. Archaeopteryx seemed to stand by itself as the first bird. Over a century later, though, the discovery of dinosaurs such as Deinonychus, an updated understanding of dinosaurs and the eventual discovery of many, many feathered dinosaurs illustrated that Archaeopteryx exhibited a number of transitional features that illustrated how the first birds evolved directly from feathered dinosaurs.
The trouble is that Archaeopteryx appears to be so close to the emergence of the very first birds. At the moment, Archaeopteryx is most often regarded as being an archaic member of the group called the Avialae, which contains all birds (Aves) and forms more closely related to them than to other dinosaurs. What this means is that, as our understanding of what a bird actually is changes, the position of Archaeopteryx might shift. The animal might have been one of the earliest birds within the avialian group, or Archaeopteryx might have been just outside the bird group among non-avian dinosaurs. This is simply how science works and is a wonderful—if frustrating—demonstration of the fact of evolution.
Birds did not simply pop out of nowhere. The earliest avians went through a long period of transformation, and the continuum between feathered, non-avian dinosaurs and the first birds, which paleontologists are now filling in, demonstrates the beauty of major evolutionary change. The debate over the position of Archaeopteryx is happening now precisely because of all the evidence for this evolutionary change that has been accumulated in the past two decades. No matter what Archaeopteryx turns out to be, the creature will remain important to both the historical development of our ideas about evolution and the actual, prehistoric transition from non-avian to avian dinosaurs.
For more on changing perspectives on long-known dinosaurs, see this week’s post on the fate of the horned dinosaur Torosaurus.
Lee, M., & Worthy, T. (2011). Likelihood reinstates Archaeopteryx as a primitive bird Biology Letters DOI: 10.1098/rsbl.2011.0884
Xu, X.; You, H.; Du, K.; Han, F. (2011). An Archaeopteryx-like theropod from China and the origin of Avialae Nature, 475, 465-470 DOI: 10.1038/nature10288