December 7, 2012
When paleontologist John Ostrom named Deinonychus in 1969, he provided the spark for our long-running fascination with the “raptors.” Similar dinosaurs had been named before–Velociraptor and Dromaeosaurus were named four decades earlier–but the skeleton of Ostrom’s animal preserved a frightening aspect of the dinosaur that had not yet been seen among the earlier finds. The assembled remains of Deinonychus included the dinosaur’s eponymous “terrible claw”–a wicked, recurved weapon held off the ground on the animal’s hyperextendable second toe. Combined with the rest of the dinosaur’s anatomy, Ostrom argued, the frightening claw indicated that Deinonychus must have been a active, athletic predator.
But how did Deinonychus and its similarly-equipped relatives use that awful toe claw? The appendage looks fearsome, but paleontologists have not been able to agree on whether the claw was using for slashing, gripping, pinning, or even climbing prey. Some researchers, such as Phil Manning and collaborators, have even argued that the claws of Velociraptor and related dinosaurs were best suited to scaling tree trunks–a conclusion consistent with the contentious hypothesis that the ancestors of birds were tree-climbing dinosaurs.
All this assumes that the claws of deinonychosaurs correspond to a special behavior, but can foot claw shapes really give away the habits of dinosaurs? That’s the question posed by a new PLoS One study by zoologist Aleksandra Birn-Jeffery and colleagues.
Based on observations of living animals, researchers have often tied particular claw shapes to certain behaviors–relatively straight, stubby claws likely belong to an animal that runs on the ground, while tree-climbing species have thin claws with small, sharp points. But nature isn’t quite so neat as to have a single, tell-tale claw shape for perchers, ground-runners, climbers, and predators. Even then, researchers don’t always interpret claw shapes the same way–depending on who you ask, the foot claws of the early bird Archaeopteryx either indicate that it was a climber or could only run on the ground.
To parse this problem, Birn-Jeffery and co-authors studied the geometry of the third toe claw–on dinosaurs, the middle toe claw–in 832 specimens of 331 species, together representing different lifestyles of birds, lizards, and extinct dinosaurs. The claw shapes didn’t strictly conform to particular behaviors. In the climber category, for example, the frill-necked lizard has lower claw curvature than expected, and, among predatory birds, the common buzzard, secretary bird, and greater sooty owl has less sharply recurved claws that anticipated for their lifestyle.
When the dinosaur data was dropped into the mix, the deinonychosaurs didn’t seem to fit in any single category. The sickle-clawed carnivores fell into the range shared by climbers, perchers, predators, and ground dwellers–these dinosaurs could be said to be anything from wholly terrestrial runners to perchers. And even though the researchers identified a general claw shape that corresponded to walking on the ground–deeper claws with less curvature–the dinosaurs did not strictly fit into this category alone.
Some dinosaurs, such as Microraptor, had claws that might have been suited to climbing. However, dinosaurs that we might regard as behaviorally similar showed differences–Velociraptor seemed to best fit the ground-dweller category, while the larger Deinonychus seemed to have claws more akin to those of predatory birds. This doesn’t mean that Microraptor was definitely a climber, or that Velociraptor wasn’t a predator. As the authors show, the different behavioral categories are not so easily distinguishable as previously thought, and saying that an animal definitely engaged in a particular behavior because of claw shape alone tempts oversimplification.
No wonder there has been such a range of interpretation about dinosaur foot claws! While the new study focused on the third toe claw rather than the famous, second deinonychosaur toe claw, the point of the analysis still applies. Claw geometry alone is not a reliable indicator of behavior. That’s to be expected–as the authors point out, claws are multi-functional, are are unlikely to represent just one type of behavior or habitat. Birds that use their claws to perch may also use them to kill prey, or birds that primarily live in the trees may also forage on the ground. Claw shape is constrained by different aspects of natural history, and reflect flexibility rather than strict adherence to a particular lifestyle. Deinonychosaur claws definitely hold clues to the natural history of dinosaurs, but drawing out those clues is a difficult, convoluted process.
Birn-Jeffery, A., Miller, C., Naish, D., Rayfield, E., Hone, D. 2012. Pedal Claw Curvature in Birds, Lizards and Mesozoic Dinosaurs – Complicated Categories and Compensating for Mass-Specific and Phylogenetic Control. PLoS ONE. 7,12: e50555. doi:10.1371/journal.pone.0050555
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 7, 2012
Sauropods were extreme dinosaurs. From the relatively small dwarfed species–still a respectable 12 feet long or so–to giants that stretched over 100 feet long, these small-headed, column-limbed, long-necked dinosaurs were among the strangest creatures ever to walk the earth. Don’t be fooled by the familiarity of species like Apatosaurus and Brachiosaurus; the anatomy of sauropods was so strange that paleontologists are still debating basic issues of their biology. How sauropods mated, fed, pumped blood from their hearts to their heads and even how they held their necks have all provided rich grounds for debate among specialists. Among the longest-running mysteries is how such enormous and undoubtedly active animals prevented themselves from overheating. Perhaps the solution lies in an anatomical quirk shared with birds.
Diplodocus and kin might have had a problem with body temperature. Multiple lines of evidence, from histology to limb proportions, have indicated that extinct dinosaurs had physiological profiles more like those of avian dinosaurs and mammals than any reptile, but maintaining an active metabolism and high body temperature came at a cost for gigantic dinosaurs. The bigger the dinosaur, the more difficult it would have been to dump excess heat. If a hot-running sauropod had to hoof it to catch up with a mate or escape a stalking theropod, the dinosaur could run the risk of overheating through exercise.
The difficulty big sauropods must have faced with shedding heat has sometimes been cited as a reason that these dinosaurs must have had an ectothermic, crocodile-like physiology, or that they were “gigantotherms” that only maintained relatively high body temperatures by virtue of their size and therefore had a little more leeway with heat generated through exercise. As paleontologist Matt Wedel argued in a 2003 review of sauropod biology, though, these positions are based upon assumptions about dinosaur respiratory systems and physiology that used crocodylians as models. Not only has evidence from bone microstructure indicated that sauropods grew at an extremely rapid pace on par with that of mammals, but paleontologists have found that sauropods had birdlike respiratory systems that combined lungs with a system of air sacs. Such a system would have been attuned to cope with an active, endothermic lifestyle, including a way to dump excess heat.
We know sauropods had air sacs because of their bones. In the neck, especially, air sacs stemming from the core of the respiratory system invaded the bone and left distinctive indentations behind. (While not always as extensive, theropod dinosaurs show evidence of these air sacs, too. To date, though, no one has found solid evidence of air sacs in the ornithischian dinosaurs, which includes the horned ceratopsians, shovel-beaked hadrosaurs and armored ankylosaurs.) In addition to lightening the skeletons of sauropods and boosting their breathing efficiency, this complex system may have played a role in allowing sauropods to dump heat through evaporative cooling in the same way that large birds do today. The concept is similar to what makes a swamp cooler work–the evaporation of water in the moist tissues of a sauropod’s trachea during exhalation would have helped the dinosaur dump heat into outgoing air.
But the role of air sacs in such a system, much less an animal 80 feet long or more, is unclear. The inference is obvious–like birds, sauropods had the anatomical hardware to cool themselves–but the mechanics of the process are still obscure given that we can’t observe a living Mamenchisaurus. Earlier this fall, however, biologist Nina Sverdlova and colleagues debuted research that may help paleontologists more closely examine sauropod breathing.
Using observations from living birds, Sverdlova created a virtual model of a chicken’s trachea and air sac with an eye towards simulating heat exchange. The researchers found that their relatively simple model was able to approximate experimental data from living birds, and so similar models may help paleobiologists estimate how sauropods dumped heat. We’ll have to wait for what future studies find. This line of evidence won’t totally resolve the debate over sauropod physiology and body temperature, but it may help paleobiologists more closely investigate the costs and benefits of being so big.
Sander, P., Christian, A., Clauss, M., Fechner, R., Gee, C., Griebeler, E., Gunga, H., Hummel, J., Mallison, H., Perry, S., Preuschoft, H., Rauhut, O., Remes, K., Tutken, T., Wings, O., Witzel, U. 2011. Biology of the sauropod dinosaurs: the evolution of gigantism. Biological Reviews 86: 117-155
Sverdlova, N., Lambertz, M., Witzel, U., Perry, S. 2012. Boundary conditions for heat transfer and evaporative cooling in the trachea and air sac system of the domestic fowl: A two-dimensional CFD analysis. PLOS One 7,9. e45315
Wedel, M. 2003. Vertebral pneumaticity, air sacs, and the physiology of sauropod dinosaurs. Paleobiology 29, 2: 243-255
September 24, 2012
Paleontologist R.T. Bird inspected many dinosaur trackways while combing Texas for the perfect set to bring back to the American Museum of Natural History. During several field seasons in the late 1930s, Bird poked around in the Early Cretaceous rock in the vicinity of the Paluxy River for a set of sauropod footprints that would fit nicely behind the museum’s famous “Brontosaurus” mount. Bird eventually got what he was after but not before poring over other intriguing dinosaur traces. One of the most spectacular seemed to be made by a swimming dinosaur.
Known as the Mayan Ranch Trackway, the roughly 113-million-year-old slab is almost entirely made up of front foot impressions. The semicircular imprints were undoubtedly left by one of the long-necked sauropod dinosaurs. But towards the end of the trail, where the dinosaur’s path makes an abrupt turn, there was a single, partial impression of a hind foot.
At the time Bird and his crew uncovered this trackway, sauropods were thought to be amphibious dinosaurs. Other than their immense bulk, what defense would they have had but to trundle into the water, where theropods feared to paddle? Under this framework, Bird thought he knew exactly how the Mayan Ranch Trackway was made. “The big fellow had been peacefully dog-paddling along, with his great body afloat, kicking himself forward by walking on the bottom here in the shallows with his front feet,” Bird wrote in his memoir. The great dinosaur then kicked off with one of its hind feet and turned.
With the exception of well-defended dinosaurs such as the ceratopsids and stegosaurs, many herbivorous dinosaurs were thought to be at least semi-aquatic. There seemed to be only two options for Mesozoic prey species–grow defenses or dive into the water. In time, though, paleontologists realized that the sauropods, hadrosaurs and other herbivores didn’t show any adaptations to swimming. Our understanding of the ecology of these dinosaurs was based on false premises and faulty evidence.
In the case of the Mayan Ranch Trackway, for example, there’s no indication that the sauropod that made the trackway was swimming. A more likely scenario has to do with evolutionary changes among sauropods. While the sauropods that dominated the Late Jurassic of North America–such as Diplodocus, Apatosaurus and Barosaurus–carried much of their weight at the hips and left deeper hindfoot impressions, the center of mass shifted among their successors–the titanosaurs–such that more of the weight was carried by the forelimbs. Hence, in some trackways, the deeper impressions made by the forefeet are more likely to stand out than those made by the hindfeet, especially if some of the top layers of the rock are eroded away to leave only “undertracks.” What seemed to be evidence of swimming sauropods instead owes to anatomy and the characteristics of the mucky substrate the dinosaur was walking on.
As far as I’m aware, no one has yet found definitive evidence of swimming sauropods or hadrosaurs–the two groups previously thought to rely on water for safety. Stranger still, paleontologists have recently uncovered good evidence that theropod dinosaurs weren’t as bothered by water as traditionally believed. In 2006, paleontologists Andrew Milner, Martin Lockley and Jim Kirkland described swim tracks made by Early Jurassic theropods at a site that now resides in St. George, Utah. Such traces weren’t the first of their kind ever discovered, but the tracksite was one of the richest ever found.
Small to medium-sized theropods made the St. George swim tracks–think of dinosaurs similar to Megapnosaurus and Dilophosaurus. Even better, the large number of smaller-size swim tracks hints that whatever dinosaurs made these tracks were moving as a group as they struggled against the current in the lake shallows. The larger dinosaurs, on the other hand, were a bit taller and able to wade where their smaller cousins splashed around.
A different team of researchers announced additional evidence for swimming theropods the following year. Paleontologist Rubén Ezquerra and co-authors described dinosaur swim traces from Early Cretaceous rock near La Rioja, Spain. Based on the details of the track and their direction, the theropod was swimming against a current that pushed the dinosaur diagonally. Along with other theropod swim tracks, the researchers noted, the discovery meant that paleontologists would have to revise their ideas about the kind of habitats theropods lived in and what carnivorous species would do. Theropod dinosaurs were not so hydrophobic, after all.
Does this mean that dinosaurs like Dilophosaurus were adapted to an amphibious lifestyle? Not at all. As Ezquerra and co-authors pointed out, the swimming strokes of these dinosaurs were exaggerated walking motions. The way the dinosaurs moved on land allowed them to be adequate swimmers while crossing rivers or lakes, but, compared with semi-aquatic animals such as crocodiles and otters, no known dinosaur shows traits indicative of a primarily waterlogged existence. (And dinosaurs found in marine sediments don’t count as evidence, as these were washed out to sea prior to burial. I can’t imagine ankylosaurs taking to life among the high seas, in any case.) Some dinosaurs could swim, but that doesn’t mean that they made the water their home. Still, thanks to special prehistoric traces, we can imagine packs of Megapnosaurus fighting to get ashore, and Dilophosaurus strutting into the shallows, aiming to snatch whatever fish were foolish enough to swim into the carnivore’s shadow.
Bird, R.T. (1985). Bones for Barnum Brown, edited by Schreiber, V. Forth Worth: Texas Christian University Press. pp. 160-161
Ezquerra, R., Doublet, S., Costeur, L., Galton, P., Pérez-Lorente, F. (2007). Were non-avian theropod dinosaurs able to swim? Supportive evidence from an Early Cretaceous trackway, Cameros Basin (La Rioja, Spain) Geology, 40 (10), 507-510 DOI: 10.1130/G23452A.1
Milner, A., Lockley, M., Kirkland, J. (2006). A large collection of well-preserved theropod dinosaur swim tracks from the Lower Jurassic Moenave Formation, St. George, Utah. New Mexico Museum of Natural History and Science Bulletin, 37, 315-328
August 28, 2012
I adore feathered dinosaurs. It feels a little strange to say that, but it’s true. Few things make me happier than seeing delicately-rendered restorations of theropods covered in fuzz and ceratopsians with some accessory bristles. The various bits of plumage–from quill-like structures to true feathers–make dinosaurs look even more wonderful and fantastic than the drab, scaly monsters I grew up with. And who wouldn’t love a fluffy like dinosaur like Sciurumimus, perhaps the cutest dinosaur of all time?
Of course, not everyone feels the same way. There are some people who want their dinosaurs to be scaly, scaly, scaly, science be damned. They weep, wail and gnash their teeth whenever a new study suggests that another branch of the dinosaur family tree might have been adorned with plumage. It’s as if they expect the Dinosauria to be consistent with an unchanging canon–sci-fi and comic fans suffer a similar apoplexy when one of their favorite characters deviates from their most cherished storyline.
io9′s “We Come From the Future” show recently debated whether science had “ruined” dinosaurs by decorating so many non-avian species with feathers. (Remember–birds are dinosaurs, too, and there have been some very scary birds in the history of life on earth). Granted, some restorations of feathery dinosaurs really do look stupid, and the minor plumes on the heads of Jurassic Park III‘s Velociraptor didn’t really help.
The show’s point-counterpoint debate on the matter isn’t totally serious, and it’s a way to get a tidbit of science out to a wider audience. That’s a good thing. All the same, I’m pretty sick of people who complain that feathers somehow detract from dinosaurian magnificence. How immature can you get? We all love the dinosaurs we first meet as kids, and, for many of us, those leviathans were drab and scaly. But those earlier versions have been slit from stem to stern by more active, colorful and complex dinosaurs, many of which had some kind of feather-like body covering. Which would you prefer? The scaly, sluggish pot-bellied Tyrannosaurus of the mid-20th century, or a svelte, agile predator that has a few patches of fuzz?
Don’t misunderstand me here. I’m not saying that all dinosaurs looked like big chickens. Dinosaurs exhibited an array of body structures–from simple, fuzzy tubes to bristles and full-on flight feathers. Some species, like modern birds, even exhibited several different types of feathers. The weird Beipiaosaurus, for one, had fuzzy protofeathers on much of its body but also had a sort of tail fan created by a different feather type. And “feathered dinosaur” doesn’t mean that the animal was entirely cloaked in plumage. Take Psittacosaurus, for example–this little ceratopsian was a very, very distant relative of birds and had a row of bristles along its tail. The structures were probably visual signals, and I have no doubt that same was true among other dinosaurs. Feathers aren’t just about flight or insulation, but they’re also important in display and communication.
And feathers are the key to dinosaur color. I’m still awestruck that we can recreate the colors of creatures that have been extinct for tens of millions of years. By comparing the microscopic details of prehistoric dinosaur feathers to the feathers of modern birds, we can finally answer that most persistent of paleo questions. That fact, alone, makes feathered dinosaurs especially magnificent.
I’m weary of this Portlandia-esque attitude that dinosaurs are over if they’re feathered. Please. New scientific discoveries are allowing us to gain unprecedented insights into the biology of dinosaurs, including the lives of the fluffy species. Feathers are just part of that bigger picture, and I’m ecstatic that paleontologists are reconstructing dinosaurs in ever-greater detail. The point is this. Feathered dinosaurs are awesome. Deal with it.