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Did Deinonychus and other “raptors” use their foot claws to restrain prey? Art by Emily Willoughby, image from Wikipedia.

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.

Left hind foot of Deinonychus antirrhopus. Image from Wikipedia.

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.

Reference:

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

I have a confession to make. Before this weekend, I’d never watched even a single episode of Doctor Who. (Shock. Horror.) I’m a bad nerd, I know. But when BBC One announced that the second episode of the show’s seventh season was titled “Dinosaurs on a Spaceship”, I knew I had to finally check out the goofy sci-fi staple.

I’m not going to say much about the plot of the show itself. When you have dinosaurs, Queen Nefertiti and a pair of insecure sentry robots voiced by David Mitchell and Robert Webb on the same ship–among other things–it’s better to simply let the program speak for itself. All you need to know is that an alien ark is harboring a number of dinosaurs rescued from earth before the non-avian varieties perished around 66 million years ago. I will say this, though: the dinosaurs in this episode of Doctor Who look infinitely better than the wonky puppets in the “Invasion of the Dinosaurs” episode of the original series. (Worst. Dinosaurs. Ever.)

Let’s start with the non-dinosaurian aspect of the alien ship’s prehistoric bestiary first. At one point, the Doctor and companions are attacked by a flock of Pteranodon. (Because where you find dinosaurs, flying monsters are never far behind.) The experts behind Pterosaur.net are better qualified to comment on these flying, non-dinosaurian archosaurs than I, but, my apologies to the Doctor, “pterodactyl” isn’t the proper term for these animals. The proper general term for these flapping archosaurs is “pterosaur.” “Pterodactyl” is an outdated term derived from the genus name of the first pterosaur recognized by science, but the term isn’t used by specialists anymore. It’s time to put “pterodactyl” to rest.

The rest of the Cretaceous cast is relatively thin. A pair of ornery ankylosaurs–modeled after Euoplocephalus–make a smashing entrance early on in the show, and our heroes soon cross a snoozing Tyrannosaurus youngster. Sadly, the juvenile tyrant is neither fuzzy nor sufficiently awkward-looking. Thanks to specimens such as “Jane“, we know that young Tyrannosaurus were leggy, slim and had relatively shallow skulls. They didn’t have the bone-crushing skull profile of their parents or the graceful bulk. And, as I’ve remarked many times before, young tyrannosaurs may very well have been fluffy flesh-rippers. The Doctor Who version, unfortunately, looks like a shrunken version of an adult.

Two different dinosaur species get most of the screen time, though. A friendly–or, at least, not overly aggressive–Triceratops helps the Doctor and friends out of a few tight spots. Like the ankylosaurs, though, the ceratopsid is a little bit too tubby and doesn’t run quite right. A Triceratops is not a horse. Likewise, the dinosaur’s tail was a bit too limp. The organ, essential to balance, flopped around like a big green sausage. All the same, the big herbivore was rather cute.

The dromaeosaurids, on the other claw, were not so friendly. They mostly keep to the shadows until the final act and are ferocious enough to temporarily endanger the crew. All the same, the unidentified “raptors” suffered the curse of the bunny hands and insufficient feathery coats. Filmmakers seem reluctant to drape feathers over dromaeosaurids, but, for any effects artists who may be reading, we know that these dinosaurs had exquisite plumage covering almost their entire body. If you’re going to have raptors, they should be intricately feathery. Nevertheless, I liked the idea that dinosaurs could ruffle their feathers to communicate with each other, and potential threats. You may want to laugh at a Deinonychus all puffed up, but that will be the last sound you ever make before it starts to eat you.

[For another take on the episode's dinosaurs, see Marc Vincent's post at Love in the Time of Chasmosaurs.]

“Feathers” by Randall Munroe, from http://xkcd.com/

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.

Deinonychus might have flapped its arms to help restrain prey. Art by Emily Willoughby, image from Wikipedia.

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.

A reconstruction of Deinonychus showing how this dinosaur walked on two-toed feet. Photo by AStrangerintheAlps, image from Wikipedia.

When I think of theropod tracks, the mental image that immediately pops up is of three-toed depressions with conspicuous indentations where the dinosaur’s claws dug into the substrate. After all, theropod means “beast foot,” and many theropod tracks seem to fit the name. But not all theropod dinosaurs balanced on three toes. The deinonychosaurs–the group of sickle-clawed dinosaurs that included the more slender troodontids and the bulky hypercarnivorous dromaeosaurids–ambled through the Mesozoic on two toes, with their curved switchblade claw held off the ground. Over the past two decades, paleontologists have found these two-toed tracks at multiple sites around the world. Most, however, have been found in China, and researchers have just issued a profile of one of the richest deinonychosaur tracksites anywhere.

The in-press Acta Palaeontologica Polonica study, written by Lida Xing and colleagues, describes an Early Cretaceous tracksite at the Liujiaxia Dinosaur National Geopark in Gansu Province, China. The site preserves 71 deinonychosaur tracks. These footprints are unique enough that the paleontologists gave them a new name–Dromaeosauripus yongjingensis. It may seem strange to name tracks, especially since we don’t know exactly what species of dinosaur made them, but trace fossil experts name particular types of tracks to keep track of the different kinds of animals that left the impressions. If a track type is unique and consistent from print to print, then it often gets its own moniker.

Of course, tracks record fossil behavior. At the new site, at least two trackways show that individual dinosaurs turned while walking. Unlike other sites, though, there isn’t any evidence for social raptors here. Based upon the spacing of the tracks and other details, the paleontologists Xing and co-authors hypothesize that the deinonychosaur trackways were made by individual animals that walked along the same surface at different times.

But what kind of deinonychosaur made the tracks? This is the Cinderella Syndrome–fitting the right trace to the proper tracemaker. Unless a dinosaur literally dies in its tracks, paleontologists can only outline the general kind of dinosaur who left the footprints. In this case, the choice is between a troodontid and a dromaeosaurid.

Footprint size isn’t especially helpful. Most troodontids were relatively small, while dromaeosaurids could grow to 20 feet long or more. The footprints at the site were made by dinosaurs ranging from two to 10 feet long, and this means that the larger animals were close to the known upper limit for troodontid size. Frustratingly, size alone is too ambiguous to distinguish between a troodontid or dromaeosaurid trackmaker.

To narrow down the possibilities, Xing and collaborators turned to another clue. The toe lengths of troodontids and dromaeosaurids are slightly different. While the fourth (or outermost) toe of troodontids is slightly shorter than the neighboring third toe, the two supporting toes in dromaeosaurids are typically just about equal in length. The tracks at the site more closely match the dromaeosaurid pattern. Thanks to this tracksite, we can imagine feathery dinosaurs akin to Deinonychus strutting across Cretaceous China.

Reference:

Xing, L., Li, D., Harris, J.D., Bell, P.R., Azuma, Y., Fujita, M., Lee, Y.-N., and Currie, P.J. (2012). A new Dromaeosauripus (Dinosauria: Theropoda) ichnospecies from the Lower Cretaceous Hekou Group, Gansu Province, China Acta Palaeontologica Polonica DOI: 10.4202/app.2011.0115

The reconstructed skeleton of a Deinonychus, a dromaeosaur, at Yale’s Peabody Museum of Natural History. Photo by the author.

From professional paleontologists to kids, there are dinosaur lovers of all ages. But one of the most enthusiastic I’ve ever seen is “the Tiny Paleontologist” – a 4-year-old dinosaur fan who shares his passion through occasional video updates on this blog. In the latest entry, the tiny paleontologists rhapsodizes about dromaeosaurs, or the group of carnivorous dinosaurs with retractable sickle claws on their toes. He’s a little off on some of the particulars, but there can be no doubt that this kid absolutely adores dinosaurs. I hope he holds on to that enthusiasm as paleontologists continue to discover more.