Smithsonian.com

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

A high-kicking Utahraptor outside the College of Eastern Utah's Prehistoric Museum in Price. Photo by the author.

Prior to the summer of 1993, “raptor” was synonymous with “bird of prey.” If you said “raptor,” whoever you were talking to knew you were talking about some kind of hawk, owl, eagle or other sharp-taloned aerial predator. Then Jurassic Park came along. Thanks to some taxonomic muddling and abbreviation, the cunning, sickle-clawed villains of the film’s third act immediately came to be known as “raptors.” Velociraptor, Deinonychus and kin had stolen the term for themselves.

Among non-avian dinosaurs, raptor might refer to the entire group of feathery coelurosaurs with grasping hands and hyperextendable toe claws—the deinonychosaurs—or to a specific subset of that group, called dromaeosaurids. It depends on where you care to draw the line. Just like its use among avian dinosaurs, the word “raptor” is informal and is a quick way to draw a conceptual outline of any dinosaur similar to Velociraptor.

But not everyone is happy with how “raptor” has been co-opted. A few months ago, paleontologist and Tetrapod Zoology author Darren Naish wrote:

Oh, and can everybody please stop using the word ‘raptor’ as a popular term for deinonychosaur, or dromaeosaurid? Admittedly, this rarely causes confusion, but it looks dumb and naive given that THE WORD RAPTOR IS ALREADY IN USE FOR ANOTHER GROUP OF ANIMALS. It would be like deciding to call sauropods ‘elephants’ or something.

And earlier this week, a reader sent me an email questioning the Los Angeles Times‘ use of the word raptor to describe a new genus of dromaeosaurid found in the Early Cretaceous rock of Utah. If birds of prey had claim to “raptor” first, and the term is just a bit of pop culture fluff, should we drop the word and push for deinonychosaur instead?

I don’t think so. Even though some informal dinosaur terms make me cringe—such as “parasaur” for Parasaurlophus and “Trike” for Triceratops—I think “raptor” provides a useful hook. To borrow a bit from another Steven Spielberg monster flick, you say “deinonychosaur,” and people say “Huh? What?” You say “raptor,” and your audience immediately has a general image of what sort of dinosaur you’re talking about. Rather than lament the reapplication of the word raptor as misappropriation or dumbing down, we might as well take advantage of the instant recognition the word triggers when trying to communicate with people who are not up on the latest theropod phylogeny. Almost twenty years after Jurassic Park debuted, it’s a little late to put “raptor” back in the cage.

More than that, I think “raptor” is a perfectly wonderful term for dromaeosaurids, if not deinonychosaurs as a whole. Not only has the “raptor” suffix been used in numerous dromaeosaurid names—Velociraptor, Utahraptor, Bambiraptor, Pyroraptor, Microraptor and so on—but these feathery dinosaurs were close cousins of the lineage which spawned the first birds. Some dromaeosaurids may have even hunted like avian raptors, using their huge tow claws to pin down prey rather than slash at it. Since “raptor” was always an informal term that applied to various lineages of avian dinosaurs anyway, I think it’s perfectly legit to use the word for the more ancient, non-avian precursors of today’s formidable falcons and eagles. Avian and non-avian raptors were dinosaurs of a feather.

A block containing the partial skeleton of Linhevenator. Abbreviations: ds, dorsal vertebrae; lf, left femur; li, left ischium; lpe, left foot; rh, right humerus; rs, right scapula; sk, skull. From Xu et al., 2011.

Raptors are total media hogs. Ever since the big screen adaptation of Jurassic Park came out in 1993, the sickle-clawed carnivores have cast a disproportionately large shadow over the rest of their dinosaurian kin, especially their close relatives the troodontids.

If you’re any kind of dinosaur fan, chances are good that you have at least seen a troodontid dinosaur before. The genus Troodon is a common staple of dinosaur books and documentaries (including Dinosaur Revolution), and it looks like a more slender version of more imposing predators such as Deinonychus. There’s a good reason for this. Together Troodon and Deinonychus represent the two branches of the sickle-clawed dinosaur group known as the deinonychosauria: Troodon represents the troodontids and Deinonychus carries the banner for the dromaeosaurids. Among the things that distinguished troodontids from their dromaeosaurid relatives were relatively big brains, big eyes, an increased number of smaller teeth, and smaller sickle claws that, in some species, could not be retracted as far as in their cousins such as Velociraptor.

Troodon is only the most famous of its kind—perhaps because it has been known for the longest time—but many other troodontid dinosaurs have been named from sites in North America, Asia, and Europe. The latest dinosaur to join the family is Linhevenator tani from Inner Mongolia, China. The dinosaur was described by paleontologists Xing Xu, Qingwei Tan, Corwin Sullivan, Fenglu Han and Dong Xiao in the journal PLoS One.

Dating back to somewhere between 84 million to 75 million years ago, Linhevenator is just one of several recently described theropod dinosaurs to be recovered and described from Inner Mongolia. (The other two, appropriately enough, were the alvarezsaurid Linhenykus and the dromaeosaurid Linheraptor.) The skeletal material which represents the new dinosaur includes the skull and jaws, several vertebrae, the right shoulder blade and upper arm bone, part of the hips, a left thigh bone, the left foot and a few other fragmentary parts. Some of these bones were found articulated with each other, others not, but as troodontid dinosaurs go, Linhevenator is one of the more complete dinosaurs yet found.

What makes Linehvenator particularly unusual are some of the details of its limbs. Compared to other troodontids, Linhevenator had a relatively long shoulder blade, a relatively short and thick humerus, and its second toe was tipped in a specialized, retractable claw like that seen in Troodon but not in some earlier members of the group. This is a curious suite of characteristics. Whereas Linhevenator appears to have had a killing claw similar to that of its dromaeosaurid cousins like Deinonychus, the newly described dinosaur may have had proportionally short and strongly muscled arms. This may hint that Linhevenator was not using its arms to capture prey in the same way as dromaeosaurids or earlier troodontid dinosaurs, even if it did have a specialized killing claw. Instead, Xu and co-authors argue that the dinosaur may have had arms adapted to digging, climbing, or something else entirely, although testing these hypotheses is difficult at present. With any luck, additional discoveries of troodontids will help flesh out what these peculiar dinosaurs were like in life.

References:

Xu, X., Tan, Q., Sullivan, C., Han, F., & Xiao, D. (2011). A Short-Armed Troodontid Dinosaur from the Upper Cretaceous of Inner Mongolia and Its Implications for Troodontid Evolution PLoS ONE, 6 (9) DOI: 10.1371/journal.pone.0022916

A tooth from Buitreraptor in multiple views. From Gianechini et al., 2010.

Thanks to plenty of good press from movies, documentaries, books and toys, over the past thirty years, Deinonychus and Velociraptor have become the quintessential dromaeosaurid (“raptor”) dinosaurs. They even rival the “Prize Fighter of Antiquity”—Tyrannosaurus rex—in fame these days. But these two sickle-clawed dinosaurs represent only part of a diverse group of animals that were widespread over the globe during the Cretaceous.

Among the strangest cousins of Velociraptor were a group of dromaeosaurids from the Southern Hemisphere called the Unenlagiinae (try saying that ten times fast). Only of handful of species—Buitreraptor, Unenlagia and Austroraptor from South America, plus Rahonavis from Madagascar—are known, but these slender, long-snouted raptors were distinct from species found anywhere else. Among the features that really set them apart are their teeth, which have just been analyzed in study by Federico Gianechini, Peter Makovicky and Sebastian Apesteguia soon to be published in the journal Acta Palaeontologica Polonica.

After looking at the preserved jaws of Buitreraptor and Austroraptor, Gianechini and co-authors were able to ascertain several specializations. Both had comparatively high numbers of teeth, small teeth for their skull size, a lack of serrations on their teeth and long grooves running up and down the tooth crown. Individually these tooth traits are also seen among various other theropod dinosaurs, but when they are all taken together they distinguish dinosaurs like Austroraptor from its dromaeosaurid relatives. Looking ahead, the recognition of these features may help paleontologists better identify which groups of raptors were present at a particular place and time on the basis of teeth alone.

The teeth may also tell us something about the feeding habits of these dinosaurs. Grooved teeth lacking serrations have are also present in a variety of dinosaurs and other prehistoric reptiles that ate fish, and it is possible that Buitreraptor and Austroraptor regularly fished for dinner. This hypothesis is not studied in detail—it is merely mentioned as an aside in the conclusion—but it is an angle worth looking into through future research.

References:

Federico A. Gianechini, Peter J. Makovicky, and Sebastián Apesteguía (2010). The teeth of the unenlagiine theropod Buitreraptor from the Cretaceous of Patagonia, Argentina, and the unusual dentition of the Gondwanan dromaeosaurids. Acta Palaeontologica Polonica, in press