At long last, Dromeosaur tracks!

For nearly as long as I can remember, artistic depictions of Deinonychus and related dromeosaurs have featured the dinosaur as a pack hunter, often pouncing on a hapless ornithischian like Tenontosaurus (see here, here, here, and here for examples). After being confronted with such imagery time and time again I didn't think twice about the pack-hunting behavior in Deinonychus as a kid, but I started to wonder on what evidence all these gory illustrations were based. The popular books in my own library treated the behavior as a fact and gave no references, the scientific articles on the subject being generally inaccessible to me until recently, and so I started to question the popular depiction of pack or group-hunting behavior in these dinosaurs. Just yesterday, however, came some momentous news; unquestionable dromoeosaurid trackways have been discovered, and they seem to support the gregariousness of these dinosaurs.

The inspiration for so many reconstructions of Deinonychus ripping the guts out of a Tenontosaurus primarily stems from the work of John Ostrom (the paleontologist who identified and named Deinonychus), a 1995 paper by W. Desmond Maxwell and Ostrom describing many shed Deinonychus teeth being closely associated with a Tenontosaurus specimen. This find, plus the initial site from which Deinonychus was described (containing fragmentary remains of several individuals and partial remains of Tenontosaurus), led the authors to hypothesize that Tenontosaurus was the preferred food of the sickle-clawed theropods. The problem with this was that the evidence for group hunting or feeding behavior is ambiguous; the dromeosaurs could have been scavenging the carcass, the available data neither confirming or refuting the popular notion of pack-hunting Deinonychus (Carpenter, 1998). In fact, a recent paper by Roach & Brinkman (2007) took a critical look at the group-hunting hypothesis, and using extant diapsids (i.e. Komodo Dragons, Varanus komodoensis) the team concluded that pack hunting in dromeosaurs was unlikely, evidence of close associations of non-avian theropods (see Currie (1998)) being more suggestive of aggression or loose, fleeting associations at best. This new study has not closed the book on the subject, however, and I am somewhat skeptical about using Komodo dragons as models for theropod dinosaurs. (Zach has a discussion of the Roach & Brinkman paper at his blog)

There is good evidence that theropods at least occasionally came into association with each other, some of the best evidences of dinosaur behavior coming from fossil trackways. In contrast to skeletons found in association with each other, the impressions in the rock capture behavior "in action" (see Lingham-Soliar, et al. (2003) and Milner, et al. (2006)). Trackways are open to interpretation, of course, but they can give us important clues as to whether animals were moving in groups, following prey, or other "vignettes" that are now lost to us outside of the fossils, and a new paper in Naturwissenschaften by Li, et al. (2007) presents for the first time undisputed dromeosaur trackways. While the identity of trackmakers is typically hard to discern, there is no doubt that the new tracks belong to dromeosaurs, two ichnogenera perhaps signaling two dromeosaur species in the same area in the same time. How can we be so sure? It's long been thought that the large recurved second digit of dinosaurs like Deinonychus and its close relatives was held up off the ground, the dinosaurs putting their weight on the other two digits. While it might not be shocking, the new tracks confirm this beautifully, finally providing the fossil proof for what has long been known. The question now, however, is how these dinosaurs used their terrible claws.

While the popular notion (backed up by Dr. Grant's soliloquy in the film Jurassic Park) is that these dinosaurs disemboweled their prey by kicking their foot and flexing this claw in a downward arc, some recent mechanical tests don't support this hypothesis. Manning, et al. (2006) found that the claws appeared to function more as "crampons," digging into flesh for a better grip, the claw compressing flesh on the lateral edge and making it difficult to remove the claw. While the huge foot claw was a formidable weapon and could cause large amount of slashing damage, perhaps the claws proved more useful in latching onto prey and pulling it down, a function that would seemingly support group hunting. In such a scenario, some members of the group pulled the victim to the ground, one or more then delivering the killing bite/slash to the throat or belly once exposed (a similar method of predation is hypothesized for sabercats, see McHenry, et al. 2007). The idea of group hunting should also be kept in consideration given the potential diets of theropods and habitat overlaps. As Van Valkenburgh and Molnar (2002) noted, theropod dinosaurs generally appear to be hypercarnivorous, their dentition not being specialized in the same way that the teeth of mammalian carnivores have to exploit different food sources. We can hypothesize, then, that different theropods had different prey preferences and behavioral tactics to obtain prey in order to supply themselves with food but not come in to direct competition from other predators, forming packs or social groups being one way to solve this ecological problem (although it has drawbacks, as well). Indeed, such a hypothesis is yet to be fully refuted or confirmed, but I see no reason to think that theropod dinosaurs were entirely solitary animals and no species ever formed a social grouping of any kind.

While the new discovery does not confirm that dromeosaurs hunted in social groups, it does reflect a certain level of gregariousness among the dinosaurs. The tracks come from Junan County, Shandong Province, China, occurring in the Lower Cretaceous rocks of the Tianjialou Formation. There are 18 large tracks assigned to the new ichnotaxa Dromaeopodus shandongensis and two smaller footprints referred to the existing (but poorly known) ichnogenus Velociraptorichnus sichuanensis, occurring in a different bedding plane than the larger prints. The tracks are very interesting in their differences, though. Dromaeopodus is larger, the digits are parallel, subequal in length, slightly curved inward, and have a "nub" on the side showing that digit II was held off the ground. The back of the foot was supported by a large pad as well, which was unexpected in these dinosaurs. Velociraptorichnus, by contrast, is smaller but superficially looks almost like a "normal" theropod foot with digit II cut off, the toes being less slender and the outside toe being projected at a little more of an angle than in Dromaeopodus.

What is most interesting about the footprints is what can be inferred about group behavior. Although there are tracks across several bedding planes, there is one particular layer in which six trackways show movement of Dromaeopodus in one direction, the sediment preserving ripple marks but no signs of drying out. Such preservational features allowed the researchers to infer that the tracks were made in a short period of time before being covered over, making it unlikely that the six trackways were created by unassociated individuals over a long period of time as they moved along the river or channel. Going into even more detail, the tracks all point in one direction, do not overlap, and are evenly spaced, the same kind of spacing that would be expected if a group were moving together. It is clear that the animals were not hunting at the time, however, their gait showing that they were walking rather than running, and whether their association was permanent, seasonal, made up of family members, or a mixed group cannot be determined. Still, the tracks do show that dromeosaurs could be gregarious in at least some circumstances, even if those circumstances are beyond our current knowledge.

As I've mentioned before on this blog, Asia and North America shared a many dinosaurian fauna through the Cretaceous, tyrannosaurids, hadrosaurs, therizinosaurs, and dromeosaurs all having a presence on both continents and indicating some amount of faunal interchange. As for dromeosaurs, it's been hypothesized that they had an origin in Asia, and the new footprints infer the presence of large and small dromeosaurs in Asia in the early Cretaceous, seemingly supporting this hypothesis. Hopefully the maker of the Dromaeopodus tracks will be found soon for comparison with North America fauna, as skeletal remains would be useful in determining evolutionary and biogeographical relationships with North American taxa like Deinonychus and Utahraptor. More study is needed, of course, especially of older deposits, but even in the light of new discoveries these footprints are going to be important in determining the biogeography and evolutionary radiation of dromeosaurs.

References;

Carpenter, K. (1998). "Evidence of predatory behavior by carnivorous dinosaurs". Gaia 15: 135-144.

Currie, P.J. (1998) "Possible Evidence of Gregarious Behavior in Tyrannosaurids." Gaia 15 pp. 271-277

Farlow, J.O. (1976) "Speculations about the Diet and Foraging Behavior of Large Carnivorous Dinosaurs." American Midland Naturalist 95 (1) pp. 186-191.

Li, R.; Lockley, M.G.; Makovicky, P.J.; Matsukawa, M.; Norell, M.A.; Harris, J.D.; Liu, M. (2007) "Behavioral and faunal implications of Early Cretaceous deinonychosaur trackways from China." Naturwissenschaften Published Online 19 October 2007.

Lingham-Soliar, T.; Broderick, T.; Ahmed, A.A.K. (2003) "Closely associated theropod trackways from the Jurassic of Zimbabwe." Naturwissenschaften 90 (12) pp. 572-576

Manning, P.L.; Payne, D.; Pennicott, J.; Barrett, P.M.; Ennos, R.A. (2006) "Dinosaur killer claws or climbing crampons?" Biology Letters 2 (1) pp. 110-112

Maxwell, W. D.; Ostrom, J.H. (1995). "Taphonomy and paleobiological implications of Tenontosaurus-Deinonychus associations". Journal of Vertebrate Paleontology 15 (4): 707-712.

McHenry, C.R.; Wroe, S.; Clausen, P.D.; Moreno, K.; Cunningham, E. (2007) "Supermodeled sabercat, predatory behavior in Smilodon fatalis revealed by high-resolution 3D computer simulation." PNAS, Published online before print October 2, 2007

Milner, A.R.C.; Lockley, M.G.; and Kirkland, J.I. (2006) "A Large Collection of Well-Preserved Theropod Dinosaur Swim Tracks From the Lower Jurassic Moenave Formation, St. George, Utah." The Triassic-Jurassic Terrestrial Transition. New Mexico Museum of Natural History and Science Bulletin 37 pp. 315-328

Roach, B.T.; Brinkman, D.L. (2007) "A Reevaluation of Cooperative Pack Hunting and Gregariousness in Deinonychus antirrhopus and Other Nonavian Theropod Dinosaurs." Bulletin of the Peabody Museum of Natural History 48 (1) pp. 103-138

Van Valkenburgh, B.; Molnar, R.E. (2002) "Dinosaurian and mammalian predators compared." Paleobiology 28 (4) pp. 527-543