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Claws on deinonychosaurs



This is a comment on an online paper published recently in Biology Letters of the Royal Society of London. I think the abstract is freely available, but the main paper requires a subscription.

Manning, P. L., et al. 2005. Dinosaur killer claws or climbing crampons? Biology Letters Online Publishing, October 11, 2005

Abstract (freely available online, I believe): Dromaeosaurid theropod dinosaurs have a strongly recurved, hypertrophied and hyperextensible ungual claw on pedal digit II. This feature is usually suggested to have functioned as a device for disembowelling herbivorous dinosaurs during predation. However, modelling of dromaeosaurid hindlimb function using a robotic model and comparison of pedal ungual morphology with extant analogue taxa both indicate that this distinctive claw did not function as a slashing weapon, but may have acted as an aid to prey capture.

News story: http://news.bbc.co.uk/1/hi/sci/tech/4332272.stm
BBC News OnLine

Here are some points from the full paper:

First of all, you have to know that this project was done in connection with a TV show for the BBC (The Truth About Killer Dinosaurs). Second, a 5-author paper that had multiple reviewers and editors and proof readers shouldn't be quoting "speeds" in meters per second squared: those units denote acceleration. It makes you wonder.

They designed a hydraulic limb based on the structure of Velociraptor and Deinonychus. The leg bones were mimicked by steel rods, controlled by hydraulics; and the hyperextension of the big claw was effected by a control cable. The claw movements were reconstructed mostly from the beautifully preserved foot of Deinonychus. The model was based on a body mass of 40 kg for the animal: roughly that of a Dobermann.

The big claw, the only reason for building the apparatus, was made from an aluminum core with a sheath of Kevlar and carbon fiber set in epoxy resin, which was sharpened appropriately. Then they banged the claw into a pig carcass mounted on a frame at speeds of 2 m "per sec2" and 11 m/sec2. Assuming these are real speeds misquoted, they represent roughly 4 mph and 20 mph.

The claw did not penetrate far into the pig, so the authors conclude it didn't cause slashing wounds. Therefore, they suggest, the claws were used like crampons to climb up the flanks of their prey until they could bite them!!!

I really have problems with this, though I'm not in a position to solve those problems.

We don't know what the "pig carcass" really was. In the photos in the paper it looks like a clean, cold, piece of pork from the local village butcher (it has one of those violet stamps on it). It's mounted as a solid lump on a metal frame: no "give" at all as there would be in a living animal. No wonder the claw didn't penetrate much. Let's go back to the abstract: if the claw is "usually suggested to have functioned as a device for disembowelling", and the device just banged the claw into a cold cleaned rigidly-mounted slab of meat, then they didn't test the disembowelling hypothesis at all. What you need is a claw applied appropriately to the guts of a warm animal. More on this below.

We don't know just how the claw impacted the meat. If it hit the meat more or less directly, of course it would try to penetrate, even if it didn't get very far. If it hit the meat in a mode in which the claw was moving to close just as it hit the meat, then the claw motion would have been dominantly slashing on impact, and would have been much more likely to tear the flesh in a cutting action. The authors don't say, but since they are so keen to tell us the "speed" of the claw, that means to me that the claw's motion wasn't dominantly slashing. The picture gives the same impression, though it's difficult to tell just how the claw was moving on impact. So again, the hypothesis was not tested.

Apart from anything else, I don't like the crampon hypothesis. There's nothing potentially lethal to bite even if you succeed in getting up on to the flanks of a big animal.

The other thing is that this whole "test" and then the subsequent inferences are predicated on the supposition that the raptors were attacking prey much larger than they were: a roughly 90 pound predator (a largish dog) attacking 500 to 1000 pounds of pig. This vision, I suppose, derives from brainwashing by the Bakker images.

My image is that it's much more likely that these raptors were taking on prey roughly their own size, or perhaps smaller. They'd knock them down then disembowel them with the foot claw, before or after tearing out their throats with their teeth. So any experiment would be better performed on a complete, warm, wriggling Bambi, with its belly skin stretched by muscular exertion rather than being relaxed in death. I'm not advocating this, of course, just making a logical point.

But wait: there are two sets of people who get close to that activity. One consists of surgeons, whether they work on humans or animals; and the other consists of hunters as they gralloch their prey. I'm none of the above, so I'd welcome feedback.

There are differences in technique, however, that have to be part of the assessment. All the above are concerned to cause minimal damage to the creature, the first because they're trying to save its life, and the second because they want a clean gralloch, without puncturing the rumen, for example:
http://wireviz.tripod.com/id3.html
with pictures and instructions!


But if you're a predator, the more damage you cause, the better, so perhaps a raptor-type claw is a good thing. At least one commercial deer-cleaning knife has a "gut-hook" on it:
"http://www.deer-uk.com/Skinner.gif


Whatever deinonychosaurs were doing, it was unique. But some interpretations seem to me to be better than others.