Reflections on the potential of human power for transportation

Tuesday, February 21, 2012

Raptors Revisited

Now for those of you who are growing bored with my ongoing discussion of the Human Powered Commuter Vehicle, something completely different, dinosaur biomechanics.
It was like being a kid again, opening Adrian Desmond’s book  “Hot Blooded Dinosaurs” and seeing the picture of a sprinting dinosaur, gracefully leaning forward with tail stretched out behind. In an eye blink, the dinosaur love of my childhood (and many others as well), Tyrannosaurus Rex was replaced by this new creature, Deinonychus Antirrhopus.
Deinonychus was discovered and named by Prof. John Ostrom of the Yale-Peabody Museum in 1969. The name for this dinosaur was descriptive of the animal’s anatomy, meaning terrible claw & counter balance. Terrible claw of course referred to the sickle-shaped claw on the second digit of each foot and counter balance referred to a tail made rigid by bony extensions on the vertebrae.
Deinonychus could possibly be the most significant dinosaur discovery of the 20th Century. The reason for this was the realization that if the sickle claw was used for attacking prey, the animal needed to be very athletic and  one that was more than likely warm blooded. To emphasize the point Robert T. Bakker, then a student of Prof. Ostrom drew a convincingly active restoration of Deinonychus, the image that graced Desmond’s book.
Prof. Ostrom also went on to observe strong similarities between the limb structure of Deinonychus and Archaeopteryx the famous proto-bird discovered in the 19th century. Based on Archaeopteryx, Thomas Huxley advocated that birds descended from dinosaurs and because of the similarities he observed, Ostrom resurrected the idea. Of course, because of numerous fossils indicating the presence of feathers on clearly flightless dinosaurs, the bird from dinosaur decent is now accepted as fact.
21 years would pass since its discovery and Michael Crichton through his book, “Jurassic Park” and the subsequent movie would introduce the world at large to Deinonychus. Well not exactly…
Ironically, the dinosaur mentioned in the book and movie was a misnamed species, Velociraptor Antirrhopus. An amateur paleontologist, Greg Paul, had decided, incorrectly that two raptors were of the same genus. They were Velociraptor Mongoliensis and Deinonychus Antirrhopus.  Velociraptor was about 18” tall, weighed about 33lb. and had a long flat skull. Deinonychus was about 40” tall, weighed about 150lb. and had a rather deep skull.  Crichton may have known the name was incorrect and used Velociraptor anyway because it conveniently shortened to “raptor”, while I don’t know how to shorten Deinonychus except to shorten it to DA.  The movie took additional liberties by significantly enlarging their raptor to make it seem more fearsome, even though DA was fearsome anyway, hunting in packs like wolves. As life imitates art, a raptor was subsequently discovered about the size of the movie raptor, Utahraptor Ostrommaysorum, that weighed about 1100lb. After “Jurassic Park” the term raptor no longer referred to birds of prey like eagles, owls and vultures but instead to medium sized carnivorous dinosaurs with sickle-shaped claws on their feet.

Skip ahead another 15 years and the image of a raptor balancing on one foot and slashing its prey with the other is under attack.  The issue of contention is the function of the sickle-shaped claw.  One fact is that the horny talon that covered the toe ungual of the living raptor is not preserved in the fossil. As a result the actual shape of the claw can only be guessed at.
 The other fact is that only one living animal, a bird, the seriema, has severely curved claws used in a flesh-slicing role, but that is after the prey has been killed by smashing the body on a hard surface
 Arial raptors and cats have hooked claws and they use them as grapples to hook on to prey or for climbing, not to cut flesh with the inside edge. Cassowaries and ostriches can inflict severe injuries with the claws on their feet, but in both cases the claws are straight and the offensive motion is a forward kick. So there is a new theory that the claw was only used to scramble up the sides of prey and hold on, while the jaws did the dispatching.

Under the direction of Prof. Phil Manning at the University of Manchester, researchers built a mechanical raptor leg and evaluated the damage the sickle claw did to pig and crocodile cadavers.  The machine was designed to duplicate the leg and sickle-claw action of a 90lb. raptor. The results would definitely not be life threatening to the large sauropods that the raptors might prey upon.  The wounds to the pig cadaver were between 1.2 and 1.6” deep and the bunching skin made it difficult to extract the claw after penetration. The claw bounced off the crocodile hide. The intent of the testing was to validate the grapple and grasp theory but the results were not at all conclusive.
Before leaving the grapple & grasp theory, allow me a few observations about stress, strain and claw penetration. Strain or the stretching of the tissue is what causes damage to the animal. Strain is related to stress, and stress in its most basic form is force divided by area. The force component in the claw-induced tissue stress is the raptor’s weight since the full weight of the animal can be applied to the claw-tissue interface. The area term in the tissue stress is related to the size and shape of the claw. I find it interesting that 90lb. was chosen as that of the raptor simulated by the machine. Recall that a Velociraptor weighed about 30lb and a Deinonychus weighed 150lb. One must question what claw size was used in the machine since a Velociraptor claw would be too small and a DA claw would be too large.
Then there is the matter of shape. A claw with a circular cross-section would produce the lowest stress in the tissue during a puncture because the tensile or hoop stress at the edge of the wound would be uniform. An oval cross-section claw would produce higher stresses since more stretching would take place along the long sides of the ellipse. Finally a teardrop shape would produce the highest stresses with the peak stresses occurring at the point of the teardrop. For example, if a knife blade had been mounted to the roboraptor’s foot, the penetration would be substantial, since the cross-sectional area is small and the stresses are very high at the edge, something referred to as a stress concentration. In reality, however, bone and horn do not have the strength of steel so a claw as sharp and narrow as a knife blade is not possible.
The claw used in the machine appeared to have an oval cross-section with the short axis orientated through the thickness of the claw. The core of the claw was aluminum and it was covered with Kevlar and carbon fiber.

As an exercise to determine if the horny talon covering the toe bone could have had a sharp edge on the inner curve, I sculpted a DA claw out of .45”-thick glass-filled polycarbonate. I started with the outline of the horny talon Prof. Ostrom sketched around the second ungual with a finely-dashed line in his 1969 monograph on DA.

What resulted was a claw whose cross-section begins at the tip as an oval with the long axis through the thickness of the claw. This transitions to a flat-sided teardrop about 1.4” along the inner edge of the claw. Since the inner edge of the claw is about 3.5” long, 2.1” of the inner edge could be considered sharp. It would be interesting to see the depth of penetration with this claw used on the roboraptor.

The grapple & gasp theory has a lot of dissension. Reasons given for this range from the fact that Velociraptor’s jaws did not have enough bite force to dispatch live prey, to the fact that raptors that were probably too large to scramble up the sides of prey (Utahraptor) still possessed sickle claws. And then how does one interpret the famous Velociraptor vs. Protoceratops fossil?

When a raptor was attacking prey that was significantly larger that it and that could injure it if given the opportunity, it would be advantageous for the raptor to get in, inflict a wound or wounds and get out quickly. The raptor could uses it sickle claws as grapples to run over the back of the prey and get off before the prey could respond. The difference between this dash & slash approach and the grapple & grasp approach is in the extent to which each claw penetration injures the prey. This is probably a more probable scenario for how the sickle claw was used.

The other interesting feature of raptors (the actual family name is dromaeosaurs, named for a dinosaur discovered in 1922 and thereby predating Velociraptor from 1924) is the structure of the tail, which was stiffened for most of its length by bony rods that held it rigid while only allowing bending near the pelvis.  Again the above picture is from Prof. Ostrom’s 1969 monograph.
He suggested that lateral motion of the rigid tail could enhance “jinking” or rapid changes in direction.
A recent Velociraptor fossil discovery exhibited a tail with a moderate S-bend from side to side, so for the purposed of the subsequent discussion it will be assumed that the rigidity contributed by the caudal rods was mainly in the vertical plane.
Many bipedal dinosaurs appear to use their tails to shift their centers of gravity (c.g.) over their back legs so they can stand with their torsos horizontal, but they don’t require caudal rods to do so. Since this type of counter balance is basically a static condition, it is logical to assume that the caudal rods were required for dynamic activities, where the forces would be significantly larger.
Of particular interest is how vertical tail motion could be used to enhance the techniques employed to injure prey.
The dash & slash approach assumes that the raptor is moving continuously over its prey. The dynamics of the raptors motion is rather complex and difficult to visualize. However one can observe some of the effects of tail motion on a raptor clinging vertically to the side of its prey. The sketch above is a simple free-body diagram of a raptor in that state.  The condition here assumes that the raptor has leapt through the air and landed on the prey and is stationary vertically.  Point a is where the sickle claw attaches to the prey, point b is where the hands attach to the prey, point c is the c.g. of the raptor without its tail, point d is where the tail attaches to the body and point e is the c.g. of the tail. (I have made the simplifying assumption that the polar inertia of the tail about point d can be reduced to Mt*L4^2). With the arms grasping the prey, the raptor is in static equilibrium. This means that all the static forces sum to zero. The vertical reaction at the sickle claw, Rv, is equal to the sum of the body weight Fb and the tail weight Ft. The moment about point a =Fb*L2+Ft*(L3+L4) is reacted by the moment Rh1*L1. So statically the maximum force causing the sickle claw to penetrate the hide of the prey is the raptors weight. Now I doubt that a raptor spent much time statically hanging off the side of his prey but it is a starting point to examine the forces involved in its support.
For  the above case, if the raptor were to rotate his tail upward with an acceleration At, the force on the sickle claw would be increased over the body weight by Mt*At.  Snapping the tail up would drive the claw deeper into the prey.
Due to conservation of angular momentum, rotation of the tail while the raptor is airborne would result in an opposite rotation of the torso. The would allow the raptor to adjust its body posture with respect to the prey the same way long jumpers use their arms to adjust their body position before landing.
As the raptor lands on the prey but before its arms can grasp, there will be a tendency for it to rotate backward because of the unreacted moment of its weight acting about the claws contact point, a. Under these circumstances, if the tail is snapped downward hard enough, the torso could be made to rotate forward, allowing the hand claws to gain purchase.  Mt*At*L3 must be larger than Fb*L2 for this to happen. Note that statically Fb=Mb (the body mass)*g, the acceleration of gravity. As the animal lands Fb would be greater =Mb*(g + dg) where dg is the added deceleration to bring the raptor to a vertical stop.
Most carnivorous dinosaurs (Therapods) have the pubic bones of their pelvis facing forward. The fact that the pubic bones of raptors face rearward could be related to increasing the muscle leverage for rotating the tail in the vertical plane.
The sickle claws and the stiffened tail are very specialized adaptations in raptors. It is logical to assume that their function significantly enhances their effectiveness as predators.

No comments:

Post a Comment