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Re: Jobaria and the Elephant Commit Suicide

Ken Clay writes:
If a sauropod was to rear up,
why would the hips or knees have to be flexed at all? It seems to me that any
degree of leg flexion would be counter-productive. Vertically oriented limb
bones with straight columnar shafts would decrease bending stress, which, as you
have pointed out, would be quite high. Also, if both caudifemoralis muscles
contracted at the same time with the feet planted on the ground and the legs
straight, would not that bring the femurs closer to the tail and thus actually
aid in rearing? Slow relaxation of them and the spinal extensors would prevent
the behemoth from crashing down and breaking its arms.

Yes, I agree with you that bending the legs is not necessarily a good idea. The reason the restorations of sauropods rearing tend to be done with the knees bent is that this is how elephants rear -- their knees are literally bent when going bipedal, as weird as that sounds. This may be because the acetabulum (hip socket) of elephants does not face out laterally as in sauropods but instead faces ventrally (down). The head of the elephant femur is spherical, and fits into the socket from below, allowing the animal to extend and flex its thigh but perhaps not allowing its pelvis to rock back. Therefore, in order to rear, it must instead bend its knees while leaning back, because at the knee joint it can get a greater degree of flexure.

In sauropods, the actetabulum faces laterally, and the femoral head inserts more or less medially into it. Therefore, what you suggest (the leg remaining columnar with the body tilting back) would seem to make sense. While the caudofemoral muscles aid in pulling the legs back during locomotion, however, this does not guarantee that they would have the power to cause the legs to pull toward the tail and generate lift. It's the back muscles (iliocostalis, longissimus, and erector spinae groups) that would have to do a lot of pulling, perhaps assisted by the caudofemoralis group. Unfortuantely, we have, as yet, no good way to measure the size and power of these muscles (beyond ball-park estimates and looking at the space between the neural spines and the transverse processes on the vertebrae). So it's difficult to test how slow or fast rearing and descending would happen.

Tall neural spines over the hips may not necessarily indicate improved back muscle or muscle power. Rather, sauropods have an awfully long tail that, if held off the ground, would require a ligament of some sort -- what better place to gain leverage on the tail than the neural spines over the hips?

As a counterpoint to saurpods keeping their legs straight while their bodies pivoted around them, I bring up the femoral adductor muscles, some of which originate on the pubis. The further you rock the pelvis back, the more you stretch these muscles as the pubis rotates forward and up. Since muscles can only stretch at a maximum about 20% of their length, how far back do you think a sauropod could get doing this? Not too far is my guess. But weighing 30-40 tons and bending your knees doing this doesn't seem as good of an idea either. Maybe this is all telling us sauropods are not really that well engineered to rear, but I stand to be corrected and would never say it was impossible.

Throwing in a theropod here, if you think sauropods could rear (one way or another), imagine an already bipedal but horizontally walking T. rex "rearing." How far up do you think it could stand? Would it have to bend its knees? If you don't think a T. rex or other predator with the primitively horizontal back could rear that well, what makes you think sauropods would be adapted to do so, even when coming from such bipedal stock? Remember, too, that a T. rex or Herrerasaurus have those long, balancing tails and nice caudofemoralis muscles. And these guys do have shorter forelimbs than hindlimbs. By the way, the "you" in here is not directed at anyone in particular, just my way of phrasing these questions.

With the differences in the hip sockets between elephants and sauropods, I again reiterate the need to be cautious about over-extending our analogs. Which you're all sick of, I'm sure. =)

Matt Bonnan

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