Re: Jobaria and the Elephant Commit Suicide

["Matthew Bonnan" <mbonnan@hotmail.com>]

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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