Thursday, February 20, 2014

Too Many Bones = Multiple Individuals

I think I've noted before that we seem to have a lot of limb bones for the amount of rock we have excavated. The first limb bone excavated, which bridged three blocks, was a nice Humerus or Femur which proved to be typical of all the limb bones. Part of this bone actually rolled free during the roadcut excavation, and was recovered by Richard Kimbell (Thanks Richard!).
It has hollow ends, a central marrow (seen in the broken surfaces in the middle) and a narrow center D-shaped shaft without a flange. During the summer we recovered a similar bone, but it was slightly shorter. It was also a mirror-image, so it had to come from the other side of the animal. The length is not due to broken ends: both bones have very thin edges at the very ends. 
So one possibility is that both  limb bones are from the same animal. In this case it would have to be a pathological bone ( one shorter than the other) or a case of front humerus / back femur being identical bones. Both cases are difficult to believe. The alternative is two animals of different ages, both juveniles at death, each contributing one bone.
But the story is not over! Here's the top of the skull with yet another humerus/femur to be removed to allow access to the skull.

This is the third bone in the series. This one is from the same side of the animal as the smaller bone above, but is smaller yet. It is also significantly more gracile (thinner) So we now have a series of three bones, progressive sizes, the large one from one side of the body and the two smaller ones from the same side. All 3 bones have been within 12 inches of each other in the block.
Now it's pretty sure we don't have three bones from the same animal unless we have a pathological condition. The bones have a wonderful progression of growth, well shaped, but all 3 still have hollow ends. The simplest explanation, though it is still unbelievable, is that we have THREE individuals of the same species, differing in ages, that died together while still juveniles!  Even in the case that two bones are from one animal (front/back) we still have TWO individuals of the same species. Combined with the fact we have two types of backbones, this means we have three or four animals mixed up in this collection of bones. What a host of questions come to mind - mass stranding of a family group, sexual dimorphism, schooling behavior, etc. Remember we have a mass stranding of Shonasaurus in Nevada from the Triassic. Perhaps this is similar behavior? 

Comments are always welcome. Sincerely, Greg Carr

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