|Um, Carpenter, that's a big Diplodocus clone.|
They don't seem to portray the actual Amphicoelias at all. They're just Diplodocus copies. They ignore the actual proportions of the animal, flying in the face of what the known fossils suggest. Let's see what's known from the Amphicoelias holotype again:
Well, we'll start with the vertebrae.
Next, the pubis.
It's massive in proportion. Amphicoelias' pubis just blows Diplodocus' one out of the water. The 10th dorsal of Diplodocus carnegii CM 84 measures about ~26.7 centimeters long, while that of Amphicoelias altus measures about ~24 centimeters long, a little shorter than the D. carnegii centrum, however, this is due to the differing shape of the centra as preserved. Overall, the centra are essentially the same size aside from width. However, the pubis of A. altus measures about ~1.17 meters long, while that of D. carnegii is about ~1 meter long. Despite having similarly-sized posterior dorsal centra, A. altus has a significantly larger pubis. This is indication of a massive hip, and subsequently, a deep torso.
Then we get to the femora.
The femura of A. altus are notably long, with the complete right femur measuring at ~1.77 meters in length. By comparison, D. carnegii CM 84 has a femur about ~1.54 meters long. Keep in mind that D. carnegii's 10th dorsal centra is similar in size to that of A. altus. This is a clear indication of longer limbs than Diplodocus proportionally, and in the case of D. carnegii, also in absolute terms.
Another way to look at it would be to scale a Diplodocus reconstruction to the provided measurements of the individual bones, treating them as separate individuals, then comparing the results to see how much they differ.
The skeleton of Diplodocus carnegii CM 84 is estimated to be approximately ~26 meters long. Since the anterior dorsal centra of CM 84 and A. altus are about the same size, scaling CM 84 to the A. altus dorsal would give an animal practically identical in size. However, scaling CM 84 to the A. altus pubis then gives a size about 17% larger in linear dimensions, or about ~30.4 meters or so. Scaling CM 84 to the A. altus femur gives an animal about ~29.9 meters long, or about ~15% larger in linear dimensions. This is another pretty good indicator that the two diplodocoids differed quite a bit in their bodily proportions.
All in all, this animal isn't really proportioned that similar to Diplodocus. It's clearly something else.
Overall, the resulting animal would be a deep-chested, long-limbed one. It would be proportionally narrower since their proportional width across the diapophyses (the "wings" of the neural arch) are similar, while the hips and the body suggested by it are deeper and the legs longer. Tall, but not rotund. At least, for Amphicoelias altus.
What about the giant Amphicoelias fragillimus? Would it have had similar proportions? Where would it differ? As of now, we lack the data needed to give a truly concrete answer. It would depend on the affinities of A. fragillimus in the sauropod family tree, and how close it would be to A. altus. That would be the subject of the next post.