It indeed has very developed tail muscles (if I had a dorsal view, you'd see the base of the tail is almost as wide as the waist). This probably helps it both with swimming and with balance, since Spinosaurus is notoriously front-heavy, but I don't see how it would help the leg bones sustain its weight.
NHMUK 16421 has different proportions than the holotype dentary and thus can be scaled in various ways. I used 32% because that makes it a perfect fit with the rostrum of MSNM v4047, no rescaling required.
Yes, that was my reference for the feet and some axial pieces of FSAC-KK 11888 that were not figured in other papers.
We know that the legs belong to the same individual as the rest of the material, and Spinosaurus must have been able to stand, or else the species wouldn't have lived long enough to leave fossils. Could it be a quadruped? Maybe, nobody has tested the possibility seriously. But it's not a chimera, so the legs surely worked. Somehow.
Thank you!