My favorite animals are whales, specifically blue whales, also known as the largest animal in known history. The species is still alive today and is bigger than any known dinosaur. One of the most interesting facts, in my opinion, about whales is the fact that if you look at a skeleton of a whale, you can sometimes see a pelvis, the only remains of the leg bone. This is because a distant ancestor of the whale used to walk on land with four legs, who lived over 50 million years ago.
This ancestor species is known as Pakicetus, and was one of the first cetaceans, a marine family that includes dolphins and whales. Pakicetus is also an ancestor to modern day hippos. It lived in what is now India and Pakistan, on the edges of lakes and rivers, until its descendants moved to living in the water. Notable descendants of the Pakicetus (and ancestors of the modern whale) are the Ambulocetus, who lived on land and water 50-48 million years ago, and the Dorudon, who lived in the water 40-33 million years ago.
On the anatomy of modern-day whales, remains of a leg remain– the pelvis. The bones themselves are not attached to the skeleton in any way, but are embedded in muscle. While they appear to have no purpose, as whales do not have legs, they do anchor reproductive muscles, primarily in males.
Sources:
https://www.nhm.ac.uk/discover/when-whales-walked-on-four-legs.html
https://evolution.berkeley.edu/what-are-evograms/the-evolution-of-whales