Final answer:
Rodhocetus is considered the first whale ancestor to exhibit the complete loss of hind legs and position of nasal openings similar to modern whales. The presence of vestigial hind leg bones in whales indicates their terrestrial ancestry. Phylogenetic trees show the evolutionary relationships and traits lost or derived during cetacean evolution.
Step-by-step explanation:
The first whale ancestor to exhibit the complete loss of hind legs and the movement of nasal openings to the position of blowholes, like those found in modern whales, was Rodhocetus. This indicates that Rodhocetus had adapted to a largely aquatic lifestyle, which is a part of the whale's evolutionary journey from land to sea. Furthermore, evidence of whales once having hindlimbs is present in their fossil record, which shows some whales, like Rodhocetus, had ankle bones similar to those of land animals. The presence of these bones is considered a vestigial feature, as they serve no function in modern cetaceans but indicate a past common ancestor that was terrestrial.
Modern day whales are descendants of even-toed ungulates, sharing a close evolutionary relationship with animals like the hippopotamus, cow, sheep, camel, and pig, as observed in phylogenetic trees. These trees help us understand the derived traits or traits lost due to evolution, such as the absence of legs in the marine mammal lineage leading to whales. Such analyses and the fossil records help to unveil the evolutionary adaptations that have occurred over millions of years, showing the transition of ancestors of whales from land-dwelling creatures to the fully aquatic animals we see today.