Final answer:
Pakicetus and Ambulocetus were transitional fossils in the evolution of whales, evidence of which includes vestigial structures and specific anatomical features linking them to land mammals. The correct answer is option 4.
Step-by-step explanation:
One of the most significant findings in evolutionary biology was the discovery of Pakicetus and Ambulocetus, which are transitional fossils in the evolution of whales. These species exhibit traits that bridge the gap between ancient land mammals and modern cetaceans, the group that includes whales, dolphins, and porpoises. Evidence for the evolutionary transition from land to water in whales is seen in the presence of vestigial hind limbs in some species and in the fossilized bones of ancient whales that show adaptations both for life on land and in water.
The fossil record indicates that cetaceans were once land-dwelling mammals. Pakicetus, initially thought to be a semi-aquatic mammal, and Rodhocetus, which was more adapted to an aquatic lifestyle, show a clear transition from land to sea. This is further supported by the presence of a specific type of ankle bone found in Rodhocetus that is unique to land mammals, highlighting its terrestrial ancestry.
Following the extinction of the dinosaurs, there was a great diversification of mammals during the Cenozoic era. Early land-dwelling mammals gradually transitioned through various forms, leading to the aquatic mammals we see today. The progression from terrestrial animals to fully marine creatures represents one of the most remarkable evolutionary stories, with Pakicetus and Ambulocetus being critical pieces of this evolutionary puzzle.
Thus, in answering the initial question, the fossils of Pakicetus and Ambulocetus were transitional forms in the evolution of whales, which is option 4 in the list provided.