Final answer:
Important features such as specific ankle bone structures in the fossils of Pakicetus and Indohyus link these extinct organisms to modern cetaceans, illustrating their evolutionary transition from land to sea. Phylogenetic trees and the study of homologous and analogous traits also support these evolutionary connections.
Step-by-step explanation:
One important feature that links extinct organisms such as Pakicetus and Indohyus to modern cetaceans, like whales and dolphins, is the presence of specific anatomical structures in their fossils that are also found in modern cetaceans. An example of such a shared feature is the structure of their ankle bones. The fossil record, including that of Rodhocetus, shows that these early aquatic mammals had a type of ankle bone that is otherwise unique to a group of land animals, which suggests a transition from land to sea in the evolutionary history of cetaceans.
This link is further supported by the fact that cetaceans are descended from even-toed ungulates and share a common ancestry with other land mammals such as the hippopotamus, cows, sheep, camels, and pigs. While Pakicetus's degree of aquatic adaptation is debated, Rodhocetus is considered to be a largely aquatic animal, reinforcing the aquatic lineage of modern whales. Furthermore, phylogenetic trees help illustrate derived traits, such as the loss of legs in marine mammals, and support the evolutionary connection between extinct species like Pakicetus and modern cetaceans.
Additionally, despite dolphins and fish having similar body shapes due to their environment, their evolutionary paths are quite separate, as demonstrated by homologous and analogous traits outlined in evolutionary biology. This distinction is important in understanding the difference between traits that arise from a common ancestry and those that are a result of similar environmental pressures, known as convergent evolution.