Final answer:
The shape of a bone in the middle ear called the involucrum links extinct organisms like Pakicetus and Indohyus to cetaceans. The evolution from terrestrial ancestors to aquatic lifestyles in cetaceans is evidenced by vestigial structures and phylogenetic relationships. Genetic and fossil records further support this evolutionary connection.Option 1 is the correct answer.
Step-by-step explanation:
One important feature that links extinct organisms such as Pakicetus and Indohyus to cetaceans is the shape of a bone in the middle ear. This specific bone is known as the involucrum, a characteristic of the auditory anatomy that is unique among mammals to cetaceans and provides a clear link between them and their terrestrial ancestors. This is part of what we've learned from the fossil record, including notable transitional forms such as Rodhocetus, which show the evolutionary process that led to modern cetaceans.
Modern whales are known to have descended from four-legged land animals, and an important piece of evidence for this is the presence of vestigial pelvis and leg bones in some species. Additionally, phylogenetic trees, which illustrate relationships among species, show us that cetaceans (like dolphins and whales) are descended from even-toed ungulates, sharing a common ancestor with animals such as hippos.
It's significant to note that while modern cetaceans do not have visibly prominent hind limbs, they share structural and genetic traits that testify to their land-dwelling heritage. This evolutionary heritage is also supported by genetic data and the geographic distribution of related fossils. An understanding of these traits provides valuable insights into the history and evolution of these fascinating marine mammals.