Final answer:
Extinct cetaceans and living even-toed ungulates shared specific structures such as the femur and pelvis. These shared structures provide evidence for an evolutionary link between the two groups, with cetaceans having descended from land-dwelling animals.
Step-by-step explanation:
The fossil record indicates that specific structures were shared by extinct cetaceans and living even-toed ungulates. Among these structures, the ones that are known to have been shared based on fossil evidence include the femur and the pelvis (which includes the type of ankle bone found in Rodhocetus), which are indicative of their common ancestry with land-dwelling even-toed ungulates like the hippopotamus, cow, sheep, camel, and pig. Although both groups have adapted to their respective environments—cetaceans to aquatic life and even-toed ungulates to terrestrial life—their evolutionary history is connected, as cetaceans are descended from terrestrial ancestors.
Cetaceans, such as whales, dolphins, and porpoises, once had ancestors that walked on land. Over evolutionary time, these land-dwelling mammals returned to the water and gradually adapted to their aquatic environment. As a result, the modern cetacean forelimbs have evolved into flippers and they have lost their hind limbs. The presence of certain bone structures supports the evolutionary link between cetaceans and even-toed ungulates. This is further confirmed by phylogenetic analyses, which show these animals share a common ancestor, and by the evidence of certain morphological traits that persist in both groups despite their disparate lifestyles.
Furthermore, the modern whale flipper is an example of both an adaptive characteristic and a vestigial structure. It has adapted to provide better swimming capabilities, and its bone structure reveals ancient links to terrestrial mammals. The similarities in bone structure between whales and other tetrapods like humans and cats suggest a common ancestral origin for these limbs, despite their different shapes and proportions reflecting their adapted functions.