Final answer:
Both a fossil pterodactyl and a living elephant have an ilium due to shared evolutionary history. Bipedal dinosaurs had long necks balanced by long tails, serving as a counterbalance for locomotion. The fossil record, including theropods and features in organisms like Archaeopteryx, supports the common ancestry of modern species.
Step-by-step explanation:
The best explanation for why both a fossil pterodactyl and a living elephant have a bone in their hip called the ilium is due to their shared evolutionary history. All vertebrates have a basic common body plan that dates back to early ancestors in the evolutionary tree. Over time, this body plan has been modified in different lineages to suit specific ways of life. The presence of the ilium in both pterosaurs and elephants shows that these animals inherited this structure from a common ancestor, despite the adaptations for flight in pterosaurs and the massive size in elephants.
Regarding bipedal dinosaurs with long necks and long tails, these features helped balance their bodies, especially while moving or standing erect. A long tail could counterbalance the weight of a long neck, improving stability and leverage. This balancing act would have been essential to maintain a center of gravity over the hips, enabling efficient bipedal locomotion.
In the case of creatures like Rodhocetus, a prehistoric aquatic mammal related to whales, having an ankle bone typically found in land animals could indicate a terrestrial ancestry, suggesting a complex evolutionary path from land to sea.
The varied anatomy seen in Therapsids transitioning towards mammalian traits and the discovery of fossils like Archaeopteryx with features of both reptiles and birds provide strong evidence for common ancestry among species, as well as illustrating both convergent and divergent traits in the evolutionary process. The fossil record, including pelvis structures, increasingly supports the theory of a shared ancestor for birds and dinosaurs, particularly within the group of theropod dinosaurs.