The following passage is adapted from an article on evolutionary biology, focusing on the fossil discoveries that linked modern cetaceans to land-dwelling mammals.
Paragraph 1 (lines 1–12)
For decades, paleontologists wrestled with the evolutionary origin of cetaceans—whales, dolphins, and porpoises. While anatomical similarities in skull structures hinted at a connection to hoofed mammals, the fossil record remained stubbornly silent on how these fully aquatic creatures transitioned from terrestrial life. This lack of physical evidence allowed competing theories to persist without resolution, leaving a critical gap in the understanding of mammalian lineages. The absence of intermediate forms meant that the early stages of cetacean evolution were largely matters of speculation, with researchers divided over whether their ancestors were closely related to extinct carnivorous mammals or modern artiodactyls.
Paragraph 2 (lines 13–28)
The discovery of Pakicetus in 1981 marked a crucial turning point in this paleontological debate. Uncovered in Pakistan, this early Eocene mammal possessed an elongated skull and teeth characteristic of early whales, yet its fossilized skeletal remains clearly showed it walked on four legs on land. Crucially, researchers identified a unique bony wall in the middle ear chamber—a structure known as the involucrum—which is found only in cetaceans and is essential for underwater hearing. Though Pakicetus was likely a terrestrial or semi-aquatic wader, this diagnostic ear structure confirmed a definitive evolutionary link between land mammals and modern marine cetaceans, providing the first concrete physical evidence of the transition.
Paragraph 3 (lines 29–42)
Subsequent discoveries of other transitional fossils, such as Ambulocetus and Rodhocetus, filled in the remaining structural gaps, detailing the gradual reduction of limbs and the migration of the nostrils to the top of the head. Collectively, these fossils dismantled the long-held notion that major evolutionary transitions occur in sudden, inexplicable leaps. Instead, they demonstrated a slow, step-by-step adaptation to marine environments, transforming cetacean evolution from a speculative hypothesis into one of the most thoroughly documented transitions in the fossil record.
Which of the following statements best summarizes the main point of the second paragraph (lines 13–28)?
- The fossil remains of Pakicetus offered the first concrete physical evidence linking terrestrial mammals to cetaceans.Cevap
- BA chronological series of fossil discoveries has fully documented the gradual stages of whale evolution.
- CThe involucrum is a specialized bone in the middle ear that is essential for underwater hearing.
- DThe fossil of Pakicetus proved that the creature was fully aquatic and could not walk on land.