Passage Summary: Whales have light, porous bones that help them float back up after diving deep. Because the extinct ichthyosaur had similar bones, it was probably a deep diver too.

Conclusion: The prehistoric ichthyosaur was likely a deep-diving marine animal.

Reasoning: Modern deep-diving mammals have porous bones to help them surface easily, and ichthyosaurs possessed this same bone structure.

Analysis: This argument relies on an analogy between modern mammals and ancient reptiles. To weaken this, we need to find a reason why porous bones might exist in an ichthyosaur for reasons unrelated to deep diving. Look for an answer that suggests this bone structure serves a different physiological purpose in reptiles or that ichthyosaurs had other traits that would have prevented deep diving regardless of their bone density.