Passage Summary: The text establishes several rules: dinosaurs only lived on land, the southern part of a specific region was underwater during the late Mesozoic, plesiosaurs were not dinosaurs, and no single dinosaur species lasted the entire era.

Reasoning: The southern region was entirely underwater during the Cretaceous; dinosaurs were strictly land animals; plesiosaurs were water-dwellers and thus not dinosaurs; and no dinosaur species existed for the duration of the Mesozoic era.

Analysis: In a 'Must be False' question, we are looking for a statement that directly contradicts the established facts. The most likely candidates for a contradiction involve the location of dinosaurs or the lifespan of a species. For instance, if an answer choice claims a dinosaur species lived in the southern part of this region during the Cretaceous, it would be false because that area was entirely water. Similarly, any claim that a specific dinosaur species survived the entire Mesozoic would violate the final premise. Look for an answer that attempts to place a dinosaur in the water or extend its species' lifespan beyond the stated limits.