Final answer:
The reappearance of sauropod dinosaurs in North America 70 million years ago likely reflects immigration from South America, facilitated by land connections after the breakup of Pangea.
Step-by-step explanation:
The sudden reappearance of sauropod dinosaurs in North America about 70 million years ago probably reflects immigration from South America. After the breakup of the supercontinent Pangea, different species of dinosaurs became geographically isolated, evolving separately. When these land bridges between South and North America became available, it could have allowed for the dinosaurs to move between these continents and thus lead to the reappearance of sauropods in North American fossil records. The option of re-evolution or convergence is less likely, as these require similar but independent developments in disconnected ecosystems, which does not as neatly explain the reappearance as does the concept of migration.