Extinction of dinosaurs at end of Cretaceous. Proposed in 1977 that Cretaceous extinction was caused by asteroid impact. Cretaceous-Tertiary border has high level of iridium , rare in earth's crust but common in meteorites. Worldwide layer of soot also defines iridium layer. Physical factors, including location of continents, limit population range.
Continental drift states that continents have slowly moved over time.
Explains close puzzle-piece fit of east coast of South America with west coast of Africa, and other continent edges. Explains distribution of seed ferns throughout southern continents. Explains distribution of early reptiles across many continents from time when land was conjoined. Explains divided distribution of mammals that evolved after continents parted. Simplest explanation is having a common ancestor whose basic forelimb plan was modified in succeeding groups as each continued along its own evolutionary pathway.