Ever Since Darwin, 4: Patterns and Punctuations in the History of Life, 16: The Great Dying

Permian extinction, 225 million years ago, wiped out half the families of marine life. What could have caused it? Ecology (more habitable area = more species) and Geology (continental drift) offer an answer.

The theory of continental drift explains that all the continents coalesced to form Pangaea during the latest Permian. This reduced the area of shallow seas in two ways:

• Plain "locking" of the continental plates eliminates all the coast at the sutures
• The locking stops the new spreading of plates, sinks the ocean ridges, withdrawing shallow seas from the continents. The overall sea level doesn't need to drop much to reduce the shallow sea area. If it drops enough to expose the continental shelf, most of the world's shallow seas would disappear.

This basic factor of space shortage led to the extinction of half the marine species.

Ever Since Darwin: Reflections in Natural History by Stephen Jay Gould