But which consequences and why? Such a fusion of fragments would produce a wide array of results, ranging from changes in weather and oceanic circulation to the interaction of previously isolated ecosystems. Here we must look to advances in evolutionary biology--to theoretical ecology and our new understanding of the diversity of living forms.

. . .

We must first understand two things about the Permian extinction and the fossil record in general. First, the Permian extinction primarily affected marine organisms. The relatively few terrestrial plants and vertebrates then living were not so strongly disturbed. Second, the fossil record is very strongly biased toward the preservation of maine life in shallow water. We have almost no fossils of organisms inhabiting the ocean depths. Thus, if we want to test the theory that reduced area played a major role in the Permian extinction, we must look to the area occupied by shallow seas.

We can identify, in a qualitative way, two reasons why a coalescence of continents would drastically reduce the area of shallow seas. The first is basic geometry: If each separate land mass of pre-Permian times were completely surrounded by shallow seas, then their union would eliminate all area at the sutures. Make a single square out of four graham crackers and the total periphery is reduced by half. The second reason involves the mechanics of plate tectonics. When oceanic ridges are actively producing new sea floor to spread outward, then the ridges themselves stand high above the deepest parts of the ocean. This displaces water from the ocean basins, world sea level rises, and continents are partly flooded. Conversely, if spreading diminishes or stops, ridges begin to collapse and sea level falls.

When continents collided in the late Permian, the plates that carried them "locked" together. This set a brake upon new spreading. Ocean ridges sank and shallow seas withdrew from the continents. The drastic reduction in shallow seas was not caused by a drop in sea level per se, but rather by the configuration of sea floor over which the drop occurred. The ocean floor does not plunge uniformly from shoreline to ocean deep. Today's continents are generally bordered by a very wide continental shelf of persistently shallow water. Seaward of the shelf lies the continental slope of much greater steepness. If sea level fell far enough to expose the entire continental shelf, then most of the world's shallow seas would disappear. This may well have happened during the late Permian.

The Great Dying

Ever Since Darwin, p 136-138.