What don't the present shapes of the continents fit perfectly into a supercontinent?

Between 320 million and 195 million years all the landmasses on Earth were joined together as a massive supercontinent called Pangaea. Over time, convection in the mantle and other forces that brought the continents together, eventually caused them to split apart and move toward their present- day location. They continue to evolve as they move in different directions, thereby no longer fitting perfectly like a puzzle for a variety of geological reasons.

First, fracturing in the crust due to rifting and erosion leads to irregular Coastlines that no longer fit like a puzzle, though the many continental edges still closely match. Secondly, as separation occurs magma rises through the rift zone to become new crust as evident in the East African rift today. Third, as Pangaea split into two halves, Gondwana and Laurasia, collisions still occurred for a few million years. Furthermore, between then and now, microcontinents and island terranes from distant places, with different fossils and rock types, collided and accreted onto the edges of continents. These collisions cause mountains to be built on the mainland and also volcanic mountains at subduction zones if ocean crust is involved in the collision, further adding new crust and extending continental edges during lava flows. Finally, surface processes such as weathering, erosion and deposition can alter coastlines and continental shelves. Wave action and hurricanes can destroy coastlines and transport the material elsewhere, as can underwater landslides. Coastlines can also be extended by deposits from ocean currents and even more so by river delta deposits as evident in the Mississippi River Delta in the Gulf of Mexico.