When I try to picture what the ocean depths must have looked like near England’s Jurassic Coast, 300 million years ago, I picture something like Van Gogh’s Starry Night (1889). I picture a space of stillness but also turbulent life, things moving ceaselessly in the restless dark; I picture everything swirling, vortices upon vortices. What I picture undoubtedly derives from my encounters with what seems to have been the region’s primary resident: the ammonite, a snail-like prehistoric shellfish with a shell in the shape of a swirl. Ammonites were everywhere in this place. If you look hard enough, you can…