The later Paleozoic (286 million to 570 million years ago) was a world that would be recognizable to us. By this time the teeming marine and land plants had expelled enough oxygen to produce an atmosphere very similar to our own. Vast forests greened the supercontinent Pangaea and supported a thriving animal population. We would be struck by the sheer size and variety of the flora and fauna: horsetails and scale trees that stood from 50 to 100 feet tall and dragonflies with 2-foot wingspans. Drippingly humid and silent, the monotonously green rain forest abounded with scuttling creatures familiar and unfamiliar...