This is a fossilized Phegopteris guyottii - a species of fern. Its genus still exists today and is known collectively as the beech ferns.
Ferns emerged in the late Devonian, some 360 million years ago. Many ferns still exists today, though most of their ancestors didn't emerge until roughly 145 million years ago in the early Cretaceous, after flowering plants came to dominate many environments. That's when most species of ferns emerged.
