A prehistoric plant (Pertica quadrifaria) was designated as the official state fossil of Maine in 1985. A primitive fern-like plant that lived during the Devonian Period (about 400 million years ago), Pertica quadrifaria was one of the largest plants of its time, growing to about 6 feet in height.

First discovered in Maine and found in only three other locations in the world, this plant is considered a scientifically important find because it represents an important step in the evolution of land plants.

Source:

Maine State Fossil: University of Maine at Presque Isle
Pertica quadrifaria - Maine state fossil: ME Geological Survey
All About Maine State Symbols - State Fossil: Maine.gov

Links:

State Fossils & Dinosaurs - 50 States