Teredo-bored petrified wood is North Dakota’s state fossil. It is found in 60 million year old Paleocene deposits, found in the southwestern area of the state. These are areas that were covered by the Cannonball Sea, now exposed at the surface. Before fossilizing, it was drift wood (ginkgo, bald cypress, metasequoia, etc.) that had been drilled (bored) into by small worm-like clams (Teredo), creating a Swiss-cheese effect.
Part of the fossilization included filling the borings with sediment or crystals, often seen as a different color or texture. If slabbed, the tiny clams can even be seen in cross section as little white crescents. The clams, also called shipworms, still pose a problem for wooden boats or docks today.
Teredo-bored petrified wood was adopted in 1967, and added to the Smithsonian Institution’s mineral collection. The selection was brought to the legislature by R. W. Carlson of Bismarck, former present of the Central Dakota Gem and Mineral Society, and H. A. Brady of Mandan. State Represenative W. G. Sanstead of Minot proposed the state fossil selection in House Bill 933.
https://www.dmr.nd.gov/ndgs/outofprint/NewsLetters/1993Spring.pdf (publication page 4 & 5)