Teredo Petrified Wood

Teredo petrified wood is a two-part fossil: the remains of a Paleocene tree, preserved as stone, and the tunnels drilled through it by Teredo, a genus of bivalve mollusk known today as the shipworm. The combination is what defines North Dakota's state fossil. Ordinary petrified wood and Teredo-bored driftwood are common individually, but finding them together in the same rock preserves a complete ecological moment from 60 million years ago.

North Dakota was a very different place during the Paleocene. Dense subtropical forests covered what are now the open plains and badlands, and slow rivers carried fallen logs toward coastal lowlands and inland seas where Teredo was active. That warm world vanished, but its trees survive as stone in the formations of the western part of the state.

The petrified wood itself is the silicified remains of Paleocene hardwoods, mainly broad-leaved subtropical trees from the Fort Union Formation. Over millions of years, groundwater carrying dissolved silica slowly replaced the original wood cell by cell, preserving the internal grain structure in quartz. A cross-section of a good specimen shows annual growth rings, vessel cells, and other wood anatomy in fine detail.

The Teredo borings appear as cylindrical tunnels, typically a quarter to half an inch across, running through the wood in all directions. In life, Teredo lined each tunnel with a thin calcium carbonate shell tube, and those tubes are often preserved inside the stone. The boring pattern gives the wood a distinctive pockmarked or honeycomb appearance on cut and polished surfaces. Teredo is not a worm but a highly modified bivalve mollusk, and the genus still exists today as a serious pest on wooden boat hulls and harbor pilings.

North Dakota designated Teredo petrified wood its official state fossil in 1967, one of the earliest such designations in the country. Petrified wood had long been a familiar feature of the western North Dakota landscape, where erosion constantly exposes new material from the badlands formations. The Teredo-bored variety is common enough that collectors throughout the region recognized it as something distinctively North Dakotan.

The choice of a plant fossil rather than a vertebrate also reflected the unusual richness of North Dakota's Paleocene plant record. The Fort Union Formation preserves one of the best-documented subtropical forest assemblages from the early Cenozoic in North America, and the bored wood is among its most striking and accessible specimens.

Teredo petrified wood is found across the badlands of western North Dakota, eroding out of the Sentinel Butte and Bullion Creek members of the Paleocene Fort Union Formation. Slope, Bowman, and Stark counties in the southwest are especially productive, as are the badlands along the Little Missouri River corridor. Erosion continually exposes fresh material, and surface collecting is permitted on much of the Bureau of Land Management land in the region.

Theodore Roosevelt National Park in Billings and McKenzie counties protects petrified wood in place and has visible specimens along several hiking trails, though collecting inside the park is not allowed. The North Dakota Heritage Center and State Museum in Bismarck holds significant collections from the Paleocene formations.