National Museum of Natural History Curator of Dinosauria, Matthew Carrano, and vertebrate fossil preparators Steve Jabo, Michelle Pinsdorf and Matthew Miller spent two weeks in Montana in July prospecting for and collecting fossils dating from the Late Cretaceous. Their trip report, written by Michelle Pinsdorf and Matthew Miller, follows.
The goal of our trip was to supply new specimens for ongoing research by Curator Matthew Carrano and Professors Ray and Kristi Rogers, geologists at Macalester College, who are using fossils and sediments to understand the ecology of an ancient ecosystem. We were searching in the Judith River Formation, a vast expanse of rock formed from sediments deposited in a broad river floodplain about 75–76 million years ago, during the Campanian part of the Cretaceous.
A panoramic view of the Upper Missouri River Breaks National Monument, a unit of Federal land managed by the Bureau of Land Management (BLM), where our field work took place. The National Monument preserves unique rock outcrops and wildlife habitats along the Missouri River (visible in the distance) in central Montana, while also allowing for recreational activities and cattle grazing. A permit from the BLM allowed us to collect fossils from this area. Click photo to zoom.
The Missouri River cuts through this rock formation in Montana and runoff from rain has helped create steep outcrops of rock where fossils can be found. Vegetation is lush along the banks of the river, but away from the water, the landscape is dry and hot. A usual day in the field for us involved loading up as much water and supplies as we could carry, and hiking to the outcrops from the closest access road, or ferrying across the river in canoes so we could reach outcrops on the other side. Our group usually prospected the outcrops together, fanning out across the steep slopes, eyes on the ground, looking for anything unusual. When we broke into smaller groups, we used walkie-talkies to communicate our locations and progress.
We were looking for sites where concentrations of fossils are weathering out of the rock. In the ancient ecosystem, these were places in floodplain where ponds and lakes had formed, depositing fine sediments and tiny bones rather than washing them away downstream. Once a site was found, its location was documented using GPS and topographic maps. Then, two kinds of sampling were done to collect specimens: surface collecting and bulk sediment collecting.
In surface collecting, we scour the ground for fossils lying out in the open. While productive, this form of collecting is skewed by the actions of erosion and weathering. The smallest and most fragile fossils get washed off of the hillside, or shattered by the freeze/thaw cycle of the changing seasons, while the more robust fossils are left behind. If only surface-collected fossils were used to determine the ecology of the site, the results would be heavily biased. A more accurate picture of what was living in the ecosystem is gained by collecting sediment dug from deeper in the hillside, where fossils are protected from the effects of surface erosion. This bulk-collected sediment will not yield its fossils without a lot of additional work back at the Museum, but the results will be worth the effort.
A weathered and broken tooth of a tyrannosaurid dinosaur, shown on the left, was one of the fossils found on the surface of the ground. The most thorough approach to surface collecting, getting your eyes as close to the ground as possible, is shown on the right. (A word to the wise: it’s always a good idea to look for rattlesnakes before lying down!) Click photos to zoom.
Bulk-collecting a sediment sample at a site high on the outcrop. Once filled with sediment, the orange 5-gallon bucket was shipped, along with others filled at different sites, back to the Smithsonian. The sediment will be washed and then searched for fossils in FossiLab. Visit the FossiLab website to learn how this is done. Click photo to zoom.
What have we found? We don’t entirely know yet! When it comes to bulk-sediment samples, one week’s worth of field work can turn into many months of work in the museum. Fossils will be washed and sorted by staff and FossiLab volunteers, identified by students and curators, catalogued and databased by collections staff, and finally made available for research. The larger specimens obtained through surface collecting are easier to immediately identify. Some of the dinosaur fossils found during the trip include a large (but shattered) tyrannosaurid tooth, a section of skull from a dome-headed pachycephalosaur, and a portion of a limb from a theropod dinosaur. Non-dinosaur fossils include teeth and scutes (thickened, bony plates) from crocodiles, and a large section of a flattened turtle shell.
Some other highlights from our two weeks of field work:
Our neighbors during our stay in the field included a herd of bighorn sheep, shown here grazing on the scarp of a large landslide (left). On the right is a chunk of sediment that, when split open, revealed a well-preserved plant seed pod. Click photos to zoom.
The first of our two campsites was under a grove of cottonwood trees along the bank of the Missouri River (left). Paleontologists don’t like rain, especially in the Judith River Formation, where water turns the dirt roads and outcrop surfaces into a sticky, slippery mud called ‘gumbo,’ but they are suckers for rainbows. This double rainbow (right) was enjoyed from our second campsite, higher on the bluffs overlooking the river – the best kind of a ‘room with a view.’ Click photos to zoom.
Photos by Michelle Pinsdorf and Steve Jabo.