Antoine Bercovici is a Peter Buck postdoctoral fellow in the Department of Paleobiology at the National Museum of Natural History. He has been in the field since early July. This is his second post from North Dakota.
For weeks now, my colleagues and I have been using GPS technology to map known fossil sites in the Hell Creek Formation with the highest possible accuracy (see my previous post). We’ve logged hundreds of miles hiking through the badlands of North Dakota and Montana to reach the old sites, and in the course of these hikes we have discovered numerous new fossil localities to add to our database.
My colleague’s dog, Kitty-Cat, may be a bone-finding expert, but this cow bone is no fossil. Click to zoom.
How does one recognize and find fossils in the immensity of the badlands? The ground here is scattered with miscellaneous rocks and concretions, and finding fossils among these can be difficult. But a trained eye will scan the rubble for the right kind of color and texture, knowing that fossil teeth and bones are not necessarily white, but also can be different shades of brown, even black.
Something shiny and brown buried in the sediments caught Erica's attention. Several minutes of digging revealed a beautifully preserved tooth from a Tyrannosaurus rex. Click to zoom.
Above left: Freshly excavated from the outcrop, a blast from the distant past, when a T. rex lost his tooth ~66 million years ago. Above right: T. rex teeth were not sharp and blade-like, but more rounded and banana-shaped -- adaptations for crushing bone. This particular specimen has lost its pointy tip and displays a worn surface that developed as the animal chewed on whatever ended up in its jaws. Note the serrated edge on the upper left portion of the tooth. Click to zoom.
Surprisingly, finding vertebrate fossils does not require any digging. We instead take advantage of the fact that weathering and erosion expose fossils naturally, by wearing away the rock that surrounds them. In the badlands of North Dakota, we have been experiencing temperature that exceed 100°F, but winters can be very harsh, with snow and ice and temperatures that can go as low as -60°F. During fall and winter, the water contained in cracks and small interstices of rocks freezes, and because ice takes more volume than water, it forces the cracks open, fragmenting even the hardest rocks into loose pieces. This process is called gelifraction, and the result is similar to what happens when a can of soda has been forgotten in the freezer. Each spring, melting snow and rainfall washes out a portion of the newly loosened sediments away, carving the badlands topography a little bit more and exposing new fossils. But these forces work to break apart exposed fossils, too, and great fossil discoveries usually start when someone finds scattered fragments of fossil bones on the ground. Everything eroding out of the badlands hillsides rolls downhill, so we search upslope from loose fossil fragments, hoping they are part of a trail of pieces leading to bones still in place within the sediments, ready to be excavated.
Right: Following the bone trail. Several large fossilized dinosaur bones are being exposed out of the rock surface at the top of the hill (look for the brown/rust colored objects just above the scale bar). Numerous scattered fragments have rolled down the hill because of erosion and weathering. Click to zoom.
Prospecting for fossils and new localities thus requires a combination of geological knowledge, organization, and good timing and luck. We use our knowledge of geology to identify the rock layers most likely to contain the fossils we seek, and to locate the places where those layers are being exposed by erosion. On the outcrops, we keep a sharp lookout for fossils lying on the surface and cover as much ground as possible in an organized fashion, so as not to miss anything,. Timing and luck come in because if we prospect an area before erosion has begun to expose a fossil, we cannot see it, but if we come through too late, it will have been destroyed completely.
Above right. Two dinosaur ribs, two very different fates. The lower one has been found just in time and is almost intact, while the upper one has been weathered into small pieces that even the most skillful puzzle master would not dare to tackle. Click to zoom.
This summer, we covered a lot of ground and found numerous new specimens, including several spectacular dinosaur skeletons and skulls. These new findings are being added to our database, increasing our knowledge on the diversity and composition of the Hell Creek ecosystem.
Above left: It took a full day of excavating, to reveal this incredibly well preserved Triceratops skull. Erosion had exposed pieces of bones that correspond to the "beak" of the animal (the darker bone in the bottom left of the photo) but the rest of the skull was still embedded in unweathered sediments. Perfect timing! Above right: The crew, which included volunteers from the Denver Museum of Nature and Science, poses happily with the freshly excavated skull. Click to zoom.
Large fossils mean heavy lifting. Before we carry them from their excavation sites, we protect the bones by molding plaster field jackets around them. Then, we roll the jackets onto a stretcher and take turns at the four handles as we make our way slowly back to our vehicle. Click to zoom.
Photos by Antoine Bercovici.