You might think that an 85-foot-deep hole where a bunch of horses, wolves, camels, elephants, and plenty of other animals accidentally plummeted to their death over tens of thousands of years would have enough red flags to make going into it yourself sound like a bad idea. But what if these unfortunate critters could tell you what their life was like and how they died? What if they could give you a warning about their death in a warming world after the last ice age and what it means for life in a warming world today? And, most importantly, what if you could fall and climb back out very slowly on a controlled rope system with an expert team of cavers and paleontologists? This past summer we decided to do just that: Barnosky lab members Eric Holt and Nick Spano with alums Susumu Tomiya and Jenny McGuire joined a crew led by Julie Meachen (Des Moines University) to descend into this “Natural Trap” Cave, excavate ice age mammal fossils, and help advance our understanding of how life responds to climate change, all without contributing any extra bones.
Natural Trap Cave is a 12-foot wide by 85-foot deep hole at the top of a hill in the Bighorn Mountains on the Wyoming side of the Montana border. The entrance to the cave is difficult to see coming down from the ridge of the hill behind it, so it’s not surprising that many Pleistocene ‘megafauna’ (animals bigger than 100 lb. or 45 kg) accidentally fell to their demise here over tens of thousands of years ago. As they fell into Natural Trap Cave, their bones formed a well-stratified and mostly undisturbed pile that has become internationally renowned since the 1970s for its paleontological significance. The cave had been closed by the Bureau of Land Management (BLM) for over 20 years to protect the fossils from theft. However advances in ancient DNA research and growing interests in what Pleistocene extinctions could tell us for conservation prompted it to be reopened by Julie Meachen’s group for further research. This site is ~42 °F at ~98% relative humidity year-round, making it an ideal refrigerator for extracting 30,000 year-old genetic material. Geographically, it is located just south of a gap that existed between the Laurentide and Cordilleran ice sheets in central North America at the last glacial maximum (LGM) ~22,000 years ago. The ice-free corridor extended all the way up to Alaska and provides a unique opportunity to investigate continental migration dynamics, population genetics with ancient DNA, and climate-driven community changes.
This past summer, Eric and I (Nick Spano) drove 18 hours from Berkeley, CA to join a volunteer crew of paleontologists and cavers led by Julie Meachen at Natural Trap Cave in Wyoming. To enter the cave, each person needs to rappel down a rope hanging 85 feet down into the cave. Even if you claim to be unafraid of heights, the first descent is still slightly nerve-wracking. Stepping backwards off of the cave’s rim into a black pit with only a constellation of faint headlamps at the bottom can be a little unsettling. Plus, easing your grip on the rope here to let out slack takes a couple days to become comfortable with.
Once you start the descent through increasingly colder temperatures, a council of packrat (Neotoma) middens along an inner rim welcomes you to the cave. After the initial shock of dangling passes and your eyes adjust to the low light, you get a sense for just how open and surreal the bell-shaped chamber is. I could only imagine what it must have been like for whole bison, horses, and wolves to fall that far down as I gracefully descended to the cave floor. Because we were searching for fossils of all sizes–from bison to mice teeth–we had to look carefully while excavating. That said, a fossil would pop out of the sediment about every ten minutes, which kept things pretty exciting.
Once discovered, each fossil needed to be tagged with information about which animal it came from, where in the cave it was found, and what kind of sediment it was found in. We then bagged the specimens and bulk sediments to be screen-washed for microfossils and hauled them back to the surface in a bucket on a rope. In that sense, we were lucky we didn’t find anything bigger than the bucket. Once the excavations were complete, the site was remediated to protect exposed sediments from further weathering and to leave the site in a pristine state for future paleontologists.
Now that the final and most recent field season has ended, Natural Trap Cave is closed again for the foreseeable future. Susumu is going through identifications and Jenny is analyzing microfossils from the site. This study will provide a greater understanding of how life was changing in a warming world at the end of the last ice age, with implications for how life might respond to current and projected warming. Eric and I are very thankful to have been volunteers involved with this project and are looking forward to some great results.