Footprints in the Mud: The Search for 23 Million Year Old Mammal Tracks

BY WHITNEY WORRELL

My path to trace fossil research started when I decided to write a summer research proposal last year. I have always wanted to do paleontology, that part was easy, but the geology department didn’t have any active paleontology research that I could take part in. Thankfully, my advisor, Professor Kena Fox-Dobbs, helped me create a project to satisfy my “must include fossils” requirement. Kena showed me a specimen that had been in our teaching collection since 1964, a nondescript chunk of sandstone, on the top of which is a tiny, yet very clear, set of footprints. On the back of the specimen is “Benham Creek” and some coordinates. The original finder of the specimen was a student at UPS in the early 1960’s who had found it on the ground during his own summer research, noted its location, and gave it to the department where it had not been investigated further until now.

ABOVE: The original trackway sample

I was lucky to have my research project funded, and I set out to study this mysterious trackway in sandstone. I can remember standing there for the first time realizing the seemingly impossible task I had agreed too: try and find more of these miniscule footprint fossils in this area, where Mount Saint Helens had famously erupted and then there was a landslide, among the many other things that had occurred in the area in the past 23 million years. The coordinates were to a spot on the side of Forest Road 25 in the Gifford-Pinchot National Forest, just northeast of Mount Saint Helens. According to other research and geologic maps, the rocks in the area are about 23 million years old. The site is a road cut, a portion of rock that had been cut away to make a road through the area, and heavily forested. My first visit to the site was daunting; I had been optimistic about the research, there was a lot of exposed rock, but there was also a turbulent river, with numerous downed trees from a landslide ten years prior.

Fortunately, I had a wonderful support team for the field work portion of my research. The department had hired a student, Audrey, to help where needed, and I was lucky to have her with me for the entire week I was going to be in the field. Additionally, my mom, Betty, and my dog, Molly, joined us at the campsite for good company. Kena and another research student would join us the next day. Audrey and I had a lot to accomplish in the first couple of days at the site before the others arrived.

To begin, we took measurements of the site, broke it up into 7 sections and measured the thicknesses of the layers of rock in each section. The layers of the site were not horizontal; they dipped in such a way that we could measure layers on top of other layers simply by walking to the next section over. The first day, Audrey and I gathered samples to bring back to the lab and classify the sedimentology -which includes the grain size and other observations about the rocks themselves- noted the thickness of layers, and broke open just about every rock we could find looking for signs of fossils. We found fossil leaves and wood fragments that day. It was exciting despite the fact that they were not the elusive footprints.

The second day, we decided that we needed to gain access to a small ledge about ten feet off the ground at the far end of the site to measure the layers and try to find more fossils. Audrey hiked up and around the outcrop and secured a rope above the ledge, then came back and was able to stand on the ledge secured by the rope. We worked on the area, broke open rocks and found numerous fossils of plant material, including cedar branchlets, leaves, and a single tiny hemlock cone. This seemed to be where we needed to be, I felt that this was going to be the place. I noticed that at the very top of this section there seemed to be defined layers, and I asked Audrey to break them off carefully and pass them down. With a rock hammer and a chisel, the layers were removed and inspected. Not long after, Audrey stopped and with cautious optimism said, “I think you need to see this.”

ABOVE: Audrey pointing out the location of trackway extractions

At this point I had not written off the possibility of actually finding the footprints, but I was aware that it was a very real needle-in-the-haystack situation. I also tried to avoid just seeing what I wanted to see, to avoid misidentification of some random shapes as a trackway. Audrey gently passed the chunk of sandstone down to me, I inspected it, and immediately we both knew we had found it. We tried to remain objective and wait to compare them to the original specimen, but we knew it was real. We found the actual layer in situ, untouched and intact for 23 million years. Carefully, we removed what we could of the trackway layer, and ended up with seven new pieces of fossil trackway, which included portions of the layer the animal had walked on and the layer that filled it in and made a cast of the footprints. With cautious confidence we returned to our campsite and showed our finds to Betty, who excitedly suggested that we celebrate, but we remained reserved and waited for Kena’s arrival on the following day.

Our findings were confirmed to be genuine, and we enthusiastically headed out to finish gathering specimens from the site. We collected over a hundred samples, including the new pieces of fossil trackway, numerous plant fossils, and various samples of rocks to study the sedimentology of the site. We brought it back to the Geology Department at UPS, where the real work was just beginning.

For the remainder of the summer, I prepared samples for analysis on petrographic microscopes, made silicone molds and resin casts of the trackway fossils, identified plant fossils, and tried to figure out who those tiny footprints belonged to. I met with a rodent paleontologist at the University of Oregon and it was determined that our track maker was a small rodent, not too dissimilar to a mouse. I have since moved on to writing a thesis on the Benham Creek site, which includes a paleoenvironmental reconstruction of this site’s animal and plant diversity, as well as the sedimentology and stratigraphy, or rock layers, of the site. After graduation in May, I will focus on graduate school applications, where I hope to earn a PhD in paleontology. This work has been able to shed light on an under studied time period in the Pacific Northwest, and could have larger implications for what we know about the animal and plant diversity in the area 23 million years ago.