ALDER, MONTANA – I’m sitting near the top of our fossil excavation site in southwest Montana, my hammer and shovel ready. I have a perfect view of the mountains. A wall of fossil-laden shale lies before me, and I’m ready to dig in.
This is our fourth day digging, and despite the early hour, I'm trembling with excitement. Today I might find something new, something no human has ever seen. It’s hard to imagine that each plant and animal we find in this deposit lived, died and was fossilized between 23 million and 33 million years ago.
I pull a small stack of shale out from the exposed outcrop and cradle the layers in one hand. The finely laminated rock readily splits into many thin layers, like pages in a book. I flip through them slowly, scrutinizing each for traces of fossilized material. What a change of pace from my desk job!
Though I earned my master’s in botany, I’m currently working as a graphic designer and illustrator at the Illinois Natural History Survey at the Prairie Research Institute. I’ve always loved both art and science, and for years I’ve struggled to choose between them. Then Sam Heads – a paleontologist at INHS – offered me the opportunity to earn a Ph.D. in paleontology. That’s how I found myself packing for this trip.
Paleontology is a unique blend of scientific research and art. It’s common for paleontologists to pair their own illustrations with fossil images in descriptive manuscripts. The added illustrations help readers interpret the fossils, particularly those that are incomplete.
Today, we are sketching stratigraphic logs. These diagrams show the height and vertical arrangement of distinct rock lithologies — groups of layers with the same rock type and texture — within an outcrop. This log provides a chronological context for the fossils. Generally, rock layers that are higher in a formation are younger than those in lower layers.
Learning to create a strat log was one of my first field lessons. We examined a section of rock layers roughly 7.5 feet tall where the weathered rock and soil had been removed. Sam patiently explained how to identify and measure the height of unique rock beds.
The outcrop we looked at consists of three main lithologies: magnesium-rich ash, fine-grained shale and coarser-grained shale. Fossils aren’t found in the ash, but are abundant in the shale layers. Each fossil’s quality is closely tied to the surrounding shale’s grain size – the finer the grains, the better the preservation.
I was lucky enough to unearth the first fossil of the trip in a fine-grained shale layer – a wonderfully preserved fly. Finding any fossil is thrilling, but I was most excited when one of my teammates found the first plant fossil.
The fossil leaves and stems we’re collecting will provide clues about this area’s ancient climate. For instance, leaves are typically smaller in arid regions than in wet, tropical environments; the smaller leaves help guard against water loss. Smooth leaves are more common in warm areas while toothed leaves dominate in temperate zones.
The density of stomata – the tiny pores on leaf surfaces that are used for gas exchange – tell us about ancient atmospheric carbon dioxide levels. In modern plants, high stomatal densities have been linked to low atmospheric CO2 concentrations; it’s thought that ancient plants follow the same trend.
The team will spend a lot of time behind a microscope when we get back to Illinois. For now, though, back to digging!