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Fossil-Finding 101: How to Spot the Right Rocks

Emily Hughes has been following her mother, paleontologist Mary Droser, into the field all her life. This summer the family is back in Australia digging up some of Earth’s oldest fossils.

A question that members of the research team often get regarding our work in the South Australian outback is simply how we know where to excavate fossil beds. It’s a good question—an important aspect of the paleontological process is simply identifying the best places to look for fossils. If we see enough of the indications that signal there are fossiliferous beds nearby, we can be confident enough to begin the excavation processes. Here are some of the signs.

First off, we need to make sure the type of rock is one that could potentially contain fossils. This means the rocks must be sedimentary (built up by the accumulation of material on Earth’s surface without having undergone too much transformation from heat or pressure, and not having bubbled up as liquid from inside the Earth). The Ediacaran fossils we find out here are preserved in quartz sandstones, which means that the rocks are formed of sand-sized quartz particles that have been compacted and cemented by other minerals. It’s on the bottoms of these rocks that we find the molds of Ediacaran fossils.

Secondly, we look for hills. Ediacaran rock layers are really, really ancient and after millions of years’ worth of plate tectonics and other geological processes most of them are either buried deep underground or have been so distorted that the stories they would tell have been erased. There’s a balance required to get the necessary amount of uplift to bring them near the surface while still ensuring they don’t greatly metamorphose in the process. This is why hilly Nilpena is one of the best localities for finding fossils, as the processes of major uplift and gentle tilt have made it possible to excavate beds of rock that are half a billion years old.

The next step is searching for “float,” or pieces of fossiliferous rock scattered on the hillside. These rocks come from fossil beds, but over time have been exposed and fallen down the landscape. If we find float, especially float with well-preserved fossils on the bottom, we know we’re close to hitting the jackpot.

So, once we’ve got the right rock type, the right locality, and we know there are fossils nearby, the next step is to search for exposed beds peeking out of the earth. Usually we look for large slabs of rock because we find it much easier to excavate large rocks than small ones (which can be easily lost!). Once we’ve found an outcrop, and generally after a bit of careful exploration, we get our tools and begin the careful process of digging, exposing, and excavating.

Here at Nilpena, we have four major sites where we’ve conducted excavations of fossil beds. The first, founded about fifteen years ago, is called Tennis Courts, so named because the laid-out beds of rock look like … tennis courts. We are very creative when it comes to naming localities.

Here’s the pit from which the beds located at the Tennis Courts site came. Photo by Emily Hughes

Our next site, founded a few years later, is just a bit south of Tennis Courts, which is why we call it South Tennis Courts. Here, the excavation pit is really pristine. There’s a layer of rock on the bottom of the pit that’s un-excavated, so it looks just like the seafloor would have looked millions of years ago, with gorgeous ripples.

The pit at STC beautifully shows each layer of bed rock. Photo by Emily Hughes

Then there’s the site we call the Bleachers (because it’s adjacent to the Tennis Courts). Here, we have a bed called “Biggest Rock in the World Bed.” This is because the bed is composed of massive and extremely heavy rocks, one of which weighs nearly a thousand pounds. It took about nine people to actually flip the rock onto the ground, and we did not manage to move it very far from the pit.

Here’s a look at BRW bed on the left, and the excavation pit from which it came on the right. Photo by Emily Hughes
Another image of BRW bed. Check out the size of that rock! Photo by Emily Hughes

Lastly, we have a site called One Tree, named after a tree standing on a nearby mountainside. We have two pits here where the rocks are really fine-grained. The average size of the rocks is much smaller than at any of our other localities, and usually we prefer dealing with larger rocks, but being of a finer grain they preserve very small fossils that we wouldn’t be able to see at other sites. As such, we’ve been working here for almost a decade.

Here’s one of the two excavation pits at the One Tree locality. Notice some scattered tools from our recent work there. Photo by Emily Hughes

All told, there are over 30 excavated beds across all the sites on these hills. We owe our years of work here to the many things that make Nilpena such a unique site for fossil preservation and excavation.

Comments

  1. Charlie Tuttle
    July 8, 9:38 am

    Nice reporting, Emily. Along with the pictures, I am getting a real sense of the work.