August 2, 2012
I’m going to Peru to see if its fossils are similar to the ones I’ve found in Nevada. I may as well tell you a bit about how we work, and what I found in Nevada. Plus, it’s nice to think about Nevada fossil hunting while I’m stuck in this sweltering Los Angeles laundry mat this morning.
The rocks are only going to tell you what the rocks are going to tell you.
This simple fact of paleontological field work best characterizes our challenge in science. It would be easy to say, “I’m going to determine what happened to bottom-dwelling marine animals after the mass extinction by studying Early Jurassic fossil crabs.” Answering this question with fossil crabs would be difficult, however. Crunchy though they are, crabs’ delicate exoskeletons often get smashed as layers of seafloor muck transform into sedimentary rocks over tens, thousands, millions of years. Consequently nice fossil crabs are rare, and if they occur sporadically in great numbers, one needs to wonder what particular conditions set that pile of crabs apart from others.
The rocks are only going to tell you what the rocks are going to tell you. So, in lieu of fossil crabs, one could look for fossilized burrows made by crabs, shrimps, and similar animals. Fossil burrows can be preserved in rocks surprisingly well. From them we can ask general questions. How did the size and abundance of burrow-makers change during a certain part of earth history? Did the diversity of types and shapes of burrows change?
My advisor Dave Bottjer is well known internationally for his extensive work on fossil burrows, “trace fossils”, and what they tell us about both specific environments and grand transitions in global biology. Here he is on a beach in San Diego, showing our paleo class some gorgeous burrows – small disturbances in the otherwise nicely layered yellow sediments.
I hit a wall early in grad school. A very large wall. Here’s a picture of me standing on it.
This cliff is made of millions of years – about two million years – of rock formed from compressed seafloor muck. I went to this particular cliff to study bottom dwelling shelly fossils. “I see some pretty large snails right after the mass extinction, which is surprising. What happened to the mollusks? How did size change in snails in the aftermath from the mass extinction? Was it gradual or sudden?” I spent some time in the field asking this question. I examined the layers of rocks, made microscope slides of them, and hunted for fossils. Sure enough I found plenty of snails – and scallops and corals and clams – but not until about 2 million years after the mass extinction. What about earlier in the Jurassic? Where the heck were all the snails?
Apparently these rocks were not going to tell me much about snails. I did find burrows though! Yes, plenty of burrows. The kind made by small shrimps that build big dividing chambered tubes deep in the muck, burrows named, “Thalassinoides”. We name the burrow because we’re rarely sure exactly who made it, but different distinctive burrow characteristics can still be tracked.
During my first years in grad school I learned how to ask the rocks different questions. For example, when burrows stand open, they can become filled with sand and bits and pieces of other fossils that sweep across the sea floor. The shrimp-like burrows I found in Nevada had a distinctive black, glassy color. Chert. That’s a rock made with tons of silica – basically it’s rock made of glass or infused with bits of silica and oxygen – the parts that make up glass and the mineral quartz. When I looked at microscope slides made from cross sections through the shrimp-like burrows, I found tons of tiny fossils! They look like little donuts and tubes.
Sponges are filled with tiny glass needles – silica again – which usually fall apart and toss around in the sediments after the sponges die. My shrimp-like burrows were filled with microscopic sponge bits! So that meant there were a ton of sponges around at the time – enough sponges that their microscopic bits filled burrows covering the seafloor for over a kilometer. Those layers of burrows became handy – I followed them in different canyons, different cliff faces, and used them to tell me I was in the right place.
It wasn’t until my third field season that I got a big break. My friend Yadi and I were following layers of storm-smashed snail shells, trying to determine which way the current was heading. When ancient storms kick up shells on the seafloor, they spread them across the deep, making a nice marker of an instant in geologic time. We kept finding odd mushroom and blob shaped cherty knobs interrupting the storm deposits. It finally dawned on me, what if there were some sponges standing on the seafloor that didn’t get smashed? What if they didn’t all get obliterated by storms and time?
The next day Yadi found them. Sponges. Beds of the seafloor life frozen in time, showing clusters and bunches of small sponges nestled together with a few scallops. We couldn’t believe our luck. Sponges rarely preserve such nice fossils – and to see the actual seafloor snapshots!
But we’re cynics, Yadi and I. We couldn’t quite believe our eyes. What if it was just an illusion? That day we decided to cut the trip short. We packed up our best samples of potential sponges, drove back to California, and started analyzing them that night in the lab. Over the next two days we poured acid on them, made microscope slides, and looked at pieces of sponge under the Scanning Electron Microscope. There was no question. These were fossil sponges. A week later I was back in Nevada with my sidekick Amir, and we found their friends – the whole damned mountain is made of sponges!
The rocks are only going to tell you what the rocks are going to tell you. Sometimes I go out to find snails, and I find sponges instead! This sponge story goes deeper – and it goes around the world. But this is enough for one blog post and my laundry’s almost dry. Next time I’ll tell you why we’re going to Peru and what we might find. But I’ve got to remember, even the rocks in Peru are only going to tell me certain things, and I don’t know until I go look.