|Modern Crinoid, |
Gulf of Mexico (click for photo credit)
I grew up reading those books about fossils and ancient seas, and tromping around the cliffs and creeks near my parents' house, which is on a cliff in the Arkansas Ozarks. I had a vague understanding that the sandstone in the cliff was once the coastal sand of a vanished sea. But it never quite seemed real. I never truly connected the landscape around me with what I was reading until I decided to go fossil hunting. I got in my car and headed about 20 miles north, to a valley near Leslie, Arkansas. I pulled over to the side of Highway 65, and walked over to a line of cliffs along the road, at the base of a noble little hill called Chicken Wilson Knob.
I had passed those cliffs hundreds of times before, on the way to my grandparents' house up the road in Marshall. It's a beautiful spot. In wet weather, springs gush from holes in the cliff face. Sometimes in the winter they freeze into beautiful ice cascades. But this was summer, so I picked my way through the kudzu, keeping an eye out for copperheads, until I found a streak of crumbling shale between two layers of limestone. Sifting through the debris, it was no time at all before I found my first fossil. It looked vaguely like a piece of petrified bamboo, about as big around as a broom handle. It was sticking about six inches out of the rocks, with the other end disappearing into the hard-packed shale deeper in the cliff. I tried to dig it out, but I ended up breaking off the part that protruded. The other end is still in that cliff, for all I know.
|Fossil Crinoids from Indiana / Indiana State Museum|
That crinoid had lived right where I was standing; at the bottom of a shallow Arkansas sea, roughly 320 million years ago. That's where it died, was buried, and slowly turned to stone. Rather a lot has happened since then. At first, more layers of muck, composed mostly of the carbonate shells of tiny sea creatures, built up and turned to limestone. Eventually, higher land rose to the north and east. As it eroded and washed into the area, it formed rivers, deltas, swamps, and beaches. The mud from the highlands turned to shale and the sand turned to sandstone--some of which forms the caprock cliffs of the southern Ozarks. The southern continent of Gondwana slowly crashed into what would become North America, buckling the old sea floor into the Ouachita Mountains to the south, and raising Ozarks as a plateau. For the next 300 million years, the area would be dry land. The dinosaurs evolved, ruled the land for ages, and then vanished. Birds, mammals, and flowers spread over the landscape. Continental glaciers came and went (though they never got this far south.) The ancestors of Native Americans migrated from east Asia after the last glacial cycle, about 100 lifetimes ago. Europeans came and spread through the hills, founding the town of Leslie. When Highway 65 was built, they dynamited the cliffs along Cove Creek, exposing that sea creature that lived so long ago. And then I came along and broke it.
I've hunted for fossils off and on since then, but I've always been more careful, and try not to disturb things too much. Now I know some of the most important fossils are unrecognizable in the field to anyone but professional fossil hunters. I started thinking I might break or displace some ancient fish that could have transformed our understanding of evolution, if I had just left it alone and let someone find it who knew what they were doing. Yesterday, I realized just how real that possibility was. I had been hearing about a fossil that's changing the way scientists think about shark evolution. Modern sharks, it seems, aren't the living fossils they're often made out to be. They're highly modified from their ancient ancestors, who had a very different jaw structure, as shown by the new fossil that was making the news a couple weeks ago. Reading up on Arkansas geology yesterday, I saw where the fossil had been found: Leslie, Arkansas.
It was just a few miles up the road from my crinoid, in a slightly older layer of rocks (the Fayetteville Shale, which is currently being "fracked" to extract its natural gas.) What's most striking about photos of the fossil is how ordinary it looks. It's just another rock, at least to the untrained eyed. The paleontologists studying it had to use high intensity x-ray scanning at the European Synchrotron to see the shark skull inside. I could have picked it up, peered at it, and tossed it aside, or worse, taken it home as a curiosity. Similar things have happened many times, I'm sure. I hate to think how many missing sections in the tree of life are sitting in a box in the back of someone's closet.
Anyway, these days I'm more careful about fossil hunting (or I would be if I didn't live in a swamp with no rocks at all). Usually I just leave them for someone else to find. Maybe they will know what they're doing, and will know if it happens to be an important find. Or maybe someone like my younger self will find it; someone who's read the books and learned the theories, but never seen for themselves that they were true.