Why I hunt sidewalk fossils

Once I started noticing these impressions, it was fun to imagine myself as a paleontologist of the urban present.

Since sidewalk fossils are essentially the same color as the surrounding concrete, they are most visible when light rakes over them; a fossil that is elusive at noon may announce itself at dawn or dusk. So I timed a second daily walk for the hour the light disappeared. Late in the afternoon I was introduced to small forked footprints that marked the scene of perhaps a bird fight. There were several more: a dog’s paws, three-quarters of a shoe. While ichnologists, who study trace fossils, may ignore leaves, I marveled at those too: the bulk of a London plane and a ginkgo, with its ruffled fan. I knelt in front of a closed snack cart until the cold concrete stung my knees. I wriggled out of my mitt and followed a leaf’s sharp, diagonal veins, its saw-toothed sides.

When scientists come across a fossil, they often try to come up with an explanation for how it got there. Perhaps an animal has become stranded or washed off its feet or chased by predators. Once I started noticing these impressions, it was fun to imagine myself as a paleontologist of the urban present. An abundance of bird feet made me wonder if someone had sprinkled seeds or dropped a bagel. How long ago? What kind? When a leaf didn’t seem to fit in with one of the nearby trees, I wondered if it was an intruder, blown in from blocks away, or if it was an ecological expulsion—a tree pulled out and replaced by another kind or swapped for sidewalk. The fossils drew my attention to something tangible, but also invited me to wander and think about city streets as collages of past and present, about how our non-human neighbors are also architects. How we all cast traces of ourselves whether we know it or not.

Of course, there is more significant evidence from the past. Mammoths sometimes pop up in farmers’ fields, their tusks curved like scythes left in the dirt. Parades of dinosaur footprints still march along the banks or bottoms of some prehistoric rivers and seas. They are great, flashy and clear. I’ll line up to see them; I yawn happily. But it was a small thrill to come across evidence from the past that was subtle and recent, evidence that there were others. The fossils on the sidewalk felt intimate—the paleontological equivalent of a series of letters tucked under a floorboard.

Only they are not really rare. When sidewalks are repaired, birds and other animals ignore efforts to keep them pristine. Leaves do what the wind asks. These fossils are easy to find and we are lucky to have them. When I got stuck in the worst parts of my brain, sidewalk fossils drove me away. Unlike the many fossils that represent the silence, the moment an animal died and the place it remained unless humans cleared it, sidewalk fossils are often a glimpse into lives that went on. The birds flew somewhere; the dogs, I hope, continued wagging their tails over many sticks and scents. As the sun set and I trudged home, the fossils—these little tail fins, these interesting accidents—were reminders of the small, exciting life.