A paleontologist once told me that city sidewalks hold snapshots. If I trained my gaze toward my feet, he said, I would find evidence of all kinds of commutes: traces of hopping birds, the soles of humans’ shoes, restless leaves that fell and sank into wet concrete at just the right moment. I might see a smattering of little paw prints zigging, zagging, doubling back, evidence of important rodent business that didn’t often overlap with mine.

These marks are too recent to pass muster with scientific sticklers, but in all respects except age, they are fossils. There are many ways to make one. Some form when a creature is entombed in sediment: Water percolates through, flush with minerals, and over time the mixture infiltrates the bones, where it settles and forms stone. Other fossils are casts, made, for instance, when a shell dissolves and leaves behind a mold that eventually fills with sediment, which hardens into rock. But not all fossils involve remains; some catalog movements. These are the kind that stipple our sidewalks — nascent trace fossils, records of fleeting contact.

Throughout the pandemic, I turned to nature to track time and step outside myself. I photographed the sweetgum tree outside my Brooklyn window, noting when it leafed into a bushy chlorophylled curtain or when it dropped fruit that fell to the ground like unshattered ornaments. Most afternoons of that first lonely spring, I roamed a cemetery. When magnolia blossoms smudged the scene pink, I stood under the canopies until wind splashed the petals against my shoulders.

I was lucky, of course, to be simply scared and lonely — not dead, not intubated, not choosing between peril and paycheck. But time was slippery, and I felt stuck in my own brain, a foggy, trembling ecosystem I had no interest in studying. By early 2022, I was cocooned in my partner’s Morningside Heights apartment. On weekend mornings, we shuffled around the neighborhood, and each volunteered to notice something new: a startling mushroom, the pale bellies of pigeons waterfalling down a facade before flocking skyward. I became fixated on sidewalk fossils. Fossil-finding outings were a relief — an invitation to crouch, touch, lose myself in evidence of skittering and scrabbling, tethering myself to a past and a future.

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

Because sidewalk fossils are essentially the same color as the surrounding concrete, they’re most visible when light rakes across them; a fossil that’s elusive at noon might announce itself at dawn or dusk. So I timed a second daily walk for the hour when the light fled. Late afternoons introduced me to tiny forked footprints that marked the scene of, perhaps, an avian skirmish. There were others: a dog’s paws, three-quarters of a shoe. Though ichnologists, who study trace fossils, might discount leaves, I marveled at those too: most of a London plane and a ginkgo, with its corrugated fan. Across from a closed-up snack cart, I knelt until the cold concrete prickled my knees. I wriggled out of my mitten and traced a leaf’s sharp, diagonal veins, its saw-toothed sides.

When scientists encounter a fossil, they often try to puzzle out an explanation of how it got there. Maybe an animal was 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. A bonanza 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 match any of the nearby trees, I wondered if it was an interloper, blown in from blocks away or if it testified to an ecological eviction — a tree yanked out and replaced with another species or swapped for sidewalk. The fossils fastened my attention to something tangible but also invited it to wander and to think about city streets as collages of past and present, about how our nonhuman neighbors are architects, too. How we all shed traces of ourselves, whether we know it or not.

Of course, there is more significant proof of the past. Mammoths sometimes turn up in farmers’ fields, their tusks curved like scythes abandoned in the dirt. Parades of dinosaur footprints still march along the banks or beds of some prehistoric rivers and seas. Those are awesome, showy and obvious. I line up to see them; I happily gawk. But it was a tiny thrill to encounter evidence of the past that was subtle and recent, proof that others were out there. The sidewalk fossils felt intimate — the paleontological equivalent of a raft of letters secreted away beneath a floorboard.

Only they’re not actually rare. When sidewalks are repaired, birds and other animals ignore attempts to keep them pristine. Leaves do whatever the wind demands. These fossils are easy to find, and we’re lucky to have them. When I was lingering in the worst parts of my brain, sidewalk fossils dislodged me. Unlike the many fossils that represent stillness, the moment when an animal died and the place it remained unless humans carved it free, sidewalk fossils are often peeks into lives that continued. The birds flew somewhere; the dogs, I hope, went on to wag over many sticks and smells. As the sun sank and I trudged home, the fossils — these little flukes, these interesting accidents — were reminders of small, exhilarating life.

Jessica Leigh Hester is a science journalist whose first book is “Sewer” (Bloomsbury Academic, 2022).