Cities have many layers. Pigeons swoop from one roof to another. Work crews descend beneath the asphalt to tend to sewers and gas pipes. And between the aboveground and subterranean realms, dogs, humans, squirrels, rats, and other creatures pad down the street, leaving traces behind. Sometimes, if you’re lucky, the concrete itself will be speckled with small, serendipitous impressions. In this way, urban sidewalks can hold modern trace fossils—proof of a creature that happened to pass by at precisely the right moment. To find those preserved prints, you’ve got to…
