Outside Freehold, N.J.—The water is icy cold and the stone is slippery as I wade in up to my calves. Along the banks of this slow-flowing stream, guarded by prickly brambles, lies one of the richest caches of fossils dating back to the extinction that claimed the dinosaurs. The remains of marine creatures buried here, kept secret to prevent looting, tell an unusual tale: rather than dying off 65 million years ago, these creatures lived on afterward, albeit briefly. The discovery is causing scientists to rethink why some creatures survived the so-called KT extinction while others did not.

Unlike this one, significant fossil sites tend to be found in exotic locales such as the searing hot Gobi Desert or the windswept pampas of Patagonia, areas remote from the kind of urban development that can ruin them. “You don’t expect to find them here in suburban New Jersey some 90 minutes away from New York City,” explains Neil Landman, curator of fossil invertebrates at the American Museum of Natural History.

The fossils here are not of dinosaurs, but ammonites. These cousins of squid and octopus were the iconic marine animals of the age of dinosaurs, flourishing worldwide for 300 million years or more before the KT extinction wiped them out. They bore shells that often resembled those of nautiluses, which rapidly evolved into hundreds of different shapes, ornamented with undulations and bumps.

Amateur paleontologist Ralph Johnson, a New Jersey park ranger, discovered ammonites in this stream in 2003 when construction workers exposed them while setting up bridge foundations. The site is now kept quiet from all but scientists—poachers have already trawled nearby areas, on the prowl for fossil shark teeth. Although this shallow, inconspicuous creek has no formal name on maps, after enduring many thorns on the way there, Landman and his team dubbed it Agony Creek.

At the time of the KT extinction, the water level at this site was some 30 meters higher than it is now. Landman investigates the iron-rich glauconite rocks here with his colleagues and students from the museum’s graduate school, using iron spikes and sledgehammers to knock off slabs that are picked apart with screwdrivers and fingers. They find the fossil bed rich with dozens of species of marine invertebrates, such as crabs, snails, clams, sea urchins, large flat oysters and ammonites, as well as fish teeth and scales.

Past digs unearthed ammonite shells up to some 35 centimeters wide. These lay amid pinna, triangular bivalves that all died here relatively undisturbed: they jut upward as they would have been posed in life. Their position suggests they were all snuffed out rapidly, “perhaps by a Pompeii-like disaster, like a pulse of mud,” Landman says. To see if these deaths were linked with the KT extinction, the researchers tested for iridium, the rare metal found throughout the world near the KT boundary, thought by most to be evidence of a cosmic impact.

Unexpectedly, the researchers discovered that the iridium was laid down before the pinna layer, which means that the ammonites and other creatures there died after the event “by 10 to maybe 100 years,” Landman concludes. Their survival runs “counter to everything we’ve been taught,” he adds. He plans to go to a site in Denmark to retrieve more potential evidence of ammonite survival past KT.

Their existence in the post-KT world raises a host of questions. “If they made it through this event like they did through other mass extinctions, why didn’t they take off again?” asks invertebrate paleontologist Peter Harries of the University of South Florida. “Why did the ancestors of the modern nautiluses make it through and not the ammonites? That’s extremely intriguing to me, and the broader message to me is that mass-extinction events are much more complex than we think.”

The trove’s proximity to cities is a bit of a double-edged sword. “In Mongolia, you don’t really have the danger that a good site today might be paved over by asphalt tomorrow, and you can’t walk into people’s backyards,” Landman remarks. “But who knows if we could have found this site otherwise without urban development.”

Note: This article was originally printed with the title, "Surviving the Suburbs."