In 2012, amateur paleontologist and dinosaur track aficionado Ray Stanford headed out to lunch with his wife Sheila, an information specialist at the Goddard Space Flight Center in Greenbelt, Maryland. After dropping her off back at work, he noticed a chunk of rock sticking out of a nearby hill that was the exact same color as a piece he had found several years ago that contained a small dinosaur print. As Kenneth Change at The New York Times reports, the tip of the rock led to the discovery of an 8.5-foot long slab of sandstone with roughly 70 tracks from eight different species. 
The rock dates back roughly 100 million years, and includes traces from both mammals and dinosaurs. It is one of the largest such concentrations of tracks ever found. Stanford and researchers from NASA/Goddard, University of Colorado, and Calvert Marine Museum published an analysis of the slab this week in the journal Scientific Reports.
“The concentration of mammal tracks on this site is orders of magnitude higher than any other site in the world,” co-author Martin Lockley, paleontologist with the University of Colorado, Denver, says in a NASA press release. In fact, it’s only one of two known sites where such an array of prints have been found together. “I don’t think I’ve ever seen a slab this size,” he says of the footprint-covered rock. “This is the mother lode of Cretaceous mammal tracks.”
So why are the remains of what looks like a Cretaceous dance party on this particular slab of stone? As Sarah Kaplan at The Washington Post reports, the area around D.C. was pretty swampy around 100 million years ago. It’s likely that this slab of stone was once part of a muddy riverbank.
Over the course of a few days—and perhaps as short as a few hours—many species crossed the spot. There are tracks of several species of mammals likely hunting worms or grubs, including a new species of squirrel-like mammal that sat on its haunches for a moment, leaving an imprint. There’s also a larger-than expected mammal track, indicating that Cretaceous mammals may have not all been the mousey little critters paleontologists previously thought.
There are four pairs of theropod dinosaur tracks, which may have been left while the crow-sized carnivores were making a coordinated hunting sweep through the area. Then there’s a nodosaur track, accompanied by its baby. A track from a the long-necked sauropod lies nearby. Another mark comes from a flying pterosaur. The slab also includes a coprolite—fossilized poop—and something that is likely a fossilized worm.
NASA Slab 2
(Scientific Reports)
“It’s a time machine,” Stanford says in the press release. “We can look across a few days of activity of these animals and we can picture it. We see the interaction of how they pass in relation to each other. This enables us to look deeply into ancient times on Earth. It’s just tremendously exciting.”
As Chang reports, without Stanford’s keen eye, the slab may have never seen the light of day. That particular parking lot and the hill where the prints were found was slated to be torn up for a new office building. Stanford alerted Compton J. Tucker, a NASA climate researcher who had experience with geophysical surveys to his find. Tucker used ground penetrating radar to identify the extent of the slab, which was later excavated by volunteers. The radar found other rocks in the area, but none of them were quite as exciting as the four-ton chunk of sandstone.
As Kaplan reports, this is not Stanford’s first find. Over the years, the 79-year-old fossil hunter has single-handedly tripled the number of dinosaurs found in Maryland. A nodosaur hatchling he found in the area is permanently displayed in the Smithsonian’s Museum of Natural History in the “Dinosaurs in Our Backyard” display.
The irony that this important fossil was found on the doorstep of one of the United States most cutting-edge science institutions isn’t lost on Stanford. “The fact this is found right under their nose,” Mr. Stanford said, “maybe it’s an omen they’re going to start finding fossil and extant life out there.”
Chang reports that a replica of the four-ton slab was recently installed in the atrium of Goddard’s Earth Sciences Center.