https://www.nytimes.com/2018/01/31/science/dinosaurs-footprints-nasa.html

More than 100 million years ago, dinosaurs roamed Maryland. So did our ancestors — small mammals the size of squirrels or badgers — and the flying reptiles known as pterosaurs.

Amazingly, the footprints of all these creatures of the Cretaceous era were preserved on a single 8.5-foot-long slab of sandstone unearthed on the grounds of NASA’s Goddard Spaceflight Center in Greenbelt, Md., not far north of the nation’s capital.

“It’s unusual to have such a large concentration of different kinds of tracks and small tracks in such a small space,” said Martin Lockley, an emeritus geology professor at the University of Colorado Denver who studied the tracks.

Dr. Lockley and his colleagues described the findings in an article published Wednesday in the journal Scientific Reports. The slab offers unique insights into the behavior of dinosaurs and early mammals; possibly some of the dinosaurs were looking to make a meal of the mammals.

All this might never have been discovered if an amateur dinosaur fossil hunter hadn’t gone to lunch with his wife not long before the construction of a new building obliterated the site.

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Even back when dinosaurs ruled the world, the Washington, D.C. area was a swamp. Somehow, the right sequence of events allowed the traipsing of the animals across a muddy surface to be preserved in stone. Millions of years later, the rock happened to be poking out to reveal its paleontological bounty.

At one end of the slab, there is a single footprint of a juvenile sauropod, a long-necked plant-eating dinosaur. At the other end is a print from a nodosaur, an armored plant-eater as heavy as a small elephant. Alongside are smaller footprints, a baby nodosaur following its parent.

There are also tracks of four theropods — relatives of the Tyrannosaurus rex that lived tens of millions of years later, but these were smaller, roughly the size of a large raven. Elsewhere on the slab, pterosaurs walked around and even left indentations in the ground where they were pecking for something to eat. The scientists even spotted at least one clump of what appears to be a coprolite, or fossilized feces. (They don’t know what pooped, but it may have been the sauropod.)