Some 110 million years ago, in the swamp that would become the Washington suburbs, a hulking, armored nodosaur trudged along a riverbank, leaving a telltale print in the mud.
Its offspring scrambled after it, slipping in the parent's print. Other dinosaurs crowded the setting: A long-necked sauropod squelched through the muck, while several theropods - smaller cousins of the fearsome T. rex - stalked the landscape.
Perhaps they were in pursuit of the small, rodent-like creatures hopping about.
Within days, a flood covered the many footprints with rock, preserving them. Millennia passed. An asteroid struck, the continents shifted, sea levels fell, mammals rose, humans climbed down from trees and launched toward the stars.
Finally, on a summer day in 2012, a self-taught fossil hunter named Ray Stanford noticed the unmistakable shape of the nodosaur's track as he drove out of a parking lot at what is now NASA's Goddard Space Flight Center in Greenbelt.
Years of excavation and analysis revealed the contours of that fossil print and dozens more on a single 8.5-foot-long (2.6 metre) slab of sandstone, Stanford and his colleagues announced Wednesday.
It is the largest and most diverse assemblage from the dinosaur age found in the Mid-Atlantic region - and it ranks among the best fossil trackways in the world.
"I like to call it the Rosetta Stone," said Martin Lockley, a dinosaur track expert at the University of Colorado at Denver who participated in the research.
The evidence on that slab surface preserves animals as they lived rather than as they died - revealing the ecology of their age in exquisite detail, he said.
And because no prints overlap, Lockley thinks the tracks were laid down and preserved in a "geologic instant" - no longer than a few days but, more likely, during a few hours.
In such a contained setting, herbivore and carnivore, reptile and mammal, predator and prey all intersected and potentially interacted.
"One could literally make a movie about everything going on in this slab," Stanford said.
The retiree had no idea of the scale of his discovery when he stumbled upon the nodosaur footprint six years ago. He had come to Goddard to eat lunch with his wife, a NASA information specialist.
He was heading home when he decided to take another look at an interesting bit of exposed sandstone stained brilliant red by large amounts of oxidised iron. Such rocks are ideal for preserving footprints, Stanford knew.
The scalloped impression of a nodosaur's four-toed foot validated his hunch, and a paleontologist from Johns Hopkins University confirmed the find.
But NASA was about to start construction on a new building in precisely that spot.
Eager to get the fossil out of the way, the agency asked Compton Tucker- a climate scientist with experience using ground-penetrating radar - to survey the area and determine the extent of the sandstone.
Then, over two chilly winter weekends, a cadre of volunteers from Goddard unearthed the whole hunk of rock.
At one point during the excavation, a NASA employee sheepishly confided that he walked past the outcrop every day without noticing what was embedded in it.
"Look, you're an astrophysicist," Stanford recalls telling him. "Your mind is out there," he said, pointing to the sky. "I'm a dinosaur hunter. My mind is down here."
Stanford's keen attention to the ground beneath his feet is legend in the palaeontology community. In a quarter-century of searching, he has tripled the number of dinosaurs and winged reptiles identified in Maryland.
He discovered a nodosaur hatchling that is now on display at the Smithsonian and has filled his living room with so many fossils that his insurance company demanded he install extra supports to keep the house from collapsing beneath their weight.
But none of that, the 79-year-old Stanford maintains, can compare to his Goddard find: "It is more than I ever expected."
The excavated slab weighed more than four tons, so the space flight center arranged to have paleontologist Stephen Godfrey make a fiberglass cast that would be easier to study.
In fall 2015, the model was installed in Stanford's basement, where he would meticulously brush fine silt grains into the mold's dips and divots to reveal the prints.
Then came hours of staring at the slab and attempting to divine what happened 110 million years ago from the faint impressions on its surface.
"I could not sleep," he recounted this week. "It was a time of total amazing discovery."
"Every time you came down and looked at it and turned the light at a different angle, you'd see something new," added his wife, Sheila, who often joined him in surveying the slab.
Like her husband, she has no formal paleontology training. But she spotted some of the slab's more interesting features, including the impression of a winged pterosaur dipping its pointed jaw into the earth in search of food, then pushing off from the ground to take flight.
In all, the slab contains about 70 footprints from at least eight types of animal, the Stanfords and their colleagues report in an article published Wednesday in the journal Scientific Reports.
Just one other discovery from the Mesozoic era (the 200 million-year span during which dinosaurs roamed Earth) bears as many mammal prints.
The report highlights additional oddities: a dark, bulbous lump called a coprolite, otherwise known as fossilised dinosaur poop, and a tubular structure that was probably the body of some prehistoric worm.
Two days before the find was to be announced, Stanford paid a visit to the fiberglass model, which Goddard installed on a wall in its Earth Science building.(The actual sandstone slab sits in a warehouse in Maryland.)
Running his hand along its rugged surface, he pointed to a set of mammal prints.
"Our ancestors," he said. And then, "look how close his steps are."
That proximity suggests the mammal was sitting on his haunches to search for food, much the way a squirrel might pose upright while nibbling a nut.
"They're foraging," Stanford said. He floated his hand over to the footprints of carnivorous theropods that appear nearby. "But someone else is foraging on them."
Looking with him, Tucker pointed out how the size, shape and spacing of the theropod tracks looked almost synchronised. "We think they could be hunting the mammals as a group," he said. "That shows some kind of social behaviour."
The fiberglass cast will remain on display at Goddard for the foreseeable future. Its reception by NASA scientists depends on their background.
Tucker, a climate researcher, looks at the fossils and considers whether the planet is headed toward a repeat of the Mesozoic, when high levels of carbon dioxide in the atmosphere heated Earth.
His colleague Melissa Trainer, who studies the environments on other planets, imagines a day when scientists uncover traces of life on alien worlds.
As for Stanford, he finds poetry in the fact that dinosaurs once walked the same landscape as astronomers and rocket scientists. Even in the Space Age, he said, Earth "still has surprises."
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This article was originally published by The Washington Post.