Scientists exploring the high-desert grasslands of northern Arizona have discovered a trove of remarkable Triassic animals, including croc-like ancestors of dinosaurs, carnivorous amphibians that grew as big as people, and — perhaps most unexpectedly — about a ton of fossilized poop.

The fossil feces, known as coprolites, were discovered in a thick, dense layer that covers at least a square kilometer, leading paleontologists to believe that the area was once the bottom of a huge, and rather putrid, stagnant pond.

“It’s just this nasty, muddy, crappy pond bottom,” said Dr. Bill Parker, chief paleontologist for Petrified Forest National Park, who led the research.

“It would’ve just been disgusting — probably the nastiest pond you’ve ever seen in your entire life.”

But all of that waste came from a rich panoply of animals that lived in the region some 233 million years ago.

Parker and his colleagues found this and other sites this summer while investigating a small section of a 4,200-acre parcel that Petrified Forest National Park is in the process of adding to its holdings. [Learn about ancient ruins discovered in the same park expansion this summer: “‘Impressive’ Pueblo Great House, Dozens of Ruins Found in Petrified Forest National Park“]

While the land had been explored in the 1920s and ’40s by paleontologists who made some impressive finds there, it has never been investigated completely or with modern techniques.

“It’s one of the reasons we wanted to acquire the lands,” Parker said. “We knew the fossils were out there — we just didn’t know how rich it was.”

Of all the finds that Parker and his team made this summer, he said, the most significant is the discovery of an ancient crocodile-like reptile known as Doswellia — the first of its kind ever found in the American West.

About the size of a modern alligator and armored with thick, pitted plates, the animal was a type of archosauriform, the oldest common ancestors of both crocodiles and dinosaurs, including modern birds.

The newfound Doswellia is only the second example of its species ever found, following one partial skeleton found in Virginia in 1980. So its discovery in Arizona “was the most exciting find” of the summer, Parker said.

But just meters away, scientists also found an exquisitely preserved and nearly complete skull of another influential founding reptile, a phytosaur.

The closest known relative of archosaurs, phytosaurs bore a superficial resemblance to modern crocodiles, although they were larger, and likely fiercer.

“Some got up to 25 to 30 feet long and pretty much ate anything that came too close,” Parker said. “It’s one of the most common animals we find out there, although complete skulls are very rare.”

“The palate [of the skull], on the bottom side, was beautiful,” he said of the specimen. “It’s probably one of the most well preserved palates I’ve ever seen on a phytosaur skull.”

And it was about a meter under these remains that two of Parker’s colleagues made the team’s most unusual find: a stratum of light-colored earth about 10 to 15 centimeters thick, packed from top to bottom with familiar-looking, organic forms.

“It became pretty obvious that it was really heavily full of coprolites,” Parker said. “These things are actually contacting each other, they’re so close.”

Feces can be a valuable source of information, yielding important clues about ancient animals’ diets, Parker noted. But they can also serve as calling cards for animals that otherwise would leave no remains.

The discovery of spiral-shaped poop, for instance, told the team that freshwater sharks once inhabited the area, because only sharks have spiral-shaped intestines.

Perhaps more important, however, the layer of feces was also packed with bits of bones and other fossilized remains — including fish scales and teeth, and the fragments of skulls and larger bones belonging to one of the era’s most unlikely apex predators: metoposaurs.

Giant amphibians that grew to more than a meter and a half in length, metoposaurs were “some of the top carnivores at the time,” Parker said, and the great diversity and density of life in the primordial pond is probably what drew them there.

“These things are just pooping like crazy, generations of these animals living and dying in this pond.

The result of all that activity was a fossil bed that Parker described as “extremely rich.”

“In some places it’s literally four inches thick of coprolites and bones. There’s a fossil every inch.” [Explore another recently discovered fossil bone bed: “Dozens of Animals Discovered in Huge Wyoming Fossil Deposit“]

And, Parker noted, this unusual biological bounty was only preserved because, by a geochemical fluke, the bottom of the pond was totally lacking in oxygen.

Oxygen allows organic matter, like feces, to decompose, and it also makes iron rust.

But without oxygen, coprolites can form — and iron in the soil can bond with other elements, like sulfur, to make the mineral pyrite, also known as fools’ gold, which the scientists found in abundance under the pond bottom.

So although it may have been a bit raw back in Triassic times, this fecund environment has left plenty of evidence to help give scientists a clearer sense of what life was like in this corner of the ancient West.

“What we’re hoping is that it will give us a nice little snapshot of what this pond environment looked like 220 million years ago,” Parker said.

But that will take a lot more work. During his team’s three weeks in the field this summer, Parker noted, they only managed to investigate “a couple of feet” of the park’s new land.

“We’ll be working this site for — who knows? — could be decades.”

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