An investigation into an ancient poop print sent researchers into the clutches of one of Earth’s most efficient killers to answer one question: was it a foot, or a penis? The resulting coprolite specimen was believed to have been imprinted sometime in the Eocene period (55.8 to 33.9 million years ago), but exactly what was impressed onto it before it fossilized proved to be something of a puzzle.

The Great Penis-Foot Poop Mystery is described in a paper published in Palaeoworld, a fitting journal title for what proved to be something of a rollercoaster of a scientific endeavor centered around a rare, fossilized poop specimen found among 100 coprolites from the Na Duong coal mine in Lang Son province, Vietnam.

“First and foremost, we made sure it was a coprolite rather than just a piece of rock,” study co-author Dr Paul Rummy told IFLScience. “For us it was easy to tell … [but] since we are writing a scientific paper, we somehow have to prove it to the audiences. 'Oh ya, that’s not a typical rock you see, but fossilized feces'.”

The hardened poop’s calcium and phosphorous content demonstrated it was the fossilized droppings of a carnivore, and its shape supported the theory that it was of crocodilian origin. Add to that, that these animals have stomach acid strong enough to dissolve bone (none of which was found in the coprolite) and you have yourself a pretty convincing ancient croc turd.

Crocodilians live their lives quite close to the ground, so it was reasonable to consider that the print in the coprolite was made by the same animal that produced it. To make sure if the print added up, the researchers turned to some of Earth's most experienced crocodilian experts: “[W]e took a trip to the crocodile farm,” said Rummy.

After getting up-close-and-personal with a Siamese crocodile, the team was able to get a look at a crocodilian footprint IRL and match it up to their specimen for comparison.

“We measured some of the footprints produced on the ground of the crocodiles,” explained Rummy. “We also made molds from the manus [fore foot] and pes [hind foot] to deduce the footprint creator on the coprolite is almost identical to it. We know that the 4th and 5th fingers of the manus are free of claws, hence you do not see any claw marks on the coprolite.”

As for the penis hypothesis, suggested to the team by a reviewer of their paper, Rummy believes it could still be a possibility but would be extremely rare. How it might come about is if a male alligator were penetrating a female as she defecated, its penis might make parallel marks similar to that seen on the coprolite.

However, the female’s muscular, multi-purpose cloaca would likely have reshaped the feces as it went on its merry way, making a lasting penis print less likely.

The paper also doubts the possibility of an ancient shark bite having created the coprolite print as bites received “before defecation” (ouch) are typically smaller than that seen in the Na Duong specimen.

Instead, the authors conclude - with the help of an illustrated reconstruction — the coprolite sculpture was most likely crafted by the fore foot of the same crocodilian that produced it. And that beyond that, it provides a rare and unique snapshot into the past where an ancient river or lake environment likely once existed in Na Duong, one which they hope to learn more about in the many coprolites they have yet to explore.

“We are in the mid of writing up our paper on the 100 over coprolites we retrieved from Na Duong,” said Rummy. “We definitely looking forward to conveying an interesting story to the World.”

[H/T: Atlas Obscura]