Take a look at this fossil and at first you might think it's simply a shark tooth embedded in a rock, which would be a pretty cool find by itself. But this is no ordinary rock; it's a coprolite, aka fossilised poop. Coprolites are rare to begin with, and this one is even more exceptional: it tells the one-of-a-kind story of an ancient shark that broke a tooth off on a turd.

This fossil shark tooth, more than 20 million years old, was discovered embedded in a 5cm (2in)-long coprolite - an ancient poop. Image: George Frandsen, Poozeum.

"I have literally held, studied and researched thousands of coprolites," said George Frandsen, "and this one was unlike anything I had ever seen before." Frandsen is the proud curator of a record-breaking collection of thousands of coprolites, as well as his online coprolite exhibit, the Poozeum.

Before Frandsen got to see this particular specimen, it was discovered by Mark Stitzer while searching for shark teeth near Charleston, South Carolina in October last year.

"Mr Stitzer knows shark teeth very well," Frandsen said in an email. "He instantly knew the embedded tooth he found belonged to a prehistoric shark named Carcharocles angustidens." We're talking about a big fish, thought to have grown as much as nine metres (30 feet) long back when it prowled the oceans between 33 and 22 million years ago. (It was a cousin of the later and even larger C. megalodon.)

Stitzer suspected he had a prehistoric poo on his hands, so he reached out to Frandsen and ultimately decided to sell the stool to the Poozeum. Granted the chance to examine it in detail, Frandsen was able to make a few more observations.

For starters, he identified it as a croc turd. Ancient crocodilian droppings aren't too dissimilar from those produced by modern crocs and gators, and Frandsen – who lives in Florida – has seen plenty of those. This coprolite also lines up well with other croc coprolites in the Poozeum collection.

But how would a shark tooth end up in croc crap? Had it passed fully through the croc's digestive tract, or did the tooth get in there after the poop was pooped? Peering at the coprolite under lighted magnification, Frandsen found a clue in the form of a thin crevice lined with striations – exactly the kind of mark you would expect to see left behind by the serrated tooth of a shark. It was a shark bite mark!

A close look at the coprolite revealed a unique striated impression left behind when a shark tooth bit into (and back out of) the poop. Image: George Frandsen, Poozeum.

For Frandsen, this was an amazing discovery. "I loved the incredible story it told about a specific moment in time millions of years ago," he said. "The unique hallmarks of this coprolite illustrate that a crocodilian pooped 33 to 22 million years ago, and that a Carcharocles angustidens shark took a bite out of that poop, but did not digest it."

It can be a challenge to identify a fossil poo. Robert Boessenecker, a palaeontologist at the College of Charleston, has spent a lot of time fossil-hunting in the same area this specimen was found, and he pointed out that those sediments contain hundreds of thousands of phosphate nodules – funky-shaped mineral chunks – that often look suspiciously similar to this new specimen. It's not unheard of for these nodules to have teeth or bone embedded inside them, too.

Bite marks, too, can be tough to confirm. Stephen Godfrey of the Calvert Marine Museum described a shark-bit coprolite from Maryland several years ago, and he explained over the phone that all sorts of things can leave marks in poo – not only teeth, but also shells, plants, or even feathers, which can all create impressions that might mimic a bite (his preferred method for exploring these marks is to fill the holes with a moulding compound to create a replica of the object that left the impression).

Even in the face of possible doody-doppelgängers, Frandsen is confident he's got this specimen properly pegged as a shark-marked poop. "The shark tooth coprolite has the same mineral composition as other coprolites from the area," he explained following his up-close investigation of the fossil. "It also has invertebrate burrows that are consistent with other coprolites from the area." As for the bite mark, the striations within match up with the serrations on the embedded tooth, he said.

So, does this mean this ancient shark was a poop-eater? I reached out to Samantha Leigh of the University of California, Irvine, who knows a thing or two about strange shark diets (she studies sharks that eat grass!). Leigh doesn't know of any modern-day sharks that dine on droppings (fun fact: eating poop is scientifically called coprophagy), but she offered a possible explanation for this shark-bit coprolite.

"There are some sharks that feed on prey items that live primarily on the sea floor," she explained in an email, "so I imagine sharks do (at least incidentally) consume some faeces."

Perhaps our ancient shark was aiming for something tasty and nabbed a turd by mistake. This fits well with the fact that the coprolite doesn't appear to have been swallowed. The shark may have snapped it up, noticed that it tasted like, well, crap, and then spat it back out. Little did the shark know that this single bite would be preserved in infamy more than 20 million years later.

It seems likely that a more typical diet for Carcharocles angustidens would have been similar to its cousin C. megalodon, which is known to have feasted on marine animals like small whales.

Frandsen also noticed another peculiarity on the coprolite: a circular starburst impression that seems to be a match for the dermal denticle – an outer scale – of a ray. How that got there is an even more intriguing mystery: did the croc digest a ray scale, or did the poo somehow get pressed up against a ray later? This question remains unsolved. 

This "starburst" impression in the coprolite seems to match the shape of a ray scale! This poop had quite a journey. Image: George Frandsen, Poozeum.

Now part of the famous Poozeum collection, this shark-bit coprolite is one of the very few examples of fossilised faeces with bite marks, or "aborted coprophagy" (that is, a failed attempt to eat a poo). It joins another such specimen with tooth impressions on the top and bottom.

Another coprolite in the Poozeum collection; this one uniquely preserves bite marks from both the top and bottom jaws of some ancient vertebrate. Image: George Frandsen, Poozeum.

Stitzer's coprolite also has the honour of being featured in a public display. From January to May, it will be part of an exhibit at the Florida Museum of Natural History called "The Scoop on Poop", where countless visitors will be able to appreciate this one-of-a-kind marvel.

Editor's note: This article has been updated to include comments from Stephen Godfrey and Robert Boessenecker.



Top header image: George Frandsen/Poozeum.