Gut check: Fossil finds give us a history of life—and what it ate

It's frustrating and gross, but we’ve all done it. We’ve all stepped in poop.

Most of us would like to forget the experience. But 33 million years ago, now-extinct life forms stepped in it, and fossilization has ensured that those events will not be forgotten.

For paleontologists, what was once repellent is now an absolute marvel, as it offers insight into extinct animals and their environments that we may not otherwise obtain. Similarly, other byproducts of life we might find disgusting—regurgitated remnants of meals, internal organs and their contents—are important clues into creatures we only know about from the fossil record.

Leaving a trace

"Coprolites" are fossil feces, and they’ve been found all over the globe from a wide range of ancient species, from enormous T. rex coprolites to those of ancient woodrats and possibly even the tiniest remnants of marine worms. Because they're evidence of a behavior—in this case, expelling waste from the digestive system—and as they are not part of an animal’s skeleton, these fossils are considered "trace fossils," a term that encompasses paw prints, nests, burrows, bite marks, and innumerable other traces left of life.

One particular coprolite caught the eye of researchers in China. It was found among 100 other coprolites by an international team in the Na Duong coal mine in Northern Vietnam, but what made this one stand out was the two fingerprints embedded within it. In other words, the scientists had discovered the rarest of the rare: trace fossils within a trace fossil.

And that seemingly small ancient bit of feces told a much larger story: A crocodilian may have stepped out of the water and crawled across the soft river bank onto land. Two of its front fingers pressed lightly into excrement, possibly left by another crocodilian, and it continued on its way.

That the coprolite exists after approximately 33 million years is one thing. That it also maintained the imprints of two crocodilian fingers is astounding. The discovery was announced in a paper published this February in Palaeoworld. Lead author and Ph.D. candidate Kazim Halaclar and paleontologist and co-author Paul Rummy—both with the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) in China—described the remarkably unique circumstances in which the feces underwent fossilization.

First, they explained, coprolites tend to survive in either caves or wet environments, such as river banks, lake shores, or swamps. The soft sediments of river banks, where water occasionally laps the ground, may have helped preserve these ancient feces, as the water kept them from drying out and breaking apart before becoming absorbed into the dirt. Just as important, however, is that an animal stepped lightly on the feces.

Had the animal in question been a cow, Rummy explained, it would have crushed the entire feces. A cow steps on its hooves with its full weight, whereas a crocodile sprawls, spreading its weight across its fingers.

At some point, the feces were gently absorbed into the soft sediment, where they remained in fossil form for millions of years.

Analyzing poop

Those details were not immediately apparent to the authors of the paper. Rather, it took different types of analysis to learn about the type of creature that left the feces, the environment in which they were deposited, and which species might have left its fingerprints on them.

One step was to analyze the chemical content of the fossil through scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS). Because two elements that were found in the coprolite (calcium and phosphorus) are indicative of meat consumption, the researchers concluded that the animal that produced the sample was a carnivore. CT scanning revealed bone remnants rather than large pieces of bone. This, the authors explained, pointed to a highly acidic digestive system, one in which much of what is eaten is digested. That’s typical of today’s crocodiles.

A visit to a crocodile farm in Beijing helped the team compare fingers and fingerprints to the traces on the coprolite. Crocodilian hind feet have webbing, which was not found on this coprolite. Neither were traces of claws. In some crocodiles, only the first three front fingers have claws; the remaining two do not. The authors deduced that the ancient crocodile pressed into the feces with its last two front fingers.

Na Duong has produced so many well-preserved fossils that the authors refer to it as a "Lagerstätte"—the term for such bountiful sites—in their paper. So far, it has produced 50 crocodilian specimens comprising at least three different species and 100 fossil turtles. This coprolite is the first to be found with crocodilian fingerprints, and it's only the second coprolite ever found with footprints of any species.

“It’s an amazing place, 33 million years ago,” Rummy noted. “A place full of crocodiles. A place full of food for the crocodiles!” he added with a laugh.

The fossil coprolite is currently housed in China for further research. Neither Rummy nor Halaclar was involved in the initial agreement, but Rummy emphasized that the project is a collaboration between the IVPP and the Vietnam Academy of Science and Technology. The researchers are part of a large team that intends to study the Na Duong in greater depth, and these co-authors are currently working on another paper about the rest of the coprolites.

“This,” Halaclar said, “is [only the] beginning.”