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 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.”
Before it could be poop
Coprolites aren’t the only fossil remnants of ancient digestive systems. The rare stomach content of a fossil was the focus of a paper recently published online in Gondwana Research.
In stark contrast to Na Duong in Vietnam, the Winton Formation in Queensland, Australia, has only produced two ancient crocodilians, although one of them is pretty special. 92.5 to 104 million years ago, the ancient crocodilian had only just devoured a young herbivorous dinosaur when the crocodile itself also died.
Incredible. But how could that sequence of events be determined millions of years later? Finding actual bones within the stomach region was the biggest clue. As previously mentioned, if ancient crocodilians had the same highly acidic digestive system of extant crocodiles, then this ornithopod meal couldn’t have been digested for long.
This fossil was preserved in a concretion—a sedimentary structure that tends to form around fossil material—and was discovered by chance at the end of a frustratingly fruitless series of digs. Matt White, an author on the paper, said that he and his team search for bones on the surface and dig whenever they find a concentration of them. In 2011, they had dug seven times, only to find nothing once they reached the fossil layer. It was approaching dusk on the last day of the dig, and their expectations were low.
“We thought this was going to be another dud site,” White said in a video interview, explaining that co-author David Elliott encouraged his son Bob to confirm that no further bones were there by scooping up a layer of dirt with the front loader the researchers used at the site.
So he did, and “there was this big crunch,” White said. They had hit bone. Moreover, they had shattered the meter-long concretion that contained the fossils. “The best discoveries are made when you break something," he said, laughing. “Well, we found it!”
In that one concretion, the team found a treasure trove: a new species of ancient crocodilian they named Confractosuchus sauroktonos, along with stomach contents that include the first skeletal remains of any ornithopod in the Winton Formation—it could be a new species in and of itself. Prior to this discovery, ornithopods were represented at the site only by fossil footprints and a single tooth.
It took the researchers six years to piece the concretion back together. Part of determining where each piece fit was done by scanning them at a local hospital. Those scans helped the researchers realize that they had a crocodilian skull, but the data couldn’t provide the detail they needed to see what else lay within the broken bits of concretion. So they decided to scan the pieces with the synchrotron at the Australian Nuclear Science and Technology Organization (ANSTO). Over three days, White compiled approximately 450,000 images. It took another 10 months of 10-hour days to digitally highlight the bone within the rock in those images. And that's when the researchers realized that there were ornithopod bones in the stomach, including one bone with a distinct tooth puncture mark.
While there are no tooth marks or shed teeth on the Confractosuchus fossil, the authors wonder whether its missing hind limbs and tail were scavenged. The tail of any crocodile, White mentioned, is the meatiest part. Winton has several fossil sites with evidence of scavenging, according to White, who explained that millions of years ago, 10- to 20-ton sauropods co-existed with theropods. “We’ve found quite a few megaraptors, and their teeth are found in almost every sauropod carcass. But we didn’t find any associated with the crocodile.”
Out the other end
Just as coprolites and stomach contents provide rich insight into the diet and digestive systems of extinct species, so, too, do the byproducts some species expel by mouth.
Remnants of the meals of two flying reptiles were described in a paper published in February in Philosophical Transactions of the Royal Society B. These remnants are the first uncontested pterosaur gastric pellets, or pellets that contain parts of food that an organism cannot digest and therefore regurgitates.
The fossils of an adult and the first juvenile Kunpengopterus sinensis—each housed in a different museum—were found in close association with gastric pellets. This suggests that either the pterosaurs died soon after expelling the pellets or that the decay process forced the pellets out.
The pellets are filled with fish scales that match a type of ray-finned fish found in that same location. The authors determined that these were gastric pellets rather than coprolites due to their shape and the location in which they were found in association with the pterosaurs. In one, the pellet was very close to the mouth, and four fish scales were scattered nearby. Lead author and paleontologist at IVPP Shunxing Jiang explained in an email that “the pellets are too large to be expelled from their cloacae. The cloaca (similar to the anus in mammals) is much smaller than the width of the hips (similar to the width of pelvic girdles)."
Both juvenile and adult Kunpengopterus were eating the same type of fish, though, based on the scales in each pellet, of considerably different sizes. This discovery changed what was previously suspected: that the pterosaur diet changed as it developed. It also indicates, according to Jiang, that juvenile and adult Kunpengopterus lived in the same environments.
The mere existence of gastric pellets indicates that these pterosaurs had the ability to produce them (the term for this type of digestive process is "efficient antiperistalsis"). That offers a window into their late Jurassic digestive system.
Taissa Rodrigues is a professor at the Federal University of Espírito Santo in Brazil; she was not involved in this research but previously studied the adult Kunpengopterus. “Antiperistalsis,” she wrote in an email, “means that the muscles of the walls of the anterior digestive system (like the esophagus) are able to produce gastric pellets regularly, and not only, say, when an animal is sick and ends up vomiting. These muscles also need to be strong because gastric pellets are often large and may [contain] hard tissues, such as bones and scales.”
No internal organs survived fossilization in these two pterosaurs, but the authors suggest that Kunpengopterus had two stomachs, something Rodrigues noted does not necessarily go hand in hand with antiperistalsis. But, she explained, the authors “build the possibility of a divided stomach upon previous work that showed the presence of gastric stones, or gastroliths, in another pterosaur genus, which suggest a two-part stomach like birds have: one dedicated to chemical digestion and the other to mechanical digestion—the gizzard—as they don’t chew food using their mouths as humans do. The other evidence to support that suggestion is based on the fact that, besides birds, crocodylians also have both a gizzard and an ‘acid’ stomach. These lineages are both archosauromorphs, as pterosaurs are, so it is possible that these animals inherited a divided stomach from a common ancestor.”
It's extraordinary that these gastric pellets survived the fossilization process, and it's remarkable that scientists can determine what they are and glean so much information from them. But the pterosaur skeletal fossils are equally astonishing in and of themselves.
“Pterosaur fossils are a pretty rare find,” Rodrigues continued, “and most specimens held in scientific collections come from just a few deposits with exceptional preservation—for instance, in China, Brazil, and Germany. Because they are difficult to find in the first place, preservation of evidence such as [gastric pellets] is even rarer. But I’d also argue that there are relatively few pterosaur experts in relation to the variety of research topics that can be done. We are still quite busy trying to understand their diversity and evolution.”
All of that makes any remnant of extinct life exceedingly important.
In some ways, these traces of ancient digestion connect us to species we know only through fossils and artistic reconstruction. We understand what these processes are, and we see them all around us. We know owls produce gastric pellets today, and, like ancient crocodilians, we sometimes step in poop. We study stomach contents in other animals as well as our own species. These examples and many others prove that, to some degree, life on Earth maintains some similarities over the ages, despite our very vast and wonderful differences.
Jeanne Timmons (@mostlymammoths) is a freelance writer with a strong passion for paleontology. Based in New Hampshire, she writes about paleontology (and some archaeology) on her blog mostlymammoths.wordpress.com.
