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Reading the Book of Life in Prehistoric Dung

“Paleoscatologist” Karen Chin has uncovered the humble origin of life after dinosaurs.

Karen Chin doesn’t have all the answers, but she is willing to make those awkward phone calls that will give her the information she needs. There was the time back in 1998 when she needed to know what size animal could produce a fecal mass of 2.4 liters in one go. So she called a physician who studied bowel movements. “I said, ‘This is going to be a funny question, but I’m a paleontologist, and I’m interested in finding out what is the largest fecal mass that a human can produce. I wonder if I can talk to the doctor about this.’ ” She pauses in her anecdote, remembering the silence on the other end of the phone line. “It was a weird question.”

It took a while to convince the receptionist that Chin wasn’t making a prank call, but eventually the message was passed on to the doctor. The physician, driven by who-knows-what mixture of courtesy and curiosity, called her later that day. He explained that in his line of work he typically focused on the health of his patients, as revealed through their bowel movements, not on the volume of matter they produced. He couldn’t give Chin a precise figure. However, he told Chin, it just didn’t make sense for a man, or a man-sized animal, to produce 2.4 liters of excrement. That answer was enough to make Chin’s heart sing. Because she was pretty sure she’d identified the first fossil of a Tyrannosaurus rex turd.

Chin isn’t the world’s only paleoscatologist—a handful of other researchers around the world study the fossilized excrement of ancient peoples to learn about their diet, health, and lifestyles, and a few others study the fossilized droppings of extinct animals. But Chin is an undisputed leader in the field, and her work has brought new insights to scientists’ understanding of Mesozoic Era, when towering reptiles walked the earth. “I think it’s fair to say I’ve studied more dinosaur feces than most,” she says modestly.

“I said, ‘This is going to be a funny question, but I’m a paleontologist, and I’m interested in finding out what is the largest fecal mass that a human can produce.’ ”

The fossilized droppings that Chin studies are called coprolites. The basic process of fossilization is not much different from the way bone fossils form: Sediment layers over the dung to protect it from air and water and consuming critters, while mineral-laden water seeps through it. Gradually the minerals replace the original material, leaving a stone facsimile.

A warm and cheerful woman, Chin is immune to jokes about her line of work—she’s heard them all. “I like working with dinosaurs because I can more easily constrain what I like to call the poopetrator!” she says. That 2.4-liter specimen from Saskatchewan, Canada, was not only large enough to suggest a king-sized beast, it also contained bone splinters indicative of a carnivorous diet. That evidence, in conjunction with the turd’s discovery in a sediment layer near T. rex bone fossils, gave Chin confidence in her groundbreaking identification.

Chin, now an associate professor of geological science at the University of Colorado, Boulder, has specialized in fossilized feces for more than 20 years, driven by their unique potential to reveal extinct ecosystems and to show animals’ places in those webs of life. She studies the leavings to imagine her way back to the beginnings.

Chin’s most recent investigation has yielded clues regarding one of the most cataclysmic events to occur on this planet: the global extinction event at the end of the Cretaceous period. When an asteroid or comet slammed into the Earth about 65 million years ago, it doomed not only the non-avian dinosaurs, but also approximately 75 percent of all species in the world: plants, insects, mammals, and so many more. Chin has immersed herself in the destruction and the geological evidence of how the world recovered. In those landscapes of death, life found a way to return, and new species emerged and flourished. Chin and her colleagues think they may have found the humble origin of that resurrection.


Chin’s recent discovery is rooted in her early training, which opened her eyes to the role of a lowly creature in the web of life. Like most budding paleontologists, she had read about William Buckland, a 19th-century Oxford University professor of geology, who coined the term coprolite from the Greek words for dung and stone. Buckland became the father of the field after studying remains in Great Britain’s Kirkdale Cave, thought to be prime evidence of Noah’s flood. In fact, Buckland discovered, the cave was a hyena den, where the scavengers dragged their meals for consumption, leaving behind deposits of their own. Buckland’s study of the hyenas’ fossilized feces had large import: It was perhaps the first paleontological study to sketch the contours of an ecological system. That ancient web of life, with its ravenous carnivores and their unlucky meals, could be traced to the droppings left behind by those repasts.

In the 1980s, Chin was getting a master’s degree in biology at Montana State University when she took a job with the legendary dinosaur hunter Jack Horner, the inspiration for the paleontologist protagonist in Jurassic Park. At first, Chin’s job was simply to slice fossilized bones into thin sections that Horner could examine under the microscope. They were working with fossils found in Montana’s Two Medicine Formation, a sandstone outcropping that Horner was exploring. The rock layers were rich with bones of a newfound duck-billed dinosaur named Maiasaura, and not just the bones—also nests and eggs, and strange blobby deposits a bit removed from the nests. Horner believed these chunky rocks, embedded with shreds of fossilized plant material, were dinosaur patties.

“That was the coolest implication of this research. Here we had evidence of interaction between the dinosaurs and the beetles.”

Chin volunteered to work on these fossils, and set to work trying to confirm that they were indeed coprolites. “The context was appropriate,” she says, in that the material hadn’t been found as a uniform layer, but rather as discrete deposits. They didn’t have particularly turd-like shapes, but then, she figured, they would have fallen some distance from the big duck-billed dinosaurs and impacted on the ground. But she found her smoking gun, she says, when she began slicing up the fossils and found what looked like small burrows inside. Could they be dung beetle burrows?

She called a leading dung beetle expert in Canada and explained her research question, and they agreed to meet when Chin was in Toronto for a conference. The meeting took place over a coffee table in a generic Toronto hotel room. “He had brought some dung beetle balls from Africa, and I had brought the coprolites,” Chin remembers. The expert compared the burrows in each of the materials, and showed Chin the obvious similarities. In the middle of this scene, the housekeeper came in to make up the room, but the two scientists barely looked up. “He was showing me why the burrows in the coprolite were characteristic—and he got really excited and I got really excited,” says Chin. The two scientists started waving their arms around, exclaiming enthusiastically that it must be dung, it just must be! All the while, the housekeeper dutifully tidied. 

While most paleontologists dream of reconstructing bones into the skeletons of whole animals, Chin was realizing that modest coprolites could tell a more interesting story. “Body fossils are vital because they tell us the types of organisms that lived in an ancient environment and what they looked like,” she says. “That can tell us that they moved in a certain way and had particular teeth that could take advantage of a particular diet. But they don’t tell us about interactions between specific organisms.”

Since a world full of dinosaurs would naturally involve a lot of dung, scientists had presumed there were organisms that recycled that material back into the ecosystem. But no direct evidence had been found of how that process occurred. Chin’s discovery was the first evidence of dung beetle activity before the Cretaceous–Paleogene (K-Pg) extinction event. “To me, that was the coolest implication of this research,” says Chin. “Here we had evidence of interaction between the dinosaurs and the beetles.”

Horner was also delighted with Chin’s identification of the coprolites, and her subsequent analysis that showed that the plant material in the dinosaur patties came from conifers. “The coprolites have the only plant material preserved at the site, and they did help us to put together the ecosystem,” Horner says. “We would always like to reconstruct the ecosystem, but usually it’s really hard to do.” It’s uncommon to find fossilized plants alongside fossilized bones, Horner explains, because plant material preserves better in an acidic environment, while bones do better in an alkaline environment. The coprolites at the Two Medicine Formation offered a way around that conundrum, and yielded surprising information. “Determining the bite-sized pieces that the dinosaurs had chewed off, that told us a lot about how the dinosaurs fed,” Horner says.

In the course of working with Horner, Chin decided to get a Ph.D. in geological science, and wrote her dissertation on herbivorous dinosaur coprolites. Still, she didn’t think that coprolites would define her career; she expected to move on to other subjects in paleontology. “No one wants to be associated with feces,” she says slowly. But she couldn’t turn away from the intriguing coprolites that paleontologists kept sending to her. “I’m not trying to reframe my research anymore, because there’s so much we can learn from fossil feces,” Chin says. “I think the questions are some of the most fascinating I’ve ever encountered.”

 

The latest question that Chin couldn’t resist was stored in a back room of a museum, in a specimen box stashed high up in a drawer.

She was in North Dakota visiting Dean Pearson, a paleontologist at the Pioneer Trails Regional Museum, who has done a great deal of work on the K-Pg. Pearson tries to create reconstructions of what the environment looked like prior to and after the big event by studying a geologically rich outcropping that contains evidence of the impact. The K-Pg boundary is marked by a thin layer of sediment containing high levels of iridium, an element that is scarce on earth but abundant in asteroids and comets, as well as two types of rock created by powerful impacts: shocked quartz and glass spherules. 

By studying the fossils below and above this geological boundary, Pearson can piece together the ancient ecosystems of North Dakota. “Before the impact it was a warm, wet environment,” he says. “You can visualize southwest North Dakota as being similar to the Florida everglades.” The scene thrummed with life: Dinosaurs, smaller reptiles, and mouse-sized mammals meandered through the grasslands, while fish filled the streams and insects buzzed in the air. The impact ended that vibrant world, and brought on an era of blazing fires and dark skies. Researchers believe the collision sent vast clouds of dust and ash into the atmosphere, enough to block the sun and prevent photosynthesis in green plants. The die-off of flora led to the destruction of the fauna that fed on it, and Pearson’s sunny grasslands became a bleak landscape of rotting vegetation and corpses. 

As part of his quest to understand how life bounced back after that calamity, Pearson has been carefully investigating the sediment above the K-Pg boundary layer to determine which plants and animals made a comeback. Just six centimeters above the boundary layer, a blink of an eye in geologic time, he found something interesting: the fossilized remains of burrows. When Chin came to visit him in 2010, Pearson got her up on a ladder to peer into the high drawer that contained that specimen, and showed her the network of tunnels in the rock. Chin told him she already had too many projects, she couldn’t possibly take on another one, but—oh, these were cool.

Reluctance gave way to curiosity. Chin began slicing the perforated rock and studying the sections under the microscope, calling on her colleague Allan A. Ekdale for expert insights on burrows. Together they determined that the burrows, which were horizontal and of uniform shape and width, were strikingly similar to the tunnels created by modern-day earthworms. When Chin found minuscule pellets of clay within the burrows, the case was closed. The dirt that passes through earthworms’ digestive tracts comes out as clay-rich feces. Chin realized she was staring at coprolites from worms that had writhed through the wreckage in the aftermath of a holocaust. 

Earthworms may have aided and abetted the survival of animals, leading to that glorious process of evolution that resulted in you.

“I think worms rebounded immediately after the impact,” says Chin. She believes they were largely protected from the initial blast in their underground burrows, and more importantly, they were able to find sources of food in the long hungry time that followed. “People have surmised that the organisms that survived were in the detritus food web,” she says.

Geologist Peter Sheehan of the Milwaukee Public Museum is one of the main proponents of this theory: His work has suggested that animals that were part of a food chain that involved green plants were in trouble. But detritivores, which fed on rotting organic matter, and the larger animals that ate those detritivores would have had a much better chance of survival. Chin’s earthworms fit well with that theory, since they would have found plenty of decomposing plant matter to eat in that rotting world. “I can just imagine them emerging from the burrows into this scene of devastation and saying, ‘This is fine for me!’” she says with a laugh. 

Not all vertebrates perished in the extinction event: Some types of birds, reptiles, and small mammals did make it through, and went on to repopulate the planet. Aquatic turtles seem to have breezed through the apocalypse, for example, and a group of burrowing, rodent-like mammals called multituberculates weren’t particularly fazed. But how did they make it though? Paleontologists have documented not just the die-off of green plants, but also of insects that fed on those plants. “Those animals that ate insects would have needed some other source of protein,” Chin says, “and worms may have been key.”

Researchers who have spent their lives in the few inches of rock that tell the story of life’s near-extinction say that Chin’s worm-burrow paper adds a nice piece of evidence to the emerging picture of how vertebrate animals survived and repopulated the planet.

“It’s a very nice story—everything seems to agree,” says David Fastovsky, a paleontologist at the University of Rhode Island who has worked with Sheehan on studies of the extinction event survivors. “The organisms that got through are animals that can find places to hide, and that could feed on the available food—and detritus would be about the only types of things that would be available,” he says. “Chin’s paper reinforces this detritivore signal very strongly,” he says.

In other words, these ancient earthworms may have aided and abetted the survival of vertebrate animals, leading to that glorious process of evolution that resulted in, among other things, you.

It’s an elegant theory: In the wastage of the world, one creature found sustenance, and provided nourishment in turn to the animals that were starting a new legacy of life. And it was the earthworm’s own refuse that told the story to the inquisitive Chin. The book of life is written in scat.


Eliza Strickland is an associate editor for the science and technology magazine IEEE Spectrum

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