The Scoop on Old Poop

Blake Edgar

One day soon I expect my curious young son to ask what dinosaur poop looks like. Now that I"ve met Karen Chin, I"ll be ready. Chin is a leading authority on, and enthusiastic champion of, paleo poop.

Fossilized feces, or coprolites, as they're called, were first recognized by Reverend William Buckland in 1823–nearly 20 years before Richard Owen even coined the word dinosaur. But it's safe to say that no one before her has shown Karen Chin"s dedication to dinosaur dung. Most paleontologists don't even know it when they see it or step on it. Chin hopes to change that, because peering inside the excrement of extinct animals, she says, "provides a new dimension of looking at ancient ecosystems."

Many summers spent as a naturalist guide in Kings Canyon, Yellowstone, and Glacier national parks taught Chin that scat may be all that one encounters of large animals during a walk through the woods. She gathered grizzly scat for biologists and even started a personal reference collection. Such leavings can reveal what an animal ate and offer a way to gauge a species" range and population size.

When she began working as a preparator and research assistant for paleontologist Jack Horner at the Museum of the Rockies, in Bozeman, Montana, Chin was amazed to learn that, under the right conditions, such soft stuff could actually turn to stone. Horner"s offhand reply that he thought he had some dinosaur scat set Chin on a ten-year journey that led to a Ph.D. from the University of California at Santa Barbara and her current position as visiting scientist with the U.S. Geological Survey in Menlo Park.

Chin investigated the collections of several museums in search of other dinosaur coprolites. In one museum, she found that a decidedly modern, though dusty, rabbit pellet had been preserved in a vial and placed among bona fide coprolites. Other suspected coprolites turn out to be concretions of silicates or coils of iron oxide, no matter how closely they resemble novelty-store dog doo. Color, size, and shape are unreliable ways to distinguish the dung of the dead, because the passage of time alters it.

Besides shape, the main criteria Chin uses to identify the real thing include the presence of phosphate and obvious bits of ingested plants or animals. Figuring out just which dinosaur defecated, however, proves to be something of a, well, crapshoot. Sorting vegetarians from meat-eaters is easy enough, but beyond that Chin must make some assumptions.

Her most recent accomplishment in teasing a tale from a turd culminated in June with a paper in Nature entitled "A king-sized theropod coprolite." In 1995, a team of paleontologists from the Royal Saskatchewan Museum was taking a break from excavating a two-thirds complete Tyrannosaurus rex skeleton, nicknamed Scotty, discovered in Canada"s Frenchman River Valley. About a mile south of the skeleton, team member Wendy Sloboda noticed dozens of chunky beige blobs eroding out of the mudstone from a larger source upslope. She called over team leader Timothy Tokaryk, and both stood silently pondering the biggest blob, each suspecting that it came from a dinosaur.

In Chin's office, I examine a dense, square chunk of the specimen in question. At 44x16x13 cubic centimeters with an estimated volume of 2.4 liters, the entire specimen is the largest carnivore coprolite ever found. Chin points out tan bits of dinosaur bone embedded throughout the matrix of solid calcium phosphate, or apatite–the building block of bone. The 65-million-year-old scat comes from deposits laid down in the Maastrichtian, near the very end of the reign of giant dinosaurs. Most carnivores running around Saskatchewan then were crocodilians and small theropod dinosaurs. But based on this coprolite"s total size, its geologic context, and the fact that it was chock-full of bone, Chin and her co-authors conclude that only one carnivore could have left such a load: Tyrannosaurus.

"I'm convinced that it was a tyrannosaur of sorts," says Chin. "It's tempting to say that Scotty defecated and then wandered off and died, but he was in a different stratigraphic layer so it definitely wasn't him."

Using some of the smaller fragments that had eroded downslope, Chin made several thin slices across the coprolite to find out who had been eaten. She enlisted Stanford University biologist Greg Erickson, an expert on bone growth who had also studied the bite force of Tyrannosaurus (see "Horizons," Winter 1997). Because the bone inside the scat lacked distinctive growth rings found in meat-eating dinosaurs and any sign of bone replacement or healed stress fractures, Erickson determined that the prey was a juvenile plant-eating dinosaur. The thickness of the bone wall told him that the many chunks most likely came from limb bones, so he could estimate that the prey was about the size of a cow and weighed up to 700 kilograms. The most common big herbivores in the vicinity were Triceratops (whose fossils were scattered not far from the coprolite) and Edmontosaurus. Tyrannosaurus bite marks have been found before on bones from both species.

Erickson was pleased to see that the coprolite corroborated his previous ideas that the shearing teeth of tyrannosaurs pulverized bone as they bit down on huge hunks of meat. Though the coprolite reveals that much bone passed through the guts undigested, other rounded pieces were partly dissolved by strong stomach acids, so the predator may have obtained nutrients from the prey's skeleton. "It tells you about the physiology of these animals," says Erickson. "They could digest bone to some degree." Chin, though, remains impressed by how little bone digestion occurred, compared with how much crocodiles dissolve swallowed bones.

"It's not a huge surprise that T. rex was eating other dinosaurs," admits Chin, "but I think it"s a big surprise that we actually found their feces." Nearly all known coprolites have come out of carnivores, whose phosphate-rich diet assists the fossilization of their feces, but carnivores, especially any the size of a tyrannosaur, are rarer than their prey. More importantly, says Chin, this find gives field workers a new fecal search image. Even Chin needs to reevaluate some specimens that she had once discounted as being coprolites. "You're not going to get these nice sausage shapes when you"re dealing with really big animals."

Few paleontologists have proper search images for the scat of their quarry. Why be a turd hunter when you can be a bone hunter? But when Jack Horner was finding skeletons, eggs, and apparent nests of an unknown dinosaur in the Two Medicine Formation of Montana, several strange angular gray blocks of rock in the same strata looked vaguely familiar from his time as a teenager spent shoveling circus elephant droppings. Horner has since garnered attention for his ideas that the duck-billed dinosaurs of Egg Mountain, which he named Maiasaura or "good mother" dinosaur, protected their young in a nest long enough for the babies to double in size from birth. If both adults and babies stayed put, then they presumably relieved themselves nearby.

Chin sets down a cardboard tray containing a half dozen coprolite chunks from the site. Rubber bands hold together the largest fragments, riddled with black flecks of wood. When Chin started making thin slices through the chunks, she realized that each coprolite was filled with conifer wood, ingested inadvertently while the dinosaur browsed leaves. It was the ultimate high-fiber diet. Eighty snails that also turned up in the specimens may have been sticking to swallowed leaves or later flocked to the feces.

Chin's close geochemical inspection provided further clues to the blocks' contents, but it turned out that the clinching clues to their fecal origin had been staring her in the face. Running throughout the Two Medicine specimens were burrows, both empty and packed with dirt and plant pieces. Among living animals, only dung beetles make burrows of similar size and form. Entomologist Bruce Gill studied the specimens and confirmed that Chin had uncovered the earliest evidence of dung beetles plus the first documented interaction between insects and dinosaurs. The burrows vary so much in diameter that Chin and Gill believe several beetle species dug this dung. With the dinosaurs' demise, dung beetles formed an alliance with mammals that continues today.

Add the conifers, and the Two Medicine turds capture three points of a food web linking two of the biggest and smallest animals in a Cretaceous community. "This was a thriving place," Chin exclaims, hefting a hunk of Maiasaura dung between her hands. These coprolites have an average volume of about six liters, though they were probably somewhat larger when fresh; for comparison, an elephant can leave behind eleven liters at a time.

As her reputation as a paleoscatologist spreads, Chin has gotten used to receiving heavy packages containing bulky and bizarre rocks. Paleontologist James Kirkland of Dinamation International Society sent her some outwardly unremarkable specimens from the Mygatt-Moore Quarry in Colorado, which has yielded a rich trove of fossils from Jurassic giants like Apatosaurus and Diplodocus. Once Chin peered inside the nodules, she discovered an exquisitely preserved array of plant remains: bits of leaves, stems, seeds, pollen, even stomata and other cellular details. One thin-section contained 17 seeds from several gymnosperms, perhaps cycads or ginkgo trees. Coiled fern sporangia showed up in other samples. The surrounding sediments preserved no similar concentration of plant remains, improving the odds that Kirkland had collected Jurassic herbivore coprolites.

From this evidence, Chin deduces that Jurassic herbivores ate a broad range of plants–though flowering plants had yet to evolve–and ingested freshly erupted volcanic ash that coated their browse. In prehistoric times, the quarry site appears to have been a waterhole, where large numbers of large herbivores gathered, drank, and dumped. Dinosaur dung may have even helped disperse seeds.

"Most people who talk about what dinosaurs ate, including me, don't have a clue," says Horner. "Only Karen knows, because she's standing behind them."

Chin aims to refine her analyses to extract more information from coprolites than just who or what was eaten. Her work has already shown that dynamics between predators and prey or among diverse plants and animals can begin to be deciphered. What other glimpses of dinosaur behavior might be gleaned from their dubious dung? "I think Karen's work is really fantastic," says Greg Erickson. "It's going to open up this field."

Blake Edgar is an Associate Editor at California Wild.

cover fall 1999

Fall 1998

Vol. 51:4