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Bonecrushers
Jorgen Ellis
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Imagine a North America before humans. A river winds its way through the wetlands and floodplains of what is now central California. A frog croaks, a goose honks, and a sabertooth salmon leaps upstream. A dog poops, then runs off with its pack mates.
Fast forward to the present day. That dog poop is now a fossil specimen from the Mehrten formation, a rock outcrop rich with fossils from 5.3 to 6.4 million years ago. A recent study by Xiaoming Wang, curator of vertebrate paleontology at the Natural History Museum of Los Angeles County (NHMLA), and colleagues shows us just how much we can learn from fossil poop – or “coprolites” in scientific lingo.
The study, published in eLife, revealed not just the species of dog responsible for the poop, but also what this fossil dog ate, because it contained bone fragments. Finding undigested food in a poop fossil is the ultimate ground-truth for paleoecology, the study of ecosystems that existed millions of years ago. We can never observe directly what extinct species were eating, so we turn to poop for clues.
But what dogs were around five million years ago? To answer this, the authors turned to another study they recently published in the Journal of Vertebrate Paleontology, describing the fossil dogs of this rock formation. From four species of fossil dog, they narrowed it down to the most likely, the bone-cracking dog Borophagous parvus, based on its size, the size of the poop, and how common B. parvus was in this rock formation. B. parvus, known from fossils in California and Arizona, was a medium-sized dog, estimated to weigh around 50 pounds. The genus Borophagus went extinct two million years ago, with no surviving descendants. (Modern dogs belong to the same family, Canidae, but descended from a different branch of the family.)
“I have studied these animals much of my adult life, but I had never seen their poop, so you can imagine how excited I was,” says Wang. “Fossilized poop is by far rarer than bones. I’m used to thinking about teeth and how these animals chewed, and how bones are built to resist cracking under the pressure of their jaws. But the poop is more visceral and requires imagination about the other end. The information these coprolites contain is a nice confirmation of what we have long-suspected: that they are bone-eaters. We just didn’t have the physical evidence of it, but now we do.”
They had more than just proof that the dogs ate bone; they had some pieces of bone that were big enough to learn more about their prey. Scientists had a guess for what size animals these dogs ate, and were excited to finally confirm it. Paleoecologist Mairin Balisi, coauthor of both papers, explains, “Scientists had deduced that not only did they crack bone, they cracked bone of animals bigger than themselves. This is important, because if you prey on animals bigger than yourself versus smaller than yourself, those are two very different niches.”
The “niche” is the role that an animal plays in an ecosystem. For each potential role, there are tradeoffs. As Balisi puts it, “larger prey requires more effort to find, chase down or ambush, and finally kill … On the other hand, larger prey, once you have it – that’s a lot of meat right there.” The theory is, if an animal is capable of hunting larger prey, they should, because it maximizes their energy spent.
Balisi used the largest of the poop bone fragments to determine the relative size of Borophagus’ prey, and thus the niche that Borophagus occupied. It was a piece of rib likely belonging to an ungulate, such as a deer.
“I had never calculated the body size of a prey species from a single rib fragment, so I had to devise a method. This involved visiting the NHMLA Mammalogy Collections, which house the skeletal remains of large land mammals that we still have today – including ribs to which I could compare the rib fragment in the fossil poop.”
The researchers determined that the rib most likely came from an animal that weighed from 75-220 pounds. Borophagus parvus is estimated to be at most 53 pounds, so these bone-cracking dogs definitely ate animals larger than themselves, and could crush and eat their bones, which no other predators in this ecosystem have been shown to do. This gives scientists a snapshot into the food web during that time, which is the ultimate goal of paleoecology.
The legacy of bone-eaters
Now that scientists know for sure there was a bone-eater in the ecosystem, they can speculate as to how that ecological role affected the rest of the ecosystem, and how things changed when that role was lost. With a bone-eater in the midst, bones were being broken down quickly, and the material that bone was made of was either incorporated into the predator’s body or was pooped out as ready-to-use nutrients. After the genus Borophagus went extinct two million years ago, that bone-eating role was never fully taken on by another large predator in North America. Bones were left to decompose by other, slower, means. They may have provided the base for a new micro-ecosystem of decomposers, which could then lead to the evolution of a different type of plant and animal community using those nutrients.
Around the same time that the genus Borophagous went extinct, Earth was plunged into an Ice Age. Paleoecologists want to know: how does the loss of a major ecological niche interact with such drastic climate change?
We don’t have all the answers yet, but these questions lead us to the dangerous territory of the present. Earth’s climate is undergoing significant change. At the same time, the extinction of large predators is already having a negative impact on modern day ecosystems, even when it’s just a local extinction. Without top predators, other animals become more abundant and wreak havoc on their own prey species. This can ultimately lead to the collapse of an ecosystem.
If fossil dog poop teaches us anything, it’s that we should appreciate species for the role that they play, and the function they serve. We can choose which species we hunt, and which we protect, in a way that protects all of the functions that are necessary to keep an ecosystem afloat. We can monitor climate change, and make sure we incorporate these changes into our projections for the future. We have the power to prevent catastrophe.
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