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Fossil Friday

Were you able to identify Monday’s fossil? Allie Valtakis was. Find out after the picture what it is. I hope it doesn’t make your weekend too crappy.


These two things are coprolites, otherwise known as fossil poop. Always a hit with kids when I show them in schools, but I always get the same questions. Do they smell? Will I get poop on my hands if I touch it? Most are tentatively reassured when I inform them that to be considered a coprolite, the poop has to be replaced with mineral. After a long period of time, there isn’t any actual poop left.

Dr. Karen Chin.

Dr. Karen Chin.

Coprolites can be quite informative. Coprolites preserve traces of what the animal that left it ate, so they can be useful for looking at the diet of prehistoric animals. Karen Chin, a curator at the Colorado University Museum at Boulder, is the leading expert on coprolites, particularly dinosaur coprolites. She found wood In some coprolites found in the Two Medicine Formation in Wyoming, which is unusual for two reasons. One, most coprolites are from carnivores, so herbivore coprolites are relatively rare. Secondly, most herbivores don’t eat wood except as a last resort when no other food sources are available. She was able to tentatively attribute these coprolites to the hadrosaur called Maiasaura (mainly due to the size and content of the coprolites, and the abundance of maiasaur bones in the area), making this the first dinosaur known to eat wood, as well as giving a unexpected perspective on the lifestyles of these “duck-billed” dinosaurs.

Probably the most famous coprolites known are also from the Two Medicine Formation and were also studied by Dr. Chin. They were uncommonly large and clearly from a carnivore. The only known carnivore from that formation big enough to create such a ponderous poop was Tyrannosaurus rex himself.

Tyrannosaur scat

Tyrannosaur scat

These coprolites told a fascinating story. The coprolites were readily identified as being from a carnivore due to elevated levels of phosphorus, which results from eating a high protein (i.e. meat) diet. The coprolites contained numerous bone chips, indicating that T. rex was not a dainty eater. T. rex had a massively built skull with powerful jaws, providing T. rex with the most powerful bite of any terrestrial animal. It put these jaws to use chomping through a carcass, bone and all. If one compares the thick, broad teeth of a tyrannosaur with the flatter, blade-like teeth and lighter skull of an allosaur, it is clear they had fundamentally different niches and eating styles.

There was bigger surprise found in the tyrannosaur coprolites. Dr. Chin found traces of undigested muscle. Obviously, it was not original muscle left in the coprolites, but mineralized remains. Why is this important? Modern reptiles have a slow metabolism. Food takes a long time to go through the digestive tract. As a result, digestion is phenomenally thorough. Crocodilians can take the enamel off teeth. Mammals, on the other hand, have notoriously inefficient digestive tracts. It is not uncommon to find recognizable bits left in the feces. Because of the elevated metabolism, food simply passes through too quickly for digestion to be complete. Meat is far easier to digest than plant matter, so carnivores, even mammalian carnivores, typically do a good job of digestion. To have traces of undigested muscle in the coprolite of a T. rex means that either the tyrannosaur was terribly sick with a bad case of the runs, or more likely, tyrannosaurs had short digestive times and a high metabolism to go along with it. It is possible to have thorough digestion with a high metabolism, but it is much harder to have incomplete digestion in a carnivore with a low metabolism.

Thus, coprolites not only tell us about the diet of extinct animals, they can also tell us about their physiology.

On the preservation side of things, one may ask how something as soft and squishy as a poop can fossilize. The answer to that is not easily. The vast majority of poops get washed away. But fecal material does have some advantages that help them get mineralized. As I stated earlier, carnivore feces is enriched in phosphorus. Phosphorus is an important nutrient, eagerly sought after by many organisms because it is not all that common in the environment, making it what is known as a limiting resource.

The other advantage is that feces is mostly made of bacteria, not really waste products. Our intestines are populated with microbes without which we can’t digest our food very well. The richer foods we eat, the more the microbes can grow and meat is a very rich food source.So why is having bacteria in the feces an advantage? Because the waste products they give off during their metabolic processes cause minerals to precipitate around them. Those bacteria are in a phenomenally rich food source in the poop, so they are growing like crazy, which means they are also precipitating minerals like crazy. In effect, they fossilize the poop while they are trying to eat it. If the poop can stay together, is not disturbed, and there is sufficient water around to allow the continued growth of the microbes, you will get a coprolite. The problem with this of course, is that poops are rarely left alone. Other animals eat them, dung beetles carry them off, they get stepped on and spread about, and rain washes them away.

dinodungIf you have a kid interested in learning more about coprolites, I recommend the book Dino Dung, by Karen Chin. The book is written for elementary school kids, but is packed with a lot of good information on the study of coprolites and provides a great introduction to the study of fossil poop.

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