Ancient Poop or Siderite Concretions? Alleged Coprolites are Picked Apart


While I was rockhounding within a limestone layer along The Cirque Trail of Rendezvous Peak, Wyoming, I may have come across something amazing…ancient poop.

Of course, my field advisor recommended I not pick up coprolites of any kind for fear they were more—recent. But that didn’t stop me, because there is nothing more fascinating than poop. We all have a natural attraction to it (even if we don’t admit it).

Paleobiologists are no exception since technically coprolites qualify as and biologic earth material after so many millions of years, 500 to be precise. Poop can tell us a lot, what was the animals last meal, the size of the animal, …. Coprolites form via natural processes (ehem)… Since its no longer “organic”, what’s in it now? What were these precious gems doing in a predominately deep marine layer of rock? As attractive as the idea of Cambrian fecal matter was there were some doubts, steaming from the fact that not many fauna could create droppings in this marine setting and in such a size. What may look like feces could very well be a siderite nodule encapsulating a recrystallized fossil, considering the age of the specimens and the location of deposition (epeiric lagoons).

How do I go about picking apart a friable mass of metal-carbonate? First, its always good practice to examine specimens with a hand lens and do a preliminary assessment with the five senses. Upon looking at it, it possesses the standard shape of a coprolite (organically clumpy). The minerals present appear to be iron rich and rusty, and smells/tastes metallic and earthy, most definitely hematite or siderite crystals. This tells me something…. there was an abundance of iron that reacted with the carbonates from skeletal remains. A preliminary examination is never enough, I must make a thin section slide (a meticulous task that takes some patience).

Before doing that I need to sand down the surface of the specimen until it completely flat while shifting from 120 grit down to 1000. Now I can take a better look inside. All of the samples contain pyrite crystals (Bottom), which are sulfide meaning that the environment was originally anoxic. These pyrite crystals serve as a nucleus for siderite that later degrade into oxidized hematite on the surface.

So is it poop or not? As of now, it looks to me as though its not poop at all but maybe something far more interesting, pieces of a dead animal. I plan on taking my samples into the lab soon and will post an update. Until then!