At one of the quarries in the Wahweap Formation we were working on with Dr. Titus, the majority of the bones we were unearthing were rotten and past the point of being able to salvage. However, surrounding some of the rotten bone were rocks called siderite concretions. In the picture below you can see that this particular concretion was quite large and after we put it somewhat back together, you could tell that it was once encasing an 80 million year old hadrosaur bone -- the dark reddish color that you see is rotten bone still attached to it. According to Dr. Titus, these concretions often form around organic material and that’s why there was so much of it in this site.
According to geology.com, concretions are formed by groundwater leaving minerals behind in soil or sediment. Many concretions form around a fossil like ours. Usually the cementing material is calcium carbonate. But, it is common to have iron carbonate (siderite) nodules like those seen here.
The concretions were part of the process that caused the bone in this location to be destroyed. Also, the siderite encased bone was not possible to extract. But at least the concretions saved enough bone to give us an estimate of the type and size of dinosaur that had been preserved there.