An iron hydroxide/oxide concretion. The common texture and morphology of this material has lead to the belief that these are actually fossilized dinosaur poop. In this case, I find that hard to believe, as the sample contains no dietary evidence (seeds, bone fragments, etc.). In addition, why should one kind of soft dinosaur material (poop) be some commonly fossilized when other soft dinosaur parts (guts) are so exceptionally rare? At the very least, a geologically interesting specimen.