Bloody marvelous! I spend a few years on here learning at the feet of the Masters. Putting in my own few thoughts on exonumia from time to time. And when it comes to my turn to shine; a chance to display my arcane knowledge; what do I get?
Dino-bloody-doodoo.
Yes coporolites.
In the early 1800s, a local girl to me, Mary Anning, was noted for finding and selling fossils from the cliffs at Lyme Regis. She is, incidentally, thought to be the subject of the tongue twister “She sells sea shells by the sea shore”
She noticed that a fossil known at that time as a bezoar stone was often found in the abdominal region of ichthyosaur skeletons found in the Lias formation at Lyme Regis. She also noted that if such stones were broken open they often contained fossilized fish bones and scales as well as sometimes bones from smaller ichthyosaurs. It was these observations by Anning that led the geologist William Buckland to propose in 1829 that the stones were fossilized feces and named them coprolites. Buckland also suspected that the spiral markings on the fossils indicated that ichthyosaurs had spiral ridges in their intestines similar to those of modern sharks.
We are, in effect, talking about rifling very much like on a modern bullet.
What we are in fact seeing is the mineralised result of a creature excreting in an extremely specific environment. Not too moist, not too dry and probably not unlike very deep leaf mould. The first thing that our dino-turd would do involved slowly losing its internal moisture, a process that required a steady temperature over a considerable period of time. After it had become dried but not quite dessiccated minerals would leach in from the earth and the surrounding leaf mould.
Millenia later, you have a very efficient paper weight and a great talking point.
The various external bumps, ridges and grooves are a co-effect of dehydration and rifling.
You can find out more on Wikipedia, but I wrote much of that also.