Scroll coprolites are fairly uncommon. As their name suggests, the fecal material appears to be wrapped around itself like a scroll. The reason for this is that the animal producing it had a scroll valve. Think of the scroll valve like a tube containing a loosely rolled up piece of tissue (known as an epithelial flap) attached along one edge on the inside. That rolled up tissue provides a greater surface area for nutrient absorption. It also creates a mold for the material passing through it. As the fecal matter is pushed through this valve, the scrolled tissue generates mucous to help its contents glide along its path. Once expelled, a mucous membrane remains, maintaining the internal structure of the valve through which the poop passed. (Whew, say that ten times fast!)
Extant (modern) animals known to have scroll valves include coelacanths and some Carcharhiniformes sharks (spadenose, hammerhead, etc.).
Dissection of a spadenose shark (Scoliodo laticaudus)
Reference:
Argyriou, T., Clauss, M., Maxwell, E. E., Furrer, H., & Sánchez-Villagra Marcelo R. (2016). Exceptional preservation reveals gastrointestinal anatomy and evolution in early actinopterygian fishes. Scientific Reports, 6(1). https://doi.org/10.1038/srep18758
