Fossil poop of this type comes from reptiles in the crocodile family. Crocodilians have very acidic digestive systems, so bone and scales from the animals they preyed upon rarely show up in their coprolites. What makes crocodilian coprolites fun is that we can estimate the length of our poopetrator based on the diameter of the coprolite.
In 2012, Jesper Milàn published a paper entitled, Crocodylian scatology – a look into morphology, internal architecture, inter- and intraspecific variation and prey remains in extant crocodylian feces. Based on his findings, the Croc's Length (cm) = [coprolite diameter (cm) - 1.10]/0.012.