UCMP MYSTERY FOSSIL #47 is . . .
the rugose coral Paleophyllum

Give yourself a pat on the back if you guessed that this was a cnidarian (phylum Cnidaria) or an anthozoan (class Anthozoa). Paleophyllum is a fossil rugose or “horn” coral (you can see why they’re called horn corals) that lived during the Ordovician Period. Though we still have corals with us today, the rugose corals went extinct at the end of the Permian. Find out more about anthozoans here.