Many groups of organisms have long histories in the fossil record, but later went extinct.  One such group is the horn corals.  These solitary corals were very common during the Paleozoic era before the time of the dinosaurs.  Below you can see horn corals from (left to right) the Ordovician, Silurian, Devonian, and Carboniferous.  All the horn corals became extinct at the end of the Permian.  How did the horn corals change through time?  How long ago did they go extinct?