There are 50 calcified fragments total on the Dreadnought (3 have yet to be found by anyone) The main use for the fragments, besides grimoire score, is for a series of quests known as "The Old Hunger". Eris will give you a chain of missions each unlocked by a certain number of fragments, eventually leading to the last mission where you need 45 of them. The end result is getting an exotic scout rifle known as Touch of Malice.