bone tissue and evolution question--please help!!

[virlomi_stone]

On my study guide for my first histology exam asks for the evidence for bone as a primitive tissue. I remember in lecture that this topic was briefly discussed but all I jotted down was that bone tissue has been highly conserved in the sense of evolution. I didn't think considering the huge amount of other information we are required to know and the fact that this is for a histology course that this was important to take detailed notes on. Please help!!