The stories in the bones
Bones are a mineral, a kind of stone. It’s called calcium phosphate by scientists. In life, bones are living things and have blood vessels, marrow and so on as well as mineral.
When animals die, the blood stops flowing and the bones stop growing. Bones and teeth of the dinosaurs are pretty hard, but all thes ame, when they get buried in the ground, they begin to change into fossils (scientists call it mineralization). This occurs on a microscopic scale and the tiny spaces of the original bone where the blood vessels were become filled with new minerals. The original hard parts of the bones and teeth may remain hardly altered, but often, the bone is replaced with new minerals, too.
Not all bones mineralise in the same way. You can get a rough idea of how fr they have changed by their weight. If bones are heavy, then they have been greratly filled with minerals. If they are light they have been only partly mineralised.
However far the process has gone, and even if the bones still look like bones, they are, strictly speaking, now stones.
Living bones, a bit like tree trunks, record the history of their growth. Like a tree rings, the limb bones show the amount of growth in a given period, and how fast the bone was growing.
By looking at the growth lines in bones we can tell that dinosaurs grew at incredibly high rates, as fast as or faster than any modern mammal or bird. They became adults in 10-20 years. In general, larger animals grow much faster to their adult size than smaller species do(think elephant). But in dinosaur times there was an important advantage to growing. The rush to reach their great adult size, was an advantage becaue they lived at a time when the land, sea and air was full of hunters of all sizes and shapes that ate anything that moved, including their own offspring. No matter what your size, something was ready to kill you if it could. So it helped to get to adult size quickly (that is, the dinosaurs who grew quickly were probably more likely to survive and so breed offspring that grew quickly, too. It was an evolutionary advantage).
This is why dinosaurs were not really like large modern reptiles even though theyr looked exactly like them. The difference is that dinosaurs grew their bones like birds and mammals, so they grew faster than other reptiles. To do this, they had to have more blood moving through the bones, which is one reason some scientists think they were warm blooded. It also tells us that dinosaurs were developing like birds, but looking like reptiles 200 million years ago.