Forensics. The marks on bones show where muscles attached and how big they were. Grooves and pits show where nerves and blood vessels ran. All animals with skeletons follow the same universal body plan, and how the bones go together is well known. Wear and tear on the bones shows what stresses they were put on when alive - from that we can calculate weight, habits, behaviors and movements.

But there is always more than just bones. Imprints of skin and soft tissues in the rock tell even more, as well as traces of proteins and DNA that survive intact. Add in parts trapped in amber, or preserved in peat or other chemical laden preserves, and even more can be known. In some cases, such as the Wooly Mammoth, entire animals were perfectly preserved in ice - explorers have literally eaten their meat.

For even older creatures - try tens of millions instead of thousands of years, such as dinosaurs - there are now examples of tails, with feathers, preserved in amber. We know exactly what dinosaur feathers looked like thanks to that. We can recover the color pigments of their feathers, skin and eggs (eggs tended to be blue-green, their feathers every color including neon shades like parrots, and skin, various browns and tans).

Add to that footprints which can tell us size, gate, weight, density and more, and we have a wealth of information!