The best form for fossils to be in is frozen solid, as the ice can persevere a whole lot about the creature. Many extinct animals such as mammoths have been found in ice, as they had existed since the last ice age. But since there has been many global warm and cold periods on Earth since life began, which means eventually the ice would melt which destroys the “perfect fossil”. Making the chance of finding any dinosaur (from any era) frozen in ice not only unlikely, but impossible.