The correct answer is - DNA.
Most of the time, the fossils provide the paleontologists with information about their structure, the environment in which they lived, the way of life they practiced, but what they usually not provide is DNA.
Even though there is the occasional case where some DNA has been preserved, though that is extremely rare, usually the DNA has not been fully preserved, so it is damaged and can not really be sued to provide more information.
Usually, the DNA is found in organism that have lived very recently, and have been preserved in permafrost, like the cave lion cub and baby woolly mammoth that were found in Russia. Maybe that DNA can be used, there's even suggestions to revive those species, but it is still not 100% sure.
