Dinosaur Blood and "Ancient"DNA:
Before the existence of supposedly"ancient" organic material had been well publicized, it was predicted that "no DNA would remain intact much beyond 10,000 years." This prediction was based upon the observed breakdown of DNA.
Not long after this prediction was made, very old DNA started turning up. For example, at the Clarkia Fossil Beds, in Idaho, a green magnolia leaf was discovered in strata that was said to be 17 million years old. Because it was so fresh-looking and even pliable, scientists decided to see if any DNA was present. And to their surprise they discovered that there was: and that it matched the DNA of modern magnolia trees.
Since then, DNA claims have been made for supposedly older material such as dinosaur bones, and insects in amber. It was said that the reason the magnolia leaf was preserved was because it was buried in clay; however, the 17 million year date is still doubtful. Likewise, scientists say that DNA from the insects was preserved because they were entombed in amber.
However, a serious problem arises when we come to the dinosaur bones; for these were not entombed in amber or clay, but in sandstone. And because sandstone and bone are both porous, this means that ground and rainwater would be able to seep into the rocks, and thus into the bones as well. The fact that the outer part of one of these bones was mineralized gives strong evidence that water -- and thus oxygen -- had access to the bones. The fact that the inside of the bones are not mineralized is an indication that they are young. The fact that the partially mineralized bone had (what looked like) redblood cells in it is a strong indication that it is young: probably less than 10,000 years old.