DNA in ancient fossils. DNA extracted from bacteria that are supposed to be 425 million years old brings into question that age, because DNA could not last more than thousands of years.
Since creationists deny most dating methods, isn't this an invalid argument?
Not really - isn't the purpose of the argument to show why dating methods are invalid. They're claiming that DNA couldn't last more than a million years, so if dating methods suggest DNA is older than this the dating methods must be wrong. It's a valid argument, even if it's not sound.