Is this a valid or fallacious argument, and why?

• If dinosaurs died millions of years ago, then scientists wouldn’t find DNA in dinosaur fossils.
• Scientists found DNA in dinosaur fossils.
• Therefore, dinosaurs didn’t die millions of years ago.

The argument form would resemble this (if such a form even exists):

• If P, then not Q.
• Q.
• Therefore, not P.

You got the form, which is a great start. Then try substitution of content to try to "break" the logic, or even use a simple argument that removes any potential biases:

If it is Wednesday (P), then it is not Monday (not Q).
It is Monday (Q).
Therefore, it is not Wednesday (not P).

Without memorizing all the possible forms, I tried a few examples and I could not break it. Now we can look at the form a bit more carefully and see that it is a valid structure.

The original example uses questionable premises that can only be argued by someone familiar with the subject. I found this interesting article: https://www.scientificamerican.com/article/possible-dinosaur-dna-has-been-found/.

It's logically valid, but the facts are wrong.

Just like :

1 ..If people are over 6 feet tall, then they are murderers.
2 ..John is 6'4
3 ..Therefore John is a murderer.

I would ask for further clarification as the initial claim is scientifically obsolete and structurally clumsy (non-sequitur) although that does not necessarily indicate fallacious reasoning, an argument can be made for a form of a false premise, or possibly a selective arrangement fallacy.

A false premise is an incorrect proposition that forms the basis of an argument or syllogism. 

"If dinosaurs died millions of years ago, then scientists wouldn’t find DNA in dinosaur fossils."

In fact, dinosaurs did not die millions of years ago, they evolved, and as Dr. Bo points out scientists have also recently discovered possible DNA remnants in a dinosaur fossil.

How Dinosaurs Shrank and Became Birds

https://www.quantamagazine.org/how-birds-evolved-from-dinosaurs-20150602/

English biologist Thomas Henry Huxley in his 1870 treatise, Further Evidence of the Affinity between the Dinosaurian Reptiles and Birds.

Huxley, a renowned anatomist perhaps best remembered for his ardent defense of Charles Darwin's theory of evolution, saw little difference between the bone structure of Compsognathus, a dinosaur no bigger than a turkey, and Archaeopteryx, which was discovered in Germany and described in 1861. When Huxley looked at ostriches and other modern birds, he saw smallish dinosaurs. If a baby chicken's leg bones were enlarged and fossilized, he noted, "there would be nothing in their characters to prevent us from referring them to the Dinosauria."

In this case, the conclusion is correct, but the internal consistency of the proposition is flawed and should be amended.

Isn't this an example of denying the consequent (modus tollens) which is a valid form? Here, the consequent is stated as a negative (not Q), so so stating the minor premise as a positive is denying the consequent. Therefore, the negative conclusion follows. In my view, this is synonymous with (the mirror image of) the more common form of modus tollens, to wit:

If P, then Q.
Not Q.
Therefore, not P.