The author concludes that a recently discovered fossil does not provide evidence that birds evolved from dinosaurs. Why? Because of the following:

In order for a fossil to serve as evidence that birds evolved from dinosaurs, it must be the case that the entire fossil is from a single animal.

But the recently discovered fossil is a compositive of bones collected from different parts of the discovery site.

We know from the first premise that if the fossil isn’t from a single animal, then it isn’t evidence that birds evolved from dinosaurs. But do we have enough to establish that the fossil isn’t from a single animal? No — all we know from the other premise is that the fossil is a composite of bones from different parts of the discovery site. But couldn’t the bones still be from the same animal?

To make the argument valid, then, we want to prove that the fossil is not from the same animal. We can do this by establishing that if the fossil is a composite from different areas of the discovery site, it must come from more than one animal.