Aside from the cursory examination of the Space Jockey, forensics is not Dallas professional specialty. He's the Captain of the Nostromo and could easily be making an incorrect judgment call based on observations in the dark he knows nothing about. Dallas is also constantly wrong and making fundamentally bad decisions throughout the movie that lead to his demise. In this regard - his error of believing the Space Jockey is "old" comes into question.
So sure, he could be providing a narrative detail to the audience about the age of the SJ; but his comments may also be considered foreshadowing of further errors he makes later in the movie. Critical errors based on poor judgment calls. Foreshadowing is also a valid narrative device; and as we are seeing in recent movies by Scott, commonplace.
What's more, define "fossilised". Once you find a definition that actually fits the condition of the Space Jockey, you'll begin to appreciate how it could easily be less than ten years old. It could easily be a recent or young sample.
The derelict may have been on Acheron for thousands of years; or it may have been a more recent event. No one knows this detail just yet. Insisting it's one way or the other is based on data that can be easily misinterpreted - and renegotiated by any upcoming film maker. This is still a very flexible detail, not yet set in stone; but as recent movies are beginning to indicate, it's a recent crash. Ten to Twenty years old.
Ten or ten thousand years old. Take your pick. But if there's one thing I have learned regarding the recent movies, it's best to be prepared to leave expectations at the door when the next movie arrives and accept the outcome.
-Windebieste.
