Fallacy

Ok try and figure out which type of fallacy this is! Dinosaurs were mortal beings. Dinosaurs no longer exist on this planet Humans are mortal beings, Therfore we will soon no longer exist on this planet.

UFO's

Now whether or not you believe in UFO's, here is a thought for you. What if UFO's were time travelers from the future? This sounds like a crappy sci-fi series, but let me make my case. If we did reach the day where time travel was possible, how would we use it? Would we go back in time and change things? Or would people of the future argue that it would be a bad idea to change the past? From the way I see it, one benefit of time travel that scientists would take advantage would be to document certain past events that need elaboration or avoided documentation. For example going back to see exactly how the Egyptians saw the pyramid. All they would do is observe. Isn't that what it seems UFO's do during sitings? They fly around as if to watch and/or observe. What do you think?