Year of the Dragon

Today is the Chinese new year and the start of the year of the dragon.  In the set of signs of the Chinese zodiac cycle, the dragon is the only mythical member.  While imaginary, the dragon is to be found in stories from cultures across the globe.  We find dragons in Greek, Western European, Chinese, Indian, and Mesoamerican mythology.  Surely, there was some cultural cross-pollination, but it seems hard to believe that there was that much of an influence.  Why then is the dragon so ubiquitous?

There seem to be three possible explanations.  One line is that the dragon is a slight variation of animals that were observed and then made larger and scarier in legend -- say, crocodiles or snakes that get made into monsters.  This seems plausible since we see this done with griffons and the like, but it is odd that with different animals in different ecosystems, the same fictional derivative would develop spontaneously in so many places. 

A second is that the notion developed as an explanation for dinosaur fossils.  If you have the ossified remains of animals larger than we see, but whose skeletons resemble observed reptiles, then it would only be reasonable to posit the existence of these larger reptilians whose details would be filled in by story.  The problem here is that while fossils did turn up from time to time, it wasn't until we started mining for coal in the industrial age that we really started unearthing significant numbers of fossils.  At the time of origin, dinosaur fossils would not be very widely known and therefore would not need explanation.

A third explanation for the commonality is evolutionary.  If disparate cultures have the same concept, perhaps it goes back to a time before we were spread all over the globe.  In coming down from the trees and living primarily on the ground, what would be the natural predation fear we as a species would develop?  Snakes and other land-dwelling carnivores including land crocs.  Since this fear is bred into all humans because of our common origin, it could be expressed cognitively in terms of fictional beings that instantiate our basic fear along the lines of a Joseph Campbell type monomyth.  It is not clear what sort of evidence one could have for such a just so story, however.

Which of these is most likely correct or is there another option?  Why are dragons cross-cultural?