Based on my first viewing of James Cameron's epic, I couldn't resist offering some thoughts on the ecosystem of Pandora.
Cameron's Avatar is not a perfect film. The plot is too derivative, not just of Dances with Wolves but of Disney's subpar Pocahontas. It was hard to keep a straight face in some of the talking tree segments.
Cameron, though, has created a visual feast with some terrifically interesting inhabitants, plant and animal. His Pandora has a lower gravity than Earth, enabling trees to grow to stupendous heights and three-meter-tall humanoids to leap and dart about the forest with the grace of spider monkeys. Cameron gives us four-winged semi-reptilian flying creatures, which is not too much of a stretch given that we know one reptile from Earth's fossil record that actually did adopt the four-wing design. It's interesting that some of his animals, such as the flying banshees, have respiration holes seemingly unconnected to the mouth or the organs of smell, wherever exactly those are. This breathing system may be a little less efficient than the multipurpose systems of Earth vertebrates, but the atmosphere of Pandora (not described except as poisonous to humans) might be rich enough in oxygen and other key gases to make it viable.
Most of the animals are hexopods (six-legged). Earth vertebrates don't have six legs because it seems the cost of maintaining extra limbs exceeds their utility, while far smaller creatures such as insects go with at least six legs (which are much simpler in construction and don't add as much to the total nutritional requirements as vertebrate legs do). It may be a combination of plentiful food and plentiful oxygen on Pandora reduces the burden. (Remember, insects got much larger in the days when Earth's atmosphere had a much higher oxygen level.)
Cameron's humanoids, the Na'vi, are an exception to the hexopod design (so are some of his flying creatures). To a degree, Cameron was hemmed in by the need to make the sentient race something human movie-goers can relate to. Really, what are the odds they would laugh and cry exactly like humans to express emotions? Cameron bent to the same necessity in giving the females some features humans would recognize as female, both facially and in having seemingly nonfunctional breasts. I kept wondering what Na'vi elbow joints are like, given that they draw their bows with the drawing hand facing palm out, which is awkward as heck for humans. I also wondered what the Na'vi evolved from: the only smaller primate-type things, the prolemuris, have six legs and not much of a resemblance. (Note to Cameron - the direhorses are too obviously horselike for all their strange features. I hate to say you didn't show enough imagination, but on this point...)
The really interesting part is that everything evolved on Pandora as part of a system: not magical but electrochemical, with trees sending messages over their root systems, the Na'vi being able to "plug" directly into the nervous systems of other creatures, and so on. The entire moon is essentially wired into one living network. There have to be some limits to this (trees wouldn't want herbivores tapping in to find the best trees to nibble on), and it's hard to figure out what evolutionary pressures might have driven this development. I'm going to have to read the Avatar book for more insights. The Na'vi can connect to both the four-legged and six-legged beasts.
Actually, I do have a thought on Pandoran evolution. I wonder if Pandoran life went from free-living microbes through a bottleneck where there was only one common ancestor to higher organisms. This might have been like a slime mold, which is made up of seemingly unrelated single cells but can somehow connect enough to organize into larger structures. Everything that came after somehow found it advantageous to keep and improve on the networking ability.
Oh, heck. Don't worry too much. Just enjoy the movie. It's a hell of a show.