Since reading Dawkins' "The Blind Watchmaker", I've been fascinated by bats. He has an excellent chapter on bat echolocation and flight development. It's pretty awesome.
I ran across this article in ScienceDaily; it's a little old (February 2008), but I still think it's really neat.
A new nicely preserved fossil of a primitive bat species shows that bats flew before developing echolocation. There had long been an argument over whether flight or the sonar system was developed first in bats (or concurrently), and this fossil shows that the echolocation was not possible in these bats- although flight was! This was the missing link between bats and their non-flying ancestors. The fossil's skull lacks the features in and around the ear that would enable echolocation. Awesome evidence!
By the way, did you know that one in five mammals living on earth is a bat?!??!?! BATS ARE SO AWESOME!