These beautiful winged water-dwellers are sea angels, floating marine slugs that belong to the same family as sea butterflies. Both sea angels and sea butterflies are pteropods, ancient creatures that first evolved in the early Cretaceous period, sharing the planet with dinosaurs and ammonites. The marine slugs are remarkably resilient; they have survived periods of major global extinctions and environmental changes, according to a recent study in the journal Proceedings of the National Academy of Sciences. In addition, they are the only living creature of their kind with a solid fossil record, so they are invaluable in helping researchers understand the effects of global warming on the marine environment.