Up: Dinosaurs' feeding
All sauropods have very high nostrils, situated above the eyes. In the extant fauna, high nostrils (except for whales) always go with a more or less long trunk.
It may also be that these high nostrils were part of an adaptation made to give out sounds (see dinosaurs' behaviour). Yet a trunk can be used to produce sounds too.
It has been pointed out that developing a trunk needs complex lips muscles, which the dinosaurs' ancestors completely lacked. However, some other dinosaurs had been able to evolve face muscles like fleshy cheeks. It is possible that a few muscles were first developed to produce sounds, and evolved in some sauropod species into the more complex manner needed to give a trunk. However, hints for trunk muscles have not yet been found on sauropod skulls.
Another hypothesis is that sauropods used their domelike skulls as a reservoir of blood, either to prevent them from anoxia when the neck was held vertical, which would have been a real problem, or to cool an overheating brain (note that almost all strange dinosaur features have some day been seen as cooling features: stegosaur plates, spinosaur spines, ceratopian frills...).
The use of a trunk is not clear: in animals with such long necks (contrary to elephants), getting some more centimeters long is not an obvious advantage. And, contrary to elephants, sauropods show various kinds of well-developed teeth in the front of the mouth, obviously used for collecting foof, while chewing was the rôle of the gizzard stones. Perhaps the trunk could have been used to feed the youth or bring them back to the nest?
A rearing sauropod with a trunk:
