I think sexual display is a very likely explanation. I'm sure they also served as at least passive predator deterrence (don't bite the pokey end!) and if they were strengthened with keratin sheathes they could have also served as more active defense in a pinch.
Greg Paul once speculated (I think it was in a Dinosaur Society bulletin in the 1990s?) that they could have even created an auditory display by clacking the horny sheaths against one another as they moved their necks - while that's obviously not testable, I have to say that the idea of an Amargasaurus clacking its neck spines to impress a mate or drive away a challenger is one of the more novel and evocative ideas I've read to date.
Greg Paul once speculated (I think it was in a Dinosaur Society bulletin in the 1990s?) that they could have even created an auditory display by clacking the horny sheaths against one another as they moved their necks - while that's obviously not testable, I have to say that the idea of an Amargasaurus clacking its neck spines to impress a mate or drive away a challenger is one of the more novel and evocative ideas I've read to date.
In fact, clak the vertebrae seems to be quite interesting, however, I read a case in which small pterosaurus spent part of their lives between the vertebrae of Amargasaurus, something I believe is plausible, however, it would be very strange. It would be an extremely symbiotic relationship.
I'm more in favor of the idea of using the vertebrae as a sexual display.
I'm more in favor of the idea of using the vertebrae as a sexual display.