Er. Why would we assume that this was evolved in basal diplodocoids
and then lost in flagellicaudatans, rather than that it is apopmorphic
for rebbachisaurids (or indeed just for Nigersaurus)?
It's currently equally parsimonious either way, AFAIK: either it evolved
once at the base of Diplodocoidea and was then lost at the base of
Diplodocidae (two steps); or it evolved convergently in Rebbachisauridae and
in Dicraeosauridae (two steps). What have I missed?