I can't find this in the description
Éric Buffetaut, Varavudh Suteethorn, Gilles Cuny, Haiyan Tong, Jean Le Loeuff, Sasidhorn Khansubha & Sutee Jongautchariyakul: The earliest known sauropod dinosaur, Nature 407, 72 -- 74 (7 September 2000).
Any sort of semi-sprawl would be highly illogic in such a large animal. Besides, Isanosaurus has straight femora.
It is very certain that they don't belong to Dinosauria. HP Mickey Mortimer posted a few synapomorphies of Dinosauria exclusive of Lago- & Marasuchus recently IIRC.
Is this what causes the unresolved, er, tetrachotomy (if I'm correctly interpreting your cladogram)?
Isanosaurus is "markedly different from prosauropods", but this is not enough evidence to dissolve Sauropodomorpha (while it may be enough to make Prosauropoda holophyletic). No phylogenetic analysis was done in that paper. Omnivorous prosauropods (if real -- I have to wait for the paper) simply conserve the plesiomorphy and don't automatically destroy Sauropodomorpha either.