Amphicoelias

Yes, Amphicoelias fragillimus does not exist and it is impossible to restorate it accurately. A. altus is the type species, and although it has been proposed as a synonym of Diplodocus, it has many symplesyomorphies shared with more basal diplodocoids.
So, Amphicoelias exists, but A. fragillimus no (like Kelmayisaurus and K. gigantus).
...I hate the scientifically unclear taxa...