The groups ancestral to the Sauropoda are theprosauropod dinosaurs.Although it has long beenrecognized that there is a close relationship betweenthese small (o100 kg) to large (3 t; Seebacher 2001)animals and the sauropods, the exact nature of thisrelationship has not yet been resolved.Pr osauropodsmay be monophyletic and in their entiretyform the sistergroup of sauropods.Alternat ively, itcould be that only certain prosauropods, namely themelanorosaurids, are the sistergroup of sauropods,the other prosauropod taxa such as plateosauridsbeing successively more distantly related to the Sauropoda(Benton et al.2000 ).Sauropod monophyly,however, is unanimously accepted, as is the monophylyof Sauropoda plus all prosauropods, which areunited in the taxon Sauropodomorpha (Upchurch 1998;Wilson 2002).