Plateosaurus is a member of a group of early herbivores known as "prosauropods".
The group is not a monophyletic group (thus given in quotation marks), and most researchers prefer the term basal sauropodomorph.
Plateosaurus was the first "prosauropod" to be described, and gives its name to the family Plateosauridae Marsh, 1895 as the type genus.
Initially, when the genus was poorly known, it was only included in Sauria, being some kind of reptile, but not in any more narrowly defined taxon.
In 1845, von Meyer created the group Pachypodes (a defunct junior synonym of Dinosauria) to include Plateosaurus, Iguanodon, Megalosaurus and Hylaeosaurus.
Plateosauridae was proposed by Othniel Charles Marsh in 1895 within Theropoda.
Later it was moved to "Prosauropoda" by von Huene, a placement that was accepted by most authors.
Before the advent of cladistics in paleontology during the 1980s, with its emphasis on monophyletic groups (clades), Plateosauridae was defined loosely, as large, broad-footed, broad-handed forms with relatively heavy skulls, unlike the smaller "anchisaurids" and sauropod-like "melanorosaurids".
Reevaluation of "prosauropods" in light of the new methods of analysis led to the reduction of Plateosauridae.
For many years the clade only included Plateosaurus and various junior synonyms, but later two more genera were considered to belong to it: Sellosaurus and possibly Unaysaurus.
Of these, Sellosaurus is probably another junior synonym of Plateosaurus.

Basal sauropodomorph phylogeny simplified after Yates, 2007.
This is only one of many proposed cladograms for basal sauropodomorphs.
Some researchers do not agree that plateosaurs were the direct ancestors of sauropods.
Plateosaurus is a member of a group of early herbivores known as "prosauropods". The group is not a monophyletic group (thus given in quotation marks), and most researchers prefer the term basal sauropodomorph. Plateosaurus was the first "prosauropod" to be described, and gives its name to the family Plateosauridae Marsh, 1895 as the type genus. Initially, when the genus was poorly known, it was only included in Sauria, being some kind of reptile, but not in any more narrowly defined taxon. In 1845, von Meyer created the group Pachypodes (a defunct junior synonym of Dinosauria) to include Plateosaurus, Iguanodon, Megalosaurus and Hylaeosaurus. Plateosauridae was proposed by Othniel Charles Marsh in 1895 within Theropoda. Later it was moved to "Prosauropoda" by von Huene, a placement that was accepted by most authors. Before the advent of cladistics in paleontology during the 1980s, with its emphasis on monophyletic groups (clades), Plateosauridae was defined loosely, as large, broad-footed, broad-handed forms with relatively heavy skulls, unlike the smaller "anchisaurids" and sauropod-like "melanorosaurids". Reevaluation of "prosauropods" in light of the new methods of analysis led to the reduction of Plateosauridae. For many years the clade only included Plateosaurus and various junior synonyms, but later two more genera were considered to belong to it: Sellosaurus and possibly Unaysaurus. Of these, Sellosaurus is probably another junior synonym of Plateosaurus.
Basal sauropodomorph phylogeny simplified after Yates, 2007. This is only one of many proposed cladograms for basal sauropodomorphs. Some researchers do not agree that plateosaurs were the direct ancestors of sauropods.
Plateosaurus