Classification
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| Cladogram of basal Ornithischia after Butler et al. (2008), showing the position of Pisanosaurus as the earliest example of an ornithischian. |
Pisanosaurus is very basal within Ornithischia; the postcrania seem to lack any good ornithischian synapomorphy and it was even suggested by Paul Sereno in 1991 that the fossil is a chimera. However, recent studies suggest that the fossils belong to a single specimen.
Over the years, Pisanosaurus has been classified as a heterodontosaurid, a fabrosaurid, a hypsilophodont and has also been considered the earliest known ornithischian. A 2008 study placed Pisanosaurus outside of (and more basal than) Heterodontosauridae. In this study, Pisanosaurus is the earliest and most primitive ornithischian. Other primitive ornithischians include Eocursor, Trimucrodon, and possibly Fabrosaurus.
Pisanosaurus is the type genus of the Pisanosauridae, a family erected by Casamiquela in the same paper which named Pisanosaurus. The pisanosauridae family has fallen into disuse, as a 1976 study considered the group synonymous with the already named Heterodontosauridae.
Read more about this topic: Pisanosaurus