Updated March 24, 2015 with a newly restored Pisanosaurus pelvis.
Pisanosaurus (Casamiquela 1967) is widely considered the most basal ornithischian dinosaur. Unfortunately Pisanosaurus is only known from a few bones. So we rely on sister taxa to help restore the missing bits and pieces.
But first, a minor problem.
Only the central portion of the Pisanosaurus pelvis (Fig. 1) is known, the area surrounding the open acetabulum. So the outline shape of the Pisanosaurus pelvis, a key trait in the identification of ornithischian dinosaurs, is not known.
I’m not sure about this…
But the placement of the possible obturator foramen and tab-like contribution of the pubis make it appear that the Sereno pelvic elements might have been misidentified — but then the other elements look correct in the Sereno orientation. Contradicting clues make this a difficult problem. Perhaps a photo or firsthand observation would show where the problems are in the drawing. Published data do not indicate where the pelvis was found in relation to the rest of the body, but I can only assume it was correctly oriented, favoring the Sereno model.
When the Pisanosaurus skull is restored to resemble the skull of Haya, the resemblance is striking. Not much difference here in the preserved elements.
The pes of Haya and Pisanosaurus
The phalanges are shorter in Haya, but the pedal morphology is otherwise similar to what we know of Pisanosaurus (Fig 4).
Pisanosaurus is a problem, but not an unsurmountable problem
Perhaps just enough clues are available. All workers nest Pisanosaurus at the base of the Ornithischia.
The origin of the predentary bone preceded Pisanosaurus, and the missing mandible tip probably marks it’s border. Even so, in the fossil a line of demarcation is present. Not sure what to make of it. It is colored yellow here (Fig. 6).
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