about the basal ornithischian, Pisanosaurus, (Casamiquela 1967), indicates it is a basal ornithischian — except the published pelvis (Fig. 1, lower left hand corner), which preserves only the circum-acetabular portions of the ilium, pubis and ischium. And these published elements give every indication that they were preserved in their in vivo positions.
that produces a rather sauropod-like pelvis when restored (Fig. 1, in the full body outline). That’s great for a basal ornithischian that had not yet developed the retroverted pubis. But the large reptile tree indicates there are more basal taxa, like Jeholosaurus (Fig. 3), that have a completely retroverted pubis.
But what if
there were some post-mortem taphonomic shifting in Pisanosaurus? It happens occasionally. Earlier we looked at the pterosaur Sordes and the problems taphonomic shifting has given paleontologists who assumed a minimum of disturbance in the fossil.
the pubis in Pisanosaurus was taphonomically rotated from its in vivo position… then when re-rotated back into position (Fig. 1) the pelvis can be restored to appear very much like that of sister taxa, like Haya (Fig. 1, lower right), with the ischium now the pubis and the pubis now the ischium.
Otherwise, the most primitive ornithischian pelvis we know
belongs to Jeholosaurus (Fig. 2) from the early Cretaceous, a sister to Daemonosaurus from the late Triassic. The pubis and ischium are quite gracile here. Perhaps that is a clue as to how and why the pubis rotated posteriorly in basal Ornithischia. Panphagia (Fig. 3) is an outgroup taxon with a similar short and gracile pubis and ischium, but apparently not yet rotated. Compare to Eoraptor a sister to Panphagia with larger ventral pelvic elements.
I think the Pisanosaurus solution is worth considering
since it solves a problem rather elegantly. If there are contra indicators, I am not aware of any. Please advise.
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