A new solution to the Pisanosaurus pelvis problem

Everything
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.

Figure 1. Pisanosaurus with new hypotheses on pelvis morphology after shifting in situ bones to in vivo positions.

Figure 1. Pisanosaurus with new hypotheses on pelvis morphology after shifting in situ bones to in vivo positions.

Unfortunately
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.

If only
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.

Figure 2. Pelvis elements of Jeholosaurus, a basal ornithischian, in situ and restored to in vivo positions. Note how gracile the pubis is. It is also lacking a prepubic process.

Figure 2. Pelvis elements of Jeholosaurus, a basal ornithischian, in situ and restored to in vivo positions. Note how gracile the pubis is. A boken bone (in yellow) may indicate a pubis prepubic process. Click to enlarge. Photo from Han et al. 2012.

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.

Figure 1. Panphagia with a closeup of the skull. This is a proximal outgroup taxon to the Ornithischia.

Figure 1. Panphagia with a closeup of the skull. This is a proximal outgroup taxon to the Ornithischia.

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.

References
Bonaparte JF 1976. Pisanosaurus mertii Casamiquela and the origin of the Ornithischia. Journal of Palaeontology 50(5):808-820.
Brusatte SL , Benton MJ , Desojo JB and Langer MC 2010. The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida), Journal of Systematic Palaeontology, 8:1, 3-47.
Casamiquela RM 1967. Un nuevo dinosaurio ornitisquio triásico (Pisanosaurus mertii; Ornithopoda) de la Formación Ischigualasto, Argentina. Ameghiniana 4 (2): 47–64.
Han, F-L, Barrettn PM, Butler RJ and  Xu X 2012. Postcranial anatomy of Jeholosaurus shangyuanensis (Dinosauria, Ornithischia) from the Lower Cretaceous Yixian Formation of China.”. Journal of Vertebrate Paleontology 32(6):1370–1395.
Irmis RB, Nesbitt SJ, Padian K, Smith ND, Turner AH, Woody D and Downs A 200a. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317 (5836): 358–361. doi:10.1126/science.1143325. PMID 17641198.
Irmis RB, Parker WG, Nesbitt SJ and Liu J 2007b. Early ornithischian dinosaurs: the Triassic record. Historical Biology 19:3-22.
Makovicky PJ, Kilbourne BM, Sadleir and Norell MA 2011. A new basal ornithopod (Dinosauria, Ornithischia) from the Late Cretaceous of Mongolia. Journal of Vertebrate Paleontology 31 (3): 626–640.
Nesbitt SJ, Irmis RB, Parker WG, Smith ND, Turner AH and Rowe T 2009. Hindlimb osteology and distribution of basal dinosauromorphs from the Late Triassic of North America. Journal of Vertebrate Paleontology 29 (2): 498–516. doi:10.1671/039.029.0218
Sereno P 1991. Lesothosaurus, “Fabrosaurids,” and the early evolution of Ornithischia. Journal of Vertebrate Paleontology 11:168-197.
Xu X, Wang and You 2000. A primitive ornithopod from the Early Cretaceous Yixian Formation of Liaoning. Vertebrata PalAsiatica 38(4:)318-325.
wiki/Pisanosaurus

 

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