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Gargoyleosaurus (Jurassic ankylosaur) pelvis

From: Ben Creisler

New in PLoS ONE:

Kenneth Carpenterl, Tony DiCroce, Billy Kinneer & Robert Simon (2013)
Pelvis of Gargoyleosaurus (Dinosauria: Ankylosauria) and the Origin
and Evolution of the Ankylosaur Pelvis.
PLoS ONE 8(11): e79887.

Discovery of a pelvis attributed to the Late Jurassic armor-plated
dinosaur Gargoyleosaurus sheds new light on the origin of the peculiar
non-vertical, broad, flaring pelvis of ankylosaurs. It further
substantiates separation of the two ankylosaurs from the Morrison
Formation of the western United States, Gargoyleosaurus and
Mymoorapelta. Although horizontally oriented and lacking the medial
curve of the preacetabular process seen in Mymoorapelta, the new ilium
shows little of the lateral flaring seen in the pelvis of Cretaceous
ankylosaurs. Comparison with the basal thyreophoran Scelidosaurus
demonstrates that the ilium in ankylosaurs did not develop entirely by
lateral rotation as is commonly believed. Rather, the preacetabular
process rotated medially and ventrally and the postacetabular process
rotated in opposition, i.e., lateral and ventrally. Thus, the dorsal
surfaces of the preacetabular and postacetabular processes are not
homologous. In contrast, a series of juvenile Stegosaurus ilia show
that the postacetabular process rotated dorsally ontogenetically.
Thus, the pelvis of the two major types of Thyreophora most likely
developed independently. Examination of other ornithischians show that
a non-vertical ilium had developed independently in several different
lineages, including ceratopsids, pachycephalosaurs, and iguanodonts.
Therefore, a separate origin for the non-vertical ilium in stegosaurs
and ankylosaurs does have precedent.