The pelvic shield of ankylosaurian dinosaurs refers to an area of osteoderms lacking differentiated transverse bands over the pelvic region and it is used as a diagnostic character for various ankylosaur groups. The pelvic shield character varies across ankylosaur taxa but is typically coded as a binary character or is excluded from phylogenetic analyses, which obscures evolutionary trends and relationships. This study investigates for the first time pelvic shield morphology in a stratigraphic and geographic context. This paper comprehensively reviews pelvic shield morphology with firsthand observations of specimens, and proposes three categories of pelvic shield morphology. Category 1 pelvic shields have un-fused but tightly interlocking osteoderms. Category 2 pelvic shields have fused osteoderms forming rosettes and are restricted to the Late Jurassic to mid Cretaceous of North America and Europe. Category 3 pelvic shields have fused polygonal osteoderms of similar size, and are found in the mid- to Late Cretaceous of North America. Although the pelvic shield is used to characterize the Polacanthidae, an interpretation supported by this review, the validity of such a clade is dependent upon a global parsimony analysis incorporating this character. Future analyses of the Ankylosauria should incorporate a more detailed treatment of the pelvic shield to determine its diagnostic value within the group.

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