Pelvis of Gargoyleosaurus (Dinosauria: Ankylosauria) and the Origin and Evolution of the Ankylosaur Pelvis
Figure 6
The origin and evolution of the ankylosaur pelvis can be approximated from this chronostratigraphic distribution of various pelves seen in dorsal view.
In the basal ornithischian condition, represented by Stormbergia (reconstructed), the ilia are near vertical plates of bone. In the earliest ankylosauromorph, respresented by Scelidosaurus, the dorsal rim of the iliac blade and postacetabulum have rotated towards the lateral side so as to overhang the femur head, and an incipient synsacrum developed. In addition, the elongation of the sacral ribs is accompanied by the medial rotation of the preacetabular process. In the earliest ankylosaur, represented by Gargoyleosaurus, the ilium has assumed a nearly horizontal position and a synsacrum was developed including both caudals and posterior dorsals. In contrast, the preacetabulum of Mymoorapelta curved ventrally for reasons not clear; regardless, this specialization suggests that Mymoorapelta is not the close sister taxon to later ankylosaurs. Further evolution of the ankylosaur pelvis resulted in divergence of the ilia, seen in nodosaurids, represented by Edmontonia, but especially in ankylosaurids, represented by Euoplocephalus. In addition, there was further elongation of the sacral ribs. Not to scale. See also Fig. 10.