24 May 2017
Tuesday there was a paper describing the pelvic armors of different ankylosaurs and Polacanthus was one of the ankylosaurs that was specifically mentioned because it possessed very unique pelvic dermal armors. Most ankylosaurs have somewhat uniform sheets, scutes, or patches of dermal bone that protect their dorsal surfaces. Polacanthus also has dermal armor along its back; however, the dermal armor along the pelvic region is uniquely constructed and protective of the dinosaur's pelvis and hips. Assuming that, as many ankylosaurs are thought to have defended themselves, Polacanthus made itself small when threats loomed, making it difficult to get at its soft underbelly, the expansive pelvic armor was capable of protecting the hips of the animal quite well as it would have served as an armored roof to that area. In many illustrations it appears as though the hips are still exposed (such as that below); however, in the skeletal reconstructions of Polacanthus we can see fairly well that the actual hip socket lies medial and ventral to the armored shelf of bone resting on the pelvis. In some line illustrations this has been exaggerated slightly, such as in the Nopsca drawing which pulls the shelf more laterally than some others, but these small errors in representation do not change the fact that the armored shelf protected the hips of Polacanthus very well and probably kept the dinosaur safe from most direct bites, slashes, and kicks to the hindlimb which, as we saw with Edmontonia, was most likely used to pivot the front shoulder spikes of Polacanthus in threatening displays or actual offensive strikes at rivals and predators.