Redescription of Gastonia burgei (Dinosauria: Ankylosauria, Polacanthidae), and description of a new species

The polacanthid ankylosaur Gastonia burgei, from the Lower Cretaceous Cedar Mountain Formation, Utah, USA, is described in detail for the first time and compared with material from a stratigraphically higher monospecific Gastonia bone bed. Taphonomy and sedimentology of the bone bed place the site within the lower part of the Ruby Ranch Member and suggest mass mortality, either due to drought or drowning while crossing a swollen river. Burial of the scavenged and disarticulated bones was by crevasse splay. Gastonia is characterized by a skull that is subtriangular in dorsal view, being nearly as wide as long, a cranial surface with a pustulate texture, and anteroventrally projecting basipterygoid processes. In the postcrania, the scapula has a well-developed, arcuate acromion flange that attaches to the midshaft. The body armor includes laterally projecting tetrahedron osteoderms that are grooved along the posterior sides, dorsoventrally compressed triangular plates along the sides of the body, and coossified pelvic shield of large, raised osteoderms surrounded by rosettes of smaller osteoderms. A new species of Gastonia, G. lorriemcwhinneyae n. sp., is identified from the bone bed material. It differs from Gastonia burgei in having a flat skull roof, short paroccipital processes that is proportionally less expanded distally, short postacetabular process of ilium that is only 36% of the length of the preacetabular process as compared to 56% in G. burgei, and an ischium that is smoothly curved ventromedially without kink at its midpoint. Its discovery adds a new species of ankylosaur to the Polacanthidae family found in the Cedar Mountain Formation.