Cedarpelta

Cedarpelta is the most basal known ankylosaurid ankylosaur (Ankylosauridae; Ankylosauria), based on material recovered from the Lower Cretaceous of North America. The skull lacks extensive cranial ornamentation, a trait which has been interpreted as plesiomorphic for ankylosaurs.

The scientific name means "Bilbey and Hall's Cedar (Mountain) shield," with the genus named for the Cedar Mountain Formation and the animal's armored plates and the species named for Sue Ann Bilbey and Evan Hall, discoverers of the type locality.

All material referrable to Cedarpelta has been recovered from the Ruby Ranch Member of the Cedar Mountain Formation. Radiometric dates place the site the specimen was collected in the (Albian) of eastern Utah.

Description
Carpenter et al. (2001) diagnose Cedarpelta by the presence of a rostrocaudally elongate pterygoid with a caudolaterally oriented, trochlear-like process, a premaxilla with six conical teeth, and a straight ischium. The presence of premaxillary teeth is a plesiomorphic character because it is present in other, primitive ornithischians. In contrast, closure of the opening on the side of the skull behind the orbit, the lateral temporal fenestra, is an advanced (apomorphic) character only known in ankylosaurid ankylosaurs.

Two skulls are known, and the skull length for Cedarpelta is estimated to have been roughly 60 centimetrts (24 in). Significantly, one of the Cedarpelta skulls was found disarticulated, a first for an ankylosaur skull, allowing paleontologists a unique opportunity to examine the individual bones instead of being limited to a cossified unit.

Holotype
Carpenter et al. (2001) designated CEUM 12360 as the holotype specimen of Cedarpelta bilbeyhallorum (CEUM is the acronym of the College of Eastern Utah Prehistoric Museum in Price, Utah). CEUM 12360 consists of an articulated, incomplete skull lacking the snout and mandibles. Carpenter et al. (2001) also designated a long list of paratype material, mostly isolated bones that could be referred to Cedarpelta bilbeyhallorum.

Phylogenetics
Cedarpelta bilbeyhallorum was believed by Carpenter et al. (2001) to be closely related to Gobisaurus domoculus of north-central China and Shamosaurus scutatus of Mongolia, and they placed the taxon within the Family Ankylosauridae. Vickaryous et al. (2004), however, interpreted the genus as the basalmost member of the Family Nodosauridae, most closely related to the nodosaurids Pawpawsaurus campbelli, Silvisaurus condrayi, and Sauropelta edwardsorum. However, new material of the skeleton confirms the original identification of Carpenter et al. of Cedarpelta being the most primitive ankylosaurid.