Cedarpelta

DiscoveryEdit

In 1990, Sue Ann Bilbey and Evan Hall discovered a quarry with the remains of ankylosaurs near the Price River in Carbon County, Utah. In 1998, the discovery was reported in the scientific literature. In 2001, the type species Cedarpelta bilbeyhallorum was named and described by Kenneth Carpenter, James Kirkland, Donald Burge and John Bird. 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 — from the Greek pelte, "small shield" — and the specific name honouring Sue Ann Bilbey and Evan Hall as the discoverers of the type locality.

Cedarpelta is known from remains recovered at the CEM and Price River II quarries (PR-2) in eastern Utah; these sites were originally thought to be within the Ruby Ranch Member of the Cedar Mountain Formation, but are now assigned to the base of the overlying Mussentuchit Member, dating to between 116 and 109 million years old (approximately the Aptian-Albian boundary).

HolotypeEdit

Carpenter et alii (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.

In 2008, Carpenter e.a. referred additional material found in Emery County. This includes previously unknown hindlimb elements.

DescriptionEdit

Cedarpelta was a large ankylosaurian. In 2010, Gregory S. Paul indicated a body length of seven metres and a weight of five tonnes. Kenneth Carpenter estimated it smaller at five metres.

Carpenter et al. (2001) established several distinguishing traits of Cedarpelta. The body of the praemaxilla, the front snout bone, is short in front of its nasal branch. The outer sides of the two praemaxillae run more parallel compared to the snouts of later forms which are strongly diverging to behind. The cutting edge of the bone core of the upper beak is limited to the front of the praemaxilla. Each praemaxilla has six (conical) teeth. The quadrate, and with it the entire back of the skull, is inclined to the front. The head of the quadrate is not fused with the paroccipital process, contrary to the situation in Shamosaurus. The neck of the occipital condyle is long and sticking out to behind, like with nodosaurids, not obliquely to below as in typical ankylosaurids. The tubera basilaria, appending processes of the rear lower braincase, form a large wedge directed to below. The pterygoid is elongated from the front to the rear and has a saddle-shaped process on its outer edge oriented to behind and sideways. The coronoid process of the rear lower jaw has an oval process at the inside. The straight ischium has a knob-shaped boss at the inside near the pubic pedicle.

Cedarpelta shows a mix of basal ("primitive") and derived traits. The presence of premaxillary teeth is a plesiomorphic character because it is inherited from earlier Ornithischia. In contrast, closure of the opening on the side of the skull behind the orbit, the lateral temporal fenestra, is an advanced, derived (apomorphic) character only known in ankylosaurid ankylosaurians.

Two skulls are known, and the skull length for Cedarpelta is estimated to have been roughly 60 centimetres (24 in). 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 an ossified unit. The skull is relatively elongated and does not show a strongly appending beak. Of the conical premaxillary teeth, the first is the largest. The maxilla bears eighteen teeth. The eye socket is surrounded by the lacrimal, a single supraorbital and a large postorbital, excluding the prefrontal and the jugal from the orbital rim. The postcranial skeleton was in 2001 not described in any detail.

The skulls, though of large and thus not juvenile individuals, do not show a distinctive pattern of fused caputegulae, head tiles. This inspired Carpenter to propose an alternative hypothesis of ankylosaur skull osteoderm formation. Formerly, it had been assumed that such armour plates were either formed by direct skin ossification into distinct scutes which later fused to the skull (the more popular theory), or by a reaction of the skull bones to the pattern of overlying scales. The lack of a clear pattern in Cedarpelta suggested to Carpenter that the ossification took place in an intermediate layer between the scales and the skull roof itself, which he surmised to have been the periosteum.

PhylogeneticsEdit

Carpenter et al. (2001) placed the taxon within the family Ankylosauridae. They offered two interpretations of the position of Cedarpelta bilbeyhallorum in the evolutionary tree. The first was that it could be the basalmost known ankylosaurid, i.e. the first discovered branch to split off from the ankylosaurid stem line. This would be in line with its plesiomorphic traits and the fact that the in 2001 supposed Barremian age made it one of the oldest known ankylosaurids. The second was that it formed an early ankylosaurid branch, or clade, Shamosaurinae together with Gobisaurus domoculus of north-central China and the eponymous Shamosaurus scutatus of Mongolia.

Vickaryous et al. (2004), however, interpreted the genus as the basalmost member of the family Nodosauridae, positioned even below 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 one of the most basal ankylosaurids. This was also a result of an analysis by Victoria Megan Arbour recovering Cedarpelta just above Gastonia, the most basal ankylosaurid in her study. No tail club of Cedarpelta is known, but Arbour stressed that early ankylosaurids might well have lacked a true club.