Pinacosaurus

***Pinacosaurus*** ("plank lizard") is a genus of medium-sized ankylosaur dinosaurs that lived from the late Santonian to the late Campanian stages of the late Cretaceous Period (roughly 80-75 million years ago), in Mongolia and China.

The type species *Pinacosaurus grangeri* was named in 1933. *Pinacosaurus mephistocephalus* named in 1999, is a second possibly valid species, differing from the type species in details of the skull armour. Of *Pinacosaurus grangeri* many skeletons have been found, more than of any other ankylosaur. These predominantly consist of juveniles that perhaps lived in herds roaming the desert landscape of their habitat.

*Pinacosaurus* was about five metres long and weighed up to two tonnes. Its body was flat and low-slung but not as heavily built as in some other members of the Ankylosaurinae. The head was protected by bone tiles, hence its name. Each nostril was formed as a large depression pierced by between three and five smaller holes, the purpose of which is uncertain. A smooth beak bit off low-growing plants that were sliced by rows of small teeth and then swallowed to be processed by the enormous hind gut. Neck, back and tail were protected by an armour of keeled osteoderms. The animal could also actively defend itself by means of a tail club.

## Description

### Size and distinguishing traits

*Pinacosaurus* was a lightly built, medium-sized ankylosaur that reached a length of five meters (sixteen ft). Paul estimated the weight of a five metres long animal at 1.9 tonnes.[2] The postcranial skeleton PIN 614 measures 366 centimetres from the first neck vertebrae to the end of the tail. In 2014, Arbour established some distinguishing traits of the genus. The upper snout armour does not consist of distinct tiles, *caputegulae*, but of a fused mass. Adult individuals have a skull that is longer than wide. This trait is shared with the distant relatives *Gobisaurus* and *Shamosaurus*, but *Pinacosaurus* differs from those in the possession of extra openings in the nostril and a pointy protruding *caputegula* on the prefrontal, directed to the front. *Pinacosaurus* differs from *Crichtonpelta* in the lack of an ornamented rear edge of the skull roof and in the cheek horn not being curved upwards.

Arbour also provided a list of traits in which *P. grangeri* and *P. mephistocephalus* differed from each other. *P. grangeri* has a notch in the snout armour just above the innermost nostril opening. *P. mephistocephalus* has squamosal horns extending to behind beyond the rear of the skull roof, their points representing the widest point of the skull, instead of the upper rims of the eye sockets. *P. mephistocephalus* also has a clear transverse narrowing of the skull roof at level of the lacrimals, just in front of the eye sockets. It had been suggested that the rear skull roof of *P. mephistocephalus* was more convex but Arbour concluded it essentially had the same curvature. The holotype of *P. mephistocephalus* has very long cheek horns but a juvenile specimen, MPC 100/1344, found as part of a *P. grangeri* group, shows a similar elongation.

### Osteoderms

The configuration of the skin ossifications, or osteoderms, of the body is partly known: no single specimen conserves a complete set. Additional information can be gleaned from the larger specimen MPC 100/1305, a possible *Pinacosaurus* exemplar. The neck is protected by two cervical halfrings, consisting of keeled rectangular segments fused to an underlying bone band. This band is completely ossified even in juvenile individuals. Godefroit assumed *Pinacosaurus* differed from other species in having three or four segments instead of the usual six, but Arbour concluded that the normal number was in fact present. The sides of the rump and the tail were occupied by moderately long, flat, recurved, triangular spikes. Parallel rows of smaller oval keeled osteoderms were present on the back. A continuous "sacral shield" on the hip, made of fused plates, is absent.