Pinacosaurus (meaning "Plank lizard") is a genus of ankylosaurid thyreophoran dinosaur that lived in Asia during the Late Cretaceous (Campanian, roughly 75 to 71 million years ago), mainly in Mongolia and China.

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.

The American Museum of Natural History sponsored several Central Asiatic Expeditions to the Gobi Desert in Mongolia in the 1920s.

Among the many paleontological finds from the "Flaming Cliffs" of the Djadokhta Formation in Shabarakh Usu (Bayn Dzak) were the original specimens of Pinacosaurus, found by Walter W. Granger in 1923.

The generic name is derived from Greek πίναξ, pinax, "plank", in reference to the small rectangular scutes covering the head.

[2][3] In 1923, a well-preserved sacrum with the attached right ilium and part of the presacral rod, caudal vertebrae, a left femur and a dermal scute was collected from the Wangshi Group in Shandong, China by H. C. T'an and Otto Zdansky and was described as Pinacosaurus cf.

At another site, Alag Teeg now considered part of the Alagteeg Formation, entire bonebeds have been uncovered of juvenile animals.

Whereas ankylosaur skeletons have often been preserved laying on their back, most Pinacosaurus juveniles are found on their belly in a resting position, with the legs tucked in.

[10] Yang Zhongjian ("C. C. Young") discovered a new specimen in the Ningxia Province at the Bayan Mandahu Formation, and described it as a new species Pinacosaurus ninghsiensis in 1935.

The specific name is a contraction of Mephistopheles and Greek κεφαλή, kephalè, "head", in reference to the "devilish" squamosal horns.

[15] It was considered a valid species by Robert Hill in 2012, based on the "secondary dermal" (squamosal) horns and narial characteristics.

[17] Pinacosaurus was a lightly built, medium-sized animal, fully grown individuals reached a total length of 5 m (16 ft).

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.

[13] 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.

A typical and remarkable element of ankylosaurine skulls is that the nostril is in the shape of a large "narial vestibule" in which several smaller oval holes are present.

[16] Maryańska in 1977 thought that this was a tabular bone, otherwise unknown in dinosaurs, proving that the Ankylosauria had independently evolved from the Aetosauria,[7] a hypothesis today entirely discarded.

Currie therefore assumed that the lower leg articulated directly with the metatarsus, the inner part of the astragalus and the entire calcaneum being absent or non-ossified cartilage elements.

The neck is protected by two cervical halfrings, consisting of keeled rectangular segments fused to an underlying bone band.

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.

As indicated by Thompson et al. 2012, the difference in the relative position of the two Pinacosaurus species between the respective analyses, is influenced by the fact that the best preserved P. grangeri skulls are from juveniles, while the holotype of P. mephistocephalus is an adult.

[22] The following cladogram is based on the 2015 phylogenetic analysis of the Ankylosaurinae conducted by Arbour and Currie:[23] Crichtonpelta Tsagantegia Zhejiangosaurus Pinacosaurus Saichania Tarchia Zaraapelta Dyoplosaurus Talarurus Nodocephalosaurus Ankylosaurus Anodontosaurus Euoplocephalus Scolosaurus Ziapelta A juvenile specimen of Pinacosaurus preserves large paraglossalia (triangular bones or cartilages located in the tongue) which show signs of muscular stress, and it is thought this was a common feature of ankylosaurs.

Though Pinacosaurus may not have fed on fibrous and woody plants, they may have had a more varied diet, including tough leaves and pulpy fruits.

[19] In the group of juveniles found together at Bayan Mandahu, the individuals were all oriented into the same direction, suggesting they represent a travelling true herd simultaneously killed and covered by a sandstorm.

This could be explained by the larger individuals being able to extract themselves from the sand, leaving the small members of the herd behind but in that case it is strange that no very young animals were found, the smallest being about one metre in length.

The concentration of fossils at Alag Teeg has been explained as caused by a drying pool, but later research showed the sediments were deposited during a flood.

The forelimbs strongly increased in robustness, while the hindlimbs did not become larger relative to the rest of the skeleton, indicating that the arms bore most of the weight.

However its mobile cricoid-arytenoid joint and long arytenoid cartilages would have enabled easy opening of the glottis, thus allowing air-flow control similar to that of birds.

This structure represents the oldest known example of a larynx preserved in a dinosaur, and suggests that Pinacosaurus was likely capable of making loud, possibly bird-like vocalizations.

It has been suggested that the relatively light build of Pinacosaurus was an adaptation to gain agility to better fight small theropods, the moderately large club being fast enough to hit these swift targets.