Pinacosaurus (meaning "plank lizard") is an extinct genus of ankylosaurid dinosaur that lived in what is now Mongolia and China during the Late Cretaceous (Campanian, roughly 75 to 71 million years ago). The genus contains two species; the type species, Pinacosaurus grangeri, was named in 1933 based on fossils found in 1923. A second species, Pinacosaurus hilwitnorum, was named in 2026. An additional species, originally placed in this genus, P. mephistocephalus, was later transferred to its own genus, Eopinacosaurus. At least 24 Pinacosaurus skeletons have been found, possibly more than of any other ankylosaur. These predominantly consist of juveniles. Adult fossils have not been found in groups. Pinacosaurus was a medium-sized ankylosaurine, about 5 metres (16 ft) long and weighed up to 1.9 metric tons (2.1 short tons). 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.
Discovery and naming
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. In 1933, Charles Whitney Gilmore named and described the type species Pinacosaurus grangeri. The generic name is derived from Greek πίναξ, pinax, "plank", in reference to the small rectangular scutes covering the head. The specific name honours Granger, who accompanied the 1923 expedition as a paleontologist. The holotype, AMNH 6523, was found in a layer of the Djadokhta Formation, dating from the Campanian. It consists of a partially crushed skull, lower jaws, the first two neck vertebrae, and dermal bones collected in 1923. The skull is still the largest known of the genus.
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. grangeri by Buffetaut (1995). Pinacosaurus is the best known Asian or worldwide ankylosaur with numerous specimens having been discovered. From the original Flaming Cliffs or Shabarakh Usu several other fossils have been reported including ZPAL MgD II/1: a nearly complete skeleton; ZPAL MgD II/9: a postcranial skeleton; ZPAL MgD II/31: the handle of a tail club; and PIN 3780/3: a skull; PIN 614: a nearly complete postcranial skeleton (= Syrmosaurus viminocaudus); and possibly MPC 100/1305, a postcranial skeleton erroneously described in 2011 as belonging to Saichania. At another site, Alag Teeg now considered part of the Alagteeg Formation, entire bonebeds have been uncovered of juvenile animals. Soviet-Mongolian expeditions in 1969 and 1970 reported thirty skeletons. Mongolian-Japanese expeditions added another thirty between 1993 and 1998. Forty were reported by Canadian expeditions between 2001 and 2006. The remains have not been all dug up and it is possible the reports partly pertain to the same material.
In Inner Mongolia at the Bayan Mandahu Formation, the Canada−China Dinosaur Project in 1987, 1988, and 1990 found specimens IVPP V16853: a skull with cervical halfrings; IVPP V16283: a partial skull, IVPP V16854: a nearly complete skeleton; IVPP V16346: a partial skull; and IVPP V16855: a skeleton. Other, as yet undescribed material included two finds of several juveniles huddled together, evidently killed by a sandstorm. 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. Because of the many finds, in principle the entire juvenile skeleton is known. Pinacosaurus especially provides information on the build of the ankylosaurian skull, as in the juveniles the head armour has not yet fused with the skull proper and the sutures of the various elements are still visible. Modern studies have not yet fully covered the abundance of data. A well-preserved juvenile skull was described by Teresa Maryańska in 1971 and 1977. In 2003, Robert Hill studied the juvenile specimen IGM 100/1014. In 2011, Currie published a study on the hand and foot, body parts often incompletely known with other ankylosaurs. The same year Michael Burns dedicated an article to four juveniles from the Bayan Mandahu. Also in 2011, the postcranial skeleton MPC 100/1305 was described in detail, though at the time referred to Saichania. Most recently, Michael Burns and colleagues described and illustrated the original Alag Teeg material from the Soviet-Mongolian expeditions in 1969 and 1970.
Additional species and synonyms
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