Tianzhenosaurus (meaning “Tianzhen lizard”) is a monospecific genus of ankylosaurid dinosaur from the Shanxi Province that lived during the Late Cretaceous (Cenomanian-Campanian, ~99-71 Ma) in what is now the Huiquanpu Formation.[1] Tianzhenosaurus may represent a junior synonym of Saichania, an ankylosaurine known from the Barun Goyot and Nemegt Formation.[2]
In 1983, Pang Qiqing and Cheng Zhengwu discovered articulated cervical vertebrae of an ankylosaurid from the Shanxi Province.[1] Numerous excavations at the site yielded more than 2,300 specimens belonging to sauropods, theropods, ornithopods and ankylosaurid specimens.[1] The holotype specimen, HBV-10001, consists of a partial skull.[1] Two paratype specimens were assigned to Tianzhenosaurus: HBV-10002, an incomplete mandible; HBV-10003, cervical vertebrae, dorsal vertebrae, caudal vertebrae, a sacral complex, ilia, pectoral girdles, pelvic girdles, fore and hind limbs with fore and hind feet, tail club and osteoderms.[1] The holotype and paratype specimens were obtained from the Huiquanpu Formation and are housed at the Geoscience Museum, Hebei GEO University, Shijiazhuang.[1]
The generic name, Tianzhenosaurus, is derived from the Tianzhen County and the Greek word “sauros” (lizard).[1] The specific name, youngi, is in honour of the late Professor Yang Zhongjian ("C. C. Young"), the founder of Vertebrate Paleontology in China.[1]
In 1999, Sullivan considered Tianzhenosaurus as a junior synonym of Saichania based on the skulls being similar in overall morphology.[3] Additionally, Sullivan also considered Shanxia as a junior synonym of Tianzhenosaurus as the only diagnosable characteristic of Shanxia is known to be variable within a single taxon such as Euoplocephalus.[3] Arbour & Currie (2015) also reaffirmed Tianzhenosaurus as a junior synonym of Saichania based on similar reasons provided by Sullivan (1999), but noted that if the humerus of Tianzhenosaurus is later shown to differ from that of Saichania, then could be considered as a distinct taxon.[2]
Pang and Cheng, 1998 distinguished Tianzhenosaurus from all other ankylosaurids based on the following traits: A flat, low, medium-sized, isosceles triangle shaped skull; skull roof covered with irregular bony tubercles; a relatively long premaxilla; a small orbit surrounded by a bony ring; horizontally elongates narial openings; septomaxilla not separating the narial openings; maxillary tooth rows slightly convergent posteriorly; short basicranium; maxilloturbinal located laterally in middle part of the palatal vault; vertical occipital region; narrow and high occipital condyle; occipital condyle not visible in dorsal view; opisthotic extends lateroventrally as a curved process; mandible is deep with a convex ventral border; no mandibular ornamentation; tooth crowns have cingula on labial sides, swollen bases, and middle ridge on lingual sides; short, amphicoelous cervical centrum; dorsal centrum is long and flat at both ends; eight fused vertebrae in sacrum; short and thick anterior caudal vertebrae; narrow and elongated posterior caudal vertebrae that ended with a tail club; rectangular, plate-like scapula; proximal and distal ends of humerus are moderately expanded and not twisted; thick femur lacking the fourth trochanter; tarsometatarsal and digits that are typical for ankylosaurs.[1]