Tarchia

Tarchia (meaning "brainy one") is a genus of ankylosaurid dinosaur from the late Cretaceous of Mongolia. It is currently the geologically youngest known of all the Asian ankylosaurid dinosaurs and is represented by five or more specimens, including two complete skulls and one nearly complete postcranial skeleton. It was discovered in the Upper Cretaceous (possibly Campanian-Maastrichtian) Barun Goyot Formation (previously known as the 'Lower Nemegt Beds') of the Nemegt Basin of Mongolia. It had a bony tail club, like many ankylosaurids.

Description
Tarchia is one of the largest known Asian ankylosaurs, with an estimated body length of 8 to 8.5 metres (26 to 28 ft), a skull length of 40 centimetres (16 in), and skull width of 45 centimetres (18 in). Tarchia may have weighed as much as 4.5 tonnes (5.0 short tons).

Named for its massive skull (Mongolian tarkhi meaning 'brain' and Latin ia), Tarchia currently includes only the type species, T. kielanae. The rocks in which they were found likely represent eolian dunes and interdune environments, with small intermittent lakes and seasonal streams. Hence, we know that Tarchia was a desert animal. The morphology of cranial sculpturing seen in Tarchia, an assortment of bulbous polygons, is reminiscent of that of Saichania chulsanensis, another ankylosaurid from the Barun Goyot Formation.

Tarchia is distinguished from Saichania on the basis of its relatively larger basicranium, an unfused paroccipital process-quadrate contact and that the premaxillary rostrum is wider than the maximum distance between the tooth rows in the maxillaries. In Tarchia, wear facets indicative of tooth-to-tooth occlusion is present (Barret, 2001).

taxonomy and phylogenetics
Vickaryous et al. (2004) state that two distinct clades of Late Cretaceous ankylosaurids are nested deep to Tarchia, one comprising North American taxa (Ankylosaurus, Euoplocephalus) and the other comprising Asian taxa (Pinacosaurus spp., Saichania, Tianzhenosaurus, Talarurus).

Dyoplosaurus giganteus was formerly considered a senior synonym of Tarchia kielanae, but recent study indicates of D. giganteus is indistinguishable from other ankylosaurs from the late Campanian-Maastrichtian of Mongolia, and hence a nomen dubium.