1923 Osborn coined the name Ankylosauria, thereby placing the ankylosaurids in their own suborder. Ankylosauria and Stegosauria are now grouped together within the clade Thyreophora. This group first appeared in the Sinemurian age, and survived for 135 million years until disappearing in the Maastrichtian. They were widespread and inhabited a broad range of environments. As more complete specimens and new genera have been discovered, theories about ankylosaurian interrelatedness have become more complex, and hypotheses have often changed between studies. In addition to Ankylosauridae, Ankylosauria has been divided into the families Nodosauridae, and sometimes Polacanthidae (these families lacked tail clubs). Ankylosaurus is considered part of the subfamily Ankylosaurinae (members of which are called ankylosaurines) within Ankylosauridae. Ankylosaurus appears to be most closely related to Anodontosaurus and Euoplocephalus. The following cladogram is based on a 2015 phylogenetic analysis of the Ankylosaurinae conducted by Arbour and Currie: Because Ankylosaurus and other Late Cretaceous North American ankylosaurids were grouped with Asian genera (in a tribe the authors named Ankylosaurini), Arbour and Currie suggested that earlier North American ankylosaurids had gone extinct by the late Albian or Cenomanian ages of the Middle Cretaceous. Ankylosaurids thereafter recolonized North America from Asia during the Campanian or Turonian ages of the Late Cretaceous, and there diversified again, leading to genera such as Ankylosaurus, Anodontosaurus, and Euoplocephalus. The theory explains a 30-million-year gap in the fossil record of North American ankylosaurids between the ages. Paleobiology Feeding Like other ornithischians, Ankylosaurus was herbivorous. Its wide muzzle was adapted for non-selective low-browse cropping, although not to the extent seen in some related genera, especially Euoplocephalus. Though ankylosaurs may not have fed on fibrous and woody plants, they may have had a varied diet, including tough leaves and pulpy fruits. Ankylosaurus probably fed on abundant ferns and low-growing shrubs. Assuming it was endothermic, Ankylosaurus would have eaten of ferns per day, similar to the amount of dry vegetation a large elephant would consume. The requirements for nutrition could have been more effectively met if Ankylosaurus ate fruit, which its small, cusp-like teeth and the shape of its beak seem well adapted for, compared to for example Euoplocephalus. Certain invertebrates, which the small teeth may have been adapted for handling, could also have provided supplemental nutrition. Fossils of Ankylosaurus teeth exhibit wear on the face of the crown rather than on the tip of the crown, as in nodosaurid ankylosaurs. In 1982 Carpenter