Anodontosaurus was a medium-sized ankylosaurid that was quadrupedal, ground-dwelling and herbivorous. Like other ankylosaurs, Anodontosaurus has armor on a majority of the dorsal surfaces of its body. It has a wide, pointed tail club at the end of its armored tail. The skull features postocular caputegulae, which are small polygonal plates of bone that are present on the cranium and are situated to the immediate rear of the eye. Coombs supported the assertion that specimen AMNH 5266 represented a juvenile by citing that the vertebral centra were not fused to their neural arches, and that sacral ribs were likewise not fused to vertebrae and to the ilium. Other morphological characters supporting that this is a juvenile specimen include long bones that feature smooth surfaces, which are not marked by the rugosities characteristic of adult bone; the head of the femur is less spherical in shape and is clearly delimited from the adjacent part of the femoral shaft; the distal ends of the tibia and the fibula are not fused to the astragalus and the calcaneum; and the ungual phalanx of the manus is not widest at the proximal articular end as is observed in adults.
Classification
In 1971, Walter Coombs concluded that there was only one species of ankylosaurid during the Campanian age of the Late Cretaceous of North America. He synonymised the species Anodontosaurus lambei, Dyoplosaurus acutosquameus, and Scolosaurus cutleri with Euoplocephalus tutus. The synonymisation of Anodontosaurus lambei and Euoplocephalus tutus was generally accepted and thus CMN 8530 was assigned to E. tutus. However, following the redescription of Dyoplosaurus as a valid genus by Arbour et al., In an abstract presented at the SVP 2010 conference, Victoria Arbour considered Anodontosaurus distinct from Euoplocephalus in distinctive skull and cervical half ring ornamentation, as well as tail club morphology, including the presence of pointed, triangular knob osteoderms in Anodontosaurus. She therefore reassigned all Horseshoe Canyon Formation ankylosaurine specimens previously referred to Euoplocephalus to Anodontosaurus. The validity of Anodontosaurus was formalized in three studies. The first, published by Paul Penkalski and William T. Blows in 2013, re-validated Scolosaurus as well. The second study, by Penkalski, named and described Oohkotokia from Montana on the basis of remains that were originally thought to be referable to Euoplocephalus. Penkalski performed a small phylogenetic analysis of some ankylosaurine specimens. The only Anodontosaurus specimen that was included in this analysis was its holotype. Anodontosaurus was placed in a polytomy with the holotype of Euoplocephalus and some specimens that are referred to it, while Oohkotokia was placed in a clade with Dyoplosaurus, and specimens that are thought to represent either Dyoplosaurus or Scolosaurus. In a study based on the results of her 2010 SVP abstract, Arbour along with Philip Currie formalized the revalidation of Anodonotsaurus, and one specimen from the Dinosaur Park Formation, TMP 1997.132.1, was referred to Anodontosaurus extending the stratigraphic range of the genus back a few million years. The DPF specimen, however, was later made the holotype of a new species, A. inceptus. The following cladogram is based on a 2015 phylogenetic analysis of the Ankylosaurinae conducted by Arbour and Currie:
