| Anodontosaurus | |
|---|---|
 | |
| A. lambeiholotype skull CMN 8530 | |
 | |
| Life restoration | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Thyreophora |
| Clade: | †Ankylosauria |
| Family: | †Ankylosauridae |
| Subfamily: | †Ankylosaurinae |
| Tribe: | †Ankylosaurini |
| Genus: | †Anodontosaurus Sternberg,1929 |
| Type species | |
| †Anodontosaurus lambei Sternberg, 1929 | |
| Other species | |
Anodontosaurus is anextinctgenus ofankylosauriddinosaurs within the subfamilyAnkylosaurinae. It is known from the entire span of theLate CretaceousHorseshoe Canyon Formation (mid LateCampanian to "middle"Maastrichtian stage, about 72.8-67 Ma ago) of southernAlberta,Canada, and is also known from theDinosaur Park Formation (75.6 Ma ago). It contains twospecies,A. lambei andA. inceptus.[1][2][3]
Anodontosaurus was named byCharles Mortram Sternberg in1928, based onholotypeCMN 8530, a partially preservedskeleton including theskull,half ring,armor and otherpostcranial remains.[4] The badly crushed[1] skeleton was collected by Sternberg in 1916 from aCanadian Museum of Nature quarry, 8 miles southwest ofMorrin.[4] It was collected from the upper part of the LowerHorseshoe Canyon Formation (unit 2), dating to the latestCampanian to the earliestMaastrichtian stage of theLate Cretaceousperiod, about 71-70.2 million years ago.[1][5] Thegeneric name means "toothless lizard" inAncient Greek. It was inspired by the fact that compression damage to the specimen had removed the teeth, at the same time shifting various flat round elements below the skull and on top of the left lower jaw, misleading Sternberg into assuming that large "trituration plates" had replaced the normal dentition.[6] Thespecific name,lambei, honoursLawrence Morris Lambe, theCanadiangeologist andpalaeontologist from theGeological Survey of Canada where the holotype was reposited.[4]
In 1986 Coombs examined specimen AMNH 5266, at the time by him referred toEuoplocephalus, and determined that it was a juvenile. It consists of five vertebralcentra, aneural arch, one dorsal and two sacral ribs, the rightischium, the complete right hindlimb, the rightpes, an incomplete left pes, and various other fragments. AMNH 5266 was discovered in 1912 at Red Deer River and was collected by Barnum Brown with assistance from Peter Kaisen, George Olsen, and Charles M Sternberg in the sediments from the Horseshoe Canyon Formation.[7]
Anodontosaurus was a medium-sized ankylosaurid, measuring up to 4–5 metres (13–16 ft) in length and 2 metric tons (2.2 short tons) in body mass.[8][9] It was a quadrupedal, ground-dwelling herbivore. 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.[1]
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 (a) long bones that feature smooth surfaces, which are not marked by the rugosities characteristic of adult bone; (b) the head of the femur is less spherical in shape and is clearly delimited from the adjacent part of the femoral shaft; (c) the distal ends of the tibia and the fibula are not fused to the astragalus and the calcaneum; and (d) the ungual phalanx of the manus is not widest at the proximal articular end as is observed in adults.[7]
In 1971,Walter Coombs concluded that there was only one species of ankylosaurid during theCampanian age of the Late Cretaceous of North America. He synonymised the speciesAnodontosaurus lambei,Dyoplosaurus acutosquameus, andScolosaurus cutleri withEuoplocephalus tutus.[10] The synonymisation ofAnodontosaurus lambei andEuoplocephalus tutus was generally accepted and thus CMN 8530 was assigned toE. tutus.[6] However, following the redescription ofDyoplosaurus as a valid genus by Arbouret al. (2009),[5] in an abstract presented at the SVP 2010 conference,Victoria Arbour consideredAnodontosaurus distinct fromEuoplocephalus in distinctive skull and cervical half ring ornamentation, as well as tail club morphology, including the presence of pointed, triangular knob osteoderms inAnodontosaurus. She therefore reassigned all Horseshoe Canyon Formation ankylosaurine specimens previously referred toEuoplocephalus toAnodontosaurus.[2]
The validity ofAnodontosaurus was formalized in three studies. The first, published byPaul Penkalski andWilliam T. Blows in 2013, re-validatedScolosaurus as well.[11] The second study, by Penkalski (2013), named and describedOohkotokia fromMontana on the basis of remains that were originally thought to be referable toEuoplocephalus. Penkalski (2013) performed a smallphylogenetic analysis of someankylosaurine specimens. The onlyAnodontosaurus specimen that was included in this analysis was its holotype.Anodontosaurus was placed in apolytomy with the holotype ofEuoplocephalus and some specimens that are referred to it, whileOohkotokia was placed in a clade withDyoplosaurus, and specimens that are thought to represent eitherDyoplosaurus orScolosaurus.[1] In a study based on the results of her 2010 SVP abstract, Arbour along with Philip Currie formalized the revalidation ofAnodontosaurus, and one specimen from the Dinosaur Park Formation, TMP 1997.132.1, was referred toAnodontosaurus extending the stratigraphic range of the genus back a few million years.[12] The DPF specimen, however, was later made the holotype of a new species,A. inceptus.[3]The followingcladogram is based on a 2015phylogenetic analysis of the Ankylosaurinae conducted by Arbour and Currie:[13]
