The sphenodontian Ankylosphenodon pachyostosus new genus and species is the second unusual sphenodontian reported from the Albian deposits of the Tlayua Formation, near Tepexi de Rodríguez, Central Mexico. The skeleton is robust with pachyostotic ribs and vertebrae. Unique long teeth are ankylosed deep into the lower jaw extending close to the edge of the Meckelian canal. Long teeth with open roots, the lack of worn-out teeth, and the presence of posterior wear surfaces exhibiting dentine suggest that tooth growth was continuous. These features combined with a propalinal action of the deep lower jaw suggest herbivory. Herbivorous specializations of Ankylosphenodon are different from the laterally expanded teeth of Toxolophosaurus and Eilenodon and may have evolved to prevent total tooth loss, a feature which is observed in sapheosaurs. A pachyostotic skeleton, a delay of the ossification of the epiphyses, and a solid structure of the vertebral column could be related to a none obligated aquatic behavior. These specializations differ greatly from those of other aquatic sphenodontians such as Pleurosaurus and Palaeopleurosaurus. A stout skeleton with swollen horizontal zygapophyses suggests affinities with sapheosaurs; however, cladistic analysis support sister-group relationship with Toxolophosaurus and/or Eilenodon on the basis of shared propalinal jaw action and deep jaws. The presence of two unique sphenodontians in the Tlayua Quarry suggests the presence of an area for the diversification of lepidosaur reptiles. The late presence of sphenodontians in the Albian also suggests that this area was a refuge for archaic forms at the time.

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