Ankylosphenodon is an extinctgenus ofsphenodontian known from multiple specimens recovered from theEarly Cretaceous deposits of theTlayúa Formation, nearTepexi de Rodriguez,Mexico.^[1] It is likely part of a sphenodontid lineage that also includesEilenodon andToxolophosaurus, though has similarities to thesapheosaur clade. Its skeleton ispachyostotic (thickened) skeleton, believed to be an adaptation for a semi-aquatic lifestyle. Its teeth appear to have grown continuously, like inrodents.

Ankylosphenodon is known from multiple specimens recovered from theTlayúa Formation, avertebrate-richLagerstätte nearTepexi de Rodriguez, in CentralMexico. Theholotype (IGM 7441) is a partial skeleton, consisting of a skull, the left forelimb, and the front half of the presacral vertebral column. Another specimen, IGM 7443, preserves most of the right forelimb and the rear half of the body, minus the tail.^[1] All specimens were recovered from the Tlayúa Quarry, which, represents the MiddleMember of theTlayúa Formation,^[2] aLagerstätte rich in vertebrate fossils.^[3] On collection, the specimens were transported to the Geological Institute of theNational Autonomous University of Mexico. In 2000, Víctor-Hugo Reynoso formally described the genus. The generic name comes fromSphenodon, the generic name of the moderntuatara, and the Greek ἀγκύλος (ankylos), meaning "fused" or "bent", referring to the thickening of its ribs and vertebrae.^[1]

The strict consensus tree in Reynoso's paper onAnkylosphenodon recovers it as part of a small, unnamed clade withinSphenodontidae, one that also includesEilenodon andToxolophosaurus. Similarities withsapheosaurs were noted, though were ascribed toconvergence.^[1] However, Sebastian Apesteguía, in 2005, suggested that it was in fact a sapheosaur.^[4]

Ankylosphenodon had an upper skull length of 8.25 cm (3.25 in). The length of the presacralvertebral column (thecervical anddorsal vertebrae), measured from the first to nineteenth vertebrae, is estimated at 28.26 cm (11.13 in).^[1]

The skull ofAnkylosphenodon is poorly preserved. Assuming the skull andmandible were equal in length, theupper temporal fenestra extended for about half the skull's length. Thepremaxillae are separated, and are the only part of the snout that is preserved in the holotype. The mandible is typical among sphenodonts, though is unusually robust. It is slenderer at the front (anteriorly) than it is at the back (posteriorly). Little in the way of fine details are preserved, save for thedentary. The teeth are unique among sphenodonts, in that they extend deep into the dentary, as far as theMeckelian groove. This is convergent with the condition seen inrhynchosaurs. The teeth ofAnkylosphenodon appear to have grown continuously, similar to some mammals, such asrodents.^[1]

Among sphenodonts,Ankylosphenodon is distinct in havingpachyostotic (thickened)vertebrae and ribs, which were massive compared to those of related genera. Its overall morphology is reflective of semi-aquatic reptiles, such ascrocodiles andmarine iguanas, suggesting that it may have adopted a similar lifestyle and swam similarly to the latter. Like the axial skeletons, the limbs were massive and heavily constructed. The hind limbs were longer than the forelimbs. The fifthpedal digit was smaller and slenderer than the others.Ankylosphenodon's tail was well-adapted tolateral motion, though resisteddorsoventral (up-and-down) motion.^[1]

The palaeoenvironment of the Tlayúa Formation, from which all specimens ofAnkylosphenodon are known, was likely a shallow coastallagoon.^[5] It may have formed part of an island, though a connection to the North American mainland cannot be ruled out.^[6] A certain degree of influence from freshwater environments is indicated by the presence of fossils fromcrocodilians and freshwater turtles.^[6]

• Rhynchocephalia
• Cretaceous reptiles of North America
• Prehistoric reptile genera

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