You don’t find very much
about Ankylosphenodon pachyostosus (Reynoso 2000) online, even though it is odd and known by a complete skeleton. It’s a basal rhynchocephalian, nesting between Gephyrosaurus and Marmoretta in the large reptile tree. The long torso and tail, along with those small limbs gives credence to the possibility that Marmoretta and Megachirella were similarly equipped (they are known from a skull and anterior torso only) at the base of the Pleurosauridae, which we looked at earlier here.
Middle Cretaceous, Mexico, considered an herbivore. Martinez et al. 2013) nested Ankylosphenodon between Sphenodon and Clevosaurus and Sphenotitan, but Marmoretta, Megachirella, and Gephyrosaurus were not included in their taxon list and neither were a host of derived rhynchocephalians. `
From Wikipedia (translated from Italian, I think).
“This animal had a body rather robust, with short legs positioned at the sides of the body and a skull from the remarkable features. These included a set of teeth unusual: there were, in fact, long teeth roots open, deeply “embedded” in the jaw and placed in the vicinity of the channel Meckel . Another feature dell’anchilosfenodonte was given by significant thickening of the ribs and vertebrae ( pachiostosi ), a feature that normally is found in aquatic vertebrates.”
Reynoso VH 1996. Early Cretaceous Lepidosaurs (Reptilia: Diapsid) from Central Mexico and the Phylogeny of Lepidosauromorphs. 369 pp. Unpublished PhD Thesis, McGill University, Montreal, Canada.
Reynoso VH 2000. An unusual aquatic sphenodontian (Reptilia: Diapsida) from the Tlayua Formation (Albian), central Mexico. Journal of Paleontology 74:133-148.