Palaeopleurosaurus posidoniae (Carroll 1985) Early Jurassic (Toarcian, 183 mya) 55 cm snout-vent length was a basal rhynchocephalian derived from Gephyrosaurus and Ankylosphenodon and not directly related to Pleurosaursus. So these pleurosaurs are convergent in their aquatic adaptations.

Distinct from the larger Ankylosphenodon, Paleopleurosaurus had a narrower cranium and smaller limbs.

Carroll RL 1985. A pleurosaur from the Lower Jurassic and the taxonomic position of the Sphenodontida. Palaeontographica Abteilung A, 189:1-28.

wiki/Palaeopleurosaurus (note the different spelling)