This post was updated December 04, 2014 with a revision of the nesting of Lacertulus as a basalmost protosquamate, close to the origin of the Tritosauria and Rhynchocephalia. This was due to the inclusion of taxa to the large reptile tree.
The paradigm in paleontology holds that lepidosaurs (lizards and sphenodontians) and archosaurs (dinosaurs and crocs) both descended from a sister to Youngina. The present reptile family tree tested this traditional hypothesis with many more taxa and found a new tree that kept Youngina with protorosaurs and archosaurs, but moved lepidosaurs over to another major branch, the lepidosauromorpha.
Paliguana, at the base of the Lepidosauriformes
A skull-only taxon, Paliguana, nested at the base of the Lepidosauriformes. It lived 250 mya during the latest Permian or earliest Triassic. It gave rise to a clade of pre-lepidosaurs including Sophineta, Palaegama, Saurosternon and the Triassic gliders.
Paleagama gave rise to the Lepidosauria: including the Rhynchocephalia (Sphenodontia), the Tritosauria and the Protosquamata (including the Squamata).
Lacertulus at the base of the Protosquamata (basal Lepidosauria)
Lepidosauria is now restricted to the last common ancestor of Squamata (lizards, snakes and amphisbaenians) and Rhynchocephalia (or Sphenodontia, represented by Sphenodon), and all descendants of that ancestor (e.g., Gauthier et al., 1988). Unfortunately there are many lepidosaurs that nest in neither of these clades.
Lacertulus, at the base of the Squamata
The living and extinct lizards all descended from a sister to Lacertulus from the Late Permian. It was described as a potential biped, but the nonfusion of the astragalus and calcaneum removed it from the Squamata in the eyes of Carroll and Thompson (1978). Since then, a few more squamates without a fused astragalus and calcaneum have been discovered including Meyasaurus and Huehuecuetzpalli. Along with Lacertulus, some of these nested at the base of a third, previously unidentified, squamate clade, the Tritosauria, which included tanystropheids and pterosaurs. The others nested with the Protosquamata, which includes the Squamata.
The Longevity of Individual Squamate Taxa
Many of these tritosaurs and protosquamates survived into the Jurassic and Cretaceous, but not beyond. All are now extinct. Given the longevity of Sphenodon into the modern era and the long-lived examples of Huehuecuetzpalli and Homoeosaurus, perhaps other lizards known from more recent times will also be found closer to the Permo-Triassic origin of squamates.
Prior “Oldest Known Lizards”
When Protorosaurus was discovered, it was described as the “oldest known lizard.” Here it nested far from the lizards. Paliguana was also described as the “oldest known lizard.” Currently the oldest known fossil lizard is Tikiguania from the Late Triassic, 220 mya. Most other old fossil lizards are known from the Jurassic and Cretaceous.
The most primitive squamate and the oldest squamate
The basal most squamate, Scandensia, lived in the Early Cretaceous, far from its origins in the Permian. A phylogenetic descendant, the TA1045 specimen is an anguimorph scleroglossan that lived in the Early Permian. It is the oldest known squamate.
Carroll and Thompson 1982. A bipedal lizardlike reptile from the Karroo. Journal of Palaeontology 56:1-10.