The Lepidosauria is one of the most successful reptile clades.
Today two living groups are known, the Rhynchocephalia (aka Sphenodontia, represented by Sphenodon), and the Squamata (composed of the Iguania, represented by Iguana (Fig. 1), Draco and Phyronosoma) and the Scleroglossa (represented by Liushusaurus (Fig. 1), Varanus and Heloderma).
In the prehistoric past there was a third lepidosaur clade, the Tritosauria, that nests just outside the Squamata in the large reptile tree. Tritosaurs became extinct at the end of the Cretaceous with the end of the Pterosauria.
Another extinct clade, one more basal to Lepidosaurs, gave rise to the so-called rib-gliders, typified by the kuehneosaurids. Altogether these taxa are considered the Lepidosauriformes with Paliguana (Fig. 1) at the base.
I thought it might be interesting to focus on the basal taxa from each of these clades. Perhaps not surprisingly, they’re quite similar to each other, yet each one gave rise to a variety of derived forms, from pterosaurs to snakes to mosasaurs to tanystropheids to rhynchosaurs to chameleons.
Let’s start at the beginning (more or less) of the Lepidosauriformes
Owenettids, like Owenetta, were basal to Lepidosauriformes according to the large reptile tree. They had a lateral termporal fenestra, but no lower temporal bar and no upper temporal fenestra. Owenettids appear to have been ground dwellers. Owenetta had wide gracile ribs giving it a wide flat body.
Paliguana (basal lepidosauriformes) is known by its skull only. It is the earliest/most basal taxon in this lineage with upper temporal fenestrae.
Long-legged Saurosternon (not shown in Fig. 1) leads a splinter lineage that became arboreal and ultimately produced so-called rib-gliders, Coelurosauravus and the Kuehneosauridae. This taxon and all subsequent forms did not have a wide torso.
Lepidosauria (Sphenodontia + Tritosauria + Squamata)
Gephyrosaurus was basal to rhychocephalians (= sphenodontians) including trilophosaurs and rhynchosaurs. The scapula was more robust and fused to the coracoid. The pelvis had a thyroid fenestra.
Tritosauria + Squamata
Dalinghosaurus was basal to the Tritosauria, a newly identified lepidosaur clade that ultimately gave rise to drepanosaurs, tanystropheids and pterosaurs. Lacertulus (late Permian) and Homoeosaurus were also basal members. Note the relatively longer, stronger hind limbs. Some of these became slow-moving arboreal forms (drepanosaurids). Others remained agile and sometimes bipedal terrestrial forms (fenestrasaurs leading to pterosaurs). Some of these later became aquatic, long-necked and gigantic (tanystropheids).
Iguana is a member of the Iguania and a basal squamate. All four limbs are robust with large unguals. Here we find a shorter neck and slender pubis/enlarged thyroid fenestra. Some were terrestrial, others arboreal. One (marine iguanas) ventured into the sea.
Liushusaurus (Fig. 1) is a basal member of the Scleroglossa and also a basal squamate. The forelimb was as large as the hind limb. Basal forms were terrestrial venturing into arboreal (Gekko). Some of these lost their legs (Lialis). Others lost their legs and became fossorial (burrowers like amphisbaenids). Another clade became fossorial (Heloderma) and also lost their legs (Cylindrophis and kin). Still another clade became marine. Some became giants (mosasaurs). Others from this clade also lost their legs (snakes and kin).
Lepidosaurs crawled, burrowed, climbed, glided, flew and swam. Some were giants. Most were not. Some were extremely tiny as adults (some gekkos and some pterosaurs). Most were cold-blooded. Pterosaurs were covered with fibers and thus were probably warm-blooded. Some laid eggs and ignored them. Others retained eggs until just before hatching. Others bore live young. Some developed a lower temporal bar. Some lost both of their temporal bars. Some sealed up their lower temporal opening.
Each of these clades started off looking pretty much alike (Fig. 1) having descended from a common ancestor and then diversifying via evolution, each according to its own niche and environs.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.