The scientific consensus is that snakes evolved from lizards – and that they had hindlimbs for millions of years during the transition. But the reason snakes have long been considered a separate kind of reptiles is because of other features they have which lizards do not. This raises the question, how much difference does it take to be considered something other than a legless lizard?
Amphisbaenians are another snake-like lineage descended from lizards. One key difference is that whereas snakes have a reduced left lung, amphisbaenians have a reduced right lung. For a long time they were considered a third squamate group, alongside the lizards and snakes, since they seem so different from both.
Then there are the legless lizards, many of which move in very snake-like ways, but are not (yet) different enough from the other lizards to be considered another “kind.” All of the Pygopodidae, all of the Dibamidae, the monogeneric Anniellidae; several genera of skinks from three different subfamilies, two of which comprise mostly skinks with limbs; one of the three subfamilies of Anguidae. yet unlike the snakes and amphisbaenians, these were never classified as different “kinds” of reptiles, but were regognized as types of lizards.
Even stranger is that the snakes are phylogenetically closer to agamids and iguanids than they are to the amphisbaenians and the scinciforms, and further still from the Gekkota, in which the Pygopodidae are. The Dibamidae seem to be basal to all the others, therefore the group most eligible to be a separate “kind.” So evolving into “snakes” appears to be a general “lizard thing” to do, occurring independently in different lineages, rather than a characteristic of a specific lineage.
I have heard of the term “carcinization,” which refers to the evolutionary tendency of various marine crustaceans to evolve into crab-like body shapes. I suppose there could be a similar term for this tendency of lizards to evolve into snake-like forms. “Ophidization”?
Nowadays we use DNA to draw out evolutionary trees, and so the snakes and amphisbaenians are nested in the lizard tree. Still, given a few more million years of evolution, one could conceive of, say, the glass lizards or the legless skinks becoming as diverse and divergent as the snakes are today, enough that a Linnean-type taxonomist might classify them as separate orders. How different would they have to be from mainstream lizards for this to happen?