Brachymeles gracilis, a "large species." From Polillo Biodiversity.
Within the genus there is an inverse relationship between size and degree of elongation. That is, the smallest species, which are chopstick-thin, have the highest numbers of vertebrae. The larger species, what I’ll call, cleverly, "large species," which have functional limbs (First photo), have 32 or 33 vertebrae in front of the sacrum (pre-sacral vertebrae). The others, henceforth referred to as "small species," with very reduced or absent limbs, have approximately 50 pre-sacral vertebrae (Second photo). The totally limbless species B. vermis has 60. Incidentally, the species can easily be divided into two distinct groups; there isn’t a gradual grading of body size and form.
Brachymeles bonitae, a "small species." From Polillo Biodiversity, in case you missed it.
The large species have a trick, or at least a trait. The relative size of the female abdomen grows faster than in males. The individual vertebrae toward the back of the body, and therefore the abdomen, increase in relative size, creating an elongated, more spacious cavity in which eggs are contained.
Diagram of gravid female Brachymeles drawn to scale showing position, size and shape of eggs, and position of the last parasternal rib (PSR).
The small species have more abdominal vertebrae, which would suggest relatively more room to carry eggs, especially if the body segments also became relatively longer in females. But this doesn’t happen. As in a juvenile lizard of either large or small species, the pre-sacral vertebrae remain approximately the same size along the length of the body. (Perhaps functionally related to a reliance on undulatory locomotion?)
Developing eggs have their own requirements and properties, based on physiological needs of the embryo and the evolutionary history of the group to which the lizards belong; one thing: they can only be so large, or so small.
A large species female, possessing what is probably closer to the body form of the common ancestor of the genus, can produce and carry up to six eggs, which are almost spherical and packed within the abdomen as far forward as beyond the last parasternal rib. The slender, small species can carry one or two cylindrical eggs, entirely posterior to the ribs.
So being a pregnant lizard is probably no fun, no matter what shape you are. Beyond that, the evolution of elongation (and this is nowhere near the degree of elongation seen in snakes, which have at least a hundred, in some, several hundred vertebrae -- and not to mention significant rearrangements of internal organs that affect available abdominal volume) is tied to changes in reproductive output, and, possibly other life history traits such as age at reproductive maturity. The small, elongate, limbless species may reproduce at an earlier age, essentially as morphological juveniles, an evolutionary change that may be a form of progenesis. There are no data to test this, unfortunately. Mostly there are pickled Brachymeles in museum jars to x-ray, measure, and muse about.
Is there a take-home message to this post, which seems to have zig-zagged as much as a panicked skink? Perhaps this: things evolve from other things, and not by changing just one thing. Things that change (size, shape, life history) are functionally and developmentally connected and historically limited.Another elongate, burrowing skink, Plestiodon egregius. This one from Florida.
Again, much smaller, more vertebrae, fewer eggs than large Plestiodons. There are rules to building these things, not yet figured out.