Chapter 1, The Reptilian Spectacle - UWSpace - University of ...

1.5.4 Evolution of the Spectacle
It is of course possible that all theories are valid and that spectacles have evolved for a number of
different reasons, particularly given that they evolved independently several times.
Snakes and geckos are the only two taxa in which the spectacle occurs throughout (again
excepting Eublepharidae) regardless of habitat and ecology. In both case, it is likely that they owe its
presence to their respective common ancestors. The majority of geckos are predominantly nocturnal,
suggesting nocturnality to be ancestral, which is in line with assertions that nocturnal species are more
likely to have spectacles to protect the eyes from injury in low-light conditions. The absence of
spectacles in Eublepharidae has thus been suggested as an ancestral trait of the taxon (Kluge 1967;
Kluge 1987). Jonniaux and Kumaza (2008) suggest, based on molecular evidence, that spectacles
either evolved independently in non-eublepharid geckos and sister taxon Pygopodidae or that eyelids
evolved independently in Eublepharidae from a spectacled ancestor, as a reversal to a more primitive
form. The significance of this latter scenario could not be overstated considering the need to re-evolve
analogous musculature and innervation for eyelid opening and closure as well as the necessary
secretory glands responsible for maintaining a tear layer. A thorough comparative study of the anatomy
and innervation of the eyelids of these geckos, as well as the source(s) of the components of their tear
layer, to ascertain similarity to or divergence from typical lacertilian eyelids, would be helpful in
verifying the stage of spectacle evolution at which this reversal might have occurred.
The oldest fossil snake found dates to ~100 mya and several studies of genomic divergence
have placed the earliest snake at 109-160 mya (reviewed in Vidal et al. 2009), so we may never know
or fully understand what prompted their common ancestor to evolve a spectacle. Walls (1940, 1942)
put forward a strong argument for the original snake having fossorial habits, based not only on the
most primitive extant snakes, the Scolecophidia, being fossorial with extremely reduced eyes, but also
on the unusual ocular anatomy of all snakes, which suggests a reduction in the need for acute vision
occurred at some juncture in their evolution. This would have been followed by the redevelopment of
functionally-analogous intraocular structures after abandoning the subterranean existence. While part
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