Embryology reveals a morphological signature of ancestral diurnality maintained in a nocturnal lineage.

Evolutionary transitions to diurnality are often associated with specialized ocular morphology, such as pits (foveae) in the retina. Foveae are required for high visual acuity and are found primarily in diurnal vertebrate taxa, including lizards. Geckos have undergone repeated evolutionary transitions between nocturnality and diurnality. Aristelliger is a nocturnal lineage embedded within a largely diurnal clade and represents a putative reversal to nocturnality. We investigated eye development and morphology of Aristelliger geckos. Despite sharing a most recent common ancestor with its diurnal sister lineage over 60 million years ago, Aristelliger exhibits a fovea. Fovea development is coordinated by dynamic changes in eye shape-such as ocular elongation and retraction-during embryonic development. In Aristelliger embryos, these elongation and retraction phases occur in the same region where the fovea forms, closely resembling the pattern seen in Anolis, a diurnal lizard with two foveae. Given the retention of a fovea in Aristelliger, we hypothesize that the transition to nocturnality is relatively recent. Incorporating these results into comparative phylogenetic methods results in a hypothesized diurnal ancestral temporal niche for sphaerodactylid geckos. We illustrate, similar to some nocturnal primates, that developmental data and robust morphological investigation can provide unique insights into macroevolutionary studies.