Are there general laws for digit evolution in squamates? The loss and re‐evolution of digits in a clade of fossorial lizards (Brachymeles, Scincinae)

Abstract

Evolutionary simplification of autopodial structures is a major theme in studies of body-form evolution. Previous studies on amniotes have supported Morse's law, that is, that the first digit reduced is Digit I, followed by Digit V. Furthermore, the question of reversibility for evolutionary digit loss and its implications for "Dollo's law" remains controversial. Here, we provide an analysis of limb and digit evolution for the skink genus Brachymeles. Employing phylogenetic, morphological, osteological, and myological data, we (a) test the hypothesis that digits have re-evolved, (b) describe patterns of morphological evolution, and (c) investigate whether patterns of digit loss are generalizable across taxa. We found strong statistical support for digit, but not limb re-evolution. The feet of pentadactyl species of Brachymeles are very similar to those of outgroup species, while the hands of these lineages are modified (2-3-3-3-2) and a have a reduced set of intrinsic hand muscles. Digit number variation suggests a more labile Digit V than Digit I, contrary to Morse's law. The observed pattern of digit variation is different from that of other scincid lizards (Lerista, Hemiergis, Carlia). Our results present the first evidence of clade-specific modes of digit reduction.

Evolution of the Gekkotan Adhesive System: Does Digit Anatomy Point to One or More Origins?

“…These taxa are relatively long-branched (see Daza et al [2012] with regard to the first two of these) and further investigation is needed. In studies in which the actual structure of purported re-evolved features have been examined (e.g., Ober 2003;Kohlsdorf and Wagner 2006;Wagner et al 2018), anatomical differences have been noted compared with the original condition. Re-evolution of entire structural complexes in their exact original configuration is generally considered unlikely (Kearney and Stuart 2004;Goldberg and Igić 2008;Wake et al 2011).…”

Venomous Snakes Reveal Ecological and Phylogenetic Factors Influencing Variation in Gut and Oral Microbiomes

“…These reptiles display a vast diversity of life history traits, particularly in dietary habits and reproductive modes (Shine and Bonnet, 2000;Vitt and Caldwell, 2013). They have also played an important role in addressing many higher-level questions in ecology and evolutionary biology across numerous fields of study that include using venom to study evolutionary key innovations (Casewell et al, 2013;Sunagar et al, 2016;Calvete, 2017) and studies of phenotypic evolution (Wagner et al, 2018;Watanabe et al, 2019), reproductive mode (Blackburn, 2006;Pyron and Burbrink, 2014), and adaptive radiation (Losos and Miles, 2002). Despite such a rich history of foundational research on the group, microbiome evolution among squamate reptiles remains poorly understood.…”

Using 3D‐digital photogrammetry to examine scaling of the body axis in burrowing skinks

“…Within skinks, the genus Brachymeles presents a particularly interesting case for investigating the evolution of the body axis, as members of the clade display a spectrum of body sizes and degree of elongation, as well as reduction of limbs and limb anatomical elements (Siler & Brown, 2011). This group of 41 species is largely endemic to the Philippines and shows notable variation in size, with a more than four‐fold difference in body length among species (Uetz, Freed, & Hošek, 2019; Wagner et al, 2018). Previous research on Brachymeles has revealed that species with reduced or absent limbs tend to have longer, slimmer bodies with more presacral vertebrae, while species with longer limbs have shorter, fatter bodies with fewer presacral vertebrae (Bergmann & Moringa, 2019; Greer, Caputo, Lanza, & Palmieri, 1998; Siler & Brown, 2011).…”

“…The complex nature of the body axis suggests that a more holistic 3D‐modeling approach would allow consideration of how cross‐sectional areas and circumference scale with size. We focus on the body axis, and not on the degree of limb development, which was addressed in Siler and Brown (2011) and Wagner et al (2018).…”