A database of the morphology, ecology and literature of the world's limb‐reduced skinks

Abstract

Aim Limb‐reduced squamates are a convenient model system to investigate macroevolutionary trends in morphology. Here, we provide morphological, ecological and literature data on all known species of limb‐reduced skinks (Scincidae) and their relatives, representing one of the most diverse and widely distributed groups of limb‐reduced squamates. Location Global. Taxon Skinks (Reptilia, Squamata: Scincidae). Limb‐reduced forms. Methods Morphological data were sourced from the primary literature, spanning a period of over 150 years. Linear body measurements were averaged across all values in the literature, preserving proportionality to body length. For digits and presacral vertebrae, we used maximum recorded counts. Ecological and biogeographical data were sourced from habitat assessments in the primary literature, online databases and field guides. Literature data were sorted according to type of study. To exemplify the applicability of the database, we used Markov‐chain ordered models to estimate the evolutionary frequency of limb reduction and loss in skinks. Results We find evidence of limb reduction and loss in a total of 394 species worldwide, representing ~23% of all skink species, and ~30% of genera. The distribution of limb‐reduced and limbless forms differs from that of fully limbed forms, as they are present in all biogeographic realms with the almost complete exclusion of the Americas. We estimate that limb reduction evolved more than 50 times in skinks, and that loss of at least one limb pair evolved at least 24 times. Main conclusions The dataset captures a broad spectrum of morphological and ecological variation in a large, globally distributed taxonomic group. It establishes a widely applicable definition of limb reduction based on limb proportions as a reference for future studies. Such an extensive collection of morphological and ecological data can pave the way for investigations of dramatic morphological transitions and their ecological drivers at a global and local scale.

Macroecological and biogeographical patterns of limb reduction in the world's skinks

“…We consider ‘limb‐reduced’ any species with miniaturised limbs (forelimb length [FLL] <15%, hindlimb length [HLL] <20% of snout‐vent length [SVL]) (following Camaiti et al, 2022), and species lacking limbs are referred to as ‘limbless’. We sourced data on FLL, HLL, SVL, head length (HdL), number of fingers (Man), toes (Pes) and presacral vertebrae numbers (PSVn) for 394 species of limb‐reduced and limbless skinks, and 211 of their fully limbed congeners and unrelated species for informed comparisons, from the dataset of Camaiti et al (2022). We added measurements for 27 species of the genus Ctenotus (all fully limbed) to increase sampling for Sphenomorphinae.…”

“…This left us with 298 species, of which 208 had presacral vertebrae counts (Supplementary Material S1). Two hundred eighty‐four species in this dataset are skinks, 168 of which are limb‐reduced or limbless (42.64% of the limb‐reduced and limbless skinks identified in Camaiti et al, 2022). Seven girdled lizards (Cordylidae), four plated lizards (Gerrhosauridae) and three night lizards (Xantusiidae) are included as the sister clade to skinks (Zheng & Wiens, 2016).…”

“…The acquisition of limb‐reduced and elongated body shapes represents one of the most dramatic morphological transitions in the history of tetrapods (Gans, 1975). This body shape exists in amphibians and mammals, but is most widespread among squamates, where it evolved at least 60 times independently (Brandley et al, 2008; Camaiti et al, 2022; Gans, 1975; Greer, 1991), including approximately 4700 species (estimated from the Reptile Database: Uetz et al, 2021) spread across various lineages (Brandley et al, 2008). Within squamates, limb reduction indicates the miniaturisation, or loss, of limbs and digits, and tends to further involve more presacral vertebrae, narrow heads and bodies (Rieppel, 1984), and elongated trunks (Camaiti et al, 2021).…”

“…Skinks (Scincidae) are a clade that underwent over 50 independent transitions towards limb reduction and shows the most divergent body shapes among limb‐reduced morphs in squamates (Camaiti et al, 2022; Greer, 1991). Thus, they are a prime example of variation in evolutionary trajectories of limb reduction.…”

“…To test our predictions, we used two approaches differing in taxonomic and geographic scope. First, to provide a clearer categorisation of burrowing ecomorphs, we examine how morphologies vary across broad substrate categories for a large sample of the world's limb‐reduced skinks, as defined by Camaiti et al (2022; also see materials and methods section of the present article). Second, to parse the specific habitat factors associated with the evolution of limb‐reduced morphologies, we perform an analysis using habitat data extracted from species' distributions on a spatially‐restricted sample (20.1% of the species) of limb‐reduced skinks and their relatives, the Australian Sphenomorphinae.…”