Variations on a bauplan: description of a new Malagasy “mermaid skink” with flipper-like forelimbs only (Scincidae,SirenoscincusSakata & Hikida, 2003)

The macroevolutionary singularity of snakes

Snakes and lizards (Squamata) represent a third of terrestrial vertebrates and exhibit spectacular innovations in locomotion, feeding, and sensory processing. However, the evolutionary drivers of this radiation remain poorly known. We infer potential causes and ultimate consequences of squamate macroevolution by combining individual-based natural history observations (>60,000 animals) with a comprehensive time-calibrated phylogeny that we anchored with genomic data (5400 loci) from 1018 species. Due to shifts in the dynamics of speciation and phenotypic evolution, snakes have transformed the trophic structure of animal communities through the recurrent origin and diversification of specialized predatory strategies. Squamate biodiversity reflects a legacy of singular events that occurred during the early history of snakes and reveals the impact of historical contingency on vertebrate biodiversity.

Slow and steady wins the race: Diversification rate is independent from body size and lifestyle in Malagasy skinks (Squamata: Scincidae: Scincinae)

Most of the unique and diverse vertebrate fauna that inhabits Madagascar derives from in situ diversification from colonisers that reached this continental island through overseas dispersal. The endemic Malagasy Scincinae lizards are amongst the most species-rich squamate groups on the island. They colonised all bioclimatic zones and display many ecomorphological adaptations to a fossorial (burrowing) lifestyle. Here we propose a new phylogenetic hypothesis for their diversification based on the largest taxon sampling so far compiled for this group. We estimated divergence times and investigated several aspects of their diversification (diversification rate, body size and fossorial lifestyle evolution, and biogeography). We found that diversification rate was constant throughout most of the evolutionary history of the group, but decreased over the last 6-4 million years and independently from body size and fossorial lifestyle evolution. Fossoriality has evolved from fully quadrupedal ancestors at least five times independently, which demonstrates that even complex morphological syndromes - in this case involving traits such as limb regression, body elongation, modification of cephalic scalation, depigmentation, and eyes and ear-opening regression - can evolve repeatedly and independently given enough time and eco-evolutionary advantages. Initial diversification of the group likely occurred in forests, and the divergence of sand-swimmer genera around 20 Ma appears linked to a period of aridification. Our results show that the large phenotypic variability of Malagasy Scincinae has not influenced diversification rate and that their rich species diversity results from a constant accumulation of lineages through time. By compiling large geographic and trait-related datasets together with the computation of a new time tree for the group, our study contributes important insights on the diversification of Malagasy vertebrates.

Hidden limbs in the "limbless skink" Brachymeles lukbani: Developmental observations

Reduced limbs and limblessness have evolved independently in many lizard clades. Scincidae exhibit a wide range of limb-reduced morphologies, but only some species have been used to study the embryology of limb reduction (e.g., digit reduction in Chalcides and limb reduction in Scelotes). The genus Brachymeles, a Southeast Asian clade of skinks, includes species with a range of limb morphologies, from pentadactyl to functionally and structurally limbless species. Adults of the small, snake-like species Brachymeles lukbani show no sign of external limbs in the adult except for small depressions where they might be expected to occur. Here, we show that embryos of B. lukbani in early stages of development, on the other hand, show a truncated but well-developed limb with a stylopod and a zeugopod, but no signs of an autopod. As development proceeds, the limb's small size persists even while the embryo elongates. These observations are made based on external morphology. We used florescent whole-mount immunofluorescence to visualize the morphology of skeletal elements and muscles within the embryonic limb of B. lukabni. Early stages have a humerus and separated ulna and radius cartilages; associated with these structures are dorsal and ventral muscle masses as those found in the embryos of other limbed species. While the limb remains small, the pectoral girdle grows in proportion to the rest of the body, with well-developed skeletal elements and their associated muscles. In later stages of development, we find the small limb is still present under the skin, but there are few indications of its presence, save for the morphology of the scale covering it. By use of CT scanning, we find that the adult morphology consists of a well-developed pectoral girdle, small humerus, extremely reduced ulna and radius, and well-developed limb musculature connected to the pectoral girdle. These muscles form in association with a developing limb during embryonic stages, a hint that "limbless" lizards that possess these muscles may have or have had at least transient developing limbs, as we find in B. lukbani. Overall, this newly observed pattern of ontogenetic reduction leads to an externally limbless adult in which a limb rudiment is hidden and covered under the trunk skin, a situation called cryptomelia. The results of this work add to our growing understanding of clade-specific patterns of limb reduction and the convergent evolution of limbless phenotypes through different developmental processes.

Trade-offs between burrowing and biting force in fossorial scincid lizards?

Trade-offs are thought to be important in constraining evolutionary divergence as they may limit phenotypic diversification. The cranial system plays a vital role in many functions including defensive, territorial, predatory and feeding behaviours in addition to housing the brain and sensory systems. Consequently, the morphology of the cranial system is affected by a combination of selective pressures that may induce functional trade-offs. Limbless, head-first burrowers are thought to be constrained in their cranial morphology as narrow heads may provide a functional advantage for burrowing. However, having a wide and large head is likely beneficial in terms of bite performance. We used 15 skink species to test for the existence of trade-offs between maximal push and bite forces, and explored the patterns of covariation between external head and body morphology and performance. Our data show that there is no evidence of a trade-off between bite and burrowing in terms of maximal force. Species that generate high push forces also generate high bite forces. Our data also show that overall head size covaries with both performance traits. However, future studies exploring trade-offs between force and speed or the energetic cost of burrowing may reveal other trade-offs.

The Cephalic Osteoderms of Varanus komodoensis as Revealed by High-Resolution X-Ray Computed Tomography

Osteoderms constitute a morphological system that plays an important role in squamate systematics. However, their study and visualization have always been difficult due to their isolated occurrence in the skin, among the first organs to be removed during the skeletonization process. High-resolution X-ray computed tomography (HRXCT) offers a nondestructive means of visualizing osteoderms both in their natural relationship to each other and to the underlying cranial bones. Although it is often stated that Varanus komodoensis has a "chain mail" of osteoderms, this morphological system was never described in this taxon. Further, given its size, it might be expected that V. komodoensis would present the extreme of osteoderm development in extant varanids, a group that tends to have weakly developed osteoderms or none at all. Indeed, our HRXCT scan of a 19-year-old captive individual reveals an elaborate mesh of cephalic osteoderms that are incredibly numerous and morphologically diverse. We describe this skeletal system and compare it to the cephalic osteoderms in other varanoids. Anat Rec, 302:1675-1680, 2019. © 2019 American Association for Anatomy.

Descriptive osteology and patterns of limb loss of the European limbless skink Ophiomorus punctatissimus (Squamata, Scincidae)

The limbless skink Ophiomorus punctatissimus is a cryptozoic species found in the Peloponnese region of Greece and on the Greek island Kythira. To provide the first thorough description of the cranial and postcranial osteology of this species, both disarticulated specimens and X-ray computed tomographies of wet-preserved specimens were examined in detail. Resulting from this, an anatomical atlas of this species is provided. Two separate considerations, an evolutionary and an ecomorphological one, are made based on the observed adaptations related to limb loss in this skink. The structure of the girdles shows a particular pattern of reduction: whereas the pelvic girdle is mostly vestigial, the pectoral girdle is instead well developed, with all the elements typical of limbed lizards except for the actual limbs. This led us to hypothesize an asynchronous pattern of limb reduction during the evolution of this species, in which the hindlimbs regressed earlier than the forelimbs. Furthermore, considerations based on overall body morphology, osteology and the structure of the inner ear led to the recognition of this species as a burrowing ecomorph. In contrast to the morphology normally displayed in this ecomorph, O. punctatissimus is characterized by the retention of autotomic vertebrae in its tail. This is consistent with the habitats in which it lives, where active burrowing would be difficult because of the hard, rocky terrain. Instead, this skink hides among rocks on the surface and is, therefore, subject to greater predation risk.

Appendicular skeletons of five Asian skink species of the genera Brachymeles and Ophiomorus, including species with vestigial appendicular structures

Vestigial biological structures provide an important line of evidence for macroevolution. They abound in the appendicular skeletons of limbless and reduced-limbed members of the skink subfamily Scincinae, which includes a predominantly Asian clade and a predominantly African clade. Reduced appendicular skeletons in the predominantly African clade have received much recent attention, but for most species in the predominantly Asian clade the appendicular skeleton has yet to be described. Here we provide descriptions of the appendicular skeletons of the reduced-limbed skinks Brachymeles bonitae and Ophiomorus blandfordi, the externally limbless skink Ophiomorus punctatissimus, and, for comparison, the pentadactyl skinks Brachymeles gracilis and B. talinis. We used x-ray radiographs to examine the skeletons of these species and to note similarities and differences in the previously-described appendicular skeletal morphology of related species. We found that in B. bonitae the pectoral and pelvic girdles are unreduced, the proximal limb elements are reduced, and the distal limb elements are vestigial. In O. punctatissimus vestigial pectoral and pelvic girdles are present. In O. blanfordi the fifth metatarsal is vestigial. The phylogenetic distribution of morphological features related to appendicular reduction shows that multiple, parallel reduction events have taken place within each of these two genera. In addition, the anatomical distribution of element reduction and loss in these genera shows that the bones are reduced and lost in the same sequence in the predominantly Asian scincine clade as they are in other squamate clades. This suggests a common evolutionary mechanism for appendicular reduction and loss across the Squamata.

A critical survey of vestigial structures in the postcranial skeletons of extant mammals

In the Mammalia, vestigial skeletal structures abound but have not previously been the focus of study, with a few exceptions (e.g., whale pelves). Here we use a phylogenetic bracketing approach to identify vestigial structures in mammalian postcranial skeletons and present a descriptive survey of such structures in the Mammalia. We also correct previous misidentifications, including the previous misidentification of vestigial caviid metatarsals as sesamoids. We also examine the phylogenetic distribution of vestigiality and loss. This distribution indicates multiple vestigialization and loss events in mammalian skeletal structures, especially in the hand and foot, and reveals no correlation in such events between mammalian fore and hind limbs.

Distinct patterns of desynchronized limb regression in malagasy scincine lizards (squamata, scincidae)

Scincine lizards in Madagascar form an endemic clade of about 60 species exhibiting a variety of ecomorphological adaptations. Several subclades have adapted to burrowing and convergently regressed their limbs and eyes, resulting in a variety of partial and completely limbless morphologies among extant taxa. However, patterns of limb regression in these taxa have not been studied in detail. Here we fill this gap in knowledge by providing a phylogenetic analysis of DNA sequences of three mitochondrial and four nuclear gene fragments in an extended sampling of Malagasy skinks, and microtomographic analyses of osteology of various burrowing taxa adapted to sand substrate. Based on our data we propose to (i) consider Sirenoscincus Sakata & Hikida, 2003, as junior synonym of Voeltzkowia Boettger, 1893; (ii) resurrect the genus name Grandidierina Mocquard, 1894, for four species previously included in Voeltzkowia; and (iii) consider Androngo Brygoo, 1982, as junior synonym of Pygomeles Grandidier, 1867. By supporting the clade consisting of the limbless Voeltzkowia mira and the forelimb-only taxa V. mobydick and V. yamagishii, our data indicate that full regression of limbs and eyes occurred in parallel twice in the genus Voeltzkowia (as hitherto defined) that we consider as a sand-swimming ecomorph: in the Voeltzkowia clade sensu stricto the regression first affected the hindlimbs and subsequently the forelimbs, whereas the Grandidierina clade first regressed the forelimbs and subsequently the hindlimbs following the pattern prevalent in squamates. Timetree reconstructions for the Malagasy Scincidae contain a substantial amount of uncertainty due to the absence of suitable primary fossil calibrations. However, our preliminary reconstructions suggest rapid limb regression in Malagasy scincids with an estimated maximal duration of 6 MYr for a complete regression in Paracontias, and 4 and 8 MYr respectively for complete regression of forelimbs in Grandidierina and hindlimbs in Voeltzkowia.

Extinction risks and the conservation of Madagascar's reptiles

Background

An understanding of the conservation status of Madagascar's endemic reptile species is needed to underpin conservation planning and priority setting in this global biodiversity hotspot, and to complement existing information on the island's mammals, birds and amphibians. We report here on the first systematic assessment of the extinction risk of endemic and native non-marine Malagasy snakes, lizards, turtles and tortoises.

Methodology/Principal Findings

Species range maps from The IUCN Red List of Threatened Species were analysed to determine patterns in the distribution of threatened reptile species. These data, in addition to information on threats, were used to identify priority areas and actions for conservation. Thirty-nine percent of the data-sufficient Malagasy reptiles in our analyses are threatened with extinction. Areas in the north, west and south-east were identified as having more threatened species than expected and are therefore conservation priorities. Habitat degradation caused by wood harvesting and non-timber crops was the most pervasive threat. The direct removal of reptiles for international trade and human consumption threatened relatively few species, but were the primary threats for tortoises. Nine threatened reptile species are endemic to recently created protected areas.

Conclusions/Significance

With a few alarming exceptions, the threatened endemic reptiles of Madagascar occur within the national network of protected areas, including some taxa that are only found in new protected areas. Threats to these species, however, operate inside and outside protected area boundaries. This analysis has identified priority sites for reptile conservation and completes the conservation assessment of terrestrial vertebrates in Madagascar which will facilitate conservation planning, monitoring and wise-decision making. In sharp contrast with the amphibians, there is significant reptile diversity and regional endemism in the southern and western regions of Madagascar and this study highlights the importance of these arid regions to conserving the island's biodiversity.