Vertebral evolution and the diversification of squamate reptiles

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.

The neck as a keystone structure in avian macroevolution and mosaicism

BACKGROUND

The origin of birds from non-avian theropod dinosaur ancestors required a comprehensive restructuring of the body plan to enable the evolution of powered flight. One of the proposed key mechanisms that allowed birds to acquire flight and modify the associated anatomical structures into diverse forms is mosaic evolution, which describes the parcelization of phenotypic traits into separate modules that evolve with heterogeneous tempo and mode. Avian mosaicism has been investigated with a focus on the cranial and appendicular skeleton, and as such, we do not understand the role of the axial column in avian macroevolution. The long, flexible neck of extant birds lies between the cranial and pectoral modules and represents an opportunity to study the contribution of the axial skeleton to avian mosaicism.

RESULTS

Here, we use 3D geometric morphometrics in tandem with phylogenetic comparative methods to provide, to our knowledge, the first integrative analysis of avian neck evolution in context with the head and wing and to interrogate how the interactions between these anatomical systems have influenced macroevolutionary trends across a broad sample of extant birds. We find that the neck is integrated with both the head and the forelimb. These patterns of integration are variable across clades, and only specific ecological groups exhibit either head-neck or neck-forelimb integration. Finally, we find that ecological groups that display head-neck and neck-forelimb integration tend to display significant shifts in the rate of neck morphological evolution.

CONCLUSIONS

Combined, these results suggest that the interaction between trophic ecology and head-neck-forelimb mosaicism influences the evolutionary variance of the avian neck. By linking together the biomechanical functions of these distinct anatomical systems, the cervical vertebral column serves as a keystone structure in avian mosaicism and macroevolution.

Evolutionary rates and shape variation along the anuran vertebral column with attention to phylogeny, body size, and ecology

The vertebral column is critical to a vertebrate species' flexibility and skeletal support, making vertebrae a clear target for selection. Anurans (frogs and toads) have a unique, truncated vertebral column that appears constrained to provide axial rigidity for efficient jumping. However, no study has examined how presacral vertebrae shape varies among anuran species at the macroevolutionary scale nor how intrinsic (developmental and phylogenetic) and extrinsic (ecological) factors may have influenced vertebrae shape evolution. We used microCT scans and phylogenetic comparative methods to examine the vertebrae of hundreds of anuran species that vary in body size as well as adult and larval ecology. We found variation in shape and evolutionary rates among anuran vertebrae, dispelling any notion that trunk vertebrae evolve uniformly. We discovered the highest evolutionary rates in the cervical vertebrae and in the more caudal trunk vertebrae. We found little evidence for selection pressures related to adult or larval ecology affecting vertebrae evolution, but we did find body size was highly associated with vertebrae shape and microhabitat (mainly burrowing) affected those allometric relationships. Our results provide an interesting comparison to vertebrae evolution in other clades and a jumping-off point for studies of anuran vertebrae evolution and development.

Mosaic adaptive peak shifts underlie body shape diversification in pelagiarian fishes (Acanthomorpha: Percomorpha)

Extreme body elongation in fishes is a major evolutionary transformation that extends the boundaries of morphological diversity and alters aspects of function, behaviour and ecology. Prior studies have identified features of the cranial and axial skeleton that characterize elongate fishes, but a lack of detailed reconstructions of anatomical evolution has limited inferences about factors that underlie major shifts in body shape. In this study, we fitted multi-peak adaptive (Ornstein–Uhlenbeck) evolutionary models to species body shape and anatomical dimensions in Pelagiaria, a radiation of open-ocean fishes whose species span a continuum from deep bodied to highly elongate. We inferred an ancestral fusiform adaptive peak that is retained by several major pelagiarian lineages (e.g. Scombridae) and found robust support for multiple transitions to deep-bodied optima (in the families Stromateidae, Bramidae and Caristiidae) and elongate-bodied optima (within Trichiuroidei), including two instances of sequential shifts towards increasingly elongate optima that followed distinct paths of anatomical evolution. Within Trichiuridae, initial increases in head length and the number of vertebrae were followed by changes in head and vertebral shape. Within an elongate-bodied subclade of taxa traditionally identified as ‘gempylids’, changes in head and vertebral shape and in the number of precaudal vertebrae preceded an increase in the number of caudal vertebrae. Altogether, this mosaic of anatomical peak shifts suggests that body shape transformations were associated with differing selective demands and developmental changes.

Coordinating tiny limbs and long bodies: Geometric mechanics of lizard terrestrial swimming

Although typically possessing four limbs and short bodies, lizards have evolved diverse morphologies, including elongate trunks with tiny limbs. Such forms are hypothesized to aid locomotion in cluttered/fossorial environments but propulsion mechanisms (e.g., the use of body and/or limbs to interact with substrates) and potential body/limb coordination remain unstudied. Here, we use biological experiments, a geometric theory of locomotion, and robophysical models to investigate body-limb coordination in diverse lizards. Locomotor field studies in short-limbed, elongate lizards (Brachymeles and Lerista) and laboratory studies of fully limbed lizards (Uma scoparia and Sceloporus olivaceus) and a snake (Chionactis occipitalis) reveal that body-wave dynamics can be described by a combination of standing and traveling waves; the ratio of the amplitudes of these components is inversely related to the degree of limb reduction and body elongation. The geometric theory (which replaces laborious calculation with diagrams) helps explain our observations, predicting that the advantage of traveling-wave body undulations (compared with a standing wave) emerges when the dominant thrust-generation mechanism arises from the body rather than the limbs and reveals that such soil-dwelling lizards propel via "terrestrial swimming" like sand-swimming lizards and snakes. We test our hypothesis by inducing the use of traveling waves in stereotyped lizards via modulating the ground-penetration resistance. Study of a limbed/undulatory robophysical model demonstrates that a traveling wave is beneficial when propulsion is generated by body-environment interaction. Our models could be valuable in understanding functional constraints on the evolutionary processes of elongation and limb reduction as well as advancing robot designs.

Ecological Drivers of Carnivoran Body Shape Evolution

AbstractMorphological diversity is often attributed as adaptations to distinct ecologies. Although biologists have long hypothesized that distinct ecologies drive the evolution of body shape, these relationships are rarely tested across macroevolutionary scales in mammals. Here, I tested hypotheses that locomotor, hunting, and dietary ecologies influenced body shape evolution in carnivorans, a morphologically and ecologically diverse clade of mammals. I found that adaptive models with ecological trait regimes were poor predictors of carnivoran body shape and the underlying morphological components that contribute to body shape variation. Instead, the best-supported model exhibited clade-based evolutionary shifts, indicating that the complexity and variation of body shape landscape cannot be effectively captured by a priori ecological regimes. However, ecological adaptations of body shapes cannot be ruled out, as aquatic and terrestrial carnivorans exhibited opposite allometric patterns of body shape that may be driven by different gravitational constraints associated with these different environments. Similar to body size, body shape is a prominent feature of vertebrate morphology that may transcend one-to-one mapping relationships between morphology and ecological traits, enabling species with distinct body shapes to exploit similar resources and exhibit similar ecologies. Together, these results demonstrate that the multidimensionality of both body shape morphology and ecology makes it difficult to disentangle the complex relationship among morphological evolution, ecological diversity, and phylogeny across macroevolutionary scales.

Body shape transformations by alternate anatomical adaptive peak shifts in blenniiform fishes

Extreme body elongation has occurred repeatedly in the evolutionary history of ray-finned fishes. Lengthening of the anterior-posterior body axis relative to depth and width can involve changes in the cranial skeleton and vertebral column, but to what extent is anatomical evolution determined by selective factors and intrinsic constraints that are shared broadly among closely related lineages? In this study, we fit adaptive (Ornstein-Uhlenbeck) evolutionary models to body shape and its anatomical determinants and identified two instances of extreme elongation by divergent anatomical peak shifts in the Blenniiformes, a radiation of small-bodied substrate-associated marine teleost fishes. Species in the genus Xiphasia (hairtail blennies) evolved toward a peak defined by a highly elongated caudal vertebral region but ancestral cranial and precaudal vertebral morphology. In contrast, a clade that includes the genera Chaenopsis and Lucayablennius (pike and arrow blennies) evolved toward a peak with a long slender skull but ancestral axial skeletal anatomy. Neither set of anatomical peak shifts aligns closely with the major axis of anatomical diversification in other blenniiform fishes. These results provide little evidence that ancestral constraints have affected body shape transformation, and instead suggest that extreme elongation arose with distinct shifts in selective factors and development.

Evolutionary and morphological patterns underlying carnivoran body shape diversity

The diversity of body shapes is one of the most prominent features of phenotypic variation in vertebrates. Biologists, however, still lack a full understanding of the underlying morphological components that contribute to its diversity, particularly in endothermic vertebrates such as mammals. In this study, hypotheses pertaining to the evolution of the cranial and axial components that contribute to the diversity of carnivoran body shapes were tested. Three trends were found in the evolution of carnivoran body shapes: (1) carnivorans exhibit diverse body shapes with intrafamilial variation predicted best by family clade age, (2) body shape is driven by strong allometric effects of body size where species become more elongate with decreasing size, and (3) the thoracic and lumbar regions and rib length contribute the most to body shape variation, albeit pathways differ between different families. These results reveal the morphological patterns that led to increased diversity in carnivoran body shapes and elucidate the similarities and dissimilarities that govern body shape diversity across vertebrates.

Locomotion and palaeoclimate explain the re-evolution of quadrupedal body form in Brachymeles lizards

Evolutionary reversals, including re-evolution of lost structures, are commonly found in phylogenetic studies. However, we lack an understanding of how these reversals happen mechanistically. A snake-like body form has evolved many times in vertebrates, and occasionally a quadrupedal form has re-evolved, including in Brachymeles lizards. We use body form and locomotion data for species ranging from snake-like to quadrupedal to address how a quadrupedal form could re-evolve. We show that large, quadrupedal species are faster at burying and surface locomotion than snake-like species, indicating a lack of expected performance trade-off between these modes of locomotion. Species with limbs use them while burying, suggesting that limbs are useful for burying in wet, packed substrates. Palaeoclimatological data suggest that Brachymeles originally evolved a snake-like form under a drier climate probably with looser soil in which it was easier to dig. The quadrupedal clade evolved as the climate became humid, where limbs and large size facilitated fossorial locomotion in packed soils.

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

Three-dimensional (3D) modeling techniques have been increasingly utilized across disciplines for the visualization and analysis of complex structures. We employ 3D-digital photogrammetry for understanding the scaling of the body axis of 12 species of scincid lizards in the genus Brachymeles. These skinks represent a diverse radiation which shows tremendous variation in body size and degree of axial elongation. Because of the complex nature of the body axis, 3D-methods are important for understanding how the body axis evolves. 3D-digital photogrammetry presents a flexible, inexpensive, and portable system for the reconstruction of biological forms. As body size increased among species, the cross-sectional area and circumference of the head and other portions of the body axis increased isometrically, which indicates that species of differing sizes possess proportionally similar head and body shapes. These results suggest that there are no substantial head and body shape changes with body size among the sampled species, but further comparative studies with larger sample sizes and functional studies of size and morphology effects on burrowing or above-ground locomotion are needed.

Neuroanatomy of the lumbosacral plexus in a highly diversified clade of South-American lizards. Evolution and phylogenetic implications

Only few published studies that describe the neuroanatomy of lizards. Here, we describe the neuroanatomy of several Iguanian species belonging to three families (species of Liolaemus and Phymaturus belonging to Liolaemidae, Tropidurus and Stenocercus as representatives of Tropiduridae, and Diplolaemus as a representative of Leiosauridae). Based on Sudan Black B staining and conventional dissections, the neuroanatomy of the lumbosacral region is described. Among the most outstanding results is the existence of a neuronal pattern of the lumbosacral plexus characteristic of Liolaemidae. In addition, it was found that in the genus Liolaemus the lumbosacral plexus is composed of five pairs of spinal nerves while in Phymaturus, Tropidurus, Stenocercus and Diplolaemus is composed from five to six pairs of spinal nerves (from pre-sacral, sacral, and caudal vertebrae). We find differences in the origin of the spinal nerves that constitute the plexus. In some cases, the pattern of nerves involved includes even the caudal vertebrae. Variation among taxa related to the zeugopodial innervation is described, and the homology of these nervous branches is discussed. Sexual differences were found in some species studied. Based on our results and available literature, we found three different patterns of innervation of the zeugopodium. The major contribution of this study is to provide a detailed description of lumbosacral plexus nerves pathways from their origins at the vertebral column to the muscles that they innervate.

Development and evolution of regionalization within the avian axial column

The origin of birds from their terrestrial antecedents was accompanied by a wholesale transformation of their skeleton as they transitioned from a terrestrial to aerial realm. Part of this dramatic transformation is the reduction of separate vertebral elements into regional fusions to limit axial flexibility. This is partially mirrored within the development of the axial column, with regions of the axial column experiencing increasing morphological modularity and the loss of skeletal elements through vertebral fusions. Using a detailed description of the morphological development of the axial column in the model domestic chicken, Gallus gallus domesticus, we present a map of axial ossification based on discrete characters. Delays in ossification are found to occur in conjunction with the formation of fusions. Our study shows that the pattern and sequence of fusion and ossification during development may reflect the presence of independent modules as subsets within the typical regions of the avian axial column. Interestingly, few of these fusion modules correspond to the initial axial Hox expression patterns, suggesting another patterning mechanism is driving axial fusion regionalization. Additionally, two regions of fusion are discovered in the synsacrum. The anterior region of seven fused synsacrals may correspond to the non-ornithuran pygostylian synsacrum of the same number of vertebrae.

The movement dynamics of autotomized lizards and their tails reveal functional costs of caudal autotomy

Autotomy has evolved independently several times in different animal lineages. It frequently involves immediate functional costs, so regeneration evolved in many instances to restore the functionality of that body part. Caudal autotomy is a widespread antipredator strategy in lizards, although it may affect energy storage, locomotion dynamics, or survival in future encounters with predators. Here, we assessed the effect of tail loss on the locomotor performance of wall lizards (Podarcis muralis), as well as the recovery of locomotor functionality of lizards with regenerated tails, and the movement dynamics of shed tails that were either intact or having regenerated portions. Tail loss had no effect on locomotion over unhindered spaces, possibly due to compensation between a negative effect on the stride of front limbs, and a positive effect of losing mass and friction force. We found a clear negative impact of tail loss on locomotion in spaces with interspersed obstacles, in which tailed lizards jumped larger distances when leaving the obstacles. Besides, lizards that used the tail to push off the ground were able to approach the obstacles from further, so that the tail seemed to be useful when used during jumping. Regeneration fully restores lizard's locomotor capacities, but tail antipredator value, as indicated by the intensity of post-autotomic movements, is only partially retrieved. From these results, we propose that, together with the recovery of post-autotomy antipredator capacities, the restoration of the organismal locomotor performance may have been an important, yet frequently neglected factor in the evolution of lizard's regeneration ability.

Convergent Evolution of Elongate Forms in Craniates and of Locomotion in Elongate Squamate Reptiles

Synopsis Elongate, snake- or eel-like, body forms have evolved convergently many times in most major lineages of vertebrates. Despite studies of various clades with elongate species, we still lack an understanding of their evolutionary dynamics and distribution on the vertebrate tree of life. We also do not know whether this convergence in body form coincides with convergence at other biological levels. Here, we present the first craniate-wide analysis of how many times elongate body forms have evolved, as well as rates of its evolution and reversion to a non-elongate form. We then focus on five convergently elongate squamate species and test if they converged in vertebral number and shape, as well as their locomotor performance and kinematics. We compared each elongate species to closely related quadrupedal species and determined whether the direction of vertebral or locomotor change matched in each case. The five lineages examined are obscure species from remote locations, providing a valuable glimpse into their biology. They are the skink lizards Brachymeles lukbani, Lerista praepedita, and Isopachys anguinoides, the basal squamate Dibamus novaeguineae, and the basal snake Malayotyphlops cf. ruficaudus. Our results support convergence among these species in the number of trunk and caudal vertebrae, but not vertebral shape. We also find that the elongate species are relatively slower than their limbed counterparts and move with lower frequency and higher amplitude body undulations, with the exception of Isopachys. This is among the first evidence of locomotor convergence across distantly related, elongate species.

Patterns of intracolumnar size variation inform the heterochronic mechanisms underlying extreme body shape divergence in microcephalic sea snakes

Sea snakes (Hydrophiinae) that specialize on burrowing eel prey have repeatedly evolved tiny heads and reduced forebody relative to hindbody girths. Previous research has found that these "microcephalic" forms have higher counts of precaudal vertebrae, and postnatal ontogenetic changes cause their hindbodies to reach greater girths relative to their forebodies. We examine variation in vertebral size along the precaudal axis of neonates and adults of three species. In the nonmicrocephalic Hydrophis curtus, these intracolumnar patterns take the form of symmetrical curved profiles, with longer vertebrae in the midbody (50% of body length) relative to distal regions. In contrast, intracolumnar profiles in the microcephalic H. macdowelli and H. obscurus are strongly asymmetrical curves (negative skewness) due to the presence of numerous, smaller-sized vertebrate in the forebody (anterior to the heart). Neonate and adult H. macdowelli and H. obscurus specimens all exhibit this pattern, implying an onset of fore- versus hindbody decoupling in the embryo stage. Based on this, we suggest plausible developmental mechanisms involving the presence and positioning of Hox boundaries and heterochronic changes in segmentation. Tests of our hypotheses would give new insights into the drivers of rapid convergent shifts in evolution, but will ultimately require studies of gene expression in the embryos of relevant taxa.

You Can't Run, but You Can Hide: The Skeleton of the Sand-Swimmer Lizard Calyptommatus leiolepis (Squamata: Gymnophthalmidae)

Squamates exhibit a vast diversity of body plans, which directly determines habitat use and preference. Here the skeleton of the sand-swimmer burrower gymnophthalmid, Calyptommatus leiolepis, is analyzed to investigate how its peculiar fossorial locomotion affects its osteology. Calyptommatus leiolepis is a limb-reduced, short-intermediate tailed lizard. Although there are other studies on its general anatomy, we performed a detailed description of its skeleton. Using high-resolution computer tomography, each bone element within the skeleton was digitally segmented and a detailed description rendered. Anatomical features related to burrowing include the head having a shovel-like snout with a well-developed horizontal soft tissue ridge, nasal cartilages that exclude sand from the nostrils, reduced eyes covered by a brille, lack of forelimbs, extreme reduction of hind limbs, and imbricated scales among others. The genus Calyptommatus has unique features such as a triradiate jugal (with digit-like posterior projections), a reduced pectoral girdle and forelimbs, parasternal processes that interconnect the ribs, and a single digit in the hind limbs. When comparing this species with other gymnophthalmid lizards including, fossorial species, it is clear that Calyptommatus exhibits the highest number of structural modifications within the family. Despite its specialized morphology, it still retains characters that link this genus to other members of Gymnophthalmidae when included in a phylogeny based solely on phenotypic data. Anat Rec, 303:1305-1326, 2020. © 2019 American Association for Anatomy.

Angles and waves: intervertebral joint angles and axial kinematics of limbed lizards, limbless lizards, and snakes

Segmentation gives rise to the anterior-posterior axis in many animals, and in vertebrates this axis comprises serially arranged vertebrae. Modifications to the vertebral column abound, and a recurring, but functionally understudied, change is the elongation of the body through the addition and/or elongation of vertebrae. Here, we compared the vertebral and axial kinematics of the robustly limbed Fire skink (Riopa fernandi) representing the ancestral form, the limbless European glass lizard (Ophisaurus apodus), and the Northern water snake (Nerodia sipedon). We induced these animals to traverse through channels and peg arrays of varied widths and densities, respectively, using high-speed X-ray and light video. We found that even though the snake had substantially more and shorter vertebrae than either lizard, intervertebral joint angles did not differ between species in most treatment levels. All three species decreased the amplitude and wavelength of their undulations as channels narrowed and the lizard species increased wave frequency in narrower channels. In peg arrays, both lizard species decreased wave amplitude, while the snake showed no differences. All three species maintained similar wavelengths and frequencies as peg density increased in most cases. Our results suggest that amplitude is decoupled from wavelength and frequency in all three focal taxa. The combination of musculoskeletal differences and the decoupling of axial kinematic traits likely facilitates the formation of different undulatory waves, thereby allowing limbless species to adopt different modes of locomotion.

Shared extremes by ectotherms and endotherms: Body elongation in mustelids is associated with small size and reduced limbs

An elongate body with reduced or absent limbs has evolved independently in many ectothermic vertebrate lineages. While much effort has been spent examining the morphological pathways to elongation in these clades, quantitative investigations into the evolution of elongation in endothermic clades are lacking. We quantified body shape in 61 musteloid mammals (red panda, skunks, raccoons, and weasels) using the head-body elongation ratio. We also examined the morphological changes that may underlie the evolution toward more extreme body plans. We found that a mustelid clade comprised of the subfamilies Helictidinae, Guloninae, Ictonychinae, Mustelinae, and Lutrinae exhibited an evolutionary transition toward more elongate bodies. Furthermore, we discovered that elongation of the body is associated with the evolution of other key traits such as a reduction in body size and a reduction in forelimb length but not hindlimb length. This relationship between body elongation and forelimb length has not previously been quantitatively established for mammals but is consistent with trends exhibited by ectothermic vertebrates and suggests a common pattern of trait covariance associated with body shape evolution. This study provides the framework for documenting body shapes across a wider range of mammalian clades to better understand the morphological changes influencing shape disparity across all vertebrates.

Rapid phenotypic evolution following shifts in life cycle complexity

Life cycle strategies have evolved extensively throughout the history of metazoans. The expression of disparate life stages within a single ontogeny can present conflicts to trait evolution, and therefore may have played a major role in shaping metazoan forms. However, few studies have examined the consequences of adding or subtracting life stages on patterns of trait evolution. By analysing trait evolution in a clade of closely related salamander lineages we show that shifts in the number of life cycle stages are associated with rapid phenotypic evolution. Specifically, salamanders with an aquatic-only (paedomorphic) life cycle have frequently added vertebrae to their trunk skeleton compared with closely related lineages with a complex aquatic-to-terrestrial (biphasic) life cycle. The rate of vertebral column evolution is also substantially lower in biphasic lineages, which may reflect the functional compromise of a complex cycle. This study demonstrates that the consequences of life cycle evolution can be detected at very fine scales of divergence. Rapid evolutionary responses can result from shifts in selective regimes following changes in life cycle complexity.

Patterns of growth in the presacral vertebral column of the leopard gecko (Eublepharis macularius)

Postnatal growth patterns within the vertebral column may be informative about body proportions and regionalization. We measured femur length, lengths of all pre-sacral vertebrae, and lengths of intervertebral spaces, from radiographs of a series of 21 Eublepharis macularius, raised under standard conditions and covering most of the ontogenetic body size range. Vertebrae were grouped into cervical, sternal, and dorsal compartments, and lengths of adjacent pairs of vertebrae were summed before analysis. Femur length was included as an index of body size. Principal component analysis of the variance-covariance matrix of these data was used to investigate scaling among them. PC1 explained 94.19% of total variance, interpreted as the variance due to body size. PC1 differed significantly from the hypothetical isometric vector, indicating overall allometry. The atlas and axis vertebrae displayed strong negative allometry; the remainder of the vertebral pairs exhibited weak negative allometry, isometry or positive allometry. PC1 explained a markedly smaller amount of variance for the vertebral pairs of the cervical compartment than for the remainder of the vertebral pairs, with the exception of the final pair. The relative standard deviations of the eigenvalues from the PCAs of the three vertebral compartments indicated that the vertebrae of the cervical compartment were less strongly integrated by scaling than were the sternal or dorsal vertebrae, which did not differ greatly between themselves in their strong integration, suggesting that the growth of the cervical vertebrae is constrained by the mechanical requirements of the head. Regionalization of the remainder of the vertebral column is less clearly defined but may be associated with wave form propagation incident upon locomotion, and by locomotory changes occasioned by tail autotomy and regeneration. Femur length exhibits negative allometry relative to individual vertebral pairs and to vertebral column length, suggesting a change in locomotor requirements over the ontogenetic size range.

A broad-scale comparison of aerobic activity levels in vertebrates: endotherms versus ectotherms

Differences in the limits and range of aerobic activity levels between endotherms and ectotherms remain poorly understood, though such differences help explain basic differences in species' lifestyles (e.g. movement patterns, feeding modes, and interaction rates). We compare the limits and range of aerobic activity in endotherms (birds and mammals) and ectotherms (fishes, reptiles, and amphibians) by evaluating the body mass-dependence of VO₂ max, aerobic scope, and heart mass in a phylogenetic context based on a newly constructed vertebrate supertree. Contrary to previous work, results show no significant differences in the body mass scaling of minimum and maximum oxygen consumption rates with body mass within endotherms or ectotherms. For a given body mass, resting rates and maximum rates were 24-fold and 30-fold lower, respectively, in ectotherms than endotherms. Factorial aerobic scope ranged from five to eight in both groups, with scope in endotherms showing a modest body mass-dependence. Finally, maximum consumption rates and aerobic scope were positively correlated with residual heart mass. Together, these results quantify similarities and differences in the potential for aerobic activity among ectotherms and endotherms from diverse environments. They provide insights into the models and mechanisms that may underlie the body mass-dependence of oxygen consumption.

Developmental mechanisms of macroevolutionary change in the tetrapod axis: A case study of Sauropterygia

Understanding how developmental processes change on macroevolutionary timescales to generate body plan disparity is fundamental to the study of vertebrate evolution. Adult morphology of the vertebral column directly reflects the mechanisms that generate vertebral counts (somitogenesis) and their regionalisation (homeotic effects) during embryonic development. Sauropterygians were a group of Mesozoic marine reptiles that exhibited an extremely high disparity of presacral vertebral/somite counts. Using phylogenetic comparative methods, we demonstrate that somitogenesis and homeotic effects evolved in a co-ordinated way among sauropterygians, contrasting with the wider pattern in tetrapods, in which somitogenetic and homeotic shifts are uncorrelated. Changes in sauropterygian body proportions were primarily enabled by homeotic shifts, with a lesser, but important, contribution from differences in postpatterning growth among somites. High body plan plasticity was present in Triassic sauropterygians and was maintained among their Jurassic and Cretaceous descendants. The extreme disparity in the body plan of plesiosaurian sauropterygians did not result from accelerated rates of evolutionary change in neck length, but instead reflect this ancestral versatility of sauropterygian axial development. Our results highlight variation in modes of axial development among tetrapods, and show that heterogeneous statistical models can uncover novel macroevolutionary patterns for animal body plans and the developmental mechanisms that control them.

Body mass scaling of passive oxygen diffusion in endotherms and ectotherms

The area and thickness of respiratory surfaces, and the constraints they impose on passive oxygen diffusion, have been linked to differences in oxygen consumption rates and/or aerobic activity levels in vertebrates. However, it remains unclear how respiratory surfaces and associated diffusion rates vary with body mass across vertebrates, particularly in relation to the body mass scaling of oxygen consumption rates. Here we address these issues by first quantifying the body mass dependence of respiratory surface area and respiratory barrier thickness for a diversity of endotherms (birds and mammals) and ectotherms (fishes, amphibians, and reptiles). Based on these findings, we then use Fick's law to predict the body mass scaling of oxygen diffusion for each group. Finally, we compare the predicted body mass dependence of oxygen diffusion to that of oxygen consumption in endotherms and ectotherms. We find that the slopes and intercepts of the relationships describing the body mass dependence of passive oxygen diffusion in these two groups are statistically indistinguishable from those describing the body mass dependence of oxygen consumption. Thus, the area and thickness of respiratory surfaces combine to match oxygen diffusion capacity to oxygen consumption rates in both air- and water-breathing vertebrates. In particular, the substantially lower oxygen consumption rates of ectotherms of a given body mass relative to those of endotherms correspond to differences in oxygen diffusion capacity. These results provide insights into the long-standing effort to understand the structural attributes of organisms that underlie the body mass scaling of oxygen consumption.

A morphospace for reef fishes: elongation is the dominant axis of body shape evolution

Tropical reef fishes are widely regarded as being perhaps the most morphologically diverse vertebrate assemblage on earth, yet much remains to be discovered about the scope and patterns of this diversity. We created a morphospace of 2,939 species spanning 56 families of tropical Indo-Pacific reef fishes and established the primary axes of body shape variation, the phylogenetic consistency of these patterns, and whether dominant patterns of shape change can be accomplished by diverse underlying changes. Principal component analysis showed a major axis of shape variation that contrasts deep-bodied species with slender, elongate forms. Furthermore, using custom methods to compare the elongation vector (axis that maximizes elongation deformation) and the main vector of shape variation (first principal component) for each family in the morphospace, we showed that two thirds of the families diversify along an axis of body elongation. Finally, a comparative analysis using a principal coordinate analysis based on the angles among first principal component vectors of each family shape showed that families accomplish changes in elongation with a wide range of underlying modifications. Some groups such as Pomacentridae and Lethrinidae undergo decreases in body depth with proportional increases in all body regions, while other families show disproportionate changes in the length of the head (e.g., Labridae), the trunk or caudal region in all combinations (e.g., Pempheridae and Pinguipedidae). In conclusion, we found that evolutionary changes in body shape along an axis of elongation dominates diversification in reef fishes. Changes in shape on this axis are thought to have immediate implications for swimming performance, defense from gape limited predators, suction feeding performance and access to some highly specialized habitats. The morphological modifications that underlie changes in elongation are highly diverse, suggesting a role for a range of developmental processes and functional consequences.

A comparative study of single-leg ground reaction forces in running lizards

The role of different limbs in supporting and propelling the body has been studied in many species with animals appearing to have either similarity in limb function or differential limb function. Differential hindlimb versus forelimb function has been proposed as a general feature of running with a sprawling posture and as benefiting sprawled postured animals by enhancing maneuvering and minimizing joint moments. Yet only a few species have been studied and thus the generality of differential limb function in running animals with sprawled postures is unknown. We measured the limb lengths of seven species of lizard and their single-limb three-dimensional ground reaction forces during high-speed running. We found that all species relied on the hindlimb for producing accelerative forces. Braking forces were forelimb dominated in four species and equally distributed between limbs in the other three. Vertical forces were dominated by the hindlimb in three species and equally distributed between the forelimb and hindlimb in the other four. Medial forces were dominated by the hindlimb in four species and equally distributed in the other three, with all Iguanians exhibiting hindlimb-biased medial forces. Relative hindlimb to forelimb length of each species was related to variation in hindlimb versus forelimb medial forces; species with relatively longer hindlimbs compared with forelimbs exhibited medial forces that were more biased towards the hindlimbs. These results suggest that the function of individual limbs in lizards varies across species with only a single general pattern (hindlimb-dominated accelerative force) being present.

A phylogeny and revised classification of Squamata, including 4161 species of lizards and snakes

BACKGROUND

The extant squamates (>9400 known species of lizards and snakes) are one of the most diverse and conspicuous radiations of terrestrial vertebrates, but no studies have attempted to reconstruct a phylogeny for the group with large-scale taxon sampling. Such an estimate is invaluable for comparative evolutionary studies, and to address their classification. Here, we present the first large-scale phylogenetic estimate for Squamata.

RESULTS

The estimated phylogeny contains 4161 species, representing all currently recognized families and subfamilies. The analysis is based on up to 12896 base pairs of sequence data per species (average = 2497 bp) from 12 genes, including seven nuclear loci (BDNF, c-mos, NT3, PDC, R35, RAG-1, and RAG-2), and five mitochondrial genes (12S, 16S, cytochrome b, ND2, and ND4). The tree provides important confirmation for recent estimates of higher-level squamate phylogeny based on molecular data (but with more limited taxon sampling), estimates that are very different from previous morphology-based hypotheses. The tree also includes many relationships that differ from previous molecular estimates and many that differ from traditional taxonomy.

CONCLUSIONS

We present a new large-scale phylogeny of squamate reptiles that should be a valuable resource for future comparative studies. We also present a revised classification of squamates at the family and subfamily level to bring the taxonomy more in line with the new phylogenetic hypothesis. This classification includes new, resurrected, and modified subfamilies within gymnophthalmid and scincid lizards, and boid, colubrid, and lamprophiid snakes.

Conservatism of lizard thermal tolerances and body temperatures across evolutionary history and geography

Species may exhibit similar thermal tolerances via either common ancestry or environmental filtering and local adaptation, if the species inhabit similar environments. We ask whether upper and lower thermal limits (critical thermal maxima and minima) and body temperatures are more strongly conserved across evolutionary history or geography for lizard populations distributed globally. We find that critical thermal maxima are highly conserved with location accounting for a higher proportion of the variation than phylogeny. Notably, thermal tolerance breadth is conserved across the phylogeny despite critical thermal minima showing little niche conservatism. Body temperatures observed during activity in the field show the greatest degree of conservatism, with phylogeny accounting for most of the variation. This suggests that propensities for thermoregulatory behaviour, which can buffer body temperatures from environmental variation, are similar within lineages. Phylogeny and geography constrain thermal tolerances similarly within continents, but variably within clades. Conservatism of thermal tolerances across lineages suggests that the potential for local adaptation to alleviate the impacts of climate change on lizards may be limited.