Morphometrics of the skeleton of Dermophis mexicanus (Amphibia: Gymnophiona). Part I. The vertebrae,with comparisons to other species

Morphometric analysis of vertebral structure in caecilians (Amphibia: Gymnophiona) is presented. Ontogenetic variation in Dermophis mexicanus is analyzed through the 100+ vertebrae composing the column. Vertebral structure in adult D. mexicanus is compared with that in Ichthyophis glutinosus and Typhlonectes compressicauda. Centra of the atlas, second, tenth, 20th, and 50th vertebrae grow at allometrically different rates in D. mexicanus, though the 20th and 50th are not significantly different, Growth appears significantly slower in several dimensions of anterior and posterior vertebrae relative to midtrunk vertebrae in all three species. Mensural patterns throughout the entire column are similar in the terrestrial burrowers D. mexicanus and I. glutinosus; patterns in the aquatic T. compressicauda differ substantially from those of the burrowing species and are strongly influenced by allometry. Of the 112 D. mexicanus examined, 13.4% had vertebral anomalies, usually fusions.     

Tail development and regeneration throughout the life cycle of the four-toed salamander Hemidactylium scutatum

The salamander tail displays different functions and morphologies in the aquatic and terrestrial stages of species with complex life cycles. During metamorphosis the function of the tail changes; the larval tail functions in aquatic locomotion while the juvenile and adult tail exhibits tail autotomy and fat storage functions. Because tail injury is common in the aquatic environment, we hypothesized that mechanisms have evolved to prevent altered larval tail morphology from affecting normal juvenile tail morphology. The hypothesis that injury to the larval tail would not affect juvenile tail morphology was investigated by comparing tail development and regeneration in Hemidactylium scutatum (Caudata: Plethodontidae). The experimental design included larvae with uninjured tails and with cut tails to simulate natural predation. The morphological variables analyzed to compare normally developing and regenerating tails were 1) tail length, 2) number of caudal vertebrae, and 3) vertebral centrum length. Control and experimental groups do not differ in time to metamorphosis or snout-vent length. Tails of experimental individuals are shorter than controls, yet they display a significantly higher rate of tail growth and less resorption of tail tissue. Anterior to the site of tail injury, caudal vertebrae in juveniles display greater average centrum lengths. Results suggest that regenerative mechanisms are functioning not only to produce structures, but also to influence growth of existing structures. Further investigation of juvenile and adult stages as well as comparative analyses of tail morphology in salamanders with complex life cycles will enhance our understanding of amphibian development and of the evolution of amphibian life cycles. J Morphol 233:15–29, 1997. © 1997 Wiley-Liss, Inc.     

Spinal nerves and their bearing on,salamander phylogeny

Examination of the vertebral columns of representatives of all families of salamanders revealed that, in contrast to the condition found in most other vertebrates, salamander spinal nerves often pass through foramina in the vertebrae. Two kinds of spinal nerve foramina were found: those in the anterior halves of vertebrae, and those in the posterior halves. In addition, many salamanders retain intervertebral nerves. However, within each family or, in a few cases, subfamily there is a characteristic pattern of spinal nerve-vertebral relationships. The first spinal nerve of all salamanders exits through a foramen in the anterior half of the atlas. All more posterior nerves are intervertebral in the families Cryptobranchidae, Hynobiidae and Proteidae. The posterior caudal nerves exit through the posterior halves of the caudal vertebrae in the family Amphiumidae, while in the subfamilies Dicamptodontinae and Rhyacotritoninae all post-sacral nerves exit through the posterior halves of the vertebrae. All but the first three nerves exit through posterior foramina in the family Plethodontidae and the subfamily Ambystomatinae, while all but the first two nerves pass through posterior foramina in the families Salamandridae and Sirenidae. Several fossil salamanders were also examined. These showed that the amphiumid and dicamptodontine-rhyacotritonine nerve patterns had evolved by the Late Cretaceous, and the sirenid pattern had probably evolved by that time. Other Cretaceous genera associated with the Ambystomatoidea still possessed the primitive intervertebral pattern. Using spinal nerve patterns and several other previously described morphological characters, a new hypothesis of the phylogeny of recent and fossil salamanders is presented and compared to earlier proposed phylogenies of the group. A new classification of salamander families is presented.     

Evolution of Sexual Dimorphism in the Number of Tail Vertebrae in Salamanders: Comparing Multiple Hypotheses

The evolution of sexual dimorphism is an important topic of evolutionary biology, but few studies have investigated the determinants of sexual dimorphism over broad phylogenetic scales. The number of vertebrae is a discrete character influencing multiple traits of individuals, and is particularly suitable to analyze processes determining morphological variation. We evaluated the support of multiple hypotheses concerning evolutionary processes that may cause sexual dimorphism in the number of caudal vertebrae in Urodela (tailed amphibians). We obtained counts of caudal vertebrae from >2,000 individuals representing 27 species of salamanders and newts from Europe and the Near East, and integrated these data with a molecular phylogeny and multiple information on species natural history. Per each species, we estimated sexual dimorphism in caudal vertebrae number. We then used phylogenetic least squares to relate this sexual dimorphism to natural history features (courtship complexity, body size dimorphism, sexual ornamentation, aquatic phenology) representing alternative hypotheses on processes that may explain sexual dimorphism. In 18 % of species, males had significantly more caudal vertebrae than females, while in no species did females have significantly more caudal vertebrae. Dimorphism was highest in species where males have more complex courtship behaviours, while the support of other candidate mechanisms was weak. In many species, males use the tail during courtship displays, and sexual selection probably favours tails with more vertebrae. Dimorphism for the number of tail vertebrae was unrelated to other forms of dimorphism, such as sexual ornamentation or body size differences. Multiple sexually dimorphic features may evolve independently because of the interplay between sexual selection, fecundity and natural selection.     

Is variation in tail vertebral morphology linked to habitat use in chameleons?

Chameleons (Chamaeleonidae) are known for their arboreal lifestyle, in which they make use of their prehensile tail. Yet, some species have a more terrestrial lifestyle, such as Brookesia and Rieppeleon species, as well as some chameleons of the genera Chamaeleo and Bradypodion. The main goal of this study was to identify the key anatomical features of the tail vertebral morphology associated with prehensile capacity. Both interspecific and intra-individual variation in skeletal tail morphology was investigated. For this, a 3D-shape analysis was performed on vertebral morphology using μCT-images of different species of prehensile and nonprehensile tailed chameleons. A difference in overall tail size and caudal vertebral morphology does exist between prehensile and nonprehensile taxa. Nonprehensile tailed species have a shorter tail with fewer vertebrae, a generally shorter neural spine and shorter transverse processes that are positioned more anteriorly (with respect to the vertebral center). The longer tails of prehensile species have more vertebrae as well as an increased length of the processes, likely providing a greater area for muscle attachment. At the intra-individual level, regional variation is observed with more robust proximal tail vertebrae having longer processes. The distal part has relatively longer vertebrae with shorter processes. Although longer, the small size and high number of the distal vertebrae allows the tail to coil around perches.     

Functional morphology and evolution of tail autotomy in salamanders

Basal tail constriction occurs in about two-thirds of the species of plethodontid salamanders. The constriction, which marks the site of tail autotomy, is a result of a reduction in length and diameter of the first caudal segment. Gross and microscopic anatomical studies reveal that many structural specializations are associated with basal constriction, and these are considered in detail. Areas of weakness in the skin at the posterior end of the first caudal segment, at the attachment of the musculature to the intermyotomal septum at the anterior end of the same segment, and between the last caudosacral and first caudal vertebrae precisely define the route of tail breakage. During autotomy the entire tail is shed, and a cylinder of skin one segment long closes over the wound at the end of the body. It is suggested that specializations described in this paper have evolved independently in three different groups of salamanders. Experiments and field observations reveal that, contrary to expectations, frequency of tail breakage is less in species with apparent provisions for tail autotomy than in less specialized species. The tail is a very important, highly functional organ in salamanders and it is suggested that selection has been for behavioral and structural adaptations for control of tail loss, rather than for tail loss per se.     

Structural features of neural spines of the caudal vertebrae of protoceratopoids (Ornithischia: Neoceratopsia)

The structure of caudal neural spines of protoceratopoids displays adaptation for aquatic and terrestrial mode of life. The increasing height of caudal neural spines in the series Leptoceratops, Udanoceratops, Protoceratops, Bagaceratops is connected with the extent of adaptation for swimming and changes in inclination of neural spines are connected with the mechanical balance of the lever. Thus, the anterior caudal vertebrae (1cd–15cd) of Protoceratops and Bagaceratops show an anticliny, which promotes extension (rise) of a heavy tail in terrestrial conditions. In the middle part of the tail (16cd–23cd), with the greatest height of neural spines, a decrease in width and increase in thickness counteract transverse loads accompanying movements on land. At the same time, the supraspinal ligament prevents divergence of neural spines caused by curvature of the tail as it is raised above the ground.     

Patterns of axial and appendicular movements during aquatic walking in the salamander Siren lacertina

Most studies of salamander locomotion have focused either on swimming or terrestrial walking, but some salamanders also use limb-based locomotion while submerged under water (aquatic walking). In this study we used video motion analysis to describe the aquatic walking gait of Siren lacertina, an elongate salamander with reduced forelimbs and no hindlimbs. We found that S. lacertina uses a bipedal-undulatory gait, which combines alternating use of the forelimbs with a traveling undulatory wave. Each forelimb is in contact with the substrate for about 50% of the stride cycle and forelimbs have little temporal overlap in contact intervals. We quantified the relative timing and frequency of limb and tail movements and found that, unlike the terrestrial gaits of most salamanders, axial and appendicular movements are decoupled during aquatic walking. We found no significant relationship between stride frequency and aquatic walking velocity, but we did find a statistically significant relationship between tailbeat frequency and aquatic walking velocity, which suggests that aquatic walking speed is mainly modulated by axial movements. By comparing axial wavespeed and distance traveled per tailbeat during swimming (forelimbs not used) and aquatic walking (forelimbs used), we found lower wavespeed and greater distance traveled per tailbeat during aquatic walking. These findings suggest that the reduced forelimbs of S. lacertina contribute to forward propulsion during aquatic walking.     

Terrestriality constrains salamander limb diversification: Implications for the evolution of pentadactyly

Patterns of phenotypic evolution can abruptly shift as species move between adaptive zones. Extant salamanders display three distinct life cycle strategies that range from aquatic to terrestrial (biphasic), to fully aquatic (paedomorphic) and to fully terrestrial (direct development). Life cycle variation is associated with changes in body form such as loss of digits, limb reduction or body elongation. However, the relationships among these traits and life cycle strategy remain unresolved. Here, we use a Bayesian modelling approach to test whether life cycle transitions by salamanders have influenced rates, optima and integration of primary locomotory structures (limbs and trunk). We show that paedomorphic salamanders have elevated rates of limb evolution with optima shifted towards smaller size and fewer digits compared to all other salamanders. Rate of hindlimb digit evolution is shown to decrease in a gradient as life cycles become more terrestrial. Paedomorphs have a higher correlation between hindlimb digit loss and increases in vertebral number, as well as reduced correlations between limb lengths. Our results support the idea that terrestrial plantigrade locomotion constrains limb evolution and, when lifted, leads to higher rates of trait diversification and shifts in optima and integration. The basic tetrapod body form of most salamanders and the independent losses of terrestrial life stages provide an important framework for understanding the evolutionary and developmental mechanisms behind major shifts in ecological zones as seen among early tetrapods during their transition from water to land.     

内蒙古上白垩统二连组一长颈的镰刀龙类(英文)

镰刀龙类 (又称“懒龙”)是一类奇特的植食性兽脚类恐龙 ,化石记录主要局限于亚洲白垩纪地层中。由于镰刀龙类极其特化的形态和化石材料的局限性 ,这类恐龙的系统位置存在较多的争议。最近的发现 (RussellandDong ,1 994;Xuetal.,1 999)表明这类恐龙属于虚骨龙类 ,但其更为具体的系统位置依然存在争议 (Sues ,1 997;MakovickyandSues,1 998;Xuetal.,1 999;Sereno,1 999)。新发现于内蒙古苏尼特左旗赛罕高毕上白垩统二连组的镰刀龙类化石材料代表这类恐龙的一个新属种。杨氏内蒙古龙 (Neimongosaurusyangigen .etsp .nov .)的正型标本为一较为完整的骨架 ,是已知镰刀龙类当中第一件在同一个体中保存了大多数脊椎和几乎所有肢骨的标本。依据以下特征将内蒙古龙归入镰刀龙超科 :U形的下颌联合部、齿骨前端向下弯曲、齿骨前部没有牙齿、牙齿有一个收缩的基部、近圆形的齿根和叶形的齿冠、前部颈椎的神经脊低矮而轴向较长、后部颈椎背视呈X形、肱骨近端角状、肱骨有后转子、肱骨的尺骨髁和挠骨髁位于前部并为一狭窄槽分开、肠骨的耻骨柄细长而坐骨柄短以及跖部短。内蒙古龙的以下特征区别于其他镰刀龙类 :前部尾椎的横突下部有一圆形的窝 ,桡骨二头肌结节非常发育 ,后足趾节近端跟部非常发育 ,胫骨的腓骨嵴长     

Evolving possibilities: postembryonic axial elongation in salamanders with biphasic (Eurcyea cirrigera,Eurycea longicauda,Eurycea quadridigitata) and paedomorphic life cycles (Eurycea nana and Ambystoma mexicanum)

Vaglia, JL., White, K, and Case, A. 2012. Evolving possibilities: postembryonic axial elongation in salamanders with biphasic (Eurcyea cirrigera, Eurycea longicauda, Eurycea quadridigitata) and paedomorphic life cycles (Eurycea nana and Ambystoma mexicanum). —Acta Zoologica (Stockholm) 93 : 2–13. Typically, the number of vertebrae an organism will have postembryonically is determined during embryogenesis via the development of paired somites. Our research investigates the phenomenon of postembryonic vertebral addition in salamander tails. We describe body and tail growth and patterns of postsacral vertebral addition and elongation in context with caudal morphology for four plethodontids (Eurycea) and one ambystomatid. Eurycea nana and Ambystoma mexicanum have paedomorphic life cycles; Eurcyea cirrigera, Eurycea longicauda and Eurycea quadridigitata are biphasic. Specimens were collected, borrowed and/or purchased, and cleared and stained for bone and cartilage. Data collected include snout‐vent length (SVL), tail length (TL), vertebral counts and centrum lengths. Eurycea species with biphasic life cycles had TLs that surpassed SVL following metamorphosis. Tails in paedomorphic species elongated but rarely exceeded body length. Larger TLs were associated with more vertebrae and longer vertebrae in all species. We observed that rates of postsacral vertebral addition varied little amongst species. Regional variation along the tail becomes prominent following metamorphosis in biphasic developers. In all species, vertebrae in the posterior one‐half of the tail taper towards the tip. We suggest that a developmental link might exist between the ability to continually add vertebrae and regeneration in salamanders.     

Morphometry, diet and habitat in the kingfishers (Aves: Alcedinidae)

Factor analysis of the linear dimensions of 92 species of kingfisher indicated that morphological differences were associated with four diet categories (aquatic, littoral, terrestrial and fossorial animals). Analysis of covariance confirmed the significance of these differences for culmen, tarsus and tail length but not for wing length. Habitat factors resulted in some significant differences but these were less easy to interpret. One case of intraspecific variation showed similar trends for culmen length.     

Caudal vertebral development and morphology in three salamanders with complex life cycles (Ambystoma jeffersonianum, Hemidactylium scutatum, and Desmognathus ocoee)

We describe caudosacral and caudal vertebral morphology across life history stages in three caudate amphibians: Ambystoma jeffersonianum (Ambystomatidae), Desmognathus ocoee (Plethodontidae: Desmognathinae), and Hemidactylium scutatum (Plethodontidae: Plethodontinae). All three species have aquatic larvae, but adults differ in habitat and predator defense strategy. Predator defense includes tail autotomy in D. ocoee and H. scutatum but not A. jeffersonianum. Of the species that autotomize, H. scutatum has a specialized constriction site at the tail base. We investigated whether aquatic larvae exhibit vertebral features similar to those previously described for aquatic adults and examined the effect of metamorphosis, if any, on vertebral morphology and the ontogeny of specialized vertebral features associated with tail autotomy. Interspecific comparisons of cleared-and-stained specimens indicate that vertebral morphology differs dramatically at hatching and that caudosacral and caudal vertebrae undergo continuous ontogenetic change throughout larval, metamorphic, and juvenile periods. Larvae and juveniles of H. scutatum do not exhibit adult vertebral features associated with constricted-base tail autotomy. The pond-type larvae of A. jeffersonianum and H. scutatum have tapering centrum lengths posterior to the sacrum. This pattern is functionally associated with aquatic locomotion. The stream-type larvae of D. ocoee undergo enhanced regional growth in the anterior tail such that the anterior caudal centra become longer than the preceding caudosacral centra. With the exception of the first two caudal vertebrae, a similar growth pattern occurs in H. scutatum adults. We hypothesize that enhanced growth of the anterior caudal segments is associated with tail elongation and autotomy.     

Head‐turning morphologies: Evolution of shape diversity in the mammalian atlas–axis complex

Mammals flex, extend, and rotate their spines as they perform behaviors critical for survival, such as foraging, consuming prey, locomoting, and interacting with conspecifics or predators. The atlas–axis complex is a mammalian innovation that allows precise head movements during these behaviors. Although morphological variation in other vertebral regions has been linked to ecological differences in mammals, less is known about morphological specialization in the cervical vertebrae, which are developmentally constrained in number but highly variable in size and shape. Here, we present the first phylogenetic comparative study of the atlas–axis complex across mammals. We used spherical harmonics to quantify 3D shape variation of the atlas and axis across a diverse sample of species, and performed phylogenetic analyses to investigate if vertebral shape is associated with body size, locomotion, and diet. We found that differences in atlas and axis shape are partly explained by phylogeny, and that mammalian subclades differ in morphological disparity. Atlas and axis shape diversity is associated with differences in body size and locomotion; large terrestrial mammals have craniocaudally elongated vertebrae, whereas smaller mammals and aquatic mammals have more compressed vertebrae. These results provide a foundation for investigating functional hypotheses underlying the evolution of neck morphologies across mammals.     

Structure and function of the hyolingual system in Hynobius and its bearing on the evolution of prey capture in terrestrial salamanders

The Hynobiidae is generally regarded as the most phylogenetically basal and least derived extant family of terrestrial salamanders. As in the other families of terrestrial salamanders, prey capture in the Hynobiidae is accomplished by lingual prehension. In Hynobius, the prey capture system appears to be a mosaic of derived and primitive features. This, in conjunction with previous studies, suggests that the hyolingual systems of all families of terrestrial salamanders have evolved various degrees of specialization since the appearance of the common ancestral condition. We propose that the generalized feeding system for the extant terrestrial salamanders includes a hyolingual skeleton comprised of one basibranchial, one pair of radial or radial-like structures, two pairs of ceratobranchials, two pairs of epibranchials, one pair of ceratohyals, and one urohyal arranged in a configuration similar to that of Hynobius; a simple, sac-like secondary tongue pad; a lift and thrust system of tongue projection; a four-part gape cycle; and a forward head and body surge. Modifications to this general plan, previously described for the disparate families, include various changes in the size, shape, and definition of the tongue pad, changes in the specific types of structures and configurations in the anterior hyolingual skeleton, secondary ossification in the posterior hyolingual skeleton, the appearance of various protrusion, projection, and flipping systems for tongue protraction, simplification of the kinematic gape profile, and loss of the forward head and body surge. The evolutionary trends in these modifications have provided a rich data set from which much phylogenetic information has been inferred. © 1996 Wiley-Liss, Inc.     

Variation in the trophic basis of production and energy flow associated with emergence of larval salamander assemblages from forest ponds

1. Larval amphibians are a dominant consumer in many freshwater systems, yet limited data on energy transfers between aquatic food resources and larvae and between metamorphosed larvae and adjacent habitats preclude an accurate assessment of their roles as links between aquatic and terrestrial food webs.
2. During 2003–04, we derived prey-specific assimilation efficiencies, analysed stomach contents, and intensively sampled ambystomatid salamander assemblages in four ponds to quantify the trophic basis of larval production. Using estimates of the contribution of each prey taxon to larval production, we constructed quantitative food webs and assessed variation in pathways of energy flow associated with emergences.
3. Overall, metamorphosed salamanders exported 3–8% of total prey production, required to account for total salamander production, to adjacent forest. Aquatic insects, zooplankton and amphibian prey were most important to energy flow associated with emergence; amounts of larval production attributed to each of these prey types shifted during development and varied among salamander taxa.
4. The majority of variation in the trophic basis of production among species was attributed to copepods (Cyclopidae) and three families of aquatic insects (Chironomidae, Chaoboridae and Culicidae). Dominant prey types contributing to the production of metamorphosed salamanders varied among ponds, representing different pathways for energy transfers between aquatic resources and forest habitats. These findings further our understanding of the ecological roles of amphibians and thus the consequences of amphibian declines and extinctions.     

Temperature‐dependent oxygen limitation and the rise of Bergmann's rule in species with aquatic respiration

Bergmann's rule is the propensity for species‐mean body size to decrease with increasing temperature. Temperature‐dependent oxygen limitation has been hypothesized to help drive temperature–size relationships among ectotherms, including Bergmann's rule, where organisms reduce body size under warm oxygen‐limited conditions, thereby maintaining aerobic scope. Temperature‐dependent oxygen limitation should be most pronounced among aquatic ectotherms that cannot breathe aerially, as oxygen solubility in water decreases with increasing temperature. We use phylogenetically explicit analyses to show that species‐mean adult size of aquatic salamanders with branchial or cutaneous oxygen uptake becomes small in warm environments and large in cool environments, whereas body size of aquatic species with lungs (i.e., that respire aerially), as well as size of semiaquatic and terrestrial species do not decrease with temperature. We argue that oxygen limitation drives the evolution of small size in warm aquatic environments for species with aquatic respiration. More broadly, the stronger decline in size with temperature observed in aquatic versus terrestrial salamander species mirrors the relatively strong plastic declines in size observed previously among aquatic versus terrestrial invertebrates, suggesting that temperature‐dependent oxygen availability can help drive patterns of plasticity, micro‐ and macroevolution.     

Fish out of water: terrestrial jumping by fully aquatic fishes

Many teleosts that live at the water's edge will voluntarily strand themselves to evade predators or escape poor conditions-this behavior has been repeatedly observed in the field for killifishes (Cyprinodontiformes). Although most killifishes are considered fully aquatic and possess no obvious morphological specializations to facilitate terrestrial locomotion, individuals from several different species have been observed moving across land via a "tail flip" behavior that generates a terrestrial jump. Like aquatic fast starts, terrestrial jumps are produced by high-curvature lateral flexion of the body (stage one), followed by contralateral flexion of the posterior body (stage two). Here, terrestrial jumps and aquatic fast starts are quantified for two littoral teleosts: Gambusia affinis (a killifish, Cyprinodontiformes) and Danio rerio (a small carp, Cypriniformes) to determine if the tail flip is produced by other (non-killifish) teleosts and to test the null hypothesis that the tail flip is a fast start behavior, performed on land. Both Danio and Gambusia produce tail flip-driven terrestrial jumps, which are kinematically distinct from aquatic escapes and characterized by (1) a prolonged stage one, during which the fish bends, lifting and rolling the center of mass over the caudal peduncle, and (2) a relatively brief stage two, wherein the caudal peduncle pushes against the substrate to launch the fish into the aerial phase. The ability of these fully aquatic fishes to employ the same structure to produce distinct kinematic patterns in disparate environments suggests that a new behavior has evolved to facilitate movement on land and that anatomical novelty is not a prerequisite for effective terrestrial locomotion.