Dorsal root ganglia, neural crest migration, and spinal cord form in snakes.

The classic view of the vertebrate dorsal root ganglion is that it arises from trunk neural crest cells that migrate to positions lateral to the spinal cord, sending axons dorsally into the spinal cord and dendrites ventrally to meet with motor axons in the ventral root to form spinal nerves. As a result, the ganglion ends up lying in the dorsal root of the spinal nerve. Serial histological sections of parts of the trunk of juveniles of different snake species revealed that the ganglia lie distal to the junction of dorsal and ventral roots of spinal nerves and outside the neural canal. The anatomical position of spinal ganglia in snakes suggests that regulation of trunk neural crest migration in snakes differs from that in the model endotherms in which it has been most thoroughly explored. Dorsal roots have no distinct rootlets and the span of root entry to the spinal cord is short compared to that of ventral rootlets in the same segment. Comparing early developmental stages to juvenile spinal cords shows an increased separation of spinal nerve root sites and ventral migration of the ganglion in later development. Dorsal rami of the spinal nerves leave directly from the dorsal edge of the ganglion, and the ventral ramus leaves from the ventral tip of the ganglion. How these features relate to the developmental regulation of ganglion form and position and the extraordinary locomotor capabilities of the snake trunk are unclear.

Interrogating Genomic-Scale Data for Squamata (Lizards, Snakes, and Amphisbaenians) Shows no Support for Key Traditional Morphological Relationships.

Genomics is narrowing uncertainty in the phylogenetic structure for many amniote groups. For one of the most diverse and species-rich groups, the squamate reptiles (lizards, snakes, and amphisbaenians), an inverse correlation between the number of taxa and loci sampled still persists across all publications using DNA sequence data and reaching a consensus on the relationships among them has been highly problematic. In this study, we use high-throughput sequence data from 289 samples covering 75 families of squamates to address phylogenetic affinities, estimate divergence times, and characterize residual topological uncertainty in the presence of genome-scale data. Importantly, we address genomic support for the traditional taxonomic groupings Scleroglossa and Macrostomata using novel machine-learning techniques. We interrogate genes using various metrics inherent to these loci, including parsimony-informative sites (PIS), phylogenetic informativeness, length, gaps, number of substitutions, and site concordance to understand why certain loci fail to find previously well-supported molecular clades and how they fail to support species-tree estimates. We show that both incomplete lineage sorting and poor gene-tree estimation (due to a few undesirable gene properties, such as an insufficient number of PIS), may account for most gene and species-tree discordance. We find overwhelming signal for Toxicofera, and also show that none of the loci included in this study supports Scleroglossa or Macrostomata. We comment on the origins and diversification of Squamata throughout the Mesozoic and underscore remaining uncertainties that persist in both deeper parts of the tree (e.g., relationships between Dibamia, Gekkota, and remaining squamates; among the three toxicoferan clades Iguania, Serpentes, and Anguiformes) and within specific clades (e.g., affinities among gekkotan, pleurodont iguanians, and colubroid families).

Interrogating genomic-scale data for squamata (lizards, snakes, and amphisbaenians) shows no support for key traditional morphological relationships

Genomics is narrowing uncertainty in the phylogenetic structure for many amniote groups. For one of the most diverse and species-rich groups, the squamate reptiles (lizards, snakes, and amphisbaenians), an inverse correlation between the number of taxa and loci sampled still persists across all publications using DNA sequence data and reaching a consensus on the relationships among them has been highly problematic. In this study, we use high-throughput sequence data from 289 samples covering 75 families of squamates to address phylogenetic affinities, estimate divergence times, and characterize residual topological uncertainty in the presence of genome-scale data. Importantly, we address genomic support for the traditional taxonomic groupings Scleroglossa and Macrostomata using novel machine-learning techniques. We interrogate genes using various metrics inherent to these loci, including parsimony-informative sites (PIS), phylogenetic informativeness, length, gaps, number of substitutions, and site concordance to understand why certain loci fail to find previously well-supported molecular clades and how they fail to support species-tree estimates. We show that both incomplete lineage sorting and poor gene-tree estimation (due to a few undesirable gene properties, such as an insufficient number of PIS), may account for most gene and species-tree discordance. We find overwhelming signal for Toxicofera, and also show that none of the loci included in this study supports Scleroglossa or Macrostomata. We comment on the origins and diversification of Squamata throughout the Mesozoic and underscore remaining uncertainties that persist in both deeper parts of the tree (e.g., relationships between Dibamia, Gekkota, and remaining squamates; among the three toxicoferan clades Iguania, Serpentes, and Anguiformes) and within specific clades (e.g., affinities among gekkotan, pleurodont iguanians, and colubroid families)

Interrogating genomic-scale data for squamata (lizards, snakes, and amphisbaenians) shows no support for key traditional morphological relationships

Genomics is narrowing uncertainty in the phylogenetic structure for many amniote groups. For one of the most diverse and species-rich groups, the squamate reptiles (lizards, snakes, and amphisbaenians), an inverse correlation between the number of taxa and loci sampled still persists across all publications using DNA sequence data and reaching a consensus on the relationships among them has been highly problematic. In this study, we use high-throughput sequence data from 289 samples covering 75 families of squamates to address phylogenetic affinities, estimate divergence times, and characterize residual topological uncertainty in the presence of genome-scale data. Importantly, we address genomic support for the traditional taxonomic groupings Scleroglossa and Macrostomata using novel machine-learning techniques. We interrogate genes using various metrics inherent to these loci, including parsimony-informative sites (PIS), phylogenetic informativeness, length, gaps, number of substitutions, and site concordance to understand why certain loci fail to find previously well-supported molecular clades and how they fail to support species-tree estimates. We show that both incomplete lineage sorting and poor gene-tree estimation (due to a few undesirable gene properties, such as an insufficient number of PIS), may account for most gene and species-tree discordance. We find overwhelming signal for Toxicofera, and also show that none of the loci included in this study supports Scleroglossa or Macrostomata. We comment on the origins and diversification of Squamata throughout the Mesozoic and underscore remaining uncertainties that persist in both deeper parts of the tree (e.g., relationships between Dibamia, Gekkota, and remaining squamates; among the three toxicoferan clades Iguania, Serpentes, and Anguiformes) and within specific clades (e.g., affinities among gekkotan, pleurodont iguanians, and colubroid families)

The suction mechanism of the pipid frog, Pipa pipa (Linnaeus, 1758).

Most suction-feeding, aquatic vertebrates create suction by rapidly enlarging the oral cavity and pharynx. Forceful enlargement of the pharynx is powered by longitudinal muscles that retract skeletal elements of the hyoid, more caudal branchial arches, and, in many fish, the pectoral girdle. This arrangement was thought to characterize all suction-feeding vertebrates. However, it does not exist in the permanently aquatic, tongueless Pipa pipa, an Amazonian frog that can catch fish. Correlating high-speed (250 and 500 fps) video records with anatomical analysis and functional tests shows that fundamental features of tetrapod body design are altered to allow P. pipa to suction-feed. In P. pipa, the hyoid apparatus is not connected to the skull and is enclosed by the pectoral girdle. The major retractor of the hyoid apparatus arises not from the pectoral girdle but from the femur, which lies largely within the soft tissue boundaries of the trunk. Retraction of the hyoid is coupled with expansion of the anterior trunk, which occurs when the hypertrophied ventral pectoral elements are depressed and the urostyle and sacral vertebra are protracted and slide forward on the pelvic girdle, thereby elongating the entire trunk. We suggest that a single, robust pair of muscles adduct the cleithra to depress the ventral pectoral elements with force, while modified tail muscles slide the axial skeleton cranially on the pelvic girdle. Combined hyoid retraction, axial protraction, and pectoral depression expand the buccopharyngeal cavity to a volume potentially equal to that of the entire resting body of the frog. Pipa may be the only tetrapod vertebrate clade that enlarges its entire trunk during suction-feeding.

Highly extensible skeletal muscle in snakes.

Many snakes swallow large prey whole, and this process requires large displacements of the unfused tips of the mandibles and passive stretching of the soft tissues connecting them. Under these conditions, the intermandibular muscles are highly stretched but subsequently recover normal function. In the highly stretched condition we observed in snakes, sarcomere length (SL) increased 210% its resting value (SL0), and actin and myosin filaments no longer overlapped. Myofibrils fell out of register and triad alignment was disrupted. Following passive recovery, SLs returned to 82% SL0, creating a region of double-overlapping actin filaments. Recovery required recoil of intracellular titin filaments, elastic cytoskeletal components for realigning myofibrils, and muscle activation. Stretch of whole muscles exceeded that of sarcomeres as a result of extension of folded terminal tendon fibrils, stretching of extracellular elastin and independent slippage of muscle fibers. Snake intermandibular muscles thus provide a unique model of how basic components of vertebrate skeletal muscle can be modified to permit extreme extensibility.

A model of the anterior esophagus in snakes, with functional and developmental implications.

The gross anatomy of the mouth of snakes has always been interpreted as an evolutionary response to feeding demands. In most alethinophidian species, their anatomy allows limited functional independence of right and left sides and the roof and floor of the mouth as well as wide separation of the tips of the mandibles. However, locations of the tongue and glottis in snakes suggest extraordinary rearrangement of pharyngeal structures characteristic of all vertebrates. Serial histological sections through the heads of a number of colubroid species show muscularis mucosal smooth muscle fibers appearing in the paratracheal gutter of the lower jaw at varying levels between the eye and ear regions. Incomplete muscularis externa elements appear beneath the paratracheal gutter more caudally but typically at otic levels. Both muscle layers encompass more of the gut wall at more posterior levels, encircling the gut at the level of the atlas or axis. The pattern in snakes suggests developmental dissociation of dorsal and ventral splanchnic derivatives and extensive topological rearrangements of some ventral pharyngeal arch derivatives typical of most tetrapods. When snakes swallow large prey, the effective oral cavity becomes extremely short ventrally. The palatomaxillary arches function as ratchets packing the prey almost directly into the esophagus. Our findings raise questions about germ layer origins and regulation of differentiation of gut regions and derivatives in snakes and suggest that significant aspects of the evolution of lepidosaurs may be difficult to recover from bones or molecular sequence data alone.

Snake lower jaw skin: extension and recovery of a hyperextensible keratinized integument.

The skin of squamates consists of a keratinized epidermis divided into thick scale and thinner, folded interscale regions underlain by a dermis containing a complex array of fibrous connective tissues. We examined the skin of the lower jaw of watersnakes (Nerodia sipedon) to determine how skin morphology changes when highly stretched during ingestion of large prey. Video records of skin behavior in the lower jaws of watersnakes feeding on fish or anesthetized watersnakes being stretched on an Instron machine showed that most skin extension involves the interscale skin. The largest intermandibular separation recorded during feeding was 7.7× resting distance, but intermandibular separation reached 10× without tissue failure during mechanical testing. Histological and anatomical analyses of lower jaws fixed in resting, moderately or highly stretched conditions showed that stretching had little effect on scale regions of the epidermis. However, stretching flattened folds of interscale regions at both gross and cellular levels and imposed changes in epidermal cell shape. Stretching of the dermis is primarily limited to realignment of collagen and stretching of elastin in the deep dermis. The configuration of dermal elastin suggests a model for passive recovery of epidermal folding following release of tension.

Mammals as prey: estimating ingestible size.

Most mammals have deformable bodies, making it difficult to measure the size of living or freshly killed ones accurately. Because small rodents are common prey of many snakes, and because nearly all snakes swallow their prey whole, we explored four methods for determining the ingestible size (the smallest cross-sectional area that the largest part of the rodent can be made into without breaking bones or dislocating joints) of 100 intact rodents, including 50 Musmusculus and 50 Rattus norvegicus. Cross-sectional areas derived from maximal height and width of specimens at rest or the same specimens wrapped snout to pelvic girdle are roughly 1.5× higher than areas calculated either by the height and width of the same specimens rolled into cylinders or by volumetric displacement. Rolling rodents into cylinders reduces cross-sectional area by straightening the vertebral column, lengthening the abdominal cavity, elevating the sternum, compressing the thoracic cavity, and protracting the shoulder joint, that is, changes similar to those seen in rodents eaten by snakes. Reduced major axis regression of the smallest attainable cross-sectional area, y, on mass, x, shows that y (in log mm(2) ) approximates 1.53x (in log grams)(0.69) for rats and 1.63x(0.64) for mice. Our results suggest that visual cues provided by live rodents might lead most predators, like snakes, to overestimate ingestible size and hence rarely attack prey too large to ingest.

Drinking in snakes: resolving a biomechanical puzzle.

Snakes have long been thought to drink with a two-phase buccal-pump mechanism, but observations that some snakes can drink without sealing the margins of their mouths suggest that buccal pumping may not be the only drinking mechanism used by snakes. Here, we report that some snakes appear to drink using sponge-like qualities of specific regions of the oropharyngeal and esophageal mucosa and sponge-compressing functions of certain muscles and bones of the head. The resulting mechanism allows them to transport water upward against the effects of gravity using movements much slower than those of many other vertebrates. To arrive at this model, drinking was examined in three snake species using synchronized ciné and electromyographic recordings of muscle activity and in a fourth species using synchronized video and pressure recordings. Functional data were correlated with a variety of anatomical features to test specific predictions of the buccal-pump model. The anatomical data suggest explanations for the lack of conformity between a buccal-pump model of drinking and the performance of the drinking apparatus in many species. Electromyographic data show that many muscles with major functions in feeding play minor roles in drinking and, conversely, some muscles with minor roles in feeding play major roles in drinking. Mouth sealing by either the tongue or mental scale, previously considered critical to drinking in snakes, is incidental to drinking performance in some species. The sponge mechanism of drinking may represent a macrostomatan exaptation of mucosal folds, the evolution of which was driven primarily by the demands of feeding.

Functional morphology of the palato-maxillary apparatus in "Palatine dragging" snakes (Serpentes: Elapidae: Acanthophis, Oxyuranus).

Elapid snakes have previously been divided into two groups (palatine erectors and palatine draggers) based on the morphology and inferred movements of their palatine bone during prey transport (swallowing). We investigated the morphology and the functioning of the feeding apparatus of several palatine draggers (Acanthophis antarcticus, Oxyuranus scutellatus, Pseudechis australis) and compared them to published records of palatine erectors. We found that the palatine in draggers does not move as a straight extension of the pterygoid as originally proposed. The dragger palato-pterygoid joint flexes laterally with maxillary rotation when the mouth opens and the jaw apparatus is protracted and slightly ventrally during mouth closing. In contrast, in palatine erectors, the palato-pterygoid joint flexes ventrally during upper jaw protraction. In draggers, the anterior end of the palatine also projects rostrally during protraction, unlike the stability of the anterior end seen in erectors. Palatine draggers differ from palatine erectors in four structural features of the palatine and its relationships to surrounding elements. The function of the palato-pterygoid bar in both draggers and erectors can be explained by a typical colubroid muscle contraction pattern, which acts on a set of core characters shared among all derived snakes. Although palatine dragging elapids share a fundamental design of the palato-maxillary apparatus with all higher snakes, they provide yet another demonstration of minor structural modifications producing functional variants.

Viper fangs: functional limitations of extreme teeth.

The fangs of vipers are extremely long, rotating, hollow teeth. Analysis of video records of more than 750 strikes recorded at 60 or 250 frames per second for 285 individuals representing 86 species in 31 genera shows that vipers reposition fangs after initial contact with prey in more than a third of the strikes. Repositioning resulted when fangs missed prey entirely or hit prey regions that did not permit adequate penetration. The prevalence of repositioning, even among species that normally release prey, suggests strong selective pressure for rapid neuromotor response to fang placement error. The rapidity of repositioning suggests the existence of (a) fine-scale sensory detection of fang penetration depth, (b) rapid modulation of contraction of antagonistic muscles, and (c) possibly neurological modifications to shorten transmission time between sensory input and motor output. Extreme fang length has apparently coevolved with extreme functions.

Dynamic changes in body form during swimming in the water snake Nerodia sipedon.

Video records of swimming water snakes show that during moderate to rapid swimming, the rear half to two-thirds of the trunk is compressed laterally, approaching the body form of some sea snakes. Body form of swimming snakes differed significantly from their shape when resting on a flat surface or when anesthetized and suspended in water. The extent of lateral flattening is positively correlated with swimming speed, a relationship generally supported by tests of trunk models in a flow tank. In Nerodia, the ability to temporarily flatten the trunk depends on kinetic costovertebral joints, a large compressible body cavity, and the absence of ventral skeletal support - features found in most snakes. Histological studies and manipulations of partially dissected preserved specimens showed that the resting angle of the ribs is maintained by localized elastic hypertrophy of the costovertebral capsular ligament. Trunk form during swimming in Nerodia is proposed to arise from anteromedial movement of the distal rib powered by deep muscles acting in concert with those proposed to generate undulation of the vertebral column.

Kinematic analysis of an appetitive food-handling behavior: the functional morphology of Syrian hamster cheek pouches.

Prodigious food hoarding in Syrian hamsters Mesocricetus auratus Waterhouse is strongly linked to appetite and is made possible by large internal cheek pouches. We provide a functional analysis of the cheek pouch and its associated retractor muscle. Frame-by-frame analysis of videotaped pouch-filling behavior revealed multiple jaw cycles for each food item pouched and the use of more jaw cycles to pouch large food items ( approximately 2.5 g chow pellets) than small (corn kernels or sunflower seed with husks). These results stand in contrast to previously reported pouching kinematics in the externally pouched Dipodomys deserti, which uses only one jaw cycle per pouching event. Comparison of pouching and mastication in the same individuals also suggests that in Syrian hamsters, feeding jaw cycles are modulated to accommodate pouch filling primarily by the addition of a pause between fast open and fast close phases, which we call ;gape phase'. Contrary to previous assertions, the retractor muscle does not merely provide structural support for the full pouch during locomotion. Video analysis of ten hamsters with unilaterally denervated retractor muscles and electrophysiological study of an anaesthetized subject confirmed that retractor muscle activity during pouch filling increases pouching efficiency for food items subsequent to the first.

Prey transport in "palatine-erecting" elapid snakes.

Cobras and mambas are members of a group of elapid snakes supposedly united by the morphology and inferred behavior of their palatine bone during prey transport (palatine erectors). The palatine erectors investigated (Dendroaspis polylepis, Naja pallida, Ophiophagus hannah, Aspidelaps scutatus, A. lubricus) show differences in the morphology of their feeding apparatus that do not affect the overall behavior of the system. We delineated the structures directly involved in producing palatine erection during prey transport. Palatine erection can be achieved by a colubroid muscle contraction pattern acting on a palato-pterygoid bar with a movable palato-pterygoid joint and a palatine that is stabilized against the snout. The palatine characters originally proposed to cause palatine erection are not required to produce the behavior and actually impede it in Naja pallida. Palatine-erecting elapids share a fundamental design of the palato-maxillary apparatus with all higher snakes. A set of plesiomorphic core characters is functionally integrated to function in prey transport using the pterygoid walk. Variant characters are either part of a structural periphery unrelated to the core structures that define function or patterns of variation are subordinate character sets operating within functional thresholds of a single system.

Feeding in Atractaspis (Serpentes: Atractaspididae): a study in conflicting functional constraints.

African fossorial colubroid snakes of the genus Atractaspis have relatively long fangs on short maxillae, a gap separating the pterygoid and palatine bones, a toothless pterygoid, and a snout tightly attached to the rest of the skull. They envenomate prey with a unilateral backward stab of one fang projected from a closed mouth. We combined structural reanalysis of the feeding apparatus, video records of prey envenomation and transport, and manipulations of live and dead Atractaspis to determine how structure relates to function in this unusual genus of snakes. Unilateral fang use in Atractaspis is similar to unilateral slashing envenomation by some rear-fanged snakes, but Atractaspis show no maxillary movement during prey transport. Loss of pterygoid teeth and maxillary movement during transport resulted in the inability to perform. 'pterygoid walk' prey transport. Atractaspis transport prey through the oral cavity using movement cycles in which mandibular adduction, anterior trunk compression, and ventral flexion of the head alternate with mandibular abduction and extension of head and anterior trunk over the prey. Inefficiencies in manipulation and early transport of prey are offset by adaptability of the envenomating system to various prey types in both enclosed and open spaces and by selection of prey that occupy burrows or tunnels in soil. Atractaspis appears to represent the evolutionary endpoint of a functional conflict between envenomation and transport in which a rear-fanged envenomating system has been optimized at the expense of most, if not all, palatomaxillary transport function.

Rhinokinetic snout of thamnophiine snakes.

Radiographic and cinegraphic behavioral data, combined with anatomical evidence, indicate that the snout in Nerodia and Thamnophis consists of four movable elements (1, premaxilla; 2, paired nasals; 3, right septomaxilla and vomer; and 4, left septomaxilla and vomer), a condition we refer to as rhinokinetic. In thamnophiine snakes, movements of the snout bones allow the teeth of the right and left sides to separate further and increase the effective stroke distance of each palatomaxillary cycle during swallowing. Histological and microdissectional analyses suggest that snout movement is keyed to the placement of the cartilaginous nasal septum and associated nasal capsules relative to the surrounding bones. The nasal septum separates the paired septomaxillae and is surrounded by loose connective tissues that extend ventrally between the vomers. The nasal capsules separate the nasal bones from the underlying septomaxillae, and also surround the anterior ends of the septomaxillae, providing a cartilaginous cushion between these bones and the premaxilla. The extraordinary rotations of the snout tip seen during swallowing in thamnophiine snakes are thus due to motion at the prokinetic joint between snout and braincase, and at all rhinokinetic joints connecting the four functional elements of the snout. © 1995 Wiley-Liss, Inc.