Intramuscular morphology and tendon geometry of the epaxial swimming muscles of dolphins

The dolphin upstroke is powered primarily by the m. multifidus and m. longissimus—robust muscles that insert serially along the vertebral column. Intramuscular and tendon morphology coupled with kinematic data are used to hypothesize regionally specific functions of these muscles. Both muscles develop equivalent forces (approximately 2kN) in the region of the thoraco-lumbar spine, but transmit those forces to different regions of the vertebral column. The m. multifidus transmits the majority of its force locally to the thoraco-lumar spine, a region of the body that undergoes no measurable bending. The action of the m. multifidus appears to be to stiffen its deep tendon of insertion, forming a temporary skeletal element for the m. longissimus. The m. longissimus transmits the majority of its force to the caudal spine, by way of a novel interaction between its insertional tendons and the subdermal connective tissue sheath. This insertional pattern confers torsional stiffness on the caudal peduncle, and allows the m. longissimus to do relatively more work (118 J) than if it had a typical mammalian insertional pattern. The caudal extension of the m. multifidus does 12 J of work on the caudal spine during an upstroke. The caudal extension of the m. longissimus transmits its force to the vertebrae in the caudal flukes and is the only epaxial muscle that acts to control the angle of attack of the fluke blade.     

Precocial development of axial locomotor muscle in bottlenose dolphins (Tursiops truncatus)

At birth, the locomotor muscles of precocial, terrestrial mammals are similar to those of adults in both mass, as a percent of total body mass, and fiber-type composition. It is hypothesized that bottlenose dolphins (Tursiops truncatus), marine mammals that swim from the instant of birth, will also exhibit precocial development of locomotor muscles. Body mass data from neonatal and adult dolphins are used to calculate Grand's (1992) Neural and Muscular Indices of Development. Using these indices, the bottlenose dolphin is a Condition "3.5" neonate, where Condition 4 is the documented extreme of precocial development in terrestrial mammals. Moreover, myosin ATPase (alkaline preincubation) analyses of the epaxial locomotor m. extensor caudae lateralis show that neonatal dolphins have fiber-type profiles very similar to those of adults. Thus, based on mass and myosin ATPase activity, muscle development in dolphins is precocial. However, succinic dehydrogenase and Nile red histochemistry demonstrate that neonatal dolphin muscle has mitochondrial and lipid distributions different from those found in adults. These data suggest that neonates have a lower aerobic capacity than adults. Dolphin neonates may compensate for an apparent lack of aerobic stamina in two ways: 1) by being positively buoyant, with a relatively increased investment of their total body mass in blubber, and 2) by "free-riding" off their mothers. This study investigates quantitatively the development of a dolphin locomotor muscle and offers suggestions about adaptations required for a completely aquatic existence.     

Morphology of the melon and its tendinous connections to the facial muscles in bottlenose dolphins (Tursiops truncatus)

The melon is a lipid‐rich structure located in the forehead of odontocetes that functions to propagate echolocation sounds into the surrounding aquatic environment. To date, the melon's ability to guide and impedance match biosonar sounds to seawater has been attributed to its unique fatty acid composition. However, the melon is also acted upon by complex facial muscles derived from the m. maxillonasolabialis. The goal of this study was to investigate the gross morphology of the melon in bottlenose dolphins (Tursiops truncatus) and to describe how it is tendinously connected to these facial muscles. Standard gross dissection (N = 8 specimens) and serial sectioning (N = 3 specimens) techniques were used to describe the melon and to identify its connections to the surrounding muscles and blubber in three orthogonal body planes. The dolphin forehead was also thin‐sectioned in three body planes (N = 3 specimens), and polarized light was used to reveal the birefringent collagen fibers within and surrounding the melon. This study identified distinct regions of the melon that vary in shape and display locally specific muscle‐tendon morphologies. These regions include the bilaterally symmetric main body and cone and the asymmetric right and left caudal melon. This study is the first to identify that each caudal melon terminates in a lipid cup that envelopes the echolocation sound generators. Facial muscles of the melon have highly organized tendon populations that traverse the melon and insert into either the surrounding blubber, the connective tissue matrix of the nasal plug, or the connective tissue sheath surrounding the sound generators. The facial muscles and tendons also lie within multiple orthogonal body planes, which suggest that the melon is capable of complex shape change. The results of this study suggest that these muscles could function to change the frequency, beam width, and directionality of the emitted sound beam in bottlenose dolphins. The echolocation sound propagation pathway within the dolphin forehead appears to be a tunable system. J. Morphol., 2008. © 2008 Wiley‐Liss, Inc.     

Caudal Fin Locomotion in Ray-finned Fishes: Historical and Functional Analyses

SYNOPSIS. Over the last 20 years, considerable progress hasbeen made in quantifying the movement of the body during locomotionby aquatic vertebrates, and in defining the role of axial musculaturein producing these kinematic patterns. Relatively little isknown, however, about how specific internal structural featuresof the axial system in fishes affect body kinematics, and howsuch structural and functional features have changed duringevolution. The major theme of this paper is that historical,phylogenetic patterns in the axial musculoskeletal system needto be integrated with experimental and functional data in orderto understand the design of the locomotor apparatus in vertebrates.To illustrate this proposition, the evolution of the tail inray-finned fishes is presented as a case study in phylogeneticand functional analysis of the vertebrate axial musculoskeletalsystem. Traditionally, the evolution of the tail in ray-finnedfishes has been viewed as a transformation from a primitivelyheterocercal (functionally asymmetrical) tail to a homocercaltail in which the axis of rotation during locomotion was vertical,generating a symmetrical thrust. Both phylogenetic and functionalapproaches are used to examine this hypothesis. Major osteologicaland myological features of the tail in ray-finned fishes aremapped onto a phylogeny of ray-finned fishes to discern historicalsequences of morphological change in the axial musculoskeletalsystem. A key event in locomotor evolution was the origin ofthe hypochordal longitudinalis muscle, the only intrinsic caudalmuscle with a line of action at an appreciable angle to thebody axis. This muscle originated prior to the origin of a caudalskeleton bearing both hypaxial and epaxial fin ray supports.The hypochordal muscle is proposed to be a key component ofthe axial musculoskeletal system that allows most fishes tomodulate caudal function and decouples external morphologicalsymmetry from functional symmetry. Experimental data (straingauge recordings from tail bones, and electromyographic recordingsfrom intrinsic and extrinsic caudal muscles) corroborate thisinterpretation and suggest that functional symmetry in the tailof ray-finned fishes is not predictable from skeletal morphologyalone, but depends on the activity of the hypochordal longitudinalismuscle and on locomotor mode. The homocercal teleost tail maythus function asymmetrically.     

Diaphragm muscle development in bottlenose dolphins (Tursiops truncatus)

Being born directly into the aquatic environment creates unique challenges for the breathing muscles of neonatal cetaceans. Not only must these muscles be active at the instant of birth to ventilate the lungs, but their activities must also be coordinated with those of the locomotor muscles such that breathing takes place only at the water's surface. At least one major locomotory muscle of bottlenose dolphins (Tursiops truncatus) has been demonstrated to be well developed and, therefore, able to power the neonatal dolphin's early movements (Dearolf et al. [2000] J Morphol 244:203-215). Thus, because of the demands for coordinated behavior with the locomotor muscles, it is hypothesized that the breathing muscles of bottlenose dolphins, represented in this study by the diaphragm, will also demonstrate adult morphology at birth. However, histochemical and biochemical analyses demonstrate that neonatal dolphins have immature diaphragms, with only 52% of the adult slow fiber-type profile (neonates: 34% slow-twitch fibers; adults: 66% slow-twitch fibers). The developmental state of the dolphin diaphragm is compared to those of other neonatal mammals, using a muscle development index (% slow-twitch fibers in neonatal muscle / % slow-twitch fibers in adult muscle). Fiber-type profiles reported in the literature are used to calculate index values for the diaphragms of altricial rats, rabbits, and cats, intermediate baboons and humans, and precocial sheep and horses. The dolphin is not unique in having an immature diaphragm at birth; however, there is a positive relationship between the developmental state of the diaphragm and the overall developmental state of the neonate. The presence of type IIc ("undifferentiated") fibers in the diaphragms of altricial developers (e.g., rats, rabbits, and cats) is correlated with the slow contraction speeds recorded from their diaphragms. The diaphragms of neonatal horses and dolphins express little to no type IIc fibers and, thus, may have the ability to contract at the speeds required for their increased ventilation rates. These results lead to the modification of the criterion for evaluating the developmental state of a muscle at birth. Thus, the developmental state of a neonatal muscle should be based on both its value of Dearolf et al.'s (2000) developmental index, as well as the percentage of type IIc fibers found in that muscle.     

Springs in Swimming Animals

Animals can lower the metabolic cost of swimming by using appropriatelytuned, elastic springs. Jet-powered invertebrates use springsthat lie in functional parallel to their swimming muscles topower half the locomotor cycle. The parallel geometry constrainsthe spring to be non-linearly elastic; muscle power is divertedto load the spring only when swimming muscles are not capableof producing maximal hydrodynamic thrust. The springs of jellyfishand scallops are forced at or near their resonant frequency,producing large energy savings. Measuring the contribution ofelastic energy storage to jet-powered locomotion has been facilitatedby the relatively simple geometries of invertebrate locomotorsystems. In contrast, complex musculoskeletal systems and kinematicshave complicated the study of springs in swimming vertebrates.Skins, tendons and axial skeletons of some vertebrate swimmershave appropriate mechanical properties to act as springs. Todate, though, there exist just a handful of studies that haveinvestigated the mechanical behaviors of these locomotor structuresin swimming vertebrates, and these data have yet to be integratedwith measures of swimming power. Integrating mechanical, kinematic,hydrodynamic and metabolic data are required to understand morefully the role of elastic springs in vertebrate swimming energetics.     

Is phylogeny driving tendon length in lizards?

Tulli, M.J., Herrel, A., Vanhooydonck, B. and Abdala, V. 2012. Is phylogeny driving tendon length in lizards?—Acta Zoologica (Stockholm) 93 : 319–329. Tendons transmit tensile forces generated by muscles and are a crucial part of the musculoskeletal system in vertebrates. Because tendons and tendon cells respond to altered mechanical load by increasing collagen synthesis, we hypothesized that a correlation between tendon morphology and the loading regime imposed by locomotor style or habitat use exists. This makes tendons an interesting model for studying the relationship between morphology and environment. In this study, we compare the general morphology of the palmar flexor plate, the length of the digital tendons, and the length of the flexor carpi ulnaris tendon in species of lizards that exploit a variety of structural habitats. The results from statistical analyses show that phylogenetic relatedness has a major impact on our ability to detect differences between habitat groups, and no differences in tendon length could be detected between iguanian species occupying different habitats when taking into account the relatedness between species. Our data for lizards diverge from the general mammalian paradigm where variation in tendon is often associated with habitat use or locomotor style.     

A novel age-group classification method for Irrawaddy dolphins based on dorsal fin shape features

The Irrawaddy dolphin is an endangered marine mammal species; therefore, there is an urgent need to take protective measures, especially in terms of population breeding and evolution. To address this, it is important to understand the age group structure of populations. Unlike biological individual identification and biological object detection based on pattern classification methods, a new age-group classification (AGC) method was developed to classify Irrawaddy dolphins into three age groups: older, middle-aged, and juvenile. Taking into account the relation between the dorsal fin shape features of Irrawaddy dolphins and their age, the AGC method constructed several dorsal fin geometric morphological features, such as leading edge length and dorsal fin height, using edge extraction and curve fitting of dolphin images. After performing a multicollinearity test on these features, nine effective features were obtained. A model was then trained to classify Irrawaddy dolphins according to their age groups. The experimental results demonstrated that the AGC method has a high classification accuracy of 80.20% for older dolphins. In contrast to individual identification and object detection methods, the proposed AGC method facilitates the analysis of population structure stability and dynamics by classifying Irrawaddy dolphins by age.     

Cruising specialists and accelerators--are different types of fish locomotion driven by differently structured myosepta?

Locomotor specialists, such as accelerators and cruisers, have clearly differing body designs. For physical reasons these designs are mutually exclusive, i.e. cruisers necessarily have poor accelerating capabilities and vice versa. For the first time, we examine whether differences in the anatomy of the musculo-tendinous system of the trunk are present in addition to the differences in external body design. We investigated the myoseptal series of two closely related locomotor specialists, the cruiser Scomber scombrus and the accelerator Channa obscura, by microdissections combined with polarized light microscopy and histology. Our comparison includes 3D-morphology of myosepta, spatial arrangement and length of myoseptal tendons, their relation to red and white muscles, rostrocaudal changes in all these aspects and the musculo-tendinous system of the caudal fin. Regarding all these features, Channa has retained the plesiomorphic condition of its actinopterygian ancestor. In contrast, the derived morphology of Scomber is characterized by (i) lateral (LT) and myorhabdoid tendons (MT) that are lengthened to up to 20% of body length (compared to a maximum of 8.2% in Channa), (ii) posterior myoseptal cones that are subsequently linked by horizontal projections of merged LTs and MTs, (iii) an increased area of red muscle fibers that insert to LTs of myosepta, (iv) the reduction of epineural (ENTs) and epipleural tendons (EPTs) that connect backbone and skin, (v) specific caudal tendons that are identified to be serial homologues of LTs and MTs of more anterior myosepta, (vi) and a partial reduction of intrinsic caudal muscles. These results suggest the following functional adaptations in the cruiser Scomber. Red muscle forces may be transmitted through LTs and posterior cones to the prominent tendons of the caudal fin. The length of LTs and the intersegmental connections along the posterior cones may facilitate posterior force transmission and may be correlated with the long propulsive wavelength generally observed in cruising carangiform swimmers. Epineural and epipleural tendons are interpreted to minimize lateral backbone displacement during high body curvatures. This is consistent with the lack of these tendons in Scomber, because high body curvatures are not displayed in stiffer-bodied carangiform swimmers. It remains to be tested whether the specializations revealed in this initial study for Scomber represent general specializations of carangiform swimmers. Taking into account the geometry of myoseptal tendons and the horizontal septum we evaluate how local bending according to beam-theory can be generated by white or red muscle activity in Channa and Scomber. In both species, the musculo-tendinous anatomy of the caudal fin explains the functional asymmetry of the caudal fin that was experimentally revealed in previous studies.     

Quantitative Comparison of Whistle Repertoires from Captive Adult Bottlenose Dolphins (Delphinidae,Tursiops truncatus): a Re-evaluation of the Signature Whistle Hypothesis

The prevailing view among researchers of dolphin communication is that bottlenose dolphins possess an individualized whistle contour; known as the ‘signature whistle’, it accounts for 74–95 % of a dolphin's whistle repertoire and functions to signal the individual identity of the whistler. This study used a new quantitative technique, termed the contour similarity technique (CS technique), and reports on the quantitative comparison of whistles from the individuals of three different social groups of bottlenose dolphins in socially interactive contexts. Results suggest that captive adult dolphins share several different whistle types including one predominant whistle type shared by all individuals across three different social groups. These analyses suggest a different interpretation of the dolphin whistle repertoire than has previously been proposed by proponents of the signature whistle hypothesis. In addition, results from our study support the results of early studies, published before the advent of the signature whistle hypothesis, in which investigators reported a large whistle repertoire within socially interactive captive and free-ranging groups and a predominant whistle type similar to that found in our study. Our results, combined with the results from earlier studies of dolphin vocal behaviour, suggest that the signature whistle hypothesis is incomplete and that dolphin whistle repertoires need to be analysed with respect to behavioural context and social relationships. In addition, these results suggest that contour discrimination and other acoustic features of whistles need to be tested in perception and categorization experiments.     

Morphology of the prosomal endoskeleton of Scorpiones (Arachnida) and a new hypothesis for the evolution of cuticular cephalic endoskeletons in arthropods

Skeletomuscular anatomy of the scorpion prosoma is examined in an attempt to explain the evolution of two endoskeletal features, a muscular diaphragm dividing the prosoma and opisthosoma and cuticular epistomal entapophyses with a uniquely complex arrangement of muscles, tendons and ligaments. Both structures appear to be derived from modifications of the mesodermal intersegmental endoskeleton that is primitive for all major arthropod groups. The scorpion diaphragm is a compound structure comprising axial muscles and pericardial ligaments of segments VI to VIII and extrinsic muscles of leg 4 brought into contact by longitudinal reduction of segment VII and integrated into a continuous subvertical sheet. This finding reconciles a long-standing conflict between one interpretation of opisthosomal segmentation based on scorpion embryology and another derived from comparative skeletomuscular anatomy. A new evolutionary-developmental mechanism is proposed to account for the complex morphology of the epistomal entapophyses. Each entapophysis receives 14 muscles and tendons that in other taxa would attach to the anterior connective endoskeleton in the same relative positions. This observation suggests that the embryological precursor to the connective endoskeleton can initiate and guide ectodermal invagination and thereby serve as a spatial template for the development of cuticular apodemes. This mesoderm-template model of ectodermal invagination is potentially applicable to all arthropods and may explain structural diversity and convergence in cephalic apodemes throughout the group. The model is used to interpret the cephalic endoskeletons of two non-chelicerate arthropods, Archaeognatha (Hexapoda) and Symphyla (Myriapoda), to demonstrate the generality of the model.     

Flexibility in the patterning and control of axial locomotor networks in lamprey

In lower vertebrates, locomotor burst generators for axial muscles generally produce unitary bursts that alternate between the two sides of the body. In lamprey, a lower vertebrate, locomotor activity in the axial ventral roots of the isolated spinal cord can exhibit flexibility in the timings of bursts to dorsally-located myotomal muscle fibers versus ventrally-located myotomal muscle fibers. These episodes of decreased synchrony can occur spontaneously, especially in the rostral spinal cord where the propagating body waves of swimming originate. Application of serotonin, an endogenous spinal neurotransmitter known to presynaptically inhibit excitatory synapses in lamprey, can promote decreased synchrony of dorsal-ventral bursting. These observations suggest the possible existence of dorsal and ventral locomotor networks with modifiable coupling strength between them. Intracellular recordings of motoneurons during locomotor activity provide some support for this model. Pairs of motoneurons innervating myotomal muscle fibers of similar ipsilateral dorsoventral location tend to have higher correlations of fast synaptic activity during fictive locomotion than do pairs of motoneurons innervating myotomes of different ipsilateral dorsoventral locations, suggesting their control by different populations of premotor interneurons. Further, these different motoneuron pools receive different patterns of excitatory and inhibitory inputs from individual reticulospinal neurons, conveyed in part by different sets of premotor interneurons. Perhaps, then, the locomotor network of the lamprey is not simply a unitary burst generator on each side of the spinal cord that activates all ipsilateral body muscles simultaneously. Instead, the burst generator on each side may comprise at least two coupled burst generators, one controlling motoneurons innervating dorsal body muscles and one controlling motoneurons innervating ventral body muscles. The coupling strength between these two ipsilateral burst generators may be modifiable and weakening when greater swimming maneuverability is required. Variable coupling of intrasegmental burst generators in the lamprey may be a precursor to the variable coupling of burst generators observed in the control of locomotion in the joints of limbed vertebrates.     

Fine‐scale habitat use in Indo‐Pacific humpback dolphins,Sousa chinensis,may be more influenced by fish rather than vessels in the Pearl River Estuary,China

Shifts in habitat use and distribution patterns in dolphins are often concerns that can result from habitat degradation. We investigated how potential changes to a habitat from human activity may alter dolphin distributions within Lingding Bay in the Pearl River Estuary, China, by studying the relationship between fish choruses, vessel presence and Indo‐Pacific humpback dolphin (Sousa chinensis) detection rates. Analyses revealed temporal and spatial variation within fish choruses, vessel presence and dolphin detection rates. After accounting for any temporal autocorrelation, correlations between fish choruses and dolphin detection rates were also found; however, no relationship between fish choruses and vessel presence or dolphin detection rates and vessel presence were observed. Furthermore, fewer dolphins were detected at sites where fish activity was less intense. Thus fish activity, rather than vessels, may be a key factor influencing the distribution of the dolphins within the estuary. These findings emphasize the risk of potential shifts in habitat use for Indo‐Pacific humpback dolphins due to detrimental changes to prey availability and dolphin feeding grounds from human activity, such as overfishing and coastal developments, within the estuary. This is a critical conservation issue for this dolphin population that is facing intense anthropogenic pressure.     

Whistle discrimination and categorization by the Atlantic bottlenose dolphin (Tursiops truncatus): a review of the signature whistle framework and a perceptual test

Dolphin whistles vary by frequency contour, changes in frequency over time. Individual dolphins may broadcast their identities via uniquely contoured whistles, "signature whistles." A recent debate concerning categorization of these whistles has highlighted the on-going need for perceptual studies of whistles by dolphins. This article reviews research on dolphin whistles as well as presenting a study in which a captive, female, adult bottlenose dolphin performed a conditional matching task in which whistles produced by six wild dolphins in Sarasota Bay were each paired with surrogate producers, specific objects/places. The dolphin subject also categorized unfamiliar exemplars produced by the whistlers represented by the original stimuli. The dolphin successfully discriminated among the group of whistles, associated them with surrogate producers, grouped new exemplars of the same dolphin's whistle together when the contour was intact, and discriminated among same-contour whistles produced by the same dolphin. Whistle sequences that included partial contours were not categorized with the original whistlers. Categorization appeared to be based on contour rather than specific acoustic parameters or voice cues. These findings are consistent with the perceptual tenets associated with the signature whistle framework which suggests that dolphins use individualized whistle contours for identification of known conspecifics.     

Features of the structural organization of the dolphin (Phocaena phocaena) lateral geniculate body in comparison to other dolphins]

The work describes a new for dolphins type of the cytoarchitectonic organization and topographo-anatomical features of the dorsal nucleus of the external geniculate body found in the dolphin Phocaena phocaena. In dolphins only the alveolar type of organization of the formation in question had been known which was principally different from the laminar and nuclear types characteristic of the terrestrial mammals. The Phocaena phocaena proved to have the nuclear type of organization of the external geniculate body, as well as in certain mammals. In addition, a fact of a sharp quantitative diminution of the external geniculate body as compared with other dolphins has been revealed which is of interest for the physiology of vision. This fact is of significance showing the total amount of nerve cells in the dorsal nucleus of the external geniculate body of the Phocaena phocaena to be equal to 6,6% as compared with the white-sided dolphin. Since the above diminution proved to be selective and did not take place in the anterior tuber of the quadrigeminum, the author puts forward the idea that a considerably less afferentation comes to the visual cortex of the Phocaena phocaena than to that of other dolphins, due to some, yet unknown, restriction of the visual function.     

Platelet-Rich Plasma and Adipose-Derived Mesenchymal Stem Cells for Regenerative Medicine-Associated Treatments in Bottlenose Dolphins (Tursiops truncatus)

Dolphins exhibit an extraordinary capacity to heal deep soft tissue injuries. Nevertheless, accelerated wound healing in wild or captive dolphins would minimize infection and other side effects associated with open wounds in marine animals. Here, we propose the use of a biological-based therapy for wound healing in dolphins by the application of platelet-rich plasma (PRP). Blood samples were collected from 9 different dolphins and a specific and simple protocol which concentrates platelets greater than two times that of whole blood was developed. As opposed to a commonly employed human protocol for PRP preparation, a single centrifugation for 3 minutes at 900 rpm resulted in the best condition for the concentration of dolphin platelets. By FACS analysis, dolphin platelets showed reactivity to platelet cell-surface marker CD41. Analysis by electron microscopy revealed that dolphin platelets were larger in size than human platelets. These findings may explain the need to reduce the duration and speed of centrifugation of whole blood from dolphins to obtain a 2-fold increase and maintain proper morphology of the platelets. For the first time, levels of several growth factors from activated dolphin platelets were quantified. Compared to humans, concentrations of PDGF-BB were not different, while TGFβ and VEGF-A were significantly lower in dolphins. Additionally, adipose tissue was obtained from cadaveric dolphins found along the Spanish Mediterranean coast, and adipose-derived mesenchymal stem cells (ASCs) were successfully isolated, amplified, and characterized. When dolphin ASCs were treated with 2.5 or 5% dolphin PRP they exhibited significant increased proliferation and improved phagocytotic activity, indicating that in culture, PRP may improve the regenerative capacity of ASCs. Taken together, we show an effective and well-defined protocol for efficient PRP isolation. This protocol alone or in combination with ASCs, may constitute the basis of a biological treatment for wound-healing and tissue regeneration in dolphins.     

The anuran Bauplan: a review of the adaptive, developmental, and genetic underpinnings of frog and tadpole morphology

Anurans (frogs, toads, and their larvae) are among the most morphologically derived of vertebrates. While tightly conserved across the order, the anuran Bauplan (body plan) diverges widely from that of other vertebrates, particularly with respect to the skeleton. Here we address the adaptive, ontogenetic, and genetic bases of three such hallmark anuran features: (1) the absence of discrete caudal vertebrae, (2) a truncated axial skeleton, and (3) elongate hind limbs. We review the functional significance of each as it relates to the anuran lifestyle, which includes locomotor adaptations to both aquatic and terrestrial environments. We then shift our focus to the proximal origins of each feature, namely, ontogeny and its molecular regulation. Drawing on relatively limited data, we detail the development of each character and then, by extrapolating from comparative vertebrate data, propose molecular bases for these processes. Cast in this light, the divergent morphology of anurans emerges as a product of evolutionary modulation of the generalised vertebrate developmental machinery. Specifically, we hypothesise that: (1) the formation of caudal vertebrae is precluded due to a failure of sclerotomes to form cartilaginous condensations, perhaps resulting from altered expression of a suite of genes, including Pax1, Pax9, Msx1, Uncx-4.1, Sonic hedgehog, and noggin; (2) anteriorised Hox gene expression in the paraxial mesoderm has led to a rostral shift of morphological boundaries of the vertebral column; and, (3) spatial and temporal shifts in Hox expression may underlie the expanded tarsal elements of the anuran hind limb. Technology is currently in place to investigate each of these scenarios in the African clawed frog Xenopus. Experimental corroboration will further our understanding of the molecular regulation of the anuran Bauplan and provide insight into the origin of vertebrate morphological diversity as well as the role of development in evolution.