The axial skeleton of the Devonian tetrapod Ichthyostega

The axial skeleton of Tiktaalik roseae

The axial columns of the earliest limbed vertebrates show distinct patterns of regionalization as compared to early tetrapodomorphs. Included among their novel features are sacral ribs, which provide linkage between the vertebral column and pelvis, contributing to body support and propulsion by the hindlimb. Data on the axial skeletons of the closest relatives of limbed vertebrates are sparce, with key features of specimens potentially covered by matrix. Therefore, it is unclear in what sequence and under what functional context specializations in the axial skeletons of tetrapods arose. Here, we describe the axial skeleton of the elpistostegalian Tiktaalik roseae and show that transformations to the axial column for head mobility, body support, and pelvic fin buttressing evolved in finned vertebrates prior to the origin of limbs. No atlas-axis complex is observed; however, an independent basioccipital-exoccipital complex suggests increased mobility at the occipital vertebral junction. While the construction of vertebrae in Tiktaalik is similar to early tetrapodomorphs, its ribs possess a specialized sacral domain. Sacral ribs are expanded and ventrally curved, indicating likely attachment to the expanded iliac blade of the pelvis by ligamentous connection. Thus, the origin of novel rib types preceded major alterations to trunk vertebrae, and linkage between pelvic fins and axial column preceded the origin of limbs. These data reveal an unexpected combination of post-cranial skeletal characters, informing hypotheses of body posture and movement in the closest relatives of limbed vertebrates.

Regionalization and morphological integration in the vertebral column of Eurasian small-bodied newts (Salamandridae: Lissotriton)

Serially homologous structures may have complex patterns of regionalization and morphological integration, influenced by developmental Hox gene expression and functional constraints. The vertebral column, consisting of a number of repeated, developmentally constrained, and highly integrated units-vertebrae-is such a complex serially homologous structure. Functional diversification increases regionalization and modularity of the vertebral column, particularly in mammals. For salamanders, three concepts of regionalization of the vertebral column have been proposed, recognizing one, two, or three presacral regions. Using three-dimensional geometric morphometrics on vertebra models acquired with microcomputerized tomography scanning, we explored the covariation of vertebrae in four closely related taxa of small-bodied newts in the genus Lissotriton. The data were analyzed by segmented linear regression to explore patterns of vertebral regionalization and by a two-block partial least squares method to test for morphological integration. All taxa show a morphological shift posterior to the fifth trunk vertebra, which corresponds to the two-region concept. However, morphological integration is found to be strongest in the mid-trunk. Taken jointly, these results indicate a highly integrated presacral vertebral column with a subtle two-region differentiation. The results are discussed in relation to specific functional requirements, developmental and phylogenetic constraints, and specific requirements posed by a biphasic life cycle and different locomotor modes (swimming vs. walking). Further research should be conducted on different ontogenetic stages and closely related but ecologically differentiated species.

How big can a walking fish be? A theoretical inference based on observations on four land-dwelling fish genera of South Vietnam

Comparative study of terrestrial locomotion of 4 fish genera including Anabas, Channa, Clarias, and Monopterus, was performed in experimental setting with the substrate surface of wet clay. No special adaptations for terrestrial locomotion were found. Every fish uses for propulsion on land what it already has. Eel-shaped Monopterus crawls by body undulations in a serpentine or sidewinding technique, the latter of which was not previously observed beyond snakes. The other 3 fish genera walk by body oscillations using stiff appendages as propulsors. When they are located anteriorly, as the serrate operculum in Anabas and the preaxial spine of the pectoral fin in Clarias, the propulsion is termed prolocomotor, when posteriorly, as the spiny anal fin in Channa-metalocomotor. Channa is the heaviest fish walking out of water in our days, quite comparable in size with first Devonian tetrapods Acanthostega and Tulerpeton. A theoretical calculation is suggested for the upper size limit of a fish capable of terrestrial walking without special locomotor adaptations. It should be roughly 20 cm in the vertical dimension of the trunk, which is just a little above the known size of Devonian tetrapodomorph fishes Panderichthys and Elpistostege. The metalocomotor walking technique of Channa is suggested as the closest extant model for terrestrial locomotion at the fish-tetrapod transition. The major difference is that the metalocomotor propulsor in Channa is represented by the anal fin, while in tetrapodomorphs by the pelvic fins. The sprawled pelvic fins were advantageous in respect of reduced requirement for side-to-side tail swinging.

Flexible Shoulder in Quadruped Animals and Robots Guiding Science of Soft Robotics

Cursorial quadrupeds have different connections to the trunk for forelimbs and hindlimbs: a flexible connection through the muscles to the forelimb, and a secure connection through the hip joint to the hindlimb. Although anatomical and biological studies have described the structure and behavior of cursorial quadrupeds by focusing on flexible shoulders, the functionality of the flexible shoulder remains unclear. In this study, we first survey the anatomical and biological studies. Second, we introduce our robotics studies, which focus on flexible connections for proximal limb joints. Further, we discuss future directions for extracting a design principle based on complex animal body structures, and we suggest the potential for interdisciplinary research between anatomy and soft robotics.

A large osteoderm-bearing rib from the Upper Triassic Kössen Formation (Norian/Rhaetian) of eastern Switzerland

An important component of the Alpine vertebrate record of Late Triassic age derives from the Kössen Formation, which crops out extensively in the eastern Alps. Here, we present an isolated and only partially preserved large rib, which carries an osteoderm on a low uncinate process. Osteological comparison indicates that the specimen likely belongs to a small clade of marine reptiles, Saurosphargidae. Members of the clade are restricted to the western (today Europe) and eastern margins of the Tethys (today China) and were so far known only from the Anisian stage of the Middle Triassic. The assignment of the new find to cf. Saurosphargidae, with potential affinities to the genus Largocephalosaurus from the Guanling Formation of Yunnan and Guizhou Provinces, China, would extend the occurrence of the clade about 35 million years into the Late Triassic.

hox gene expression predicts tetrapod-like axial regionalization in the skate, Leucoraja erinacea

The axial skeleton of tetrapods is organized into distinct anteroposterior regions of the vertebral column (cervical, trunk, sacral, and caudal), and transitions between these regions are determined by colinear anterior expression boundaries of Hox5/6, -9, -10, and -11 paralogy group genes within embryonic paraxial mesoderm. Fishes, conversely, exhibit little in the way of discrete axial regionalization, and this has led to scenarios of an origin of Hox-mediated axial skeletal complexity with the evolutionary transition to land in tetrapods. Here, combining geometric morphometric analysis of vertebral column morphology with cell lineage tracing of hox gene expression boundaries in developing embryos, we recover evidence of at least five distinct regions in the vertebral skeleton of a cartilaginous fish, the little skate (Leucoraja erinacea). We find that skate embryos exhibit tetrapod-like anteroposterior nesting of hox gene expression in their paraxial mesoderm, and we show that anterior expression boundaries of hox5/6, hox9, hox10, and hox11 paralogy group genes predict regional transitions in the differentiated skate axial skeleton. Our findings suggest that hox-based axial skeletal regionalization did not originate with tetrapods but rather has a much deeper evolutionary history than was previously appreciated.

The Evolution of Appendicular Muscles During the Fin-to-Limb Transition: Possible Insights Through Studies of Soft Tissues, a Perspective

The evolution of the appendages during the fin-to-limb transition has been extensively studied, yet the majority of studies focused on the skeleton and the fossil record. Whereas the evolution of the anatomy of the appendicular musculature has been studied, the changes in the muscular architecture during the fin-to-limb transition remain largely unstudied, yet may provide important new insights. The fin-to-limb transition is associated with the appearance of a new mode of locomotion and the associated shift from pectoral to pelvic dominance. Here, we propose ways to investigate this question and review data on muscle mass and muscle architecture of the pectoral and pelvic muscles in extant vertebrates. We explore whether changes in appendage type are associated with changes in the muscular architecture and the relative investment in different muscle groups. These preliminary data show a general increase in the muscle mass of the appendages relative to the body mass during the fin-to-limb transition. The locomotor shift suggested to occur during the fin-to-limb transition appears supported by our preliminary data since in “fish” the pectoral fins are heavier than the pelvic fins, whereas in tetrapods, the forelimb muscles are less developed than the hind limb muscles. Finally, a shift in the investment in different muscle groups with an increase of the contribution of the superficial groups in tetrapods compared to “fish” appears to take place. Our study highlights the potential of investigating quantitative features of the locomotor muscles, yet also demonstrates the lack of quantitative data allowing to test these ideas.

Variation in Articular Cartilage Thickness Among Extant Salamanders and Implications for Limb Function in Stem Tetrapods

The size and shape of articular cartilage in the limbs of extant vertebrates are highly variable, yet they are critical for understanding joint and limb function in an evolutionary context. For example, inferences about unpreserved articular cartilage in early tetrapods have implications for how limb length, joint range of motion, and muscle leverage changed over the tetrapod water-land transition. Extant salamanders, which are often used as functional models for early limbed vertebrates, have much thicker articular cartilage than most vertebrate groups, but the exact proportion of cartilage and how it varies across salamander species is unknown. I aimed to quantify this variation in a sample of 13 salamanders representing a broad range of sizes, modes of life, and genera. Using contrast-enhanced micro-CT, cartilage dimensions and bone length were measured non-destructively in the humerus, radius, ulna, femur, tibia, and fibula of each specimen. Cartilage correction factors were calculated as the combined thickness of the proximal and distal cartilages divided by the length of the bony shaft. Articular cartilage added about 30% to the length of the long bones on average. Cartilage was significantly thicker in aquatic salamanders (42 ± 14% in the humerus and 35 ± 8 in the femur) than in terrestrial salamanders (21 ± 7% in both humerus and femur). There was no consistent relationship between relative cartilage thickness and body size or phylogenetic relatedness. In addition to contributing to limb length, cartilage caps increased the width and breadth of the epiphyses by amounts that varied widely across taxa. To predict the effect of salamander-like cartilage correction factors on muscle leverage, a simplified model of the hindlimb of the Devonian stem tetrapod Acanthostega was built. In this model, the lever arms of muscles that cross the hip at an oblique angle to the femur was increased by up to six centimeters. Future reconstructions of osteological range of motion and muscle leverage in stem tetrapods and stem amphibians can be made more rigorous by explicitly considering the possible effects of unpreserved cartilage and justifying assumptions based on available data from extant taxa, including aquatic and terrestrial salamanders.

The postcranial anatomy of Whatcheeria deltae and its implications for the family Whatcheeriidae

Here we describe the postcranial skeleton and present the first full-body reconstruction of the early tetrapod Whatcheeria deltae from the Viséan of Iowa. The skeletal proportions, including an elongate neck and large limbs, are unlike those of other Devonian and Mississippian tetrapods. The robust limbs of Whatcheeria appear adapted for a walking gait, but the lateral lines of the cranium are fundamentally unsuited for sustained subaerial exposure. Thus, although Whatcheeria bears a general resemblance to certain terrestrially adapted Permian and Triassic members of crown tetrapod lineages, its unusual form signals a broader range of early amphibious morphologies and habits than previously considered. From the exceptionally rich collection it is evident that most Whatcheeria specimens represent immature individuals. Rare specimens suggest an adult body size of at least 2 m, over twice that of the holotype. Further comparison suggests that the Pederpes holotype might also be a juvenile and reveals a combination of hindlimb characters unique to Whatcheeria and Pederpes. These new data contribute to a revised diagnosis of the family Whatcheeriidae and a re-evaluation of fragmentary Devonian–Carboniferous fossils reported as ‘whatcheeriid’ but sharing no synapomorphies with the more precisely defined clade.

Evolution of forelimb musculoskeletal function across the fish-to-tetrapod transition

One of the most intriguing questions in vertebrate evolution is how tetrapods gained the ability to walk on land. Although many hypotheses have been proposed, few have been rigorously tested using the fossil record. Here, we build three-dimensional musculoskeletal models of the pectoral appendage in Eusthenopteron, Acanthostega, and Pederpes and quantitatively examine changes in forelimb function across the fin-to-limb transition. Through comparison with extant fishes and tetrapods, we show that early tetrapods share a suite of characters including restricted mobility in humerus long-axis rotation, increased muscular leverage for humeral retraction, but not depression/adduction, and increased mobility in elbow flexion-extension. We infer that the earliest steps in tetrapod forelimb evolution were related to limb-substrate interactions, whereas specializations for weight support appeared later. Together, these results suggest that competing selective pressures for aquatic and terrestrial environments produced a unique, ancestral "early tetrapod" forelimb locomotor mode unlike that of any extant animal.

Ontogenetic plasticity in cranial morphology is associated with a change in the food processing behavior in Alpine newts

Background

The feeding apparatus of salamanders consists mainly of the cranium, mandible, teeth, hyobranchial apparatus and the muscles of the cranial region. The morphology of the feeding apparatus in turn determines the boundary conditions for possible food processing (i.e., intraoral mechanical reduction) mechanisms. However, the morphology of the feeding apparatus changes substantially during metamorphosis, prompting the hypothesis that larvae might use a different food processing mechanism than post-metamorphic adults. Salamandrid newts with facultative metamorphosis are suitable for testing this hypothesis as adults with divergent feeding apparatus morphologies often coexist in the same population, share similar body sizes, and feed on overlapping prey spectra.

Methods

We use high-speed videography to quantify the in vivo movements of key anatomical elements during food processing in paedomorphic and metamorphic Alpine newts (Ichthyosaura alpestris). Additionally, we use micro-computed tomography (μCT) to analyze morphological differences in the feeding apparatus of paedomorphic and metamorphic Alpine newts and sort them into late-larval, mid-metamorphic and post-metamorphic morphotypes.

Results

Late-larval, mid-metamorphic and post-metamorphic individuals exhibited clear morphological differences in their feeding apparatus. Regardless of the paedomorphic state being externally evident, paedomorphic specimens can conceal different morphotypes (i.e., late-larval and mid-metamorphic morphotypes). Though feeding on the same prey under the same (aquatic) condition, food processing kinematics differed between late-larval, mid-metamorphic and post-metamorphic morphotypes.

Conclusions

The food processing mechanism in the Alpine newt changes along with morphology of the feeding apparatus during ontogeny, from a mandible-based to a tongue-based processing mechanism as the changing morphology of the mandible prevents chewing and the tongue allows enhanced protraction. These results could indicate that early tetrapods, in analogy to salamanders, may have developed new feeding mechanisms in their aquatic environment and that these functional innovations may have later paved the way for terrestrial feeding mechanisms.

Flexibility of intraoral food processing in the salamandrid newt Triturus carnifex: effects of environment and prey type

Intraoral food processing mechanisms are known for all major vertebrate groups, but the form and function of systems used to crush, grind or puncture food items can differ substantially between and within groups. Most vertebrates display flexible mechanisms of intraoral food processing with respect to different environmental conditions or food types. It has recently been shown that newts use cyclical loop-motions of the tongue to rasp prey against the palatal dentition. However, it remains unknown whether newts can adjust their food processing behavior in response to different food types or environmental conditions. Newts are interesting models for studying the functional adaptation to different conditions because of their unique and flexible lifestyle: they seasonally change between aquatic and terrestrial habitats, adapt their prey-capture mode to the respective environment, and consume diverse food types with different mechanical properties. Using X-ray high-speed recordings, anatomical investigations, behavioral analyses and mechanical property measurements, we tested the effects of the medium in which feeding occurs (water/air) and the food type (maggot, earthworm, cricket) on the processing behavior in Triturus carnifex We discovered that food processing, by contrast to prey capture, differed only slightly between aquatic and terrestrial habitats. However, newts adjusted the number of processing cycles to different prey types: while maggots were processed extensively, earthworm pieces were barely processed at all. We conclude that, in addition to food mechanical properties, sensory feedback such as smell and taste appear to induce flexible processing responses, while the medium in which feeding occurs appears to have less of an effect.

The smallest known Devonian tetrapod shows unexpectedly derived features

A new genus and species of Devonian tetrapod, Brittagnathus minutus gen. et sp. nov., is described from a single complete right lower jaw ramus recovered from the Acanthostega mass-death deposit in the upper part of the Britta Dal Formation (upper Famennian) of Stensiö Bjerg, Gauss Peninsula, East Greenland. Visualization by propagation phase contrast synchrotron microtomography allows a complete digital dissection of the specimen. With a total jaw ramus length of 44.8 mm, Brittagnathus is by far the smallest Devonian tetrapod described to date. It differs from all previously known Devonian tetrapods in having only a fang pair without a tooth row on the anterior coronoid and a large posterior process on the posterior coronoid. The presence of an incipient surangular crest and a concave prearticular margin to the adductor fossa together cause the fossa to face somewhat mesially, reminiscent of the condition in Carboniferous tetrapods. A phylogenetic analysis places Brittagnathus crownward to other Devonian tetrapods, adjacent to the Tournaisian genus Pederpes. Together with other recent discoveries, it suggests that diversification of 'Carboniferous-grade' tetrapods had already begun before the end of the Devonian and that the group was not greatly affected by the end-Devonian mass extinction.

The Spine: A Strong, Stable, and Flexible Structure with Biomimetics Potential

From its first appearance in early vertebrates, the spine evolved the function of protecting the spinal cord, avoiding excessive straining during body motion. Its stiffness and strength provided the basis for the development of the axial skeleton as the mechanical support of later animals, especially those which moved to the terrestrial environment where gravity loads are not alleviated by the buoyant force of water. In tetrapods, the functions of the spine can be summarized as follows: protecting the spinal cord; supporting the weight of the body, transmitting it to the ground through the limbs; allowing the motion of the trunk, through to its flexibility; providing robust origins and insertions to the muscles of trunk and limbs. This narrative review provides a brief perspective on the development of the spine in vertebrates, first from an evolutionary, and then from an embryological point of view. The paper describes functions and the shape of the spine throughout the whole evolution of vertebrates and vertebrate embryos, from primordial jawless fish to extant animals such as birds and humans, highlighting its fundamental features such as strength, stability, and flexibility, which gives it huge potential as a basis for bio-inspired technologies.

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.

Aquatic-terrestrial transitions of feeding systems in vertebrates: a mechanical perspective

Transitions to terrestrial environments confront ancestrally aquatic animals with several mechanical and physiological problems owing to the different physical properties of water and air. As aquatic feeders generally make use of flows of water relative to the head to capture, transport and swallow food, it follows that morphological and behavioral changes were inevitably needed for the aquatic animals to successfully perform these functions on land. Here, we summarize the mechanical requirements of successful aquatic-to-terrestrial transitions in food capture, transport and swallowing by vertebrates and review how different taxa managed to fulfill these requirements. Amphibious ray-finned fishes show a variety of strategies to stably lift the anterior trunk, as well as to grab ground-based food with their jaws. However, they still need to return to the water for the intra-oral transport and swallowing process. Using the same mechanical perspective, the potential capabilities of some of the earliest tetrapods to perform terrestrial feeding are evaluated. Within tetrapods, the appearance of a mobile neck and a muscular and movable tongue can safely be regarded as key factors in the colonization of land away from amphibious habitats. Comparative studies on taxa including salamanders, which change from aquatic feeders as larvae to terrestrial feeders as adults, illustrate remodeling patterns in the hyobranchial system that can be linked to its drastic change in function during feeding. Yet, the precise evolutionary history in form and function of the hyolingual system leading to the origin(s) of a muscular and adhesive tongue remains unknown.

Anatomical integration of the sacral-hindlimb unit coordinated by GDF11 underlies variation in hindlimb positioning in tetrapods

Elucidating how body parts from different primordia are integrated during development is essential for understanding the nature of morphological evolution. In tetrapod evolution, while the position of the hindlimb has diversified along with the vertebral formula, the mechanism responsible for this coordination has not been well understood. However, this synchronization suggests the presence of an evolutionarily conserved developmental mechanism that coordinates the positioning of the hindlimb skeleton derived from the lateral plate mesoderm with that of the sacral vertebrae derived from the somites. Here we show that GDF11 secreted from the posterior axial mesoderm is a key factor in the integration of sacral vertebrae and hindlimb positioning by inducing Hox gene expression in two different primordia. Manipulating the onset of GDF11 activity altered the position of the hindlimb in chicken embryos, indicating that the onset of Gdf11 expression is responsible for the coordinated positioning of the sacral vertebrae and hindlimbs. Through comparative analysis with other vertebrate embryos, we also show that each tetrapod species has a unique onset timing of Gdf11 expression, which is tightly correlated with the anteroposterior levels of the hindlimb bud. We conclude that the evolutionary diversity of hindlimb positioning resulted from heterochronic shifts in Gdf11 expression, which led to coordinated shifts in the sacral-hindlimb unit along the anteroposterior axis.

Terrestrial capture of prey by the reedfish, a model species for stem tetrapods

Due to morphological resemblance, polypterid fishes are used as extant analogues of Late Devonian lobe‐finned sarcopterygians to identify the features that allowed the evolution of a terrestrial lifestyle in early tetrapods. Previous studies using polypterids showed how terrestrial locomotion capacity can develop, and how air ventilation for breathing was possible in extinct tetrapodomorphs. Interestingly, one polypterid species, the reedfish Erpetoichthys calabaricus, has been noted being capable of capturing prey on land. We now identified the mechanism of terrestrial prey‐capture in reedfish. We showed that this species uses a lifted trunk and downward inclined head to capture ground‐based prey, remarkably similar to the mechanism described earlier for eel‐catfish. Reedfish similarly use the ground support and flexibility of their elongated body to realize the trunk elevation and dorsoventral flexion of the anterior trunk region, without a role for the pectoral fins. However, curving of the body to lift the trunk may not have been an option for the Devonian tetrapodomorphs as they are significantly less elongated than reedfish and eel‐catfish. This would imply that, in contrast to the eel‐like extant species, evolution of the capacity to capture prey on land in early tetrapods may be linked to the evolution of the pectoral system to lift the anterior part of the body.

Evolution of the patellar sesamoid bone in mammals

The patella is a sesamoid bone located in the major extensor tendon of the knee joint, in the hindlimb of many tetrapods. Although numerous aspects of knee morphology are ancient and conserved among most tetrapods, the evolutionary occurrence of an ossified patella is highly variable. Among extant (crown clade) groups it is found in most birds, most lizards, the monotreme mammals and almost all placental mammals, but it is absent in most marsupial mammals as well as many reptiles. Here, we integrate data from the literature and first-hand studies of fossil and recent skeletal remains to reconstruct the evolution of the mammalian patella. We infer that bony patellae most likely evolved between four and six times in crown group Mammalia: in monotremes, in the extinct multituberculates, in one or more stem-mammal genera outside of therian or eutherian mammals and up to three times in therian mammals. Furthermore, an ossified patella was lost several times in mammals, not including those with absent hindlimbs: once or more in marsupials (with some re-acquisition) and at least once in bats. Our inferences about patellar evolution in mammals are reciprocally informed by the existence of several human genetic conditions in which the patella is either absent or severely reduced. Clearly, development of the patella is under close genomic control, although its responsiveness to its mechanical environment is also important (and perhaps variable among taxa). Where a bony patella is present it plays an important role in hindlimb function, especially in resisting gravity by providing an enhanced lever system for the knee joint. Yet the evolutionary origins, persistence and modifications of a patella in diverse groups with widely varying habits and habitats-from digging to running to aquatic, small or large body sizes, bipeds or quadrupeds-remain complex and perplexing, impeding a conclusive synthesis of form, function, development and genetics across mammalian evolution. This meta-analysis takes an initial step toward such a synthesis by collating available data and elucidating areas of promising future inquiry.

A fish that uses its hydrodynamic tongue to feed on land

To capture and swallow food on land, a sticky tongue supported by the hyoid and gill arch skeleton has evolved in land vertebrates from aquatic ancestors that used mouth-cavity-expanding actions of the hyoid to suck food into the mouth. However, the evolutionary pathway bridging this drastic shift in feeding mechanism and associated hyoid motions remains unknown. Modern fish that feed on land may help to unravel the physical constraints and biomechanical solutions that led to terrestrialization of fish-feeding systems. Here, we show that the mudskipper emerges onto land with its mouth cavity filled with water, which it uses as a protruding and retracting 'hydrodynamic tongue' during the initial capture and subsequent intra-oral transport of food. Our analyses link this hydrodynamic action of the intra-oral water to a sequence of compressive and expansive cranial motions that diverge from the general pattern known for suction feeding in fishes. However, the hyoid motion pattern showed a remarkable resemblance to newts during tongue prehension. Consequently, although alternative scenarios cannot be excluded, hydrodynamic tongue usage may be a transitional step onto which the evolution of adhesive mucosa and intrinsic lingual muscles can be added to gain further independence from water for terrestrial foraging.

Homeotic transformations and number changes in the vertebral column of Triturus newts

We explored intraspecific variation in vertebral formulae, more specifically the variation in the number of thoracic vertebrae and frequencies of transitional sacral vertebrae in Triturus newts (Caudata: Salamandridae). Within salamandrid salamanders this monophyletic group shows the highest disparity in the number of thoracic vertebrae and considerable intraspecific variation in the number of thoracic vertebrae. Triturus species also differ in their ecological preferences, from predominantly terrestrial to largely aquatic. Following Geoffroy St. Hilaire's and Darwin's rule which states that structures with a large number of serially homologous repetitive elements are more variable than structures with smaller numbers, we hypothesized that the variation in vertebral formulae increases in more elongated species with a larger number of thoracic vertebrae. We furthermore hypothesized that the frequency of transitional vertebrae will be correlated with the variation in the number of thoracic vertebrae within the species. We also investigated potential effects of species hybridization on the vertebral formula. The proportion of individuals with a number of thoracic vertebrae different from the modal number and the range of variation in number of vertebrae significantly increased in species with a larger number of thoracic vertebrae. Contrary to our expectation, the frequencies of transitional vertebrae were not correlated with frequencies of change in the complete vertebrae number. The frequency of transitional sacral vertebra in hybrids did not significantly differ from that of the parental species. Such a pattern could be a result of selection pressure against transitional vertebrae and/or a bias towards the development of full vertebrae numbers. Although our data indicate relaxed selection for vertebral count changes in more elongated, aquatic species, more data on different selective pressures in species with different numbers of vertebrae in the two contrasting, terrestrial and aquatic environments are needed to test for causality.

Primordial germ cells: the first cell lineage or the last cells standing?

Embryos of many animal models express germ line determinants that suppress transcription and mediate early germ line commitment, which occurs before the somatic cell lineages are established. However, not all animals segregate their germ line in this manner. The 'last cell standing' model describes primordial germ cell (PGC) development in axolotls, in which PGCs are maintained by an extracellular signalling niche, and germ line commitment occurs after gastrulation. Here, we propose that this 'stochastic' mode of PGC specification is conserved in vertebrates, including non-rodent mammals. We postulate that early germ line segregation liberates genetic regulatory networks for somatic development to evolve, and that it therefore emerged repeatedly in the animal kingdom in response to natural selection.

A Diverse Tetrapod Fauna at the Base of 'Romer's Gap'

The lack of fossil tetrapod bearing deposits in the earliest Carboniferous (‘Romer’s Gap’) has provoked some recent discussions regarding the proximal cause, with three explanations being offered: environmental, taphonomic, and collection failure. One of the few, and earliest, windows into this time is the locality of Blue Beach exposed in the Tournaisian deposits at Horton Bluff lying along the Avon River near Hantsport, Nova Scotia, Canada. This locality has long been known but, because the fossils were deposited in high energy settings they are almost always disarticulated, so the fauna has not been described in detail. Recent intensive collection has revealed a diverse assemblage of material, including for the first time associated elements, which permits an evaluation of the faunal constituents at the locality. Although not diagnosable to a fine taxonomic level, sufficient apomorphies are present to identify representatives from numerous clades known from more complete specimens elsewhere. The evidence suggests a diverse fauna was present, including whatcheeriids and embolomeres. A single humerus previously had been attributed to a colosteid, but there is some uncertainty with this identification. Additional elements suggest the presence of taxa otherwise only known from the late Devonian. Depositional biases at the locality favor tetrapod fossils from larger individuals, but indirect evidence from trackways and tantalizing isolated bones evidences the presence of small taxa that remain to be discovered. The fossils from Blue Beach demonstrate that when windows into the fauna of ‘Romer’s Gap’ are found a rich diversity of tetrapods will be shown to be present, contra arguments that suggested this hiatus in the fossil record was due to extrinsic factors such as atmospheric oxygen levels. They also show that the early tetrapod fauna is not easily divisible into Devonian and Carboniferous faunas, suggesting that some tetrapods passed through the end Devonian extinction event unaffected.

Animal evolution and atmospheric pO2: is there a link between gradual animal adaptation to terrain elevation due to Ural orogeny and survival of subsequent hypoxic periods?

Considering evolution of terrestrial animals as something happening only on flat continental plains seems wrong. Many mountains have arisen and disappeared over the geologic time scale, so in all periods some areas of high altitude existed, with reduced oxygen pressure (pO2) and increased aridity. During orogeny, animal species of the raising terrain can slowly adapt to reduced oxygen levels.This review proposes that animal evolution was often driven by atmospheric oxygen availability. Transitions of insect ancestors and amphibians out of water are here interpreted as events forced by the lack of oxygen in shallow and warm water during Devonian. Hyperoxia during early Carboniferous allowed giant insects to be predators of lowlands, forcing small amphibians to move to higher terrains, unsuitable to large insects due to reduced pO2. In arid mountainous habitats, ascended animals evolved in early reptiles with more efficient lungs and improved circulation. Animals with alveolar lungs became the mammalian ancestors, while those with respiratory duct lungs developed in archosaurs. In this interpretation, limb precursors of wings and pneumatised bones might have been adaptations for moving on steep slopes.Ural mountains have risen to an estimated height of 3000 m between 318 and 251 Mya. The earliest archosaurs have been found on the European Ural side, estimated 275 Myr old. It is proposed that Ural orogeny slowly elevated several highland habitats within the modern Ural region to heights above 2500 m. Since this process took near 60 Myr, animals in these habitats fully to adapted to hypoxia.The protracted P-Tr hypoxic extinction event killed many aquatic and terrestrial animals. Devastated lowland areas were repopulated by mammaliaformes that came down from mountainous areas. Archosaurs were better adapted to very low pO2, so they were forced to descend to the sea level later when the lack of oxygen became severe. During the Triassic period, when the relative content of O2 reduced to near 12%, archosaurs prevailed as only animals that could cope with profound hypoxia at the sea level. Their diverse descendants has become dominant terrestrial animals, until the K-Pg extinction due to meteor impact.

Stochastic specification of primordial germ cells from mesoderm precursors in axolotl embryos

A common feature of development in most vertebrate models is the early segregation of the germ line from the soma. For example, in Xenopus and zebrafish embryos primordial germ cells (PGCs) are specified by germ plasm that is inherited from the egg; in mice, Blimp1 expression in the epiblast mediates the commitment of cells to the germ line. How these disparate mechanisms of PGC specification evolved is unknown. Here, in order to identify the ancestral mechanism of PGC specification in vertebrates, we studied PGC specification in embryos from the axolotl (Mexican salamander), a model for the tetrapod ancestor. In the axolotl, PGCs develop within mesoderm, and classic studies have reported their induction from primitive ectoderm (animal cap). We used an axolotl animal cap system to demonstrate that signalling through FGF and BMP4 induces PGCs. The role of FGF was then confirmed in vivo. We also showed PGC induction by Brachyury, in the presence of BMP4. These conditions induced pluripotent mesodermal precursors that give rise to a variety of somatic cell types, in addition to PGCs. Irreversible restriction of the germ line did not occur until the mid-tailbud stage, days after the somatic germ layers are established. Before this, germline potential was maintained by MAP kinase signalling. We propose that this stochastic mechanism of PGC specification, from mesodermal precursors, is conserved in vertebrates.

Lungfish axial muscle function and the vertebrate water to land transition

The role of axial form and function during the vertebrate water to land transition is poorly understood, in part because patterns of axial movement lack morphological correlates. The few studies available from elongate, semi-aquatic vertebrates suggest that moving on land may be powered simply from modifications of generalized swimming axial motor patterns and kinematics. Lungfish are an ideal group to study the role of axial function in terrestrial locomotion as they are the sister taxon to tetrapods and regularly move on land. Here we use electromyography and high-speed video to test whether lungfish moving on land use axial muscles similar to undulatory swimming or demonstrate novelty. We compared terrestrial lungfish data to data from lungfish swimming in different viscosities as well as to salamander locomotion. The terrestrial locomotion of lungfish involved substantial activity in the trunk muscles but almost no tail activity. Unlike other elongate vertebrates, lungfish moved on land with a standing wave pattern of axial muscle activity that closely resembled the pattern observed in terrestrially locomoting salamanders. The similarity in axial motor pattern in salamanders and lungfish suggests that some aspects of neuromuscular control for the axial movements involved in terrestrial locomotion were present before derived appendicular structures.

Pelvic girdle and fin of Tiktaalik roseae

A major challenge in understanding the origin of terrestrial vertebrates has been knowledge of the pelvis and hind appendage of their closest fish relatives. The pelvic girdle and appendage of tetrapods is dramatically larger and more robust than that of fish and contains a number of structures that provide greater musculoskeletal support for posture and locomotion. The discovery of pelvic material of the finned elpistostegalian, Tiktaalik roseae, bridges some of these differences. Multiple isolated pelves have been recovered, each of which has been prepared in three dimensions. Likewise, a complete pelvis and partial pelvic fin have been recovered in association with the type specimen. The pelves of Tiktaalik are paired and have broad iliac processes, flat and elongate pubes, and acetabulae that form a deep socket rimmed by a robust lip of bone. The pelvis is greatly enlarged relative to other finned tetrapodomorphs. Despite the enlargement and robusticity of the pelvis of Tiktaalik, it retains primitive features such as the lack of both an attachment for the sacral rib and an ischium. The pelvic fin of Tiktaalik (NUFV 108) is represented by fin rays and three endochondral elements: other elements are not preserved. The mosaic of primitive and derived features in Tiktaalik reveals that the enhancement of the pelvic appendage of tetrapods and, indeed, a trend toward hind limb-based propulsion have antecedents in the fins of their closest relatives.

Movement in a gravitational field: The question of limb interarticular coordination in terrestrial vertebrates

In this paper, we demonstrated that interarticular coordination of terrestrial tetrapods emerges from an environment highly constrained by friction and the gravitational field. We briefly review recent works on the jumping behavior in squamates, lemurs and amphibians. We then explore previously published work as well as some unpublished experimental data on human jumping. Finally, we end by inferring locomotion in some of the first limbed vertebrates using a simulation procedure. All these data show that despite changes in shape, structure, and motor controls of taxa, the same spatio-temporal sequence of joint displacements always occurs when the movement is executed in a terrestrial environment. Comparison with aquatic locomotion argues for the hypothesis that this pattern emerged in early terrestrial tetrapods as a response to the gravitational constraint and the terrestrial frictional environment.

Comparative pelvic development of the axolotl (Ambystoma mexicanum) and the Australian lungfish (Neoceratodus forsteri): conservation and innovation across the fish-tetrapod transition

Background

The fish-tetrapod transition was one of the major events in vertebrate evolution and was enabled by many morphological changes. Although the transformation of paired fish fins into tetrapod limbs has been a major topic of study in recent years, both from paleontological and comparative developmental perspectives, the interest has focused almost exclusively on the distal part of the appendage and in particular the origin of digits. Relatively little attention has been paid to the transformation of the pelvic girdle from a small unipartite structure to a large tripartite weight-bearing structure, allowing tetrapods to rely mostly on their hindlimbs for locomotion. In order to understand how the ischium and the ilium evolved and how the acetabulum was reoriented during this transition, growth series of the Australian lungfish Neoceratodus forsteri and the Mexican axolotl Ambystoma mexicanum were cleared and stained for cartilage and bone and immunostained for skeletal muscles. In order to understand the myological developmental data, hypotheses about the homologies of pelvic muscles in adults of Latimeria, Neoceratodus and Necturus were formulated based on descriptions from the literature of the coelacanth (Latimeria), the Australian Lungfish (Neoceratodus) and a salamander (Necturus).

Results

In the axolotl and the lungfish, the chondrification of the pelvic girdle starts at the acetabula and progresses anteriorly in the lungfish and anteriorly and posteriorly in the salamander. The ilium develops by extending dorsally to meet and connect to the sacral rib in the axolotl. Homologous muscles develop in the same order with the hypaxial musculature developing first, followed by the deep, then the superficial pelvic musculature.

Conclusions

Development of the pelvic endoskeleton and musculature is very similar in Neoceratodus and Ambystoma. If the acetabulum is seen as being a fixed landmark, the evolution of the ischium only required pubic pre-chondrogenic cells to migrate posteriorly. It is hypothesized that the iliac process or ridge present in most tetrapodomorph fish is the precursor to the tetrapod ilium and that its evolution mimicked its development in modern salamanders.

The evolutionary history of the development of the pelvic fin/hindlimb

The arms and legs of man are evolutionarily derived from the paired fins of primitive jawed fish. Few evolutionary changes have attracted as much attention as the origin of tetrapod limbs from the paired fins of ancestral fish. The hindlimbs of tetrapods are derived from the pelvic fins of ancestral fish. These evolutionary origins can be seen in the examination of shared gene and protein expression patterns during the development of pelvic fins and tetrapod hindlimbs. The pelvic fins of fish express key limb positioning, limb bud induction and limb outgrowth genes in a similar manner to that seen in hindlimb development of higher vertebrates. We are now at a point where many of the key players in the development of pelvic fins and vertebrate hindlimbs have been identified and we can now readily examine and compare mechanisms between species. This is yielding fascinating insights into how the developmental programme has altered during evolution and how that relates to anatomical change. The role of pelvic fins has also drastically changed over evolutionary history, from playing a minor role during swimming to developing into robust weight-bearing limbs. In addition, the pelvic fins/hindlimbs have been lost repeatedly in diverse species over evolutionary time. Here we review the evolution of pelvic fins and hindlimbs within the context of the changes in anatomical structure and the molecular mechanisms involved.

Evo-Devo of the Human Vertebral Column: On Homeotic Transformations, Pathologies and Prenatal Selection

Homeotic transformations of vertebrae are particularly common in humans and tend to come associated with malformations in a wide variety of organ systems. In a dataset of 1,389 deceased human foetuses and infants a majority had cervical ribs and approximately half of these individuals also had missing twelfth ribs or lumbar ribs. In ~10 % of all cases there was an additional shift of the lumbo-sacral boundary and, hence, homeotic transformations resulted in shifts of at least three vertebral boundaries. We found a strong coupling between the abnormality of the vertebral patterns and the amount and strength of associated malformations, i.e., the longer the disturbance of the vertebral patterning has lasted, the more associated malformations have developed and the more organ systems are affected. The germ layer of origin of the malformations was not significantly associated with the frequency of vertebral patterns. In contrast, we find significant associations with the different developmental mechanisms that are involved in the causation of the malformations, that is, segmentation, neural crest development, left-right patterning, etc. Our results, thus, suggest that locally perceived developmental signals are more important for the developmental outcome than the origin of the cells. The low robustness of vertebral A-P patterning apparent from the large number of homeotic transformations is probably caused by the strong interactivity of developmental processes and the low redundancy of involved morphogens during early organogenesis. Additionally, the early irreversibility of the specification of the A-P identity of vertebrae probably adds to the vulnerability of the process by limiting the possibility for recovery from developmental disturbances. The low developmental robustness of vertebral A-P patterning contrasts with a high robustness of the A-P patterning of the vertebral regions. Not only the order is invariable, also the variation in the number of vertebrae per region is small. This robustness is in agreement with the evolutionary stability of vertebral regions in tetrapods. Finally, we propose a new hypothesis regarding the constancy of the presacral number of vertebrae in mammals.

Tetrapod-like axial regionalization in an early ray-finned fish

Tetrapods possess up to five morphologically distinct vertebral series: cervical, thoracic, lumbar, sacral and caudal. The evolution of axial regionalization has been linked to derived Hox expression patterns during development and the demands of weight-bearing and walking on land. These evolutionary and functional explanations are supported by an absence of similar traits in fishes, living and extinct. Here, I show that, Tarrasius problematicus, a marine ray-finned fish from the Mississippian (Early Carboniferous; 359-318 Ma) of Scotland, is the first non-tetrapod known to possess tetrapod-like axial regionalization. Tarrasius exhibits five vertebral regions, including a seven-vertebrae 'cervical' series and a reinforced 'sacrum' over the pelvic area. Most vertebrae possess processes for intervertebral contact similar to tetrapod zygapophyses. The fully aquatic Tarrasius evolved these morphologies alongside other traits convergent with early tetrapods, including a naked trunk, and a single median continuous fin. Regional modifications in Tarrasius probably facilitated pelagic swimming, rather than a terrestrial lifestyle or walking gait, presenting an alternative scenario for the evolution of such traits in tetrapods. Axial regionalization in Tarrasius could indicate tetrapod-like Hox expression patterns, possibly representing the primitive state for jawed vertebrates. Alternately, it could signal a weaker relationship, or even a complete disconnect, between Hox expression domains and vertebrate axial plans.

Development and evolution of the muscles of the pelvic fin

Locomotor strategies in terrestrial tetrapods have evolved from the utilisation of sinusoidal contractions of axial musculature, evident in ancestral fish species, to the reliance on powerful and complex limb muscles to provide propulsive force. Within tetrapods, a hindlimb-dominant locomotor strategy predominates, and its evolution is considered critical for the evident success of the tetrapod transition onto land. Here, we determine the developmental mechanisms of pelvic fin muscle formation in living fish species at critical points within the vertebrate phylogeny and reveal a stepwise modification from a primitive to a more derived mode of pelvic fin muscle formation. A distinct process generates pelvic fin muscle in bony fishes that incorporates both primitive and derived characteristics of vertebrate appendicular muscle formation. We propose that the adoption of the fully derived mode of hindlimb muscle formation from this bimodal character state is an evolutionary innovation that was critical to the success of the tetrapod transition.

Acoustic communication at the water's edge: evolutionary insights from a mudskipper

Coupled behavioural observations and acoustical recordings of aggressive dyadic contests showed that the mudskipper Periophthalmodon septemradiatus communicates acoustically while out of water. An analysis of intraspecific variability showed that specific acoustic components may act as tags for individual recognition, further supporting the sounds' communicative value. A correlative analysis amongst acoustical properties and video-acoustical recordings in slow-motion supported first hypotheses on the emission mechanism. Acoustic transmission through the wet exposed substrate was also discussed. These observations were used to support an "exaptation hypothesis", i.e. the maintenance of key adaptations during the first stages of water-to-land vertebrate eco-evolutionary transitions (based on eco-evolutionary and palaeontological considerations), through a comparative bioacoustic analysis of aquatic and semiterrestrial gobiid taxa. In fact, a remarkable similarity was found between mudskipper vocalisations and those emitted by gobioids and other soniferous benthonic fishes.

Evolution of the axial system in craniates: morphology and function of the perivertebral musculature

The axial musculoskeletal system represents the plesiomorphic locomotor engine of the vertebrate body, playing a central role in locomotion. In craniates, the evolution of the postcranial skeleton is characterized by two major transformations. First, the axial skeleton became increasingly functionally and morphologically regionalized. Second, the axial-based locomotion plesiomorphic for craniates became progressively appendage-based with the evolution of extremities in tetrapods. These changes, together with the transition to land, caused increased complexity in the planes in which axial movements occur and moments act on the body and were accompanied by profound changes in axial muscle function. To increase our understanding of the evolutionary transformations of the structure and function of the perivertebral musculature, this review integrates recent anatomical and physiological data (e.g., muscle fiber types, activation patterns) with gross-anatomical and kinematic findings for pivotal craniate taxa. This information is mapped onto a phylogenetic hypothesis to infer the putative character set of the last common ancestor of the respective taxa and to conjecture patterns of locomotor and muscular evolution. The increasing anatomical and functional complexity in the muscular arrangement during craniate evolution is associated with changes in fiber angulation and fiber-type distribution, i.e., increasing obliqueness in fiber orientation and segregation of fatigue-resistant fibers in deeper muscle regions. The loss of superficial fatigue-resistant fibers may be related to the profound gross anatomical reorganization of the axial musculature during the tetrapod evolution. The plesiomorphic function of the axial musculature -mobilization- is retained in all craniates. Along with the evolution of limbs and the subsequent transition to land, axial muscles additionally function to globally stabilize the trunk against inertial and extrinsic limb muscle forces as well as gravitational forces. Associated with the evolution of sagittal mobility and a parasagittal limb posture, axial muscles in mammals also stabilize the trunk against sagittal components of extrinsic limb muscle action as well as the inertia of the body's center of mass. Thus, the axial system is central to the static and dynamic control of the body posture in all craniates and, in gnathostomes, additionally provides the foundation for the mechanical work of the appendicular system.

Evolution of the germ line-soma relationship in vertebrate embryos

The germ line and soma together maintain genetic lineages from generation to generation: the germ line passes genetic information between generations; the soma is the vehicle for germ line transmission, and is shaped by natural selection. The germ line and somatic lineages arise simultaneously in early embryos, but how their development is related depends on how primordial germ cells (PGC) are specified. PGCs are specified by one of two means. Epigenesis describes the induction of PGCs from pluripotent cells by signals from surrounding somatic tissues. In contrast, PGCs in many species are specified cell-autonomously by maternally derived molecules, known as germ plasm, and this is called preformation. Germ plasm inhibits signaling to PGCs; thus, they are specified cell-autonomously. Germ plasm evolved independently in many animal lineages, suggesting convergent evolution, and therefore it would be expected to convey a selective advantage. But, what this is remains unknown. We propose that the selective advantage that drives the emergence of germ plasm in vertebrates is the disengagement of germ line specification from somatic influences. This liberates the evolution of gene regulatory networks (GRNs) that govern somatic development, and thereby enhances species evolvability, a well-recognized selective advantage. We cite recent evidence showing that frog embryos, which contain germ plasm, have modified GRNs that are not conserved in axolotls, which represent more basal amphibians and employ epigenesis. We also present the correlation of preformation with enhanced species radiations, and we discuss the mutually exclusive trajectories influenced by germ plasm or pluripotency, which shaped chordate evolution.

Limb-bone histology of temnospondyls: implications for understanding the diversification of palaeoecologies and patterns of locomotion of Permo-Triassic tetrapods

The locomotion of early tetrapods has long been a subject of great interest in the evolutionary history of vertebrates. However, we still do not have a precise understanding of the evolutionary radiation of their locomotory strategies. We present here the first palaeohistological study based on theoretical biomechanical considerations among a highly diversified group of early tetrapods, the temnospondyls. Based on the quantification of microanatomical and histological parameters in the humerus and femur of nine genera, this multivariate analysis provides new insights concerning the adaptations of temnospondyls to their palaeoenvironments during the Early Permian, and clearly after the Permo-Triassic crisis. This study therefore presents a methodology that, if based on a bigger sample, could contribute towards a characterization of the behaviour of species during great evolutionary events.