Tetrapodomorph Fish Research Articles

Reconstructing the subcephalic musculature in Pucapampella and Ichthyostega.

We present new reconstructions of subcephalic musculature for the stem chondrichthyan Pucapampella, the tetrapodomorph fish Eusthenopteron, and the Devonian tetrapod Ichthyostega. These reconstructions are based on macroscopic dissections of the head muscles of an archaic shark Heptranchias and an archaic actinopterygian Polypterus, that are combined with functional considerations and a reappraisal of not widely known theoretical concepts from the past. The subcephalic, as well as the supracephalic, musculature is formed by four anterior myomeres. They are continuous with subsequent myomeres of the trunk, but are innervated by ventral nerve roots of the medulla oblongata and thus belong to the head. The fourth subcephalic myomere ends with its posterior myoseptum on the occiput in osteichthyans, but on the first vertebra in chondrichthyans. The original function of subcephalic and supracephalic muscles in basal gnathostomes supposedly was to hold together anterior and posterior parts of the neurocranium during interaction with prey, such as the backward-ripping prey dissection, hypothesized for Pucapampella. In sarcopterygian osteichthyans, subcephalic musculature is involved in active depression of the anterior part of the neurocranium; specialization of this mechanism resulted in a complete separation of m. subcephalicus from trunk myomeres in Latimeria. Fusion of anterior and posterior parts of the neurocranium has resulted in reduction of the subcephalic musculature in the majority of cartilaginous and bony fishes. However, hexanchid sharks retain three posterior subcephalic myomeres for backward-ripping prey dissection. Polypterus and Chauliodus have retained the subcephalic musculature, but its function has shifted to a depression of the whole neurocranium.

A fresh look at Cladarosymblema narrienense, a tetrapodomorph fish (Sarcopterygii: Megalichthyidae) from the Carboniferous of Australia, illuminated via X-ray tomography.

BackgroundThe megalichthyids are one of several clades of extinct tetrapodomorph fish that lived throughout the Devonian–Permian periods. They are advanced “osteolepidid-grade” fishes that lived in freshwater swamp and lake environments, with some taxa growing to very large sizes. They bear cosmine-covered bones and a large premaxillary tusk that lies lingually to a row of small teeth. Diagnosis of the family remains controversial with various authors revising it several times in recent works. There are fewer than 10 genera known globally, and only one member definitively identified from Gondwana. Cladarosymblema narrienense Fox et al. 1995 was described from the Lower Carboniferous Raymond Formation in Queensland, Australia, on the basis of several well-preserved specimens. Despite this detailed work, several aspects of its anatomy remain undescribed.MethodsTwo especially well-preserved 3D fossils of Cladarosymblema narrienense, including the holotype specimen, are scanned using synchrotron or micro-computed tomography (µCT), and 3D modelled using specialist segmentation and visualisation software. New anatomical detail, in particular internal anatomy, is revealed for the first time in this taxon. A novel phylogenetic matrix, adapted from other recent work on tetrapodomorphs, is used to clarify the interrelationships of the megalichthyids and confirm the phylogenetic position of C. narrienense.ResultsNever before seen morphological details of the palate, hyoid arch, basibranchial skeleton, pectoral girdle and axial skeleton are revealed and described. Several additional features are confirmed or updated from the original description. Moreover, the first full, virtual cranial endocast of any tetrapodomorph fish is presented and described, giving insight into the early neural adaptations in this group. Phylogenetic analysis confirms the monophyly of the Megalichthyidae with seven genera included (Askerichthys, Cladarosymblema, Ectosteorhachis, Mahalalepis, Megalichthys, Palatinichthys, and Sengoerichthys). The position of the megalichthyids as sister group to canowindrids, crownward of “osteolepidids” (e.g.,Osteolepis and Gogonasus), but below “tristichopterids” such as Eusthenopteron is confirmed, but our findings suggest further work is required to resolve megalichthyid interrelationships.

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.

Brain Reconstruction Across the Fish-Tetrapod Transition; Insights From Modern Amphibians

The fish-tetrapod transition (which incorporates the related fin-limb and water-land transitions) is celebrated as one of the most important junctions in vertebrate evolution. Sarcopterygian fishes (the “lobe-fins”) are today represented by lungfishes and coelacanths, but during the Paleozoic they were much more diverse. It was some of these sarcopterygians, a lineage of the tetrapodomorph fishes, that gave rise to tetrapods (terrestrial vertebrates with limbs bearing digits). This spectacular leap took place during the Devonian Period. Due to the nature of preservation, it is the hard parts of an animal’s body that are most likely to fossilize, while soft tissues such as muscular and brain tissues, typically fail to do so. Thus, our understanding of the adaptations of the hard skeletal structures of vertebrates is considerably greater than that of the soft tissue systems. Fortunately, the braincases of early vertebrates are often ossified and thereby have the potential to provide detailed morphological information. However, the correspondence between brain and endocast (an internal mold of the cavity) has historically been considered poor in most “lower” vertebrates and consequently neglected in such studies of brain evolution. Despite this, recent work documenting the spatial relationship in extant basal sarcopterygians (coelacanth, lungfish, axolotl, and salamander) has highlighted that this is not uniformly the case. Herein, we quantify and illustrate the brain-endocast relationship in four additional extant basal tetrapod exemplars: neobatrachian anurans (frogs) Breviceps poweri and Ceratophrys ornata; and gymnophionans (caecilians) Gegeneophis ramaswamii and Rhinatrema bivittatum. We show that anurans and caecilians appear to have brains that fill their endocasts to a similar degree to that of lungfishes and salamanders, but not coelacanth. Ceratophrys has considerably lower correspondence between the brain and endocast in the olfactory tract and mesencephalic regions, while Breviceps has low correspondence along its ventral endocranial margin. The brains of caecilians reflect their endocasts most closely (vol. ∼70%). The telencephalon is tightly fitted within the endocast in all four taxa. Our findings highlight the need to adequately assess the brain-endocast relationship in a broad range of vertebrates, in order to inform neural reconstructions of fossil taxa using the Extant Phylogenetic Bracket approach and future studies of brain evolution.

A New Look at Carboniferous Rhizodontid Humeri (Sarcopterygii; Tetrapodomorpha)

ABSTRACT The Rhizodontida are a group of tetrapodomorph sarcopterygian fishes with a worldwide distribution in the Devonian and Carboniferous periods. Rhizodontids were first described in the U.K., with Carboniferous taxa such as Rhizodus, Strepsodus, and Archichthys each based on isolated teeth (probably symphysial tusks) with distinctive morphologies. Some isolated fin material has also been described, including humeri, but this material has generally been assigned to either Rhizodus or Strepsodus purely on the basis of its size (with the larger material assigned to Rhizodus) or lithology (with specimens in coalshale assigned to Strepsodus). As a first step towards resolving this situation, we have utilized micro-CT scans of U.K. rhizodontid humeri, including newly discovered specimens from Scotland, to establish four distinct humeral morphogroups. This provides a robust framework for interpreting future discoveries, with the hope that these morphogroups can ultimately be securely linked with the distinctive tooth morphologies. The histology of the endochondral bone of the rhizodontid humerus is also described for the first time. http://zoobank.org/urn:lsid:zoobank.org:pub:DABDD50C-9072-4932-BECF-2F6D2AF50C94

The Upper Devonian tetrapodomorph Gogonasus andrewsae from Western Australia: Reconstruction of the shoulder girdle and opercular series using X-ray Micro-Computed Tomography

The tetrapodomorph fish, Gogonasus andrewsae is a three dimensionally well-preserved sarcopterygian from the Gogo Formation (Frasnian, early Upper Devonian, ∼380 million years ago) in Western Australia. High-resolution X-ray Micro-Computed Tomography and 3D printouts were used to obtain a digital reconstruction of its shoulder girdle and opercular series. Our new findings show the opercular series in a close fit against the upper bones of the shoulder girdle only if the anocleithrum, supracleithrum and post-temporal are aligned more horizontally than in previous reconstructions. The lowermost subopercular bone also differs, in partly covering the clavicle of the shoulder girdle. The ascending process of the clavicle, and the ventral process of the anocleithrum, do not fit closely inside the cleithrum, and perhaps functioned for ligamentous attachment. A rugose area on the anocleithral process is in a similar relative position to the attachment of a muscle ligament on the shoulder girdle of various living actinopterygians. Our manipulation of 3D printouts permits testing of the morphological fit of extremely fragile acid-etched bones, and indicates a new way to investigate the constructional morphology of one or more mechanical units of the vertebrate skeleton. It is suggested that Micro-CT imaging, reconstruction, visualisation and 3D printing techniques will provide a rigorous new test leading to modification of previous reconstructions of extinct vertebrates that were based on graphical methods and 2D imaging.

Evolution of Hindlimb Muscle Anatomy Across the Tetrapod Water-to-Land Transition, Including Comparisons With Forelimb Anatomy.

Tetrapod limbs are a key innovation implicated in the evolutionary success of the clade. Although musculoskeletal evolution of the pectoral appendage across the fins-to-limbs transition is fairly well documented, that of the pelvic appendage is much less so. The skeletal elements of the pelvic appendage in some tetrapodomorph fish and the earliest tetrapods are relatively smaller and/or qualitatively less similar to those of crown tetrapods than those of the pectoral appendage. However, comparative and developmental works have suggested that the musculature of the tetrapod forelimb and hindlimb was initially very similar, constituting a "similarity bottleneck" at the fins-to-limbs transition. Here, we used extant phylogenetic bracketing and phylogenetic character optimization to reconstruct pelvic appendicular muscle anatomy in several key taxa spanning the fins-to-limbs and water-to-land transitions. Our results support the hypothesis that transformation of the pelvic appendages from fin-like to limb-like lagged behind that of the pectoral appendages. Compared to similar reconstructions of the pectoral appendages, the pelvic appendages of the earliest tetrapods had fewer muscles, particularly in the distal limb (shank). In addition, our results suggest that the first tetrapods had a greater number of muscle-muscle topological correspondences between the pectoral and pelvic appendages than tetrapodomorph fish had. However, ancestral crown-group tetrapods appear to have had an even greater number of similar muscles (both in terms of number and as a percentage of the total number of muscles), indicating that the main topological similarity bottleneck between the paired appendages may have occurred at the origin of the tetrapod crown group. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc. Anat Rec, 303:218-234, 2020. © 2018 American Association for Anatomy.

New insights into the origins and radiation of the mid-Palaeozoic Gondwanan stem tetrapods

ABSTRACTThe earliest tetrapodomorph fishes appear in Chinese deposits of Early Devonian age, and by the Middle Devonian they were widespread globally. Evidence for the earliest digitated tetrapods comes from largely uncontested Middle Devonian trackways and Late Devonian body fossils. The East Gondwana Provence (Australasia, Antarctica) fills vital gaps in the phylogenetic and biogeographic history of the tetrapods, with the Gondwanan clade Canowindididae exhibiting a high degree of endemism within the early part of the stem tetrapod radiation. New anatomical details of Koharalepis, from the Middle Devonian Aztec Siltstone of Antarctica, are elucidated from synchrotron scan data. These include the position of the orbit, the condition of the hyomandibular, the shape of the palate and arrangement of the vomerine fangs. Biogeographical and phylogenetic models of stem tetrapod origins and radiations are discussed.

Reconstructing pectoral appendicular muscle anatomy in fossil fish and tetrapods over the fins-to-limbs transition.

The question of how tetrapod limbs evolved from fins is one of the great puzzles of evolutionary biology. While palaeontologists, developmental biologists, and geneticists have made great strides in explaining the origin and early evolution of limb skeletal structures, that of the muscles remains largely unknown. The main reason is the lack of consensus about appendicular muscle homology between the closest living relatives of early tetrapods: lobe-finned fish and crown tetrapods. In the light of a recent study of these homologies, we re-examined osteological correlates of muscle attachment in the pectoral girdle, humerus, radius, and ulna of early tetrapods and their close relatives. Twenty-nine extinct and six extant sarcopterygians were included in a meta-analysis using information from the literature and from original specimens, when possible. We analysed these osteological correlates using parsimony-based character optimization in order to reconstruct muscle anatomy in ancestral lobe-finned fish, tetrapodomorph fish, stem tetrapods, and crown tetrapods. Our synthesis revealed that many tetrapod shoulder muscles probably were already present in tetrapodomorph fish, while most of the more-distal appendicular muscles either arose later from largely undifferentiated dorsal and ventral muscle masses or did not leave clear correlates of attachment in these taxa. Based on this review and meta-analysis, we postulate a stepwise sequence of specific appendicular muscle acquisitions, splits, and fusions that led from the ancestral sarcopterygian pectoral fin to the ancestral tetrapod forelimb. This sequence largely agrees with previous hypotheses based on palaeontological and comparative work, but it is much more comprehensive in terms of both muscles and taxa. Combined with existing information about the skeletal system, our new synthesis helps to illuminate the genetic, developmental, morphological, functional, and ecological changes that were key components of the fins-to-limbs transition.

How did our face evolve from fish-like ancestors?

Face, the frontal portion of the head of vertebrates, plays a crucial role in the physical and social life in human and other animals. The face of human is thought to represent one’s identity and personality. Indeed, the word “person” or “people” in modern English is the etymological descendant of the ancient Greek word “πρόσωπον” or prosopon . It was only later realized that similar facial features shared by human and other vertebrates have common origins. Since then, the evolution of face has long been the spotlight issue in evolutionary biology. A series of important advances in evolutionary genomics, evolutionary developmental biology and paleontology has renewed our understanding of the vertebrate face evolution from fish to human. In this paper we briefly reviewed some of these advances following the steps of the vertebrate facial evolution from the architecture of craniates, “a new head”, to the emergence of the expressive mask in human. The “new head”, with supporting skull and novel sensory organs, vastly improved the sensory and overall evolutionary potentials of vertebrates. The developmental foundation of this innovation is the migration of neural crest cells and the formation of placodes. The origin of jaw is another key innovation in the rise of modern jawed vertebrate face. The traditional hypothesis that takes the jaw as the derivate of the anterior gill arch is under challenge. The intersection of developmental biology and paleontological evidences contributes to a well-supported new model, in which the disassociation of the naso-hypophyseal complex is a prerequisite for the origin of the jaw. The similarity between jaws and gill arches might be secondary. The recent discovery of Silurian maxillate placoderms illuminates the origin of marginal dermal jaw bones that leads to the primary functional jaw bones in most modern vertebrates including ourselves. We now have a clear picture of how facial organs such as noses, eyes, ears and teeth adapt to great transitions in vertebrate evolution such as that from water to land. The nose, originally an olfactory organ, secondarily adapts to become a part of the respiration system in the rise of terrestrial vertebrates. Early Devonian tetrapodomorph fish Kenichthys shows a rare transitional condition in the evolution of the nose, in which the premaxilla and maxilla split to display a notch, representing the posterior external nostril before it moved into the oral cavity to become the choana. The ear, after various innovative transformations, especially the incorporation of the novel middle ear and pinna, is modified to become an exquisite sound detection apparatus from its original function of balance. In contrast, the design of the camera-type eyes in all vertebrates is hardly changed since its initial emergence, with minor changes in its auxiliary organs. Competing hypotheses are forwarded to explain the origin and evolution of tooth and dentition. The enamel, the hardest tissue in vertebrate body first emerged on the surface of dermal scales in early bony fishes as a protection, and only covered the teeth secondarily. The pharyngeal muscles, which first evolved for feeding and respiration, immigrate to the frontal region of face in mammals to become mimetic muscles that control the facial gestures. All these shifts in the structures and functions of vertebrate faces finally result in the advent of facial features in modern Homo sapiens . In light of the facial evolution in our mammalian close relatives, the polishing of our hominine mask with personality has dual evolutionary controlling factors, physical and social. In addition, the foundation of the dominant importance of face in human social life lies in the modification of the facial traits in the common ancestors of human and other animals over deep time.

Changes in pectoral appendicular muscle anatomy and function over the water‐land transition in tetrapods

The question of how tetrapod limbs evolved from fins is one of the great puzzles of vertebrate biology. While paleontologists, developmental biologists, and geneticists have made great strides in explaining the origin and early evolution of limb skeletal structures, that of the muscles remains largely unknown. We sought to fill this gap by reconstructing pectoral appendicular muscles in three fossil taxa and measuring changes in muscle leverage about the shoulder and elbow joints over the fin‐limb and water‐land transitions. Building upon hypotheses of muscle homology recently published by our group and osteological correlates from the literature, we used extant phylogenetic bracketing and character optimization to reconstruct pectoral appendicular muscles in fossil taxa spanning the fin‐limb and water‐land transitions: the tetrapodomorph fish Eusthenopteron and the stem tetrapods Acanthostega and Pederpes. Then, we built 3D biomechanical models of the three fossil taxa, plus an extant lungfish and an extant coelacanth to compare the leverage of various muscle groups over a range of limb postures. Our preliminary results support the idea that one of the first major functional changes in the ancestors of tetrapods was the division of the superficial dorsal muscle mass of the shoulder into deltoid and latissimus dorsi muscles. This division would have provided leverage for forelimb protraction and retraction over a larger range of motion, likely facilitating tetrapod‐like gaits.Support or Funding InformationAAA Post‐doctoral Fellowship to J. Molnar

A large, anatomically primitive tristichopterid (Sarcopterygii: Tetrapodomorpha) from the Late Devonian (Frasnian) Alves Beds, Upper Old Red Sandstone, Moray, Scotland

Synopsis A nearly complete mandible of a large tetrapodomorph sarcopterygian from the Frasnian Alves Beds is studied with the aid of computed tomography. The absence of cosmine, a small parasymphysial dental plate bearing dentition that is discontinuous with that of the coronoids, and the presence of an elongated posterior coronoid that bears two fang pairs restrict placement of the tetrapodomorph from the Alves Beds to Tristichopteridae, and suggest that is a member of the clade that is sister to the anatomically primitive Givetian genus Tristichopterus . Taxa from this segment of tristichopterid phylogeny include the Givetian–Frasnian Eusthenopteron and Frasnian Jarvikina , but the tristichopterid from the Alves Beds is distinguished from both by the presence of an organized row of well-developed teeth on the posterior coronoid. This character has been proposed as a synapomorphy of taxa crownward of Trischopteridae, but is widely distributed among tetrapodomorph fishes. Our reinterpretation of lower jaw material of Tinirau clackae suggests this species is a member of the Tristichopteridae rather than the ‘Elpistostegalia’, and likely includes material assigned to a second species and genus of tetrapodomorph from the Red Hill site, Bruehnopteron murphyi . Proposal of a formal taxonomic name for the tristichopterid from the Alves Beds is withheld pending the discovery or analysis of further material from these deposits.

Feeding biomechanics inAcanthostegaand across the fish–tetrapod transition

Acanthostega is one of the earliest and most primitive limbed vertebrates. Its numerous fish-like features indicate a primarily aquatic lifestyle, yet cranial suture morphology suggests that its skull is more similar to those of terrestrial taxa. Here, we apply geometric morphometrics and two-dimensional finite-element analysis to the lower jaws of Acanthostega and 22 other tetrapodomorph taxa in order to quantify morphological and functional changes across the fish-tetrapod transition. The jaw of Acanthostega is similar to that of certain tetrapodomorph fish and transitional Devonian taxa both morphologically (as indicated by its proximity to those taxa in morphospace) and functionally (as indicated by the distribution of stress values and relative magnitude of bite force). Our results suggest a slow tempo of morphological and biomechanical changes in the transition from Devonian tetrapod jaws to aquatic/semi-aquatic Carboniferous tetrapod jaws. We conclude that Acanthostega retained a primitively aquatic lifestyle and did not possess cranial adaptations for terrestrial feeding.

Do mudskippers and lungfishes elucidate the early evolution of four-limbed vertebrates?

BackgroundRecently, the origin of four-limbed vertebrates (tetrapods) was re-assessed, based on footprints and trackways that were made by early land animals almost 400 million years (my) ago. That is 18 my earlier than the oldest known tetrapod body fossils, such as Acanthostega and Ichthyostega from Greenland, and 10 my older than the earliest ‘tetrapodomorph fishes’ (Panderichthys, Tiktaalik etc.). These and other facts suggest that the first tetrapods may have thrived in the marine-influenced intertidal and/or lagoon zone as well as in brackish and freshwater environments associated with land vegetation, as previously thought.ResultsHere, we discuss the controversial question whether or not extant air-breathing fishes, such as mudskippers, amphibious gobioids that inhabit mangrove swamps, can be interpreted as living model organisms, with reference to the earliest land plants. In addition, recent developmental and behavioural studies on lungfishes (Dipnoi) are summarized and evaluated.ConclusionsWe conclude that mudskipping ‘walking fishes’ (Periophthalmus sp.) and Dipnoi (Protopterus sp.) shed light on the gradual evolutionary transition of ancient fishes to early tetrapods that occurred during the Devonian in muddy, salty waters. However, they are not the ancestors of tetrapods, because extant organisms cannot be progenitors of other living beings.

Vertebral architecture in the earliest stem tetrapods

The construction of the vertebral column has been used as a key anatomical character in defining and diagnosing early tetrapod groups. Rhachitomous vertebrae--in which there is a dorsally placed neural arch and spine, an anteroventrally placed intercentrum and paired, posterodorsally placed pleurocentra--have long been considered the ancestral morphology for tetrapods. Nonetheless, very little is known about vertebral anatomy in the earliest stem tetrapods, because most specimens remain trapped in surrounding matrix, obscuring important anatomical features. Here we describe the three-dimensional vertebral architecture of the Late Devonian stem tetrapod Ichthyostega using propagation phase-contrast X-ray synchrotron microtomography. Our scans reveal a diverse array of new morphological, and associated developmental and functional, characteristics, including a possible posterior-to-anterior vertebral ossification sequence and the first evolutionary appearance of ossified sternal elements. One of the most intriguing features relates to the positional relationships between the vertebral elements, with the pleurocentra being unexpectedly sutured or fused to the intercentra that directly succeed them, indicating a 'reverse' rhachitomous design. Comparison of Ichthyostega with two other stem tetrapods, Acanthostega and Pederpes, shows that reverse rhachitomous vertebrae may be the ancestral condition for limbed vertebrates. This study fundamentally revises our current understanding of vertebral column evolution in the earliest tetrapods and raises questions about the presumed vertebral architecture of tetrapodomorph fish and later, more crownward, tetrapods.

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

BackgroundThe 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).ResultsIn 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.ConclusionsDevelopment 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.

Pectoral girdle and fin anatomy of Gogonasus andrewsae long, 1985: Implications for tetrapodomorph limb evolution

Recently discovered material has yielded new information on the pectoral girdle and fin endoskeleton of Gogonasusandrewsae (Frasnian Gogo Formation, Kimberley Region, Western Australia). These elements permit the first comprehensive description of the anocleithrum, cleithrum, scapulocoracoid, and lepidotrichia. New autapomorphies of Gogonasus include a square exposed region on the supracleithrum, an unusual knob-like process on the scapulocoracoid, a relatively small entepicondyle, and lepidotrichia with I-beam-shaped cross sections. Several poorly ossified regions on the scapulocoracoid and humerus indicate an early ontogenetic state, as with other immature tetrapodomorph fish specimens. A phylogenetic analysis indicates a more stemward position for Gogonasus in a weakly supported clade with other "osteolepidid" taxa, compared to other recent studies placing Gogonasus crownward of osteolepidid fishes and the Tristichopteridae, as the sister taxon to the "Elpistosteglia" + Tetrapoda. A phylogenetic position among megalichthyid fishes is suggested for Sterropterygion, while radiographs of the megalichthyid Cladarosymblema show a scythe-like radius terminating distally with that of the intermedium. New data on the scapulocoracoid of the rhizodontid Barameda reveals a coroacoid crest and small supraglenoid foramen.

Three-dimensional limb joint mobility in the early tetrapod Ichthyostega

The origin of tetrapods and the transition from swimming to walking was a pivotal step in the evolution and diversification of terrestrial vertebrates. During this time, modifications of the limbs—particularly the specialization of joints and the structures that guide their motions—fundamentally changed the ways in which early tetrapods could move. Nonetheless, little is known about the functional consequences of limb anatomy in early tetrapods and how that anatomy influenced locomotion capabilities at this very critical stage in vertebrate evolution. Here we present a three-dimensional reconstruction of the iconic Devonian tetrapod Ichthyostega and a quantitative and comparative analysis of limb mobility in this early tetrapod. We show that Ichthyostega could not have employed typical tetrapod locomotory behaviours, such as lateral sequence walking. In particular, it lacked the necessary rotary motions in its limbs to push the body off the ground and move the limbs in an alternating sequence. Given that long-axis rotation was present in the fins of tetrapodomorph fishes, it seems that either early tetrapods evolved through an initial stage of restricted shoulder and hip joint mobility or that Ichthyostega was unique in this respect. We conclude that early tetrapods with the skeletal morphology and limb mobility of Ichthyostega were unlikely to have made some of the recently described Middle Devonian trackways.

Evolution of vertebrate appendicular structures: Insight from genetic and palaeontological data

The new body of evidence from fossils and comparative-developmental analysis of subset of appendicular patterning genes has revealed that limb elements seen in tetrapods are assembled in fish fin over evolutionary time. However, despite of deep homology in basic structure and underlying developmental system, there remains a large morphological gap between distal elements of tetrapod limb and distal fin skeleton of tetrapodomorph fish. Understanding the genetic basis of major transformations in distal-limb morphology is the next challenge for evolutionary developmental biologists. Here by integrating data from fossils, comparative-developmental and genetic studies, models are proposed describing the evolution of cis-regulatory elements as a basis for diversification of appendicular architecture. Instead of emphasizing the subset of developmental genes, for instance Hoxd genes, the focus here is on the significance of elucidating cis-regulatory elements for multiple other key molecular players of limb/fin development and genetic/molecular interactions among them, for a better understanding of the developmental and genetic basis of limb evolution.

Air-breathing adaptation in a marine Devonian lungfish

Recent discoveries of tetrapod trackways in 395 Myr old tidal zone deposits of Poland (Niedźwiedzki et al. 2010 Nature 463, 43-48 (doi:10.1038/nature.08623)) indicate that vertebrates had already ventured out of the water and might already have developed some air-breathing capacity by the Middle Devonian. Air-breathing in lungfishes is not considered to be a shared specialization with tetrapods, but evolved independently. Air-breathing in lungfishes has been postulated as starting in Middle Devonian times (ca 385 Ma) in freshwater habitats, based on a set of skeletal characters involved in air-breathing in extant lungfishes. New discoveries described herein of the lungfish Rhinodipterus from marine limestones of Australia identifies the node in dipnoan phylogeny where air-breathing begins, and confirms that lungfishes living in marine habitats had also developed specializations to breathe air by the start of the Late Devonian (ca 375 Ma). While invasion of freshwater habitats from the marine realm was previously suggested to be the prime cause of aerial respiration developing in lungfishes, we believe that global decline in oxygen levels during the Middle Devonian combined with higher metabolic costs is a more likely driver of air-breathing ability, which developed in both marine and freshwater lungfishes and tetrapodomorph fishes such as Gogonasus.