Romer's Gap Plugged: 5 new Tetrapods From Tournaisian Mississippian Carboniferous Scotland

Phylogenetic and environmental context of a Tournaisian tetrapod fauna

Authors:

Clack et al

Abstract:

The end-Devonian to mid-Mississippian time interval has long been known for its depauperate palaeontological record, especially for tetrapods. This interval encapsulates the time of increasing terrestriality among tetrapods, but only two Tournaisian localities previously produced tetrapod fossils. Here we describe five new Tournaisian tetrapods (Perittodus apsconditus, Koilops herma, Ossirarus kierani, Diploradus austiumensis and Aytonerpeton microps) from two localities in their environmental context. A phylogenetic analysis retrieved three taxa as stem tetrapods, interspersed among Devonian and Carboniferous forms, and two as stem amphibians, suggesting a deep split among crown tetrapods. We also illustrate new tetrapod specimens from these and additional localities in the Scottish Borders region. The new taxa and specimens suggest that tetrapod diversification was well established by the Tournaisian. Sedimentary evidence indicates that the tetrapod fossils are usually associated with sandy siltstones overlying wetland palaeosols. Tetrapods were probably living on vegetated surfaces that were subsequently flooded. We show that atmospheric oxygen levels were stable across the Devonian/Carboniferous boundary, and did not inhibit the evolution of terrestriality. This wealth of tetrapods from Tournaisian localities highlights the potential for discoveries elsewhere.

pop sci write up.

Irish Middle Devonian Trackways Were Deposited in Semi Arid Environs

Ichnology and depositional environment of the Middle Devonian Valentia Island tetrapod trackways, south-west Ireland

Authors:

Stössel et al

Abstract:

Nine tetrapod trackways are described from the Middle Devonian continental Valentia Slate Formation on the north-eastern coast of Valentia Island in County Kerry, Ireland. The trackways occur at three sites – Dohilla, Coosadillisk and Culoo Head – the latter two being recorded for the first time. Morphological and taphonomic analyses of the trackways suggest they were made by populations of similar tetrapods, but of varying size (body length = 0.5 to 1 m; width = 0.15 to 0.30 m), that moved in some cases by terrestrial locomotion and in others possibly by a slow “paddling gait” while the substrate was submerged by very shallow floodwater. Sedimentological facies analysis of the trackway-bearing sequences has been carried out for the first time and shows that the preserved trackways are associated with the late stages of (1) a principal river channel margin (Coosadillisk), (2) a minor floodplain drainage channel (Culoo Head) and (3) a sand-rich crevasse splay/medial alluvial-ridge environment (Dohilla). The Valentia Island tetrapods inhabited marginal environments of proximal river channels less than 20 km from the northern margin of the Munster Basin, where hinterland drainage basins probably provided reliable sources of water in an overall semi-arid climate. It is proposed that the more perennial river channel belts crossing the basin would have provided conducive route ways that allowed ancestral tetrapods to migrate (greater than 170 km) northwards into the continent from postulated shallow marine habitats as they evolved this capability over a potential 4–5 million year period during the Middle Devonian.

Ichthyostegid-like & Whatcheeriid-like Tetrapods Found in Famennian Devonian Belgium

New discoveries of tetrapods (ichthyostegid-like and whatcheeriid-like) in the Famennian (Late Devonian) localities of Strud and Becco (Belgium)

Authors:

Olive et al

Abstract:

The origin of tetrapods is one of the key events in vertebrate history. The oldest tetrapod body fossils are Late Devonian (Frasnian–Famennian) in age, most of them consisting of rare isolated bone elements. Here we describe tetrapod remains from two Famennian localities from Belgium: Strud, in the Province of Namur, and Becco, in the Province of Liège. The newly collected material consists of an isolated complete postorbital, fragments of two maxillae, and one putative partial cleithrum, all from Strud, and an almost complete maxilla from Becco. The two incomplete maxillae and cleithrum from Strud, together with the lower jaw previously recorded from this site, closely resemble the genus Ichthyostega, initially described from East Greenland. The postorbital from Strud and the maxilla from Becco do not resemble the genus Ichthyostega. They show several derived anatomical characters allowing their tentative assignment to a whatcheeriid-grade group. The new tetrapod records show that there are at least two tetrapod taxa in Belgium and almost certainly two different tetrapod taxa at Strud. This locality joins the group of Devonian tetrapod-bearing localities yielding more than one tetrapod taxon, confirming that environments favourable to early tetrapod life were often colonized by several tetrapod taxa.

The Life History of Stem Tetrapod Acanthostega

Life history of the stem tetrapod Acanthostega revealed by synchrotron microtomography

Authors:

Sanchez et al

Abstract:

The transition from fish to tetrapod was arguably the most radical series of adaptive shifts in vertebrate evolutionary history. Data are accumulating rapidly for most aspects of these events, but the life histories of the earliest tetrapods remain completely unknown, leaving a major gap in our understanding of these organisms as living animals. Symptomatic of this problem is the unspoken assumption that the largest known Devonian tetrapod fossils represent adult individuals. Here we present the first, to our knowledge, life history data for a Devonian tetrapod, from the Acanthostega mass-death deposit of Stensiö Bjerg, East Greenland. Using propagation phase-contrast synchrotron microtomography (PPC-SRμCT)8 to visualize the histology of humeri (upper arm bones) and infer their growth histories, we show that even the largest individuals from this deposit are juveniles. A long early juvenile stage with unossified limb bones, during which individuals grew to almost final size, was followed by a slow-growing late juvenile stage with ossified limbs that lasted for at least six years in some individuals. The late onset of limb ossification suggests that the juveniles were exclusively aquatic, and the predominance of juveniles in the sample suggests segregated distributions of juveniles and adults at least at certain times. The absolute size at which limb ossification began differs greatly between individuals, suggesting the possibility of sexual dimorphism, adaptive strategies or competition-related size variation.

How Basal is Endothermy to Tetrapods?

A phenology of the evolution of endothermy in birds and mammals

Author:

Lovegrove

Abstract:

Recent palaeontological data and novel physiological hypotheses now allow a timescaled reconstruction of the evolution of endothermy in birds and mammals. A three-phase iterative model describing how endothermy evolved from Permian ectothermic ancestors is presented. In Phase One I propose that the elevation of endothermy – increased metabolism and body temperature (Tb) – complemented large-body-size homeothermy during the Permian and Triassic in response to the fitness benefits of enhanced embryo development (parental care) and the activity demands of conquering dry land. I propose that Phase Two commenced in the Late Triassic and Jurassic and was marked by extreme body-size miniaturization, the evolution of enhanced body insulation (fur and feathers), increased brain size, thermoregulatory control, and increased ecomorphological diversity. I suggest that Phase Three occurred during the Cretaceous and Cenozoic and involved endothermic pulses associated with the evolution of muscle-powered flapping flight in birds, terrestrial cursoriality in mammals, and climate adaptation in response to Late Cenozoic cooling in both birds and mammals. Although the triphasic model argues for an iterative evolution of endothermy in pulses throughout the Mesozoic and Cenozoic, it is also argued that endothermy was potentially abandoned at any time that a bird or mammal did not rely upon its thermal benefits for parental care or breeding success. The abandonment would have taken the form of either hibernation or daily torpor as observed in extant endotherms. Thus torpor and hibernation are argued to be as ancient as the origins of endothermy itself, a plesiomorphic characteristic observed today in many small birds and mammals.

Thai Cavefish Cryptotora thamicola Climbs, Walks Like a Salamander

Tetrapod-like pelvic girdle in a walking cavefish

Authors:

Flammang et al

Abstract:

Fishes have adapted a number of different behaviors to move out of the water, but none have been described as being able to walk on land with a tetrapod-like gait. Here we show that the blind cavefish Cryptotora thamicola walks and climbs waterfalls with a salamander-like diagonal-couplets lateral sequence gait and has evolved a robust pelvic girdle that shares morphological features associated with terrestrial vertebrates. In all other fishes, the pelvic bones are suspended in a muscular sling or loosely attached to the pectoral girdle anteriorly. In contrast, the pelvic girdle of Cryptotora is a large, broad puboischiadic plate that is joined to the iliac process of a hypertrophied sacral rib; fusion of these bones in tetrapods creates an acetabulum. The vertebral column in the sacral area has large anterior and posterior zygapophyses, transverse processes, and broad neural spines, all of which are associated with terrestrial organisms. The diagonal-couplet lateral sequence gait was accomplished by rotation of the pectoral and pelvic girdles creating a standing wave of the axial body. These findings are significant because they represent the first example of behavioural and morphological adaptation in an extant fish that converges on the tetrapodal walking behaviour and morphology.

pop sci link.

Evolving Legs was Surprisingly Easy for Tetrapod Ancestors

New research reveals that the limbs of the earliest four-legged vertebrates, dating back more than 360 million years ago, were no more structurally diverse than the fins of their aquatic ancestors.

The new finding overturns long-held views that the origin of vertebrates with legs (known as tetrapods) triggered an increase in the anatomical diversity of their skeletons.

The research was carried out by Dr Marcello Ruta from the School of Life Sciences at the University of Lincoln and Professor Matthew Wills from the Milner Centre for Evolution at the University of Bath in the UK. The authors found that fish and early tetrapods developed similar levels of anatomical diversity within their fins and limbs, despite the fact that their skeletons were constructed in very different ways.

Published in the leading scientific journal Palaeontology, the findings challenge some long-standing assumptions about evolution. It is generally expected that when organisms evolve new features - or 'key innovations' - that enable them to exploit new environments, the rate of evolution and diversification will speed up. This is believed to have happened with the evolution of birds from dinosaurs and, most iconically of all, in the transition from finned aquatic fish to limbed tetrapods.

The evolution of limbs was thought to have opened up a whole new realm of possibilities for tetrapods, so the scientists set out to examine just how substantial the evolutionary transition from fish to tetrapods really was by analysing a variety of different fin and limb skeletons from the fossil record.

Dr Marcello Ruta said: "Our work investigated how quickly the first legged vertebrates blossomed out to explore new skeletal constructions, with surprising results. We might expect that early tetrapods evolved limbs that were more complex and diverse than the fins of their aquatic predecessors. However, although radically different from limbs, the fins of the distant fish-like forerunners of tetrapods display a remarkable array of subtly varying traits.

"This variation may point to a previously unsuspected range of biomechanical functions in their fins, despite the fact that those ancestors lived exclusively in water."

link.

Consider Romer's Gap Plugged! New Floodplain Siltstone Deposit From Scotland Preserves Tetrapods From Tournaisian Carboniferous

Early Mississippian sandy siltstones preserve rare vertebrate fossils in seasonal flooding episodes

Authors:

Bennett et al

Abstract:

Flood-generated sandy siltstones are under-recognised deposits that preserve key vertebrate (actinopterygians, rhizodonts, and rarer lungfish, chondrichthyans and tetrapods), invertebrate and plant fossils. Recorded for the first time from the Lower Mississippian Ballagan Formation of Scotland, more than 140 beds occur throughout a 490 m thick core succession characterised by fluvial sandstones, palaeosols, siltstones, dolostone ‘cementstones’ and gypsum from a coastal–alluvial plain setting. Sandy siltstones are described as a unique taphofacies of the Ballagan Formation. They are matrix-supported siltstones with millimetre-sized siltstone and very fine sandstone lithic clasts. Common bioclasts include plants and megaspores, fish, ostracods, eurypterids and bivalves. Fossils have a high degree of articulation compared with those found in other fossil-bearing deposits such as conglomerate lags at the base of fluvial channel sandstones. Bed thickness and distribution varies throughout the formation, with no stratigraphic trend. The matrix sediment and clasts are sourced from the reworking of floodplain sediments including desiccated surfaces and palaeosols. Secondary pedogenic modification affects 30% of the sandy siltstone beds and most (71%) overlie palaeosols or desiccation cracks. Sandy siltstones are interpreted as cohesive debris flow deposits that originated by the overbank flooding of rivers and due to localised floodplain sediment transport at times of high rainfall; their association with palaeosols and desiccation cracks indicates seasonally wet to dry cycles throughout the Tournaisian. Tetrapod and fish fossils derived from floodplain lakes and land surfaces are concentrated by local erosion and reworking and are preserved by deposition into temporary lakes on the floodplain; their distribution indicates a local origin, with sediment distributed across the floodplain in seasonal rainfall episodes. These deposits are significant new sites that can be explored for the preservation of rare non-marine fossil material and provide unique insights into the evolution of early terrestrial ecosystems.

Extraordinary Claim: Mesozoic Tetrapod PaleoDiversity Stagnated

Near-Stasis in the Long-Term Diversification of Mesozoic Tetrapods

Authors:

Benson et al

Abstract:

How did evolution generate the extraordinary diversity of vertebrates on land? Zero species are known prior to ~380 million years ago, and more than 30,000 are present today. An expansionist model suggests this was achieved by large and unbounded increases, leading to substantially greater diversity in the present than at any time in the geological past. This model contrasts starkly with empirical support for constrained diversification in marine animals, suggesting different macroevolutionary processes on land and in the sea. We quantify patterns of vertebrate standing diversity on land during the Mesozoic–early Paleogene interval, applying sample-standardization to a global fossil dataset containing 27,260 occurrences of 4,898 non-marine tetrapod species. Our results show a highly stable pattern of Mesozoic tetrapod diversity at regional and local levels, underpinned by a weakly positive, but near-zero, long-term net diversification rate over 190 million years. Species diversity of non-flying terrestrial tetrapods less than doubled over this interval, despite the origins of exceptionally diverse extant groups within mammals, squamates, amphibians, and dinosaurs. Therefore, although speciose groups of modern tetrapods have Mesozoic origins, rates of Mesozoic diversification inferred from the fossil record are slow compared to those inferred from molecular phylogenies. If high speciation rates did occur in the Mesozoic, then they seem to have been balanced by extinctions among older clades. An apparent 4-fold expansion of species richness after the Cretaceous/Paleogene (K/Pg) boundary deserves further examination in light of potential taxonomic biases, but is consistent with the hypothesis that global environmental disturbances such as mass extinction events can rapidly adjust limits to diversity by restructuring ecosystems, and suggests that the gradualistic evolutionary diversification of tetrapods was punctuated by brief but dramatic episodes of radiation.

Phylogenetic Analysis of Paleozoic Tetrapods

Reevaluation of the largest published morphological data matrix for phylogenetic analysis of Paleozoic limbed vertebrates 

Authors:

Marjanović​ et al

Abstract:

The largest data matrix for phylogeny of early limbed vertebrates (Ruta M, Coates MI. 2007. J. Syst. Palaeont. 5:69–122) has supported controversial hypotheses; e.g., it has recovered Seymouriamorpha, Diadectomorpha and (in some trees) Caudata as paraphyletic and found the “temnospondyl hypothesis” on the origin of Lissamphibia (TH) to be one step more parsimonious than the “lepospondyl hypothesis” (LH). Scrutiny of the matrix reveals thousands of suboptimal scores (many clearly due to typographic and similar errors) as well as logically linked (redundant) characters, characters with only one described state, and even cases where taxa were scored after presumed relatives. Moreover, all characters – even obviously continuous ones – were unordered, effects of ontogeny were not sufficiently taken into account, and the authors mostly excluded data published after 2001, even their own. Our revised version – we document and justify all changes – yields much longer trees with a different topology, e.g. monophyletic Caudata, Diadectomorpha and (sometimes) Seymouriamorpha, Ichthyostega more rootward than Acanthostega, Anthracosauria more rootward than Temnospondyli, and the LH, which is 10 steps more parsimonious than the TH and 15 more than the “polyphyly hypothesis” (PH). Bootstrap values, though, are low, and few of the topologies are statistically distinguishable. For another set of analyses, we added 48 OTUs to the original 102. This destabilizes parts of the tree, e.g. the relationships of Anthracosauria and Temnospondyli. However, many of the added taxa have a fully resolved position or nearly so; this concerns the well-known Chroniosaurus (sister to a clade containing Solenodonsaurus, Seymouriamorpha, Diadectomorpha, Amniota and Amphibia), but also isolated lower-jaw material from the Devonian and Carboniferous. Despite the addition of Gerobatrachus, Micropholis and Tungussogyrinus and the extremely peramorphic salamander Chelotriton, the difference between LH and TH only shrinks to 9 steps, that between LH and PH to 13 steps. The “lepospondyl” Brachydectes is neither found as sister to Lissamphibia nor in the “microsaur” grade. Bootstrap values plummet, though, and all three hypotheses become statistically indistinguishable at p = 0.05. We then duplicated all analyses after coding all losses of bones as irreversible. Anthracosauria is then consistently placed more rootward than Temnospondyli; given the original taxon sample, the LH is 12 steps shorter than the “temnospondyl hypothesis” and 17 steps shorter than the PH, while the expanded taxon sample makes the LH 10 steps shorter than the TH and only 12 steps shorter than the PH. More robust results could likely be obtained by adding the many characters used in other analyses or discussed in the literature. We discuss phylogeny, approaches to coding, and certain character complexes, in particular the supposed middle ear of temnospondyls.

Salamander-like Limb Regeneration Evolved Before 290 Million Years Ago in Tetrapods

Deep-time evolution of regeneration and preaxial polarity in tetrapod limb development

Authors:

Fröbisch et al

Abstract:

Among extant tetrapods, salamanders are unique in showing a reversed preaxial polarity in patterning of the skeletal elements of the limbs, and in displaying the highest capacity for regeneration, including full limb and tail regeneration. These features are particularly striking as tetrapod limb development has otherwise been shown to be a highly conserved process. It remains elusive whether the capacity to regenerate limbs in salamanders is mechanistically and evolutionarily linked to the aberrant pattern of limb development; both are features classically regarded as unique to urodeles. New molecular data suggest that salamander-specific orphan genes play a central role in limb regeneration and may also be involved in the preaxial patterning during limb development. Here we show that preaxial polarity in limb development was present in various groups of temnospondyl amphibians of the Carboniferous and Permian periods, including the dissorophoids Apateon and Micromelerpeton, as well as the stereospondylomorph Sclerocephalus. Limb regeneration has also been reported in Micromelerpeton, demonstrating that both features were already present together in antecedents of modern salamanders 290 million years ago. Furthermore, data from lepospondyl ‘microsaurs’ on the amniote stem indicate that these taxa may have shown some capacity for limb regeneration and were capable of tail regeneration, including re-patterning of the caudal vertebral column that is otherwise only seen in salamander tail regeneration. The data from fossils suggest that salamander-like regeneration is an ancient feature of tetrapods that was subsequently lost at least once in the lineage leading to amniotes. Salamanders are the only modern tetrapods that retained regenerative capacities as well as preaxial polarity in limb development.

Tetrapod Ossinodus pueri was Definitely Terrestrial: Fell, Broke its leg

Oldest Pathology in a Tetrapod Bone Illuminates the Origin of Terrestrial Vertebrates

Authors:

Bishop et al

Abstract:

The origin of terrestrial tetrapods was a key event in vertebrate evolution, yet how and when it occurred remains obscure, due to scarce fossil evidence. Here, we show that the study of palaeopathologies, such as broken and healed bones, can help elucidate poorly understood behavioural transitions such as this. Using high-resolution finite element analysis, we demonstrate that the oldest known broken tetrapod bone, a radius of the primitive stem tetrapod Ossinodus pueri from the mid-Viséan (333 million years ago) of Australia, fractured under a high-force, impact-type loading scenario. The nature of the fracture suggests that it most plausibly occurred during a fall on land. Augmenting this are new osteological observations, including a preferred directionality to the trabecular architecture of cancellous bone. Together, these results suggest that Ossinodus, one of the first large (>2m length) tetrapods, spent a significant proportion of its life on land. Our findings have important implications for understanding the temporal, biogeographical and physiological contexts under which terrestriality in vertebrates evolved. They push the date for the origin of terrestrial tetrapods further back into the Carboniferous by at least two million years. Moreover, they raise the possibility that terrestriality in vertebrates first evolved in large tetrapods in Gondwana rather than in small European forms, warranting a re-evaluation of this important evolutionary event.

Also with the evolution of tetrapods lecture from the Royal Tyrrell Museum's Lecture Series, some of the earliest tetrapods known, like Ichthyostega and Acanthostega, were secondarily aquatic...meaning, yes, they returned to the water, this makes for some interesting implications.

Ward Takes a hit, Romer's Gap not Real: Diverse Tournaisian Mississipian Carboniferous Tetrapods From Nova Scotia Include Several Devonian Taxa

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

Authors:

Anderson et al

Abstract:

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.

One Big Step for Fish, One Giant Leap for Tetrapod-kind

Famennian Devonian Acanthostega gunnari Skull Reconstructed

360 million-year-old tetrapods may have been more like modern crocodiles than previously thought, according to 3D skull reconstruction. The results publish March 11, 2015 in the open-access journal PLOS ONE by Laura Porro from University of Bristol, UK, and colleagues.

Acanthostega gunnari was a 'four-footed' vertebrate, also known as a tetrapod, that invaded land during one of the great evolutionary transitions in Earth's history, 380-360 million years ago. This species is crucial for understanding the anatomy and ecology of the earliest tetrapods; however, after hundreds of millions of years buried in the ground, fossils are often damaged and deformed. To try to reconstruct the skull of this species from numerous skull pieces, the authors of the study applied high-resolution X-ray computed tomography (CT) scanning to several specimens of Acanthostega gunnari from East Greenland.

Researchers found that the reconstructed skull had a longer postorbital region and a more strongly hooked lower jaw than previously thought. They also found clues as to how the species fed. The size and distribution of its teeth and the shape of contacts between individual bones of the skull (called sutures) suggest Acanthostega may have initially seized prey at the front of its jaws using its large front teeth and hook-shaped lower jaw.

link.

Early Tetrapods Lived in Shallow Tideless Lagoons in Eifelian Devonian Poland

Palaeonvironments of the Eifelian dolomites with earliest tetrapod trackways (Holy Cross Mountains, Poland)

Authors:

Narkiewicz et al

Abstract:

The Eifelian dolomites in the Zachełmie Quarry (Holy Cross Mountains, Poland) contain trackways and tracks of tetrapods 390-391 Ma old, and thus the oldest known so far. The environments of the trackway-bearing beds have been investigated using sedimentological, palaeontological, geochemical and palaeomagnetic methods. The reconstructed tetrapod habitats comprised shallow-water lagoons separated from an open marine basin by sparsely vegetated islands and spits. The lagoonal waters were well-aerated and a few-meters deep at most, undergoing periodic desiccation. The dolomitic sediments, primarily of microbial origin, formed in tropical waters of slightly modified marine composition. Oxygen isotope data obtained from the dolomicrites suggest water temperatures around 30 °C. The seasonal semi-arid to subhumid climate, deduced from paleosol characteristics, was probably of a tropical monsoonal type. The degree of restriction of the lagoonal system evolved from relatively open, evaporation-dominated towards increasingly closed, fresh-water influenced.

The detailed observations of the footprint-bearing beds, as well as the characteristics of the tracks, indicate that they were formed mostly under subaqueous conditions, by wading, walking on the bottom or swimming animals. Lack of tidal indicators in the restricted Zachełmie lagoons argues against previous concept that tidal flats served as a food source for the early tetrapods. Nor is a hypothesis of flooded woodlands confirmed as a habitat promoting the “fish-to-tetrapod” transition. We propose that functional limbs emerged among aqueous animals that acquired their locomotional capabilities in a shallow lagoonal water before attempting longer excursions on land.

Weberepeton sondalensis: a new Stem Tetrapod From Frasnian Devonian Russia

Revision of the early tetrapod Obruchevichthys Vorobyeva, 1977 from the Frasnian (Upper Devonian) of the North-western East European Platform

Authors:

Clément et al

Abstract:

Species composition of the genus Obruchevichthys Vorobyeva, 1977, previously based on two specimens from the Upper Frasnian (Upper Devonian) of Latvia and Leningrad Region of Russia is revised. The precise locality of the latter specimen was considered by the author of this taxon as unknown. Archives recently found in collection and field research in the presumable locality allowed the rediscovery of this important locality along the Sondala River (east of the Leningrad Region). This provenance is furthermore supported by spectrometric testing. Despite the presence of several subadult features hindering its attribution by earlier authors to a separate taxon, new observations on its morphology revealed that several characters of the material from Russia significantly differ from those of the type specimen that resulted in its attribution to a new genus and species Weberepeton sondalensis gen. et sp. nov. These differences mainly consist in a number of lower jaw characters, such as relative size of marginal teeth, development rate of the adsymphysial plate, orientation of the precoronoid fossa, and dermal ornamentation.

A Hypothesis on the Evolution of Tetrapod Limbs

Urodelans, Ichthyostega and the origin of the tetrapod limb

Author:

Mednikov

Abstract:

Available information on the development of primitive urodele (Hynobiidae) limbs and limb structure in Devonian tetrapods provide the basis for formulating the hypothesis on the existence of a special phase, the phase of the biserial limb in tetrapod history. The limb of the Devonian amphibian Ichthyostega with two groups of digits, preaxial and postaxial, corresponds well to this phase. Based on the structure of the Ichthyostega limb, it is suggested that the fin type ancestral to terrestrial limbs was asymmetrical biserial, possessed a short axis, unbranched jointed preaxial radials deviating one by one from each mesomere of the axis, and unbranched jointed postaxial radials deviating from the distal mesomeres of the axis in two or more radials.

Insights into the Evolution of Scales in Tetrapods From Tulerpeton curtum

New insights into the scales of the Devonian tetrapod Tulerpeton curtum Lebedev, 1984

Authors:

Mondéjar-Fernández et al

Abstract:

The Devonian origin of tetrapods and their transition from aquatic to terrestrial habitats is one of the most important episodes in vertebrate evolutionary history (e.g., Coates, 1996; Janvier, 1996; Jarvik, 1996; Daeschler et al., 2006; Clack, 2012). Among the multiple changes that took place during the so-called ‘fish-tetrapod transition,’ those concerning the integumentary dermal skeleton have received little attention. Nonetheless, the skin and associated dermal ossifications of early tetrapods determine key aspects of their metabolism, way of life, and locomotion (e.g., Bystrow, 1947; Castanet et al., 2003; Markey and Marshall, 2007; Witzmann, 2007, 2011).

The main integumentary ossifications of early tetrapods are scales and osteoderms. Osteoderms are plates of dermal bone made by intradermal ossification that often bear a pitted outer surface. By contrast, dermal scales originate from the mesodermal layer of the dermis, are thinner than osteoderms, are often round or elongate ovals in outline, and may overlap (Castanet et al., 2003). Most Palaeozoic tetrapods were broadly covered with ossified dermal scales, mainly in their ventral and lateral regions (Romer, 1956; Janvier, 1996). However, dermal scales were lost in several tetrapod groups during the late Palaeozoic and Mesozoic, and replaced by osteoderms as the main integumentary mineralized structures (Vickaryous and Sire, 2009; Witzmann and Soler-Gijón, 2010).

Devonian tetrapods (such as Ichthyostega and Acanthostega) are considered as mainly aquatic animals, with limited walking abilities (Ahlberg et al., 2005; Pierce et al., 2012), as evidenced by the retention of primitive traits such as a caudal fin supported by osseous fin rays, a lateral line system, internal gills, and a dermal scale covering (Coates, 1996; Jarvik, 1996; Clack, 2012). Within Tetrapoda, the transition to land (or terrestrialization) during the Devonian and Carboniferous (Clack, 2012; Steyer, 2012) affected the squamation, modifying size, shape, overlapping pattern, and the bone tissue of the scales (see Witzmann, 2011, for a thorough review). However, the current lack of knowledge on the squamation of the earliest tetrapods does not allow testing or drawing of broader evolutionary scenarios. Here we furnish the first highly detailed three-dimensional reconstructions of the scales of the Devonian tetrapod Tulerpeton, thereby providing new interpretations on the morphological and microstructural evolution of the squamation in early tetrapods.

Just What WAS the "Parrsboro jaw" From Pennsylvanian Carboniferous Nova Scotia

Redescription and Phylogenetic Analysis of the Mandible of an Enigmatic Pennsylvanian (Late Carboniferous) Tetrapod from Nova Scotia, and the Lability of Meckelian Jaw Ossification

Authors:

Sookias et al

Abstract:

The lower jaw of an unidentified Pennsylvanian (Late Carboniferous) tetrapod from Nova Scotia – the “Parrsboro jaw”- is redescribed in the light of recent tetrapod discoveries and work on evolution of tetrapod mandibular morphology and placed for the first time in a numerical cladistics analysis. All phylogenetic analyses place the jaw in a crownward polytomy of baphetids, temnospondyls, and embolomeres. Several features resemble baphetids and temnospondyls including dermal ornamentation, absence of coronoid teeth, and presence of coronoid shagreen. Dentary dentition is most similar to Baphetes. An adsymphysial toothplate may not preclude temnospondyl affinity. An apparent large exomeckelian fenestra, with the dorsal foraminal margins formed by an unossified element, echoes the morphology of the stem tetrapod Sigournea and is unusually primitive given the other features of the jaw. The jaw may thus provide an example of an intermediate stage in Meckelian element evolution.