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The known diversity of tetrapods of the Devonian period has increased markedly in recent decades, but their fossil record consists mostly of tantalizing fragments1–15. The framework for interpreting the morphology and palaeobiology of Devonian tetrapods is dominated by the near complete fossils of Ichthyostega and Acanthostega; the less complete, but partly reconstructable, Ventastega and Tulerpeton have supporting roles2,4,16–34. All four of these genera date to the late Famennian age (about 365–359 million years ago)—they are 10 million years younger than the earliest known tetrapod fragments5,10, and nearly 30 million years younger than the oldest known tetrapod footprints35. Here we describe Parmastega aelidae gen. et sp. nov., a tetrapod from Russia dated to the earliest Famennian age (about 372 million years ago), represented by three-dimensional material that enables the reconstruction of the skull and shoulder girdle. The raised orbits, lateral line canals and weakly ossified postcranial skeleton of P. aelidae suggest a largely aquatic, surface-cruising animal. In Bayesian and parsimony-based phylogenetic analyses, the majority of trees place Parmastega as a sister group to all other tetrapods. Three-dimensionally preserved fossils of Parmastega aelidae, a newly described tetrapod from the earliest Famennian (Late Devonian) of Russia, provide detailed insights into the morphology and palaeobiology of the earliest tetrapods.

International audience; 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 fish-to-tetrapod transition—followed later by terrestrialization—represented a major step in vertebrate evolution that gave rise to a successful clade that today contains more than 30,000 tetrapod species. The early tetrapod Ichthyostega was discovered in 1929 in the Devonian Old Red Sandstone sediments of East Greenland (dated to approximately 365 million years ago). Since then, our understanding of the fish-to-tetrapod transition has increased considerably, owing to the discovery of additional Devonian taxa that represent early tetrapods or groups evolutionarily close to them. However, the aquatic environment of early tetrapods and the vertebrate fauna associated with them has remained elusive and highly debated. Here we use a multi-stable isotope approach (δ13C, δ18O and δ34S) to show that some Devonian vertebrates, including early tetrapods, were euryhaline and inhabited transitional aquatic environments subject to high-magnitude, rapid changes in salinity, such as estuaries or deltas. Euryhalinity may have predisposed the early tetrapod clade to be able to survive Late Devonian biotic crises and then successfully colonize terrestrial environments. An approach using multiple stable isotopes reveals that early tetrapods of the Devonian period were euryhaline animals that inhabited aquatic environments of highly variable salinity.

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.

This article introduces the tetrapods, both modern and extinct, and sets out what we know or can infer about the origin of the group. It introduces fossil members of the group that gave rise to tetrapods and discusses some of the conditions in which tetrapods developed limbs and walking. Terrestrial locomotion followed the evolution of limbs with digits once weight-bearing and flexible joints were developed.

This chapter describes the ear regions of tetrapods from the Paleozoic. In the past couple of decades, understanding of their morphology has been increased by discoveries of fossils from the Late Devonian and early Carboniferous (Mississippian) periods. The primitive condition, as found in several only distantly related taxa, consisted of a bulky stapes with a large footplate and a stout wing-shaped distal portion. In some, the stapes seems to have been a strut supporting the braincase. The hearing capabilities of these taxa were probably poor, confined to low-frequency detection primarily in water but, possibly, also in air. Ichthyostega had a highly modified version probably specialized for aquatic audition. Taxa from the later Carboniferous (Pennsylvanian) and Permian show a great deal of variation, especially among more terrestrially adapted forms. These include the larger stem amniotes, such as seymouriamorphs and diadectomorphs. All these taxa have some kind of a notch or embayment at the back of the skull, which in early forms was probably part of a spiracular mechanism but, in later ones, might have housed a tympanic membrane that closed off an air-filled middle ear cavity. The small recumbirostran microsaurs and the earliest amniotes had no such notch. Microsaur stapes had large footplates and short, stubby shafts. These and the earliest amniotes probably had no middle ear cavity. The earliest amniote stapes were robust, long, and laterally or downwardly projecting. In some cases they contacted the cheek bones or the jaw joint, likely precluding good aerial sound reception.

A new genus and species of Devonian tetrapod has been identified from material collected in 1947 from the southern slope of Mt. Celsius, Ymer phi, North-East Greenland. The specimen preserves both ...

The Late Devonian early tetrapods in East Greenland occur in the Celsius Bjerg Group. Key occurrences are located in a detailed stratigraphic section used here to interpret the sedimentary palaeoenvironments. The palaeoenvironment for the Britta Dal Formation (which contains both Ichthyostega and Acanthostega) is reinterpreted. The Britta Dal Formation channels have flat bases, are poorly channelized, are of low sinuosity and are part of a very major distributory system that periodically experienced extreme flooding. The tetrapod fossils were recovered from an ephemeral system that was not permanently habitable in the immediate area. Plant megafossils are poorly preserved casts and impressions dominated by lycopsids and fern-like plants. The overbank siltstones are dominated by arid soil forming processes and comprise a spectacular sequence of vertisols. The 1174 m in situ Ichthyostega locality in Paralleldal was relocated and occurs just below the midpoint of the second megacycle in the Britta Dal Formation.

After a brief historical review of the fin to limb transition and consideration of a theoretical “prototetrapod,” this article considers new ideas generated from recent fossil finds and from developmental biology that bear on the question of how limbs, digits, limb joints, and pentadactyly evolved. Among the first changes to take place were those to the humerus, in concert with those to the breathing apparatus, and these adaptations were acquired while the animals were still basically aquatic with the evolution of digits occurring during this phase. Studies from developmental biology of modern taxa can be integrated with information from fossils to produce a fuller picture. The acquisition of pentadactyly was among the last changes to occur in the modification of a fin into a limb. This vision differs radically from older theoretical ideas which perceived land locomotion as the prime evolutionary force driving the transition.

Study of Devonian tetrapods and their relatives spanning the so-called ‘fish–tetrapod’ transition has expanded almost exponentially in the last 15 years or so. This evolutionary event is now represented by at least nine named genera of Devonian tetrapod, several new ‘near-tetrapods’ and a number of new tetrapods from the Early Carboniferous. The anatomy of Acanthostega has radically changed ideas about how this transition took place, and more recently the anatomy of Ichthyostega is being reassessed with some startling conclusions such as the unique construction of its ear region. The current state of research on this range of animals is summarized, followed by a consideration of the acquisition of limbs and digits among tetrapods including their possible forerunners, the development of digits and their original function, and the onset of pentadactyly. The faunal relations and palaeoecological contexts of the Devonian tetrapods are brought together in an initial though necessarily brief survey, followed by an assessment of Devonian tetrapod diversity, which is seen to be much greater than previously realised. Finally, some hopes and ambitions for the future are set out.

Ichthyostega was the first Devonian tetrapod to be subject to a whole-body reconstruction. It remains, together with Acanthostega, one of only two Devonian tetrapods for which near-complete postcranial material is available. It is thus crucially important for our understanding of the earliest stages of tetrapod evolution and terrestrialization. Here we show a new reconstruction of Ichthyostega based on extensive re-examination of original material and augmented by recently collected specimens. Our reconstruction differs substantially from those previously published and reveals hitherto unrecognized regionalization in the vertebral column. Ichthyostega is the earliest vertebrate to show obvious adaptations for non-swimming locomotion. Uniquely among early tetrapods, the presacral vertebral column shows pronounced regionalization of neural arch morphology, suggesting that it was adapted for dorsoventral rather than lateral flexion.

The presence of a petaloid pattern (previously known as ‘dark dentine’) in crosssections of teeth of the embolomere Pholiderpeton attheyi has been used as a synapomorphy of theembolomeres or of the embolomeres plus the stem tetrapod, Crassigyrinus scoticus . Among the taxastudied, dentine that appears dark results from closely packed dentine tubules and can be found inany part of a tooth section in which such crowding occurs. The petaloid pattern is restricted to toothsections of a particular diameter, and is obliterated in larger sections of teeth that show complexfolding. Petaloid dentine has been found in all tetrapod teeth with plicidentine that were sectioned inthis study, whether from stem tetrapods, the Embolomeri, Temnospondyli, or Stereospondyli, andhas been recognised in some sarcopterygian sh, an extant actinopterygian sh, ichthyosaurs, andVaranus . The presence of petaloid dentine is neither a synapomorphy of the tetrapod node nor of anynode within tetrapods.KEY WORDS: dark dentine, Embolomeri, petaloid dentine, Rhizodontida, stem tetrapods,Stereospondyli, Temnospondyli, tooth histology.The pattern known as ‘dark dentine’ in sections of earlytetrapod teeth with folded dentine (plicidentine) was rstrecognised in 1876 by Thomas Atthey when describing teeth ofthe Carboniferous embolomere, Pholiderpeton attheyi (Clack1987; =‘ Anthracosaurus russelli ’ in error in Atthey 1876 p162,pl. 11; Fig. 1a). Atthey described the appearance of thePholiderpeton attheyi tooth sections as stellate (=petaloid inthe present paper) with a number of fusiform bodies of lightcoloured dentine radiating from the pulp cavity to the circum-ference and embedded in dentine of a dark colour. The rstreference to ‘dark dentine’ was to the interstices between thefusiform bodies, but Panchen (1977, 1985) noted that thecolour arrangement could be reversed. Tooth sections thatshow ‘dark dentine’ may resemble either a light coloured daisyon a dark background when seen in transmitted light (e.g.Fig. 1a) or a dark coloured daisy on a light background whenseen in re ected light (e.g. Fig. 3a).The petaloid pattern is known in the early tetrapodliterature as ‘dark dentine’ (e.g. Panchen 1985). The presenceof ‘dark dentine’ has been used as a synapomorphy ofCrassigyrinus scoticus and embolomeres (Panchen 1985;Panchen & Smithson 1988), and of Ichthyostega stensioei andmost other tetrapods (Coates 1996), although Smithson(1980) implied that the phenomenon might be size-related,having been described in only the larger anthracosauroids(most embolomeres) with the condition unknown in smallerforms. The purpose of this paper is to review the pattern oftooth folding in early tetrapods, and to determine thedistribution of petaloid dentine among them. The Ducabrooklocality from the Lower Carboniferous of Queensland,Australia (Thulborn et al . 1996), yields histologically intactspecimens of both the stem tetrapod Ossinodus pueri(Warren & Turner 2004) and the rhizodontid Strepsodus sp.(Johanson et al . 2000) with which to test the hypothesis ofthe phylogenetic usefulness of tooth dentine characters, inparticular the phenomenon of petaloid dentine among earlytetrapods.

Lepidotrichia are dermal elements located at the distal margin of osteichthyan fins. In sarcopterygians and actinopterygians, the term has been used to denote the most distal bony hemisegments and also the more proximal, scale-covered segments which overlie endochondral bones of the fin. In certain sarcopterygian fishes, including the Rhizodontida, these more proximal, basal segments are very long, extending at least half the length of the fin. The basal segments have a subcircular cross section, rather than the crescentic cross section of the distal lepidotrichial hemisegments, which lack a scale cover and comprise short, generally regular, elements. In rhizodonts and other sarcopterygians, e.g. Eusthenopteron, the basal elements are the first to appear during fin development, followed by the endochondral bones and then the distal lepidotrichia. This sequence contradicts the 'clock-face model' of fin development proposed by Thorogood in which the formation of endochondral bones is followed by development of lepidotrichia. However, if elongate basal 'lepidotrichia' are not homologous with more distal, jointed lepidotrichia and if the latter form within a distal fin-fold and the former outside this fold, then Thorogood's 'clock-face' model remains valid. This interpretation might indicate that the fin-fold has been lost in early digited stem-tetrapods such as Acanthostega and Ichthyostega and elongate basal elements, but not true lepidotrichia, occur in the caudal fins of these taxa.

Two morphologically distinct assemblages of the Late Devonian tetrapod Ichthyostega from East Greenland are described on the basis of a large collection of skulls and postcranial elements. Skull specimens collected on Gauss Halvo show that the assemblage from the lower Aina Dal Formation has proportionally narrower skulls and finer skull roof sculpture than the assemblage from the higher Britta Dal Formation. The assemblages are compared with the stratigraphically unconstrained type material from the north side of Celsius Bjerg on Ymer O, allowing a taxonomic revision. From the original description of seven taxa, three species are recognized and related to the assemblages from Gauss Halvo. I. stensioei is comparable with the Aina Dal Formation assemblage while I. eigili and I. watsoni are valid for specimens found in the Britta Dal Formation. Ichthyostegopsis wimani is not a valid genus or species and may be regarded as a juvenile I. eigili. This example of species-level variability is the earliest known from the fossil record of early tetrapods.

The fossil record of early tetrapods has been increased recently by new finds from the Devonian period1 and mid–late Early Carboniferous period2. Despite this, understanding of tetrapod evolution has been hampered by a 20-million-year gap (‘Romer's Gap’3) that covers the crucial, early period when many key features of terrestrial tetrapods were acquired. Here I describe the only articulated skeleton of a tetrapod, Pederpes, yet found from the Tournaisian epoch (354–344 million years ago (Myr)). The new taxon includes a pes with five robust digits, but a very small, possibly supernumerary digit preserved on the manus suggests the presence of polydactyly. Polydactylous early tetrapods may have survived beyond the end of the Devonian and pentadactyly cannot be assumed for the pes. However, the pes has characteristics that distinguish it from the paddle-like feet of the Devonian forms and resembles the feet of later, more terrestrially adapted Carboniferous forms. Pederpes is the earliest-known tetrapod to show the beginnings of terrestrial locomotion and was at least functionally pentadactyl. With its later American sister-genus, Whatcheeria4,5, it represents the next most primitive tetrapod clade after those of the Late Devonian, bridging the temporal, morphological and phylogenetic gaps that have hitherto separated Late Devonian and mid-Carboniferous tetrapod faunas.

The lower jaw of the Devonian tetrapodAcanthostegais described for the first time. Redescriptions are provided for the lower jaws of the elpistostegidPanderichthys, the Devonian tetrapodsElginerpeton, Obruchevichthys, Metaxygnathus, VentastegaandIchthyostega, and the Carboniferous tetrapodsCrassigyrinus, MegalocephalusandGephyrostegus. The character distri- butions thus revealed differ considerably from previous accounts, particularly in the wide distribution of certain primitive characters. Meckelian ossification in the middle part of the jaw is widespread among Devonian tetrapods, being demonstrably absent only inAcanthostega.Among Carboniferous tetrapods, a tooth-bearing parasymphysial plate is shown to be present inCrassigyrinusandMegalocephalus(having already been demonstrated by other authors inWhatcheeriaandGreererpeton).A phylogenetic analysis of 26 early tetrapods including all the aforementioned genera, scored for 51 lower jaw characters, produces at least 2,500 equally parsimonious trees. However, the lack of resolution lies largely in a big top end polychotomy containing anthracosaurs, temnospondyls, seymouriamorphs, microsaurs and a nectridean-amniote clade. Below this polycho- tomy, which may correspond approximately to the tetrapod crown group, there is a well-resolved stem-group containing, in descending order,Megalocephalus, Greererpeton, Crassigyrinus, (jaws associated with)Tulerpeton, Whatcheeria, Acanthostega, Metaxygnathus, Ichthyostega, VentastegaandMetaxygnathus(unresolved), anElginerpeton-Obruchevichthysclade, andPanderichthys.This conflicts with recently published phylogenies by Coates and Lebedev & Coates, which placeTulerpetonand all post-Devonian tetrapods in the amphibian or amniote branches of the tetrapod crown group.

A new reconstruction of the skull ofCrassigyrinus scoticus(Lydekker) is provided, based on newly prepared material. A revised interpretation of the naris shows it in most respects to be like that of other Carboniferous tetrapods, lacking both an anterior tectal and a lateral rostral. However it does incorporate an enlarged septomaxilla, and part of the dorsal surface of the vomer is also visible within the naris of most specimens. Prominent paired buttresses run along the midline of the snout, and a single bowl-shaped interpremaxillary fenestra pierces the snout and palate but is not confluent with the naris or choana. A preopercular is probably absent. A new cladistic analysis placesCrassigyrinuswith the Viséan genusWhatcheeria, and puts those two taxa as a sister-clade to the anthracosaursProterogyrinusandPholiderpeton.Temnospondyls form a clade with loxommatids, and these two arms form a dichotomy crownward toGreererpeton, IchthyostegaandAcanthostegawhich are successive stemward plesions.

The postcranial skeleton ofAcanthostega gunnarifrom the Famennian of East Greenland displays a unique, transitional, mixture of features conventionally associated with fishand tetrapod-like morphologies. The rhachitomous vertebral column has a primitive, barely differentiated atlas-axis complex, encloses an unconstricted notochordal canal, and the weakly ossified neural arches have poorly developed zygapophyses. More derived axial skeletal features include caudal vertebral proliferation and, transiently, neural radials supporting unbranched and unsegmented lepidotrichia. Sacral and post-sacral ribs reiterate uncinate cervical and anterior thoracic rib morphologies: a simple distal flange supplies a broad surface for iliac attachment. The octodactylous forelimb and hindlimb each articulate with an unsutured, foraminate endoskeletal girdle. A broad-bladed femoral shaft with extreme anterior torsion and associated flattened epipodials indicates a paddle-like hindlimb function. Phylogenetic analysis placesAcanthostegaas the sister-group ofIchthyostegaplus all more advanced tetrapods.Tulerpetonappears to be a basal stemamniote plesion, tying the amphibian-amniote split to the uppermost Devonian.Caerorhachismay represent a more derived stem-amniote plesion. Postcranial evolutionary trends spanning the taxa traditionally associated with the fish-tetrapod transition are discussed in detail. Comparison between axial skeletons of primitive tetrapods suggests that plesiomorphic fish-like morphologies were re-patterned in a cranio-caudal direction with the emergence of tetrapod vertebral regionalisation. The evolution of digited limbs lags behind the initial enlargement of endoskeletal girdles, whereas digit evolution precedes the elaboration of complex carpal and tarsal articulations. Pentadactylous limbs appear to have stabilised independently in amniote and amphibian lineages; the colosteidGreererpetonhas a pentadactylous manus, indicating that basal amphibian forelimbs may not be restricted to patterns of four digits or less.

The shoulder girdle of Panderichthys rhombolepis ( Gross ) shows a complex of tetrapod-like; fish-like and specialized features. The scapulocoracoid; which is located very ventrally; has two attachment areas to the cleithrum. It is characterized by the well-developed coracoid plate (comparable with the scapular plate of Ichthyostega) and by the presence of a large subscapular foramen surrounded by the cleithrum and the clavicle. The clavicle is pocket-shaped and overlappes partly the ventral and dorsal surfaces of the coracoid plate. The glenoid fossa is saddle-like and is oriented posteriorly. The weakly developed joint between the skull and the shoulder girdle is confined to the region of the lateral extrascapular and posttemporal.

Ventastega curonica, from the Upper Famennian Ketleri Formation, is the first tetrapod find from the Upper Devonian of Latvia, and only the fourth adequately represented Devonian tetrapod genus to be described. The taxon is represented by disarticulated cranial and postcranial elements from two localities, Ketleri on the Venta River and Pavari on the Ciecere River. A second tetrapod, represented by a single mandibular fragment, appears to be present at Ketleri. The lower jaw of Ventastega is strikingly primitive in retaining fangs on the coronoid series, but shares many characters with those of other known Devonian tetrapods. Some of these features are interpreted as basal tetrapod synapomorphies; they provide a new data set for the identification of isolated tetrapod jaw fragments, and confirm the (previously disputed) tetrapod status of Metaxygnathus. The upper jaw bones of Ventastega are broadly similar to those of Acanthostega, Ichthyostega and Tulerpeton, as is the narial region. The lateral rostral bone is either very small or absent. A preopercular bone is present in the cheek, and the lacrimal is excluded from the orbit. The palate is closed. Palatine and vomer bear fangs which are set in the marginal tooth row. An isolated iliac blade from Pavari, probably attributable to Ventastega, resembles that of Acanthostega but may not have carried a dorsal process. Two clavicles from Pavari and Ketleri which may also belong to Ventastega are of a typical early tetrapod pattern, similar to Greerpeton but with a broader ventral blade. Non-attributable or doubtfully attributable bones from Ketleri include a probable tetrapod postorbital and a possible limb bone. Ventastega appears to be a tetrapod of the same broad `grade' as Ichthyostega and Acanthostega, but is arguably more primitive than either.

Transitional forms of the recent classes of vertebrates are only known in paleontology. The well described examples are: Eusthenopteron foordi (Crossopterygii), Ichthyostega and Acanthostega (Labyrinthodontia) between Osteichthyes and Amphibia, Seymouria baylorensis (Amphibiosaria) between Amphibia and Reptilia, Archaeopteryx lithographica (Archaeornithes) between Reptilia and Aves, and the mammal-like reptiles Pelycosauria, Therapsida and Cynodontia between Reptilia and Aves, and the description of their phylogenetical heterochronies in terms of peramorphosis and paedomorphosis shows the progressive role of the motorial, especially the locomotorial organ systems and their functions in comparison with the retarded evolution of the axial system, especially the skull and central nervous system. The evolution of the Hominidae shows the same rule. The evaluation of these transitional forms in their fossil context reveals them as inhabitants of biotopes situated in the border areas of coastal and shore landscapes of marine, brackish or fresh water. These biotopes have obviously favoured the innovations on the high taxonimic level of macro-evolutionary characteristics.

Several discoveries of Late Devonian tetrapods (limbed vertebrates) have been made during the past two decades, but each has been confined to one locality. Here we describe a tetrapod jaw of about 365 million years (Myr) old from the Famennian of Belgium, which is the first from western continental Europe. The jaw closely resembles that of Ichthyostega, a Famennian tetrapod hitherto known only from Greenland. The environment of this fossil provides information about the conditions that prevailed just before the virtual disappearance of tetrapods from the fossil record for 20 Myr.

An early tetrapod fossil from the Upper Devonian of Pennsylvania (Catskill Formation) extends the temporal range of tetrapods in North America and suggests that they attained a virtually global equatorial distribution by the end of the Devonian. Derived features of the shoulder girdle indicate that appendicular mechanisms of support and propulsion were well developed even in the earliest phases of tetrapod history. The specialized morphology of the pectoral skeleton implies that the diversity of early tetrapods was great and is suggestive of innovative locomotor patterns in the first tetrapods.

Acanthostega gunnari is a tetrapod from the Upper Devonian of East Greenland, and is therefore one of the earliest tetrapod taxa for which skeletal remains are known. It was exactly contemporary with Ichthyostega (Jarvik 1952, 1980), a much more widely known form, but recent discoveries in Greenland (Bendix-Almgreen, Clack, and Olsen 1988) have shown that in many ways Ichthyostega and Acanthostega are quite different (Clack 1988a,b). Apart from primitive characters, they have little in common, suggesting early diversification among tetrapods.

Among the most tantalizing paleontologic discoveries in the 20th century were the Late Devonian (ca. 370 million-year-old) tetrapods Ichthyostega and Acanthostega from East Greenland in the Arctic....

Palynological dates unambiguously resolve the stratigraphic age of the East Greenland sedimentary rocks containing the earliest well-preserved tetrapod remains. This is the first time that spore samples have been discovered in the sedimentary succession that has yielded Acanthostega and Ichthyostega , two tetrapods that are regarded as critically important taxa for our understanding of the origin and early evolution of the tetrapods. These palynological assemblages conclusively show that the rocks are Devonian in age. The evidence resolves a 60-year-old dispute regarding the age of these rocks and contradicts a recent controversial study suggesting a much younger (Carboniferous, Visean) age for these tetrapods. Spore samples bracketing the in situ occurrences of both tetrapod genera place them securely within the Famennian Age of the Devonian Period and at least as old as Famennian 2b. The ages of all known Devonian tetrapods are reviewed and related to a common palynological standard. This review places Ichthyostega and Acanthostega as the earliest of the Famennian tetrapods.

New paleomagnetic and isotopic data from East Greenland indicate that this classical “Devonian” basin was partly formed in Carboniferous. The basin preserves a stratigraphically linked magnetic reversal pattern of primary character. Paleomagnetic data indicate that the two stratigraphically lowermost intrabasinal angular unconformities, identified on each side of the basin, in fact correlate as one unconformity. This implies a 2 km reduction of the estimated basin thickness, and thus that the unconformity represents a major depositional hiatus. Successions below the unconformity are taken to be Devonian (Givetian) in age, on the basis of correlation with paleomagnetic reference poles. However, we argue that the overlying strata are Carboniferous, rather than Devonian, in age, on the basis of a ca. 336 Ma 40Ar/39Ar extrusive age for a basalt flow and paleomagnetic data. A Carboniferous age for the strata has significant implications for vertebrate evolution; fossils of a terrestial tetrapod, Ichthyostega , are found above the unconformity. Ichthyostega is regarded as the earliest fossil of an animal known to walk on land; however, our data suggest that these dry footsteps are much younger than previously believed. Our results are also significant for plate reconstructions. Paleomagnetic data indicate that the lower part of the basin was deposited at low southerly latitudes. Sediments above our Early Carboniferous unconformity were deposited approximately at lat 4°N, indicating that the continent had drifted northward. A minor pole-longitude misfit between Devonian and Carboniferous poles from East Greenland and North America implies (1) a closer pre–Labrador Sea Greenland–North America fit; (2) counterclockwise block rotations (10°–15°) of the study area; or (3) a combination of both. The East Greenland “Devonian” basin formed along the Caledonian spine of Euramerica, and counterclockwise block rotation may have occurred between sinistral faults resulting from continued relative movement between Baltica and Laurentia during Devonian and Carboniferous time.

Late Devonian faunal and facies relationships are examined in seven around the North Atlantic — in eastern North America, Greenland, western Europe and northwest Africa. A shallow marine (“littoral”) environment, characterized by the genus Cyrtospirifer, is distinguished from a deeper water (“bathyal”) goniatite-conodont milieu on the one hand, and from the “Old Red Sandstone” terrestial facies bearing plant an d fresh-water fish remains on the other. Current or source directions indicate that an “Acadian Divide” existed, separating west-flowing drainage systems in North America from those flowing to the east on the Afro-European side. All species of the osteolepid Latvius, and the majority of species of Glyptopomus are found on the eastern flank. Conversely, the earliest amphibian, Ichthyostega, may have been confined to the western side of the divide. Palaeogeographic reconstruction places northwest Africa fairly close to the Catskill Delta in the Late Devonian, thus accounting for the presence of an “American fauna” in the former. North—south migration of littoral faunas along the Afro-European shores was, however, apparently inhibited.

A locality in the Late Devonian Cloghnan Shale (Nangar Subgroup) near Forbes, N.S.W., has yielded a rich vertebrate fauna including the antiarchs Bothriolepis and Remigolepis, the phyllolepid Phyllolepis, the dipnoan Soederberghia, and the jaw of a new ichthyostegaliid amphibian Metaxygnathus denticulus gen. et sp. nov. The amphibian is the oldest known member of the group with the possible exception of Elpistostege Westoll, and it retains characters previously associated with the rhipidistian fishes. The musculatures of rhipidistian and amphibian jaws are analysed and are shown to be functionally related to distinctive morphologies of the articulatory region in each group, Metaxygnathus being of amphibian type. Comparison is made with the Greenland ichthyostegids, and rhipidistians such as Eusthenodon.