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.

Meemannia eos: The Earliest Known Ray-Finned Fish From Lochkovian Devonian China

Osteichthyans, or bony fishes, comprise two categories, each containing over 32,000 living species: Sarcopterygii (lobe-finned fishes and tetrapods) and Actinopterygii (ray-finned fishes). Nevertheless, actinopterygians have an obscure early evolutionary history. The earliest definitive actinopterygian is the Middle Devonian (Eifelian) Cheirolepis, with earlier candidates generally represented by fragments subject to differing phylogenetic interpretations. By contrast, earliest Devonian deposits yield a diversity of lobe-finned fishes and recent discoveries from China extend their origin into the late Silurian.

The Early Devonian (Lochkovian) Xitun Formation of Yunnan, China, provides remarkable fossils to illustrate the evolutionary origins of individual sarcopterygian lineages, but apparently lacks any actinopterygians. Meemannia is the newest--and least understood--member of this fauna. Represented by four isolated skull roofs and a referred jaw, Meemannia presents an intriguing mosaic of characteristics: histology interpreted as a precursor to the "cosmine" of rhipidistian sarcopterygians (lungfishes plus tetrapods) combined with an undivided braincase and skull roof resembling that of actinopterygians. Previous phylogenetic analyses placed Meemannia as the earliest-diverging sarcopterygian, based on histological features.

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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."

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Cooling in an Early Devonian Greenhouse Climate? From a VERY hot Lochkovian to a hot & Humid Pragian

Warming or cooling in the pragian? Sedimentary record and petrophysical logs across the lochkovian-pragian boundary in the Spanish Central Pyrenees

Authors:

Slavik et al

Abstract:

High-resolution petrophysical correlation methods were applied, for the first time, to mid-Paleozoic rocks of the Pyrenees. The methods included magnetic susceptibility measurements (MS), gamma-ray spectrometry (GRS), and alignment of MS logs using the dynamic time warping (DTW) algorithm. Conodont biostratigraphy provided the basic framework necessary for work with the GRS and MS logs. In spite of differences in the sediment patterns and accumulation/erosion rates, the logs from two selected sections in the Spanish Central Pyrenees show a striking symmetry that correlates well with the previously published logs from the Barrandian area in the Czech Republic. The high similarity between the petrophysical records from paleogeographically related but distant areas has the potential to contribute to the current discussions about the eustatic and climatic changes that took place in the overall greenhouse settings, where evidence for any polar ice sheets is still absent. In addition to the extant evidence of a major Lochkovian-Pragian offlap, combined with sea level lowering known from the North America, evidence about a major sea level fall and drastic reconstruction of the climatic system in the Pragian is now expanding over significant parts of the peri-Gondwanan belt. The data from Spain provide many new details about this change, including the phenomenon of a stratigraphically condensed upper Lochkovian, a dramatically different type of sedimentation in the Pragian, and a GRS-Th-and-MS anomalous stratigraphic interval in the middle Pragian. The multifaceted evidence allows us to determine the essential parts of the Pragian as a still “hot and humid” period, even with the strong differences from the possibly “extremely hot” Lochkovian.

The Frasnian Devonian Tropical Forest Found in Svalbard, Norway

UK researchers have unearthed ancient fossil forests, thought to be partly responsible for one of the most dramatic shifts in the Earth's climate in the past 400 million years.

The fossil forests, with tree stumps preserved in place, were found in Svalbard, a Norwegian archipelago situated in the Arctic Ocean. They were identified and described by Dr Chris Berry of Cardiff University's School of Earth and Ocean Science.

Prof John Marshall, of Southampton University, has accurately dated the forests to 380 million years.

The forests grew near the equator during the late Devonian period, and could provide an insight into the cause of a 15-fold reduction in levels of carbon dioxide (CO2) in the atmosphere around that time.

Current theories suggest that during the Devonian period (420-360 million years ago) there was a huge drop in the level of CO2 in the atmosphere, thought to be largely caused by a change in vegetation from diminutive plants to the first large forest trees.

Forests pulled CO2 out of the air through photosynthesis - the process by which plants create food and tissues - and the formation of soils.

Although initially the appearance of large trees absorbed more of the sun's radiation, eventually temperatures on Earth also dropped dramatically to levels very similar to those experienced today because of the reduction in atmospheric CO2.

Because of the high temperatures and large amount of rainfall on the equator, it is likely that equatorial forests contributed most to the drawdown of CO2. Svalbard was located on the equator around this time, before the tectonic plate drifted north by around 80° to its current position in the Arctic Ocean.

"These fossil forests shows us what the vegetation and landscape were like on the equator 380 million years ago, as the first trees were beginning to appear on the Earth," said Dr Berry.

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Giant Sharks From the Devonian/Carboniferous Boundary in Texas

Previously, giant sharks had only been recovered from rock dating back 130 million years, during the age of the dinosaurs. The largest shark that ever lived, commonly called "Megalodon", is much younger, with an oldest occurrence at about 15 million years ago. This means the new fossils from Texas indicate giant sharks go much further back into the fossil record.

After the generous donation of these fossils and careful study with Dr. John Maisey of the American Museum of Natural History in New York, the team was able to estimate how big the entire sharks would have been by comparison with smaller and more complete fossils of closely related sharks. The results were very impressive.

The size range estimated for these two Texas 'supersharks' was between 18 and 26 feet in length (5.5 to 8 meters). The largest of these specimens was 25% bigger than today's largest predatory shark, the Great White. Although not nearly as large as Megalodon, which might have reached up to 67 feet in length (about 20 meters), the fossil sharks from Texas would have been by far the biggest sharks in the sea.

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Severe Selenium Depletion Detected During Ordovician, Devonian and Triassic Mass Extinctions

Severe Selenium depletion in the Phanerozoic oceans as a factor in three global mass extinction events

Authors:

Long et al

Abstract:

Selenium (Se) is one of the key trace elements required by all animal and most plant life, and Se deficiencies in the food chain cause pathologies or death. Here we show from new geochemical analyses of trace elements in Phanerozoic marine pyrite that sustained periods of severe Se depletion in the past oceans correlate closely with three major mass extinction events, at the end of the Ordovician, Devonian and Triassic periods. These represent periods of Se depletion > 1.5-2 orders of magnitude lower than current ocean abundances, being within the range to cause severe pathological damage in extant Se-reliant organisms. Se depletion may have been one of several factors in these complex extinction scenarios. Recovery from the depletion/extinction events is likely part of a natural marine cycle, although rapid rises in global oxygen from sudden major increases in marine productivity and plant biomass after each extinction event may also have played a crucial role.

The Evolution of Devonian Trees Impact on Soils, Climate and Weather

Investigating Devonian trees as geo-engineers of past climates: linking palaeosols to palaeobotany and experimental geobiology

Authors:

Morris et al

Abstract:

We present the rationale for a cross-disciplinary investigation addressing the ‘Devonian plant hypothesis’ which proposes that the evolutionary appearance of trees with deep, complex rooting systems represents one of the major biotic feedbacks on geochemical carbon cycling during the Phanerozoic. According to this hypothesis, trees have dramatically enhanced mineral weathering driving an increased flux of Ca2+ to the oceans and, ultimately, a 90% decline in atmospheric CO2 levels through the Palaeozoic. Furthermore, experimental studies indicate a key role for arbuscular mycorrhizal fungi in soil–plant processes and especially in unlocking the limiting nutrient phosphorus in soil via Ca-phosphate dissolution mineral weathering. This suggests co-evolution of roots and symbiotic fungi since the Early Devonian could well have triggered positive feedbacks on weathering rates whereby root–fungal P release supports higher biomass forested ecosystems. Long-standing areas of uncertainty in this paradigm include the following: (1) limited fossil record documenting the origin and timeline of the evolution of tree-sized plants through the Devonian; and (2) the effects of the evolutionary advance of trees and their in situ rooting structures on palaeosol geochemistry. We are addressing these issues by integrating palaeobotanical studies with geochemical and mineralogical analyses of palaeosol sequences at selected sites across eastern North America with a particular focus on drill cores from Middle Devonian forests in Greene County, New York State.

Revising the Frasnian–Famennian Devonian Boundary and Kellwasser Events

Revised correlation of the Frasnian–Famennian boundary and Kellwasser Events (Upper Devonian) in shallow marine paleoenvironments of New York State

Authors:

Bush et al

Abstract:

The Frasnian–Famennian boundary (Upper Devonian) is exposed in a 200 km-long outcrop belt in New York State, with deeper paleoenvironments to the west and shallower ones to the east. Geochronology in the eastern end of the outcrop belt has been based primarily on lithostratigraphic correlation with western sections, which were dated using conodonts. We collected conodonts and brachiopods from several measured sections and numerous other localities, and these collections suggest that these east–west lithostratigraphic correlations require revision. We correlate the Wiscoy Formation with the upper Angola Formation and the Canaseraga with the upper Hanover. Thus, the Canaseraga Formation contains the Frasnian–Famennian boundary and Upper Kellwasser Event, and the dark shale above the Wiscoy is equivalent to the Pipe Creek Formation and Lower Kellwasser Event. These new correlations imply that the Lower Kellwasser Event had greater impact on the shelly benthos of New York than the Upper Kellwasser, at least for the subset of taxa examined here. All strophomenid brachiopods and rugose corals were extirpated at the Lower Kellwasser, along with numerous other brachiopods. The final species of atrypid brachiopod persisted to the Upper Kellwasser.

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.

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.

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Famennian Devonian to Mississippian Carboniferous Events of the Western USA

Uppermost Devonian (Famennian) to Lower Mississippian events of the western U.S.: Stratigraphy, sedimentology, chemostratigraphy, and detrital zircon geochronology

Authors:

Cole et al

Abstract:

Upper Devonian to Lower Mississippian strata in Utah and Montana record global events through this important interval in Earth history. Late Famennian strata of the Beirdneau, Leatham, and Pilot Shale formations in Utah, and Three Forks and Sappington formations in Montana, record widespread deposition of generally fine-grained siliciclastic and carbonate strata. Integration of sedimentology, physical stratigraphy, chemostratigraphy, and published biostratigraphy allows for the recognition of important disconformities and regional stratigraphic patterns. These enable the reconstruction of uppermost Devonian to lowermost Mississippian depositional, tectonic, and eustatic history of the region.

Carbon isotopic data allows for stratigraphic evaluation of the presence and absence of global bioevents of the Late Devonian, including the Annulata, Dasberg, and Hangenberg events, some of which are clearly recorded in hinterland deposits to the east in Colorado. While the Devonian–Mississippian boundary is also missing in our sections, a significant positive shift in δ13Ccarb in Lower Mississippian strata in Utah and Montana represents one of the largest positive δ13Ccarb isotope excursions of the Phanerozoic, linked to drawdown of atmospheric CO2 and glaciation in the Kinderhookian.

Detrital zircon spectra from latest Devonian to Early Mississippian strata of Utah and Colorado include populations representing derivation from the Mazatzal and Yavapai provinces, Middle Proterozoic anorogenic granite bodies, and a small influx of Grenville and presumed Appalachian–Caledonian grains. Minor Paleoproterozoic and Late Archean peaks in Utah are likely multiple generation grains originally derived from the Peace River Arch of northwestern Canada and recycled in Ordovician rocks of Nevada. These patterns in detrital zircon geochronological data reflect, in part, changes in sediment dispersal patterns due to tectonic and eustastic variability within the Antler foreland basin during the Devonian–Mississippian boundary interval. This variability also led to irregular spatial patterns of unconformity development, as well as complicated physical stratigraphic and chemostratigraphic architecture.

Detection of the Devonian Hangenberg Event in an Open Oceanic Island Arc

Climate instability and tipping points in the Late Devonian: Detection of the Hangenberg Event in an open oceanic island arc in the Central Asian Orogenic Belt

Authors:

Carmichael et al

Abstract:

Sedimentary petrology and trace element geochemistry indicate that the Late Devonian to Early Carboniferous Heishantou Formation near Boulongour Reservoir (NW Xinjiang, China) was deposited on a steep slope, mid-latitude accreting island arc complex in an open oceanic system. Bulk 87Sr/86Sr ratios show excursion patterns that are consistent with excursions at the Devonian-Carboniferous (D-C) boundary in epicontinental margin sediments. Sedimentation rates for the Boulongour Reservoir sediments show highly variable rates that range from 0.5 cm/ky to 10 cm/ky, consistent with other Late Devonian sections and modern arc environments. Multiple whole rock geochemical proxies for anoxia and the size and distribution of pyrite framboids suggest the presence of the Hangenberg Event in the sediments associated with the D-C boundary, despite the lack of visible black shale. The presence of anoxia in an open ocean, island arc environment cannot be explained by upwelling of anoxic bottom waters at this paleolatitude, but can be explained by the global infliction of oceanic shallow water eutrophication on to a climate system in distress.

Whoa-Awesome Level: Fossil Ripple Marks Indicate Middle Devonian Wind Direction During dry Season (April to July)

Ripple marks indicate Mid-Devonian palaeo-wind directions in the Orcadian Basin (Orkney Isles, Scotland)

Authors:

De Vleeschouwer et al

Abstract:

Few climate proxies provide information on palaeo-wind directions. However, fossilized bedform elements can provide insight into the direction of flow that formed them. Wave-formed ripple marks, for example, which developed in shallow waters, extend transverse to the wind direction. In this study, Middle Devonian palaeo-wind directions are reconstructed by measuring the orientation of 511 fossilized wave ripple marks in the Rousay Flagstone Formation on the island of Westray, Orkney, Scotland. The orientation of ripple marks was measured in four different localities on the island. A chi-squared test demonstrates that these four ripple-mark subsets show the same distribution of ripple mark orientation and thus indicates that the studied ripple marks display the same distribution of palaeo-winds. Two dominant ripple mark orientations were observed at all studied localities. The most abundant ripple mark orientation suggests palaeo-winds from the present-day North or South. Second most abundant are ripple marks that suggest palaeo-winds from the present-day ENE or WSW (70° or 250°). When a 40° clockwise rotation of Orkney since the Middle Devonian is taken into account, the ripple marks suggest that the dominant wind directions were south-southeast (SSE) and north-northeast (NNE) in the tropical setting of the Orcadian basin. The SSE wind-direction is represented by the abundant ripple marks, with a present-day near N-S orientation of the line perpendicular to the crests. These ripple marks are also characterized by the largest wavelengths. The abundance of the latter ripple marks with long wavelengths is the result of a lower water level and thus a larger wave-exposed surface during the dry winter season, from April to July, which was mainly characterized by SSE palaeo-trade winds.

The PaleoEcology of a Frasnian Devonian Coral Reef From Russia

Paleoecology and sedimentary environment of the Late Devonian coral biostrome from the Central Devonian Field, Russia

Authors:

Zaton et al

Abstract:

The upper Frasnian coral biostrome, well-exposed in the Russkiy Brod quarry, Central Devonian Field, Russia, has been studied in detail with respect to paleoecology and sedimentary environment. The biostrome, formed by auloporid tabulates and solitary and colonial rugose corals, originated in an offshore environment characterized by calmer periods with slow or halted sedimentation, and more energetic periods when sedimentation rate increased. The episodic, higher sediment influx and stronger hydrodynamic regime are not only well-expressed in the microfacies, but also in the variability of colony integration of the rugose corals observed even within single coralla. Distinct development of constrictions, rejuvenescences and deflection of growth directions in rugose corallites may also indicate unstable sedimentary conditions. The latter features, however, may have, in part, also resulted from syn vivo biotic interaction with the associated auloporids. Both the facies and paleontological observations suggest that the biostrome originated by the colonization of deposited bioclasts by pioneering auloporids, creating the framework for settlement of later generations of auloporids and rugose corals. Apart from abundant auloporids representing a single species and associated rugose corals, the other encrusters are not numerous and poorly diversified, represented by dominating foraminifers, followed by single species of productid brachiopods, stromatoporoids, microconchids and cornulitids. The coral-associated macrobenthos has similarly a low diversity, being represented by single species of spiriferid and rhynchonellid brachiopods, and gastropods. Being developed in an offshore carbonate sedimentary system devoid of any organic-rich deposits, and characterized by extremely low abundance and diversity of suspension-feeding organisms, the biostrome is considered to have originated in a low productivity, oligotrophic environment strictly dominated by heterozoan coral communities. Its development in a well-oxygenated, oligotrophic environment during the time when organic-rich, black Kellwasser facies developed elsewhere, additionally attests for multi-causal scenarios for the Frasnian-Famennian event, during which other factors were responsible in different paleogeographic and facies settings.

A new Large Tetrapodomorph Sarcopterygian from the Late Devonian of Iran

A new large tetrapodomorph sarcopterygian from the Late Devonian of Iran

Authors:

Davesne et al

Abstract:

Remains of a large sarcopterygian were collected in the Middle-Late Devonian Zarand Formation of Southeastern Iran. These remains consist of incomplete jaw bones of uncertain identification, preserved in a dense and heterogeneous rock matrix. The use of CT scan microtomography and 3D visualization enabled unraveling their overall anatomy as well as the microstructure of one tooth. This large tooth shows eusthenodont-type plicidentine, found only in Devonian tetrapodomorph sarcopterygians and more specifically suggesting affinities with the family Tristichopteridae. This discovery constitutes the first occurrence of a large tetrapodomorph in the Devonian of Iran and the Middle East.

Diversity patterns of Devonian Plant Zosterophyllopsida Tied to PaleoGeography

Diversity patterns of the vascular plant group Zosterophyllopsida in relation to Devonian palaeogeography

Authors:

Cascales-Miñana et al

Abstract:

The Zosterophyllopsida originated in the Silurian and became prominent vascular components of Early Devonian floras worldwide. An updated dataset of zosterophyllopsids at species level is analysed to compare the taxic composition of five putative palaeophytogeographic units, Laurussia, Siberia, northwestern Gondwana, Kazakhstan and northeastern Gondwana (i.e. Australia, China and the Shan-Thai block). The high level of endemicity shown by each unit confirms the phytogeographic differentiation and the occurrence of geographical barriers preventing massive floral exchanges between the corresponding regions for the Late Silurian-Early Devonian time interval. Statistical analyses conducted on the three largest datasets, those corresponding to Laurussia, Siberia and northeastern Gondwana, indicate that the diversity dynamics of the group followed the same pattern in these regions. Almost all taxic diversity measures show that specific diversity was greatest in the middle and late Pragian. Diversity dropped significantly thereafter; however, residual diversity reveals genuine regional patterns. From this, we show that the radiation of the Zosterophyllopsida may have stopped earlier in northeastern Gondwana and Siberia than Laurussia. We propose that the onset of these extinctions resulted from the competitive replacement of the zosterophyllopsids by increasingly diversified lycopsids and basal euphyllophytes whose evolution could have been favoured by external factors.