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A time of great transition. In the sea ammonoids and fish evolve and quickly diversify. On land trees and forests appear for the first time. The first insects, spiders, and tetrapods evolve.

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In the southern hemisphere the great supercontinent of (including what is now southern Europe) moves steadily north. But most of the action is happening in the north, where the two continents of and , closing the and forming a mountain range where sea once was. This is known as the . At the same time other mountain ranges are thrown up - in southern Laurentia the Acadian/Appalachian, to the west the Antler/Cordillerian, to the north the Ellesmere (along the north margin of Laurentia) and to the far east the Uralian (in eastern Baltica). The new continent that results from this collision is called Laurussia or Euramerica. During the Devonian the equatorial region was dominated by this newly formed supercontinent, sometimes called the "Old Red continent". It is so called because of its prevailingly reddish, erosion-produced sediments that were deposited in England, Scotland, the Ardennes, and the Rhenish Mountains. The great shallow sandy bays, deltas, and inlets of the Old Red Continent provided a prosperous home for strange armoured jawless fishes, as well as the placoderms which had appeared at this time. To the north again lies the Siberian terraine.

The whole of Euramerica starts to drift northward, whereas Gondwanaland underwent a counterclockwise rotation around the Australian axis. Some of the Chinese blocks and Armorica have started to rift away from the Gondwanan margin. Siberia and the Kazakhstan terranes continued to drift northward.

Both Gondwana and Euramerica are surrounded by subduction zones. They are set on a collision course that will culminate in the formation of a single supercontinent of Pangea during the Permo-Carboniferous.

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If the Devonian was the Age of Fishes, Devonian climatology is the Age of Baloney. Reported results vary strongly depending on what climate proxies are used and where they are studied. Historically, the Devonian has been regarded as largely warm and equable, with a disastrous drop in temperatures in the Late Devonian leading to the Frasnian-Famennian "mass extinction(s)." The reason for this impression may be that most work was traditionally done on the "Old Red Continent," i.e., the shallow marine sediments of the seas around Euramerica.

A careful examination of the paleoclimate maps at the Paleomap Project site suggests a different global picture. See climate maps of the Early Devonian, Middle Devonian, and Late Devonian. The climate of the Early Devonian is rather strongly zonal, with a narrow equatorial tropical belt, broad subtropical arid zones extending to about 35° latitude, and a temperate zones extending essentially to the poles. There is little change in this general picture at any time in the Devonian. In the Late Devonian, the southern "cool temperate" zone expands, with indications of glacial ice in parts of far western Gondwana (northern South America). However, the northern temperate zone appears to retreat before a subtropical zone which extends almost to 60° N. So, although parts of the south were cooler, parts of the north, which had very little land area, were becoming warmer. In short, we are not looking at a simple pattern of planetary cooling.

Instead, we would suggest that the observed effects can be accounted for by a modest drop in sea level combined with a series of local changes related to the formation of the Pangean supercontinent and the spread of land plants. To appreciate the problems, we need to briefly review the tectonics of the period. As we approach the Late Devonian, Pangea is beginning to take shape. This involved pressure on the Laurentian continent from three sides, as well as gradual closure of the seaway between the Rheic and Paleotethys Oceans. As the pressure on the Laurentian plate increased, huge mountain ranges were thrust up around the periphery of the continent. At about this same time, plants were also beginning to make an impact on the land surface and on atmospheric chemistry. Carbon dioxide levels were still several times higher than in present times, but may have dropped as much as 80% from the Silurian. In addition, the Late Devonian saw the evolution of large trees with deep root systems. These strongly increased terrestrial weathering, with a corresponding draw-down of carbon dioxide.

With these generalities in mind, it is easier to appreciate what was happening on a local level. For most of the Devonian, South America had been invaded by a very shallow sea. Further, the broad connection between the largely equatorial Paleotethys and the deep southern Rheic Oceans probably moderated climates all along the northern coast of Gondwana. In the Late Devonian, that connection remained open, but it was constricted, and deep ocean circulation was probably cut off entirely. The Rheic became colder and more thermally isolated. The flow of warm water from the Paleotethys decreased along the north coast, and the falling sea levels drained the central shallow sea. In addition, as South America began to move north, it emerged from the south polar zone of air circulation into a zone dominated by the trade winds passing east to west. Instead of receiving relatively warm, moist air from the Rheic which might create seasonal rains, northern South America would be exposed to cold air dehydrated by the long passage across the entire Gondwanan continent. Thus, it is not surprising that we observe periods of glaciation at high altitudes in northern South America.

Laurentia was also in the southern trade winds. These winds would carry moisture from the Paleotethys. However the mass of the continent lay in the rain shadow of the mountains raised by the subduction of the Gondwanan and Baltican plates, as well as numerous microplates around the eastern and southern margins. The internal geography of the continent was dominated by desert, with an accumulation of evaporites which, when used as climate proxies, may well suggest a hotter climate than was actually present.

Along the well-studied coasts of Baltica and Laurentia, marine chemistry would have undergone enormous changes. The rain which was not falling on central Laurentia and South America was falling on the eastern and southern slopes of the ring of mountains around Laurentia. Forests were beginning to grow here, with deep-rooted trees stirring up soil ions which would be swept into the narrow oceans with torrential flows of fresh water. While the precise results of this process are impossible to reconstruct, it almost certainly meant great changes in ocean chemistry and plankton populations, as well as the usual result of excess runoff -- algal blooms.

While it seems unlikely that this extended exercise in geochemical speculation hits very much closer to the truth than anything else, it may serve as a reminder that local conditions often matter a great deal more than global generalities.

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The most famous of the Devonian Lagerstätten is the Rhynie Chert from Scotland. This deposit is dated as from the Pragian Age of the early Devonian. Paleogeographic reconstructions and other evidence suggest the environment was tropical to subtropical. This deposit is a petrified peat bog preserving the plants in exquisite anatomical detail in the place where they grew and died.

Fossils from the Rynie Chert were buried in short-lived freshwater deposits that later were subjected to replacement of organic material with silica, forming a chert deposit that preserved even details of the cells of the organisms.

The peat species include Aglaophyton (formerly Rhynia major), Horneophyton, Nothia and the proto-lycopod Asteroxylon, but the only plant preserved exactly in its growth position is Rhynia gwynne-vaughanii.

The preservation of all these plants is so fine that individual cells can be seen. The detail of preservation shows, for example, that the stomata of Rhynia were connected to an extensive intercellular system of air spaces, essential for the ventilation of a land plant, and that groundwater was absorbed through unicellular hairs on the horizontal stems. The plant assemblage itself is interesting for the Early Devonian in that its members are not recognized or recorded elsewhere in Euramerica.

It is impossible to determine how typical the Rhynie Chert flora was of the wetter areas of Euramerica. Other Early Devonian assemblages contain plants with far greater amounts of thick-walled structural tissues, and are thus thought to have lived in places subjected to much drier periods.

As well as a number of types of land-plants, Fungi, including mycorhizal fungi, have been recovered from the Rhynie Chert. Wefts of fine, sparingly septate hyphae, some terminating in vesicles, which occur within degraded tissue of vascular plants, are usually identified as a saprotrophic fungus (Phycomycetes), but thick-walled spore-like bodies superficially similar to those of endomycorrhiza (Endogone) suggest that the fungal hyphae lived in symbiotic association with the vascular plants even at that early stage of terrestrial evolution, just as they do today.

Also found are algae, including mats of filamentous blue-green algae, a charophyte green alga called Palaconitella, and filamentous green algae.

Small arthropods are exquisitely preserved between the plant stems and within sporangia. They include crustaceans, a springtail (Class Colembella), several small mites, the first spider and numerous larger extinct mite-like arachnids called trigonotarbids. The trigonotarbids probably preyed on other arthropods while the insects and mites ate spores, leaf-litter, and microorganisms or sucked plant sap, as the associated wounded plant stems suggest.

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- Ongoing research into the stratigraphy, sedimentology and paleontology of an Early Devonian hot spring, by The Rhynie Chert Research Group: Aberdeen - Best on the Web

- good coverage by University of California Museum of Paleontology site.

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The warm tropical oceans of the Devonian period abound in fish, nautiloids, corals, echinoderms, trilobites, and conodonts.

Types of marine life: In Devonian seas, sponges were represented by newly evolved siliceous forms, many of which were similar to the modern Venus flower basket. The association between algae, sponges, and corals that began in the Ordovician continued, with flourishing reefs, such as the one illustrated in the above diorama, thriving in the warm shallow seas. During this time not only the hylaesponges, rugose and tabulate corals (shown above) but also the brachiopods reached their zenith in number and diversity. The spiriferid brachiopods (left) were particularly abundant. Among molluscs, while gastropods, bivalves, and nautiloids continue with little change from the Silurian, the first ammonoids mark the beginning of an important new phase of molluscan evolution. Trilobites were generally on the decline, but a few groups remained abundant, and some giant forms evolved, such as the huge spiny Terataspis grandis (30 to 60 cm). The increase in swimming predators (such as new forms of fish and cephalopods) may have contributed to the trilobite decline.

The Devonian saw the rapid evolution diversification of fish, especially the Placodermi, primitive sharks, [[Sarcopterygii}} (lobe-finned fish and lungfish) and Actinopterygii (conventional bony fish or ray-finned fish). So pronounced is this evolutionary radiation that the Devonian has been called "the Age of Fish".

Terrestrial life: Many arthropods, including eurypterids, arachnids (spiders and their kin) and primitive wingless insects invaded the land. Towards the end of the period the first fish-like tetrapods move ashore. Seed-bearing plants (Gymnosperms) also appeared during the latest Devonian. Seeds mean a freedom from dependence on moist habitats for reproduction, and allowed plants to expand into drier areas.

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There is a major mass extinction during the Late Devonian, the so-called Frasnian-Famennian event at around 364 . The tabulate-stromatoporoid reefs disappear completely, with corals so seriously decimated that extensive reef building did not happen until the Triassic with the evolution of a new group of reef-building corals, the scleractinians. Brachiopods, trilobites and primitive fish groups either were either diminished or completely killed off, as were many planktonic and nektonic (floating and swimming) animals. The planktonic graptolites and enigmatic tentaculites die out and trilobites are much reduced. Tropical taxa were the most severely affected. The effect on terrestrial ecosystems was not as marked.

Various causes have been suggested. Global cooling tied to Gondwanan glaciation has been proposed as the cause of the Devonian extinction, as it was also suspected of in the case of the terminal Ordovician extinction. Support for this hypothesis comes from the fact that the forms of marine life most affected by the extinction were the warm water to tropical ones. Another hypothesis is that environmental sea-level and climatic change in conjunction with an extraterrestrial impact (comet/asteroid) caused a global cooling. There are several impact sites known to be of the right potential age to have been involved in this extinction. But neither the glaciation or the impact hypothesis is unequivocally supported by the available data.

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The subject of Devonian plants is one that has occupied numerous scholars for their entire professional lives. Obviously, we are not going to be able to do it justice here. We have discussed various aspects of the matter in connection with the Rhynie Chert. At Paleozoic Plants, we include a few paragraphs specifically on Devonian plants, with links to more extensive treatment of individual taxa.. Finally, we include links to several of the many good web sites on this topic below. Consequently, there is no obvious need for yet another summary of this topic here -- assuming we cared whether or not there was a need. As the astute reader will already have perceived, that sort of utilitarian calculus is rarely involved in decision-making at Palaeos -- assuming an astute reader would be reading Palaeos at all, instead of some more authoritative source. But enough of this dizzying ontological circuity. Let us attend to the subject at hand.

The Devonian Period was, for plants, a sort of Cambrian explosion. Plants began the Devonian, just as animals began the Cambrian, with a small amount of important, but largely cryptic diversity. That is, some of the important groups had already diverged, but we have not yet found much of that divergence in the fossil record. Just as almost all kinds of animals looked more or less like flatworms in the earliest Cambrian, almost all land plants looked more or less like Cooksonia at the end of the Silurian. Important specializations had already occurred, but they are hidden by a poor fossil record of small plants which all look more or less the same. However, by the end of the Devonian, plants had adapted to land in many different forms. They had evolved structures capable of raising dense forests up to 30 meters against the force of gravity (e.g., Archaeopteris); and were making far more effective use of the resources available to them. However, the "explosion" of forms consisted largely in developing and refining the key evolutionary innovations already present at the beginning of the Period.

The structure of this revolution is revealed by comparing Cooksonia with Late Devonian plants. Cooksonia itself was already a vascular plant with the key features of the true vascular land plants (Tracheophyta) [1]. That is, it had a specialized vascular system (tracheids) composed of the cell walls of dead cells (xylem) to transport water and nutrients upward. The walls of the tracheids were somewhat thickened to support the stem against gravity. However, the amount of reinforcing material was small and it may not have been the lignin (woody tissue) of tracheophytes. Cooksonia also had tiny, adjustable vents (stoma) for gas exchange, and well-developed sporangia (spore-bearing reproductive structures). Although it lacked a massively reticulated root system, it did have a sort of taproot and ground-level side branches (rhizomes), both bearing root hairs. What it lacked were leaves, the massive lignin supports of more derived plants, and seeds. These were acquired in approximately that phylogenetic order.

Cooksonia itself is a member of the Rhyniopsida, the most basal group of tracheophytes (and quite likely paraphyletic). In broad outline, the Devonian progress of the group looks like this:

|-- = Rhyniopsida: includes Cooksonia)
`--+--: lycopods (club moss) and zosterophylls
  `--+--: horsetails & ferns
     `--+--: Trimerophyton, Psilophyton & Pertica
        `--: seed plants and a few others

Some of the rhyniophytes had already developed "spikes" and various other excuses for increasing surface area to catch more sunlight. The problem is that more surface area also means faster water loss by evaporation. It took a bit longer to evolve the waxy covering that allows plants to form broad leaves. Leaves are present in all of the more derived groups, and seem to have developed at first by growing "webbing" of photosynthetic tissues between small twigs.

Wood is also a Devonian innovation. Wood means axial strength, which means the ability to grow taller to reach open sunlight and to carry a greater weight of branches and leafy, photosynthetic surface per meter of height. Thus it is no surprise that we go from the rather flaccid stems of Cooksonia to the true wood of progymnosperms by the Middle Devonian. As soon as the environment of land plants came to be dominated by other land plants, the race would be on to join one of the four great plant guilds: (a) trees (tall-growing plants that shade out the competition), (b) shrubs (low, shade tolerant, densely growing plants that crowd out competition), (c) weeds (fast-growing, opportunistic, adventitious plants that outrun competition by spreading quickly through temporarily open spaces) and (d) survivalists (hardy plants that colonize marginal environments where no competitors can live). Wood is plainly an essential for members of the tree guild.

Moniliforms and trimerophytes developed refinements of the vascular system, particularly secondary xylem and, in progymnosperms, phloem, the specialized vascular tissue that moves the products of photosynthesis down from the leaves to other regions of the plant. This suggests that these plants first developed as shrubs, selected for dense, efficient growth.

It was left to the Carboniferous to develop the seed, a device which, like the amniotic egg of vertebrates, allowed plants to spread far from open water. However, by the end of Devonian, both plants and vertebrates were solidly established on the terrestrial margins and poised to colonize the interior highlands.

Links: ; ; ; .

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- the best over-all introduction.

- nice basic intro - easy to understand - actually a class project

- all about the first tetrapods (four-legged animals). Gives an excellent coverage of the new paradigm that the first tetrapods amphibians were not so much crossopterygian fish crawling on land to a new pond to escape drought (and only evolving legs afterwards), but rather (i.e. legs evolved before moving on land).

in what is now West Australia

- a small selection of Devonian fossils from Nova Scotia

- the fossils of this time and two illustrations

- some photos of fossils from Victoria (South-East Gondwana)

by Martin J. S. Rudwick - the history of science, relating to the 19th century discovery of Devonian-age rocks

[1] Phylogenetic taxonomy has not caught on completely among paleobotanists, with the result that there is still a certain amount of pointless debate about exactly what characters should be used to define the Tracheophyta. We respectfully submit that characters should never be used to define taxa at all. Tracheophyta ought to be defined as the last common ancestor of Cooksonia and cabbage, with all of its descendants (or some equivalent). Then we could move on to the real job of figuring out what they have in common and who belongs to the group. See discussion at Cladograms.

Text by M. Alan Kazlev 1998-2002 Unless otherwise attributed, text on this page may be used only under the terms of a Creative Commons License.

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