Portal:Devonian

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The Devonian Portal

Map of the Earth during the late Devonian, c. 370 Ma.

The Devonian (/dəˈvoʊni.ən, dɛ-/ də-VOH-nee-ən, deh-) is a geologic period and system of the Paleozoic era during the Phanerozoic eon, spanning 60.3 million years from the end of the preceding Silurian period at 419.2 million years ago (Ma), to the beginning of the succeeding Carboniferous period at 358.9 Ma. It is named after Devon, South West England, where rocks from this period were first studied.

The first significant evolutionary radiation of life on land occurred during the Devonian, as free-sporing land plants (pteridophytes) began to spread across dry land, forming extensive coal forests which covered the continents. By the middle of the Devonian, several groups of vascular plants had evolved leaves and true roots, and by the end of the period the first seed-bearing plants (pteridospermatophytes) appeared. This rapid evolution and colonization process, which had begun during the Silurian, is known as the Silurian-Devonian Terrestrial Revolution. The earliest land animals, predominantly arthropods such as myriapods, arachnids and hexapods, also became well-established early in this period, after beginning their colonization of land at least from the Ordovician period.

Fishes, especially jawed fish, reached substantial diversity during this time, leading the Devonian to often be dubbed the Age of Fishes. The armored placoderms began dominating almost every known aquatic environment. In the oceans, cartilaginous fishes such as primitive sharks became more numerous than in the Silurian and Late Ordovician. Tetrapodomorphs, which include the ancestors of all four-limbed vertebrates (i.e. tetrapods), began diverging from freshwater lobe-finned fish as their more robust and muscled pectoral and pelvic fins gradually evolved into forelimbs and hindlimbs, though they were not fully established for life on land until the Late Carboniferous. (Full article...)

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Selected Devonian Article

The Late Devonian extinction consisted of several extinction events in the Late Devonian Epoch, which collectively represent one of the five largest mass extinction events in the history of life on Earth. The term primarily refers to a major extinction, the Kellwasser event, also known as the Frasnian-Famennian extinction, which occurred around 372 million years ago, at the boundary between the Frasnian stage and the Famennian stage, the last stage in the Devonian Period. Overall, 19% of all families and 50% of all genera became extinct. A second mass extinction called the Hangenberg event, also known as the end-Devonian extinction, occurred 359 million years ago, bringing an end to the Famennian and Devonian, as the world transitioned into the Carboniferous Period.

Although it is well established that there was a massive loss of biodiversity in the Late Devonian, the timespan of this event is uncertain, with estimates ranging from 500,000 to 25 million years, extending from the mid-Givetian to the end-Famennian. Some consider the extinction to be as many as seven distinct events, spread over about 25 million years, with notable extinctions at the ends of the Givetian, Frasnian, and Famennian stages.

By the Late Devonian, the land had been colonized by plants and insects. In the oceans, massive reefs were built by corals and stromatoporoids. Euramerica and Gondwana were beginning to converge into what would become Pangaea. The extinction seems to have only affected marine life. Hard-hit groups include brachiopods, trilobites, and reef-building organisms; the latter almost completely disappeared. The causes of these extinctions are unclear. Leading hypotheses include changes in sea level and ocean anoxia, possibly triggered by global cooling or oceanic volcanism. The impact of a comet or another extraterrestrial body has also been suggested, such as the Siljan Ring event in Sweden. Some statistical analysis suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions. This might have been caused by invasions of cosmopolitan species, rather than by any single event. Placoderms were hit hard by the Kellwasser event and completely died out in the Hangenberg event, but most other jawed vertebrates were less strongly impacted. Agnathans (jawless fish) were in decline long before the end of the Frasnian and were nearly wiped out by the extinctions. (Full article...)

List of selected Devonian articles
Sarcopterygii Trilobites Marcellus Formation Acadian Orogeny Bactritida List of Early Devonian land plants Dunkleosteus

Selected Devonian land plant article

Lycopodiella inundata

A pteridophyte is a vascular plant (with xylem and phloem) that reproduces by means of spores. Because pteridophytes produce neither flowers nor seeds, they are sometimes referred to as "cryptogams", meaning that their means of reproduction is hidden.

Ferns, horsetails (often treated as ferns), and lycophytes (clubmosses, spikemosses, and quillworts) are all pteridophytes. However, they do not form a monophyletic group because ferns (and horsetails) are more closely related to seed plants than to lycophytes. "Pteridophyta" is thus no longer a widely accepted taxon, but the term pteridophyte remains in common parlance, as do pteridology and pteridologist as a science and its practitioner, for example by the International Association of Pteridologists and the Pteridophyte Phylogeny Group. (Full article...)

List of selected Devonian articles
Sigillaria Pteridospermatophyta Lycophyte Equisetidae Progymnosperm Rhyniophyte Trimerophytopsida Lycopodiopsida

Selected Devonian formation

The Hamilton Group is a Devonian-age geological group which is located in the Appalachian region of the United States. It is present in New York, Pennsylvania, Maryland, Ohio, West Virginia, northwestern Virginia and Ontario, Canada, and is mainly composed of marine shale with some sandstone.

There are two main formations encompassed by the group: the Mahantango Formation and the Marcellus Shale. In southwestern Virginia, where the two sub-units are not easily distinguishable, the Hamilton Group is broadly equivalent to the Millboro Shale or Millboro Formation.

The group is named for the village of Hamilton, New York. These rocks are the oldest strata of the Devonian gas shale sequence. (Full article...)

List of selected Devonian formation articles
Tully Formation Tioga Bentonites Windom shale Rhinestreet Formation Oriskany Formation Bakken formation Traverse Group New Albany Shale Old Red Sandstone Grey Downtonian

Selected Devonian fish article

Lepidaspis serrata ("serrated scaly shield") is an extinct heterostracan jawless fish from Early Devonian Canada. Its scientific name refers to the fact that the armor is composed of hundreds of tiny scales with serrated edges.

Although it is regarded as a heterostracan, its exact placement within Heterostraci is regarded as incertae sedis, as the fact that it is currently impossible to discern where its dorsal and ventral plates (which are made up of a mosaic of the aforementioned tiny scales) end or begin. Some experts regard it as being descended from the basal heterostracans of the Silurian. (Full article...)

List of selected Devonian fish articles
Ctenaspis Brochoadmones Cheirolepis Holoptychius Hyneria Scaumenacia Tinirau clackae Placoderm Diplocercides Dipterus Elpistostegalia Gooloogongia Ventastega Tristichopteridae Phoebodus Stethacanthidae Xenacanthus

Selected Devonian invertebrate

Fossil specimen of Eurypterus remipes housed at the State Museum of Natural History Karlsruhe in Karlsruhe, Germany

Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 million years ago. The group is likely to have appeared first either during the Early Ordovician or Late Cambrian period. With approximately 250 species, the Eurypterida is the most diverse Paleozoic chelicerate order. Following their appearance during the Ordovician, eurypterids became major components of marine faunas during the Silurian, from which the majority of eurypterid species have been described. The Silurian genus Eurypterus accounts for more than 90% of all known eurypterid specimens. Though the group continued to diversify during the subsequent Devonian period, the eurypterids were heavily affected by the Late Devonian extinction event. They declined in numbers and diversity until becoming extinct during the Permian–Triassic extinction event (or sometime shortly before) 251.9 million years ago.

Although popularly called "sea scorpions", only the earliest eurypterids were marine; many later forms lived in brackish or fresh water, and they were not true scorpions. Some studies suggest that a dual respiratory system was present, which would have allowed for short periods of time in terrestrial environments. The name Eurypterida comes from the Ancient Greek words εὐρύς (eurús), meaning 'broad' or 'wide', and πτερόν (pterón), meaning 'wing', referring to the pair of wide swimming appendages present in many members of the group.

The eurypterid order includes the largest known arthropods ever to have lived. The largest, Jaekelopterus, reached 2.5 meters (8.2 ft) in length. Eurypterids were not uniformly large and most species were less than 20 centimeters (8 in) long; the smallest eurypterid, Alkenopterus, was only 2.03 centimeters (0.80 in) long. Eurypterid fossils have been recovered from every continent. A majority of fossils are from fossil sites in North America and Europe because the group lived primarily in the waters around and within the ancient supercontinent of Euramerica. Only a handful of eurypterid groups spread beyond the confines of Euramerica and a few genera, such as Adelophthalmus and Pterygotus, achieved a cosmopolitan distribution with fossils being found worldwide. (Full article...)