112-590: The Munster Basin is a late Middle to Upper Devonian age extensional (rift) sedimentary basin in the south-west of Ireland . The basin fill comprises fluvial Old Red Sandstone (ORS) magnafacies with minor silicic volcanic and mafic sub-volcanic centres. The depocentre of the basin is located between the MacGillycuddy's Reeks and the Kenmare River on the Iveragh peninsula where the succession
224-452: A Silurian placoderm, Wangolepis of Silurian China and possibly Vietnam, is known only from a few fragments that currently defy attempts to place them in any of the recognized placoderm orders. So far, only three officially described Silurian placoderms are known from more than scraps: The first officially described Silurian placoderm is an antiarch, Shimenolepis , which is known from distinctively ornamented plates from Hunan , China. It
336-539: A feature shared by birds and some ichthyosaurs . Early arthrodires, such as the genus Arctolepis , were well-armoured fishes with flattened bodies. The largest member of this group, Dunkleosteus , was a true "superpredator" of the latest Devonian period, reaching 3 to as much as 8 metres in length. In contrast, the long-nosed Rolfosteus measured just 15 cm. Fossils of Incisoscutum have been found containing unborn fetuses, indicating that arthrodires gave birth to live young. Antiarchi ("opposite anus") were
448-417: A few species of the order Proetida . The subsequent end-Devonian extinction , which occurred at around 359 Ma, further impacted the ecosystems and completed the extinction of all calcite sponge reefs and placoderms. Devonian palaeogeography was dominated by the supercontinent Gondwana to the south, the small continent of Siberia to the north, and the medium-sized continent of Laurussia to
560-468: A group of chimaera-like placoderms closely related to the rhenanid placoderms. Superficially, acanthoracids resembled scaly chimaeras or small, scaly arthrodires with blunt rostrums . They were distinguished from chimaeras by a pair of large spines that emanate from their chests, the presence of large scales and plates, tooth-like beak plates, and the typical bone-enhanced placoderm eyeball. They were distinguished from other placoderms due to differences in
672-533: A late Givetian chronostratigraphic age on recent Devonian time scales. The general Late Devonian age of the basin fill is also given by miospore and fish fossil records. Devonian The Devonian ( / d ə ˈ v oʊ n i . ən , d ɛ -/ 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
784-763: A low-latitude archipelago to the north of Gondwana. They were separated from the southern continent by an oceanic basin: the Paleo-Tethys . Although the western Paleo-Tethys Ocean had existed since the Cambrian, the eastern part only began to rift apart as late as the Silurian. This process accelerated in the Devonian. The eastern branch of the Paleo-Tethys was fully opened when South China and Annamia (a terrane equivalent to most of Indochina ), together as
896-529: A massive extinction event . ( See Late Devonian extinction ). Primitive arthropods co-evolved with this diversified terrestrial vegetation structure. The evolving co-dependence of insects and seed plants that characterized a recognizably modern world had its genesis in the Late Devonian Epoch. The development of soils and plant root systems probably led to changes in the speed and pattern of erosion and sediment deposition. The rapid evolution of
1008-513: A mosaic of tubercles. Like Stensioella heintzi , the pseudopetalichthids' placement within Placodermi is suspect. The matter is not easy to resolve because there are no complete, undamaged and articulated specimens. The anatomical studies done on the crushed specimens that have been found indicate that if they are placoderms, they may be a group more advanced than the ptyctodonts . As such, placoderm experts consider Pseudopetalichthyida to be
1120-407: A movable joint between armour surrounding the head and body. As the lower jaw moved down, the head shield moved, allowing for a larger opening. All arthrodires, save for Compagopiscis , lacked teeth, and used instead the sharpened edges of a bony plate, termed a "tooth plate," as a biting surface ( Compagopiscis had true teeth in addition to tooth plates). The eye sockets are protected by a bony ring,
1232-504: A primitive placoderm, though some paleontologists believe the rationale for the placement is inadequate. The paleontologist Philippe Janvier , as well as other paleontologists, has suggested that Stensioella is not a placoderm, but instead is a holocephalian . If this is true, then the holocephalians diverged from sharks before the Chondrichthyan Devonian radiation. Critics of Janvier's position say that aside from
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#17327764813741344-420: A range of evidence, such as plant distribution, points to a Late Devonian warming. The climate would have affected the dominant organisms in reefs ; microbes would have been the main reef-forming organisms in warm periods, with corals and stromatoporoid sponges taking the dominant role in cooler times. The warming at the end of the Devonian may even have contributed to the extinction of the stromatoporoids. At
1456-484: A skinny Gemuendina with thin, strap-like pectoral fins. Similar to those of the Rhenanida, its armour was a complex mosaic of small, scale-like tubercles. The shoulder joints of its armour are similar to other placoderms, and there are superficial similarities in skull plates, and even more superficial similarities between its tubercles and the tubercles of the rhenanids . It is tentatively placed within Placodermi as
1568-664: A terrestrial ecosystem that contained copious animals opened the way for the first vertebrates to seek terrestrial living. By the end of the Devonian, arthropods were solidly established on the land. The Late Devonian extinction is not a single event, but rather is a series of pulsed extinctions at the Givetian-Frasnian boundary, the Frasnian-Famennian boundary, and the Devonian-Carboniferous boundary. Together, these are considered one of
1680-541: A unified continent, detached from the northeastern sector of Gondwana. Nevertheless, they remained close enough to Gondwana that their Devonian fossils were more closely related to Australian species than to north Asian species. Other Asian terranes remained attached to Gondwana, including Sibumasu (western Indochina), Tibet, and the rest of the Cimmerian blocks. While the South China-Annamia continent
1792-602: A volcanically active region during the Devonian, as it continued to assimilate smaller island arcs. The island arcs of the region, such as the Balkhash-West Junggar Arc, exhibited biological endemism as a consequence of their location. Siberia was located just north of the equator as the largest landmass in the Northern Hemisphere. At the beginning of the Devonian, Siberia was inverted (upside down) relative to its modern orientation. Later in
1904-622: A warm temperate climate . In the Late Devonian, by contrast, arid conditions were less prevalent across the world and temperate climates were more common. The Devonian Period is formally broken into Early, Middle and Late subdivisions. The rocks corresponding to those epochs are referred to as belonging to the Lower, Middle and Upper parts of the Devonian System. The Early Devonian lasted from 419.2 to 393.3 Ma. It began with
2016-620: Is at least ca . 6 km thick. The non-marine ORS is conformably succeeded by latest Devonian coastal plain and shallow marine clastic deposits (the Toe Head Sandstone and Old Head Sandstone Formations, and equivalents), followed by shallow to deeper marine Carboniferous sandstones, mudstones and limestones of the South Munster Basin . During the Late Palaeozoic Variscan (or Hercynian) orogeny
2128-578: Is good evidence that Rheic oceanic crust experienced intense subduction and metamorphism under Mexico and Central America. The closure of the eastern part of the Rheic Ocean is associated with the assemblage of central and southern Europe. In the early Paleozoic, much of Europe was still attached to Gondwana, including the terranes of Iberia , Armorica (France), Palaeo-Adria (the western Mediterranean area), Bohemia , Franconia , and Saxothuringia . These continental blocks, collectively known as
2240-415: Is likely monophyletic . The first identifiable placoderms appear in the fossil record during the late Llandovery epoch of the early Silurian. They eventually outcompeted the previously dominant marine arthropods (e.g. eurypterids ) and cephalopod molluscs (e.g. orthocones ), producing some of the first and most infamous vertebrate apex predators such as Eastmanosteus , Dinichthys and
2352-598: Is not clear, as none of the fossil specimens found have preserved mouth parts. Pseudopetalichthyida ("false petalichthyids") is a group of elongated, possibly flattened fishes comprising three, poorly preserved and poorly studied genera. It is known only from rare fossils in Lower Devonian strata in Hunsrück , Germany. Like Stensioella heintzi , and the Rhenanida , the pseudopetalichthids had armour made up of
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#17327764813742464-430: Is observed in many of those plants. Some of the early land plants such as Drepanophycus likely spread by vegetative growth and spores. The earliest land plants such as Cooksonia consisted of leafless, dichotomous axes with terminal sporangia and were generally very short-statured, and grew hardly more than a few centimetres tall. Fossils of Armoricaphyton chateaupannense , about 400 million years old, represent
2576-641: The Antler orogeny , which extended into the Carboniferous. Mountain building could also be found in the far northeastern extent of the continent, as minor tropical island arcs and detached Baltic terranes re-join the continent. Deformed remnants of these mountains can still be found on Ellesmere Island and Svalbard . Many of the Devonian collisions in Laurussia produce both mountain chains and foreland basins , which are frequently fossiliferous. Gondwana
2688-565: The Frasnian , 382.7 to 372.2 Ma, during which the first forests took shape on land. The first tetrapods appeared in the fossil record in the ensuing Famennian subdivision, the beginning and end of which are marked with extinction events. This lasted until the end of the Devonian, 358.9 Ma. The Devonian was a relatively warm period, although significant glaciers may have existed during the Early and Middle Devonian. The temperature gradient from
2800-468: The Gogo Formation of Western Australia, had streamlined, bullet-shaped head armor, and Amazichthys , with morphology like that of other fast-swimming pelagic organisms , strongly supporting the idea that many, if not most, arthrodires were active swimmers, rather than passive ambush-hunters whose armor practically anchored them to the sea floor. Some placoderms were herbivorous, such as
2912-483: The Late Carboniferous . The first ammonites , a subclass of cephalopod molluscs , appeared. Trilobites , brachiopods and the great coral reefs were still common during the Devonian. The Late Devonian extinction , which started about 375 Ma, severely affected marine life, killing off most of the reef systems, most of the jawless fish, half of all placoderms, and nearly all trilobites save for
3024-640: The Lochkovian Stage 419.2 to 410.8 Ma, which was followed by the Pragian from 410.8 to 407.6 Ma and then by the Emsian , which lasted until the Middle Devonian began, 393.3 Ma. During this time, the first ammonoids appeared, descending from bactritoid nautiloids . Ammonoids during this time period were simple and differed little from their nautiloid counterparts. These ammonoids belong to
3136-562: The Silurian and the Devonian periods . While their endoskeletons are mainly cartilaginous , their head and thorax were covered by articulated armoured plates (hence the name), and the rest of the body was scaled or naked depending on the species . Placoderms were among the first jawed fish (their jaws likely evolved from the first pair of gill arches ), as well as the first vertebrates to have true teeth . They were also
3248-475: The cladoxylopsids and progymnosperm Archaeopteris . These tracheophytes were able to grow to large size on dry land because they had evolved the ability to biosynthesize lignin , which gave them physical rigidity and improved the effectiveness of their vascular system while giving them resistance to pathogens and herbivores. In Eifelian age, cladoxylopsid trees formed the first forests in Earth history. By
3360-508: The hox genes hoxd13, the origin of the mating organs in placoderms most likely relied on different sets of hox genes and were structures that developed further down the body as an extra and independent pair of appendages, but which during development turned into body parts used for reproduction only. Because they were not attached to the pelvic fins, as are the claspers in fish like sharks, they were much more flexible and could probably be rotated forward. A study on Kolymaspis showcases that
3472-452: The " Big Five " mass extinctions in Earth's history. The Devonian extinction crisis primarily affected the marine community, and selectively affected shallow warm-water organisms rather than cool-water organisms. The most important group to be affected by this extinction event were the reef-builders of the great Devonian reef systems. Amongst the severely affected marine groups were the brachiopods, trilobites, ammonites, and acritarchs , and
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3584-710: The Armorican Terrane Assemblage, split away from Gondwana in the Silurian and drifted towards Laurussia through the Devonian. Their collision with Laurussia leads to the beginning of the Variscan Orogeny , a major mountain-building event which would escalate further in the Late Paleozoic. Franconia and Saxothuringia collided with Laurussia near the end of the Early Devonian, pinching out the easternmost Rheic Ocean. The rest of
3696-536: The Armorican terranes followed, and by the end of the Devonian they were fully connected with Laurussia. This sequence of rifting and collision events led to the successive creation and destruction of several small seaways, including the Rheno-Hercynian, Saxo-Thuringian, and Galicia-Moldanubian oceans. Their sediments were eventually compressed and completely buried as Gondwana fully collided with Laurussia in
3808-511: The Carboniferous. Sea levels in the Devonian were generally high. Marine faunas continued to be dominated by conodonts, bryozoans , diverse and abundant brachiopods , the enigmatic hederellids , microconchids , and corals . Lily-like crinoids (animals, their resemblance to flowers notwithstanding) were abundant, and trilobites were still fairly common. Bivalves became commonplace in deep water and outer shelf environments. The first ammonites also appeared during or slightly before
3920-533: The Devonian Period. The newly evolved forests drew carbon out of the atmosphere, which were then buried into sediments. This may be reflected by a Mid-Devonian cooling of around 5 °C (9 °F). The Late Devonian warmed to levels equivalent to the Early Devonian; while there is no corresponding increase in CO 2 concentrations, continental weathering increases (as predicted by warmer temperatures); further,
4032-527: The Devonian extinction events were caused by an asteroid impact. However, while there were Late Devonian collision events (see the Alamo bolide impact ), little evidence supports the existence of a large enough Devonian crater. Placoderm Placoderms (from Greek πλάξ ( plax , plakos ) ' plate ' and δέρμα ( derma ) 'skin') are vertebrate animals of the class Placodermi , an extinct group of prehistoric fish known from Paleozoic fossils during
4144-408: The Devonian seas. The first abundant genus of cartilaginous fish, Cladoselache , appeared in the oceans during the Devonian Period. The great diversity of fish around at the time has led to the Devonian being given the name "The Age of Fishes" in popular culture. The Devonian saw significant expansion in the diversity of nektonic marine life driven by the abundance of planktonic microorganisms in
4256-645: 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
4368-434: 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
4480-506: The Early Devonian and the existence of fossils such as Protichnites suggest that amphibious arthropods may have appeared as early as the Cambrian ). By far the largest land organism at the beginning of this period was the enigmatic Prototaxites , which was possibly the fruiting body of an enormous fungus, rolled liverwort mat, or another organism of uncertain affinities that stood more than 8 metres (26 ft) tall, and towered over
4592-471: The Early and Middle Devonian, the west coast of Laurussia was a passive margin with broad coastal waters, deep silty embayments, river deltas and estuaries, found today in Idaho and Nevada . In the Late Devonian, an approaching volcanic island arc reached the steep slope of the continental shelf and began to uplift deep water deposits. This minor collision sparked the start of a mountain-building episode called
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4704-637: The Late Devonian the tectonic situation had relaxed and much of South America was covered by shallow seas. These south polar seas hosted a distinctive brachiopod fauna, the Malvinokaffric Realm, which extended eastward to marginal areas now equivalent to South Africa and Antarctica. Malvinokaffric faunas even managed to approach the South Pole via a tongue of Panthalassa which extended into the Paraná Basin . The northern rim of Gondwana
4816-682: The Late Devonian. The Altai-Sayan region was shaken by volcanism in the Early and Middle Devonian, while Late Devonian magmatism was magnified further to produce the Vilyuy Traps , flood basalts which may have contributed to the Late Devonian Mass Extinction. The last major round of volcanism, the Yakutsk Large Igneous Province, continued into the Carboniferous to produce extensive kimberlite deposits. Similar volcanic activity also affected
4928-434: The Middle Devonian of Australia, suggesting that the basipterygia were used in copulation. The placoderm claspers are not homologous with the claspers in cartilaginous fishes . The similarities between the structures has been revealed to be an example of convergent evolution . While the claspers in cartilaginous fishes are specialized parts of their paired pelvic fins that have been modified for copulation due to changes in
5040-565: The Middle to Late Devonian arthrodire Holonema , and some were planktivores , such as the gigantic arthrodire Titanichthys , various members of Homostiidae , and Heterosteus . Extraordinary evidence of internal fertilization in a placoderm was afforded by the discovery in the Gogo Formation, near Fitzroy Crossing , Kimberley , Western Australia, of a small female placoderm, about 25 cm (10 in) in length, which died in
5152-640: The Silurian and Devonian, it decreased across the span of the Devonian, particularly during the Pragian, and that the overall diversity of nektonic taxa did not increase significantly during the Devonian compared to during other geologic periods, and was in fact higher during the intervals spanning from the Wenlock to the Lochkovian and from the Carboniferous to the Permian. The study's authors instead attribute
5264-560: 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
5376-418: The anatomy of their skulls, and due to patterns on the skull plates and thoracic plates that are unique to this order. From what can be inferred from the mouthplates of fossil specimens, acanthothoracids were shellfish hunters ecologically similar to modern-day chimaeras. Competition with their relatives, the ptyctodont placoderms, may have been one of the main reasons for the acanthothoracids' extinction prior to
5488-448: The antiarchs, with their highly modified, jointed bony pectoral fins, were highly successful inhabitants of Middle-Late Devonian freshwater and shallow marine habitats, with the Middle to Late Devonian genus , Bothriolepis , known from over 100 valid species. The vast majority of placoderms were predators , many of which lived at or near the substrate . Many, primarily the arthrodires , were active, nektonic predators that dwelled in
5600-661: The beginning of the Mississippian subperiod of the Carboniferous. In 19th-century texts the Devonian has been called the "Old Red Age", after the red and brown terrestrial deposits known in the United Kingdom as the Old Red Sandstone in which early fossil discoveries were found. Another common term is "Age of the Fishes", referring to the evolution of several major groups of fish that took place during
5712-684: The continent (such as Greenland and Ellesmere Island ) established tropical conditions, most of the continent was located within the natural dry zone along the Tropic of Capricorn , which (as nowadays) is a result of the convergence of two great air-masses, the Hadley cell and the Ferrel cell . In these near-deserts, the Old Red Sandstone sedimentary beds formed, made red by the oxidised iron ( hematite ) characteristic of drought conditions. The abundance of red sandstone on continental land also lends Laurussia
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#17327764813745824-548: The continents Laurentia (modern day North America) and Baltica (modern day northern and eastern Europe). The tectonic effects of this collision continued into the Devonian, producing a string of mountain ranges along the southeastern coast of the continent. In present-day eastern North America, the Acadian Orogeny continued to raise the Appalachian Mountains . Further east, the collision also extended
5936-473: The date of first viviparity back some 200 million years earlier than had been previously known. Specimens of the arthrodire Incisoscutum ritchei , also from the Gogo Formation, have been found with embryos inside them indicating this group also had live bearing ability. The males reproduced by inserting a long clasper into the female. Elongated basipterygia are also found on the phyllolepid placoderms, such as Austrophyllolepis and Cowralepis , both from
6048-715: The deposits in the basin were subjected to compressional deformation that resulted in pressure solution cleavage formation, buckle folding and contractional faulting under very low-grade metamorphic conditions. The oldest deposits found in the Munster Basin belong to the Valentia Slate Formation from which a silicic air-fall tuff bed (the Keel-Enagh Tuff) was radiometrically dated as 384.9 ± 0.7 Ma, which can be linked to local miospore biostratigraphic records. In combination, this corresponds to
6160-405: The early Devonian Period around 400 Ma. Bactritoids make their first appearance in the Early Devonian as well; their radiation, along with that of ammonoids, has been attributed by some authors to increased environmental stress resulting from decreasing oxygen levels in the deeper parts of the water column. Among vertebrates, jawless armored fish ( ostracoderms ) declined in diversity, while
6272-662: The east. Major tectonic events include the closure of the Rheic Ocean , the separation of South China from Gondwana, and the resulting expansion of the Paleo-Tethys Ocean . The Devonian experienced several major mountain-building events as Laurussia and Gondwana approached; these include the Acadian Orogeny in North America and the beginning of the Variscan Orogeny in Europe. These early collisions preceded
6384-569: The end of the Devonian, the first seed-forming plants had appeared. This rapid appearance of many plant groups and growth forms has been referred to as the Devonian Explosion or the Silurian-Devonian Terrestrial Revolution. The 'greening' of the continents acted as a carbon sink , and atmospheric concentrations of carbon dioxide may have dropped. This may have cooled the climate and led to
6496-516: 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 the fourth period of both the Paleozoic and the Phanerozoic. 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
6608-474: The environmental catastrophes of the Late Devonian and end-Devonian extinctions . The earliest identifiable placoderm fossils are of Chinese origin and date to the early Silurian . At that time, they were already differentiated into antiarchs and arthrodires , as well as other, more primitive, groups. Earlier fossils of basal placoderms have not yet been discovered. The Silurian fossil record of
6720-521: The equally enigmatic Pseudopetalichthyida . These orders are considered to be basal or primitive groups within Placodermi, though their precise placement within the class remains unsure. Fossils of both are currently known only from the Hunsruck lagerstatten . Arthrodira ("jointed neck") were the most diverse and numerically successful of the placoderm orders, occupying roles from giant apex predators to detritus -nibbling bottom dwellers . They had
6832-412: The equator to the poles was not as large as it is today. The weather was also very arid, mostly along the equator where it was the driest. Reconstruction of tropical sea surface temperature from conodont apatite implies an average value of 30 °C (86 °F) in the Early Devonian. Early Devonian mean annual surface temperatures were approximately 16 °C. CO 2 levels dropped steeply throughout
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#17327764813746944-449: The exquisitely preserved placoderm fossils from Gogo reef changed the picture again. They showed that placoderms shared anatomical features not only with chondrichthyans but with other gnathostome groups as well. For example, Gogo placoderms show separate bones for the nasal capsules as in gnathostomes; in both sharks and bony fish those bones are incorporated into the braincase. Placoderms also share certain anatomical features only with
7056-489: The extinct and related acanthothoracids , and the living and unrelated holocephalians, most of the ptyctodontids are thought to have lived near the sea bottom and preyed on shellfish . On account of their lack of armour, some paleontologists have suggested that the Ptyctodontida were not placoderms, but holocephalians or the ancestors of holocephalians. Anatomical examinations of whole fossil specimens have shown that
7168-638: The extinction event at the Frasnian – Famennian boundary, the Late Devonian extinctions. The remaining species then died out during the end-Devonian extinction; not a single placoderm species has been confirmed to have survived into the Carboniferous . The earliest studies of placoderms were published by Louis Agassiz , in his five volumes on fossil fishes, 1833–1843. In those days, placoderms were thought to be shelled jawless fish akin to ostracoderms . Some naturalists even suggested that they were shelled invertebrates or even turtle -like vertebrates. In
7280-544: The first fish clade to develop pelvic fins , the second set of paired fins and the homologous precursor to hindlimbs in tetrapods . 380-million-year-old fossils of three other genera, Incisoscutum , Materpiscis and Austroptyctodus , represent the oldest known examples of live birth . Placoderms are thought to be paraphyletic , consisting of several distinct outgroups or sister taxa to all living jawed vertebrates , which originated among their ranks. In contrast, one 2016 analysis concluded that Placodermi
7392-459: The formation of the single supercontinent Pangaea in the Late Paleozoic. The period is named after Devon , a county in southwestern England, where a controversial argument in the 1830s over the age and structure of the rocks found throughout the county was resolved by adding the Devonian Period to the geological timescale. The Great Devonian Controversy was a lengthy debate between Roderick Murchison , Adam Sedgwick and Henry De la Beche over
7504-492: The free water column as well as high ecological competition in benthic habitats, which were extremely saturated; this diversification has been labeled the Devonian Nekton Revolution by many researchers. However, other researchers have questioned whether this revolution existed at all; a 2018 study found that although the proportion of biodiversity constituted by nekton increased across the boundary between
7616-744: The increased overall diversity of nekton in the Devonian to a broader, gradual trend of nektonic diversification across the entire Palaeozoic. A now-dry barrier reef, located in present-day Kimberley Basin of northwest Australia , once extended 350 km (220 mi), fringing a Devonian continent. Reefs are generally built by various carbonate -secreting organisms that can erect wave-resistant structures near sea level. Although modern reefs are constructed mainly by corals and calcareous algae , Devonian reefs were either microbial reefs built up mostly by autotrophic cyanobacteria or coral-stromatoporoid reefs built up by coral-like stromatoporoids and tabulate and rugose corals . Microbial reefs dominated under
7728-425: The increasing competition, predation, and diversity of jawed fishes . The shallow, warm, oxygen-depleted waters of Devonian inland lakes, surrounded by primitive plants, provided the environment necessary for certain early fish to develop such essential characteristics as well developed lungs and the ability to crawl out of the water and onto the land for short periods of time. Finally, the Late Devonian started with
7840-403: The jawed fish (gnathostomes) simultaneously increased in both the sea and fresh water . Armored placoderms were numerous during the early ages of the Devonian Period and became extinct in the Late Devonian, perhaps because of competition for food against the other fish species. Early cartilaginous ( Chondrichthyes ) and bony fishes ( Osteichthyes ) also become diverse and played a large role within
7952-400: The jawless osteostracans ; because of this, the theory that placoderms are the sister group of chondrichthyans has been replaced by the theory that placoderms are a group of basal gnathostomes. Currently, Placodermi are divided into eight recognized orders . There are two further controversial orders: One is the monotypic Stensioellida, containing the enigmatic Stensioella ; the other is
8064-481: The land lay under shallow seas, where tropical reef organisms lived. The enormous "world ocean", Panthalassa , occupied much of the Northern Hemisphere as well as wide swathes east of Gondwana and west of Laurussia. Other minor oceans were the Paleo-Tethys Ocean and Rheic Ocean . By the early Devonian, the continent Laurussia (also known as Euramerica ) was fully formed through the collision of
8176-520: The late 1920s, Dr. Erik Stensiö , at the Swedish Museum of Natural History in Stockholm , established the details of placoderm anatomy and identified them as true jawed fishes related to sharks . He took fossil specimens with well-preserved skulls and ground them away, one tenth of a millimeter at a time. After each layer had been removed, he made an imprint of the next surface in wax . Once
8288-515: The limbs is still not perfectly understood, but most hypothesize that they helped their owners pull themselves across the substrate, as well as allowing their owners to bury themselves into the substrate. Brindabellaspis (" Brindabella's shield") was a long-snouted placoderm from the Early Devonian . When it was first discovered in 1980, it was originally regarded as a weejasperaspid acanthothoracid due to anatomical similarities with
8400-575: The low, carpet-like vegetation during the early part of the Devonian. Also, the first possible fossils of insects appeared around 416 Ma, in the Early Devonian. Evidence for the earliest tetrapods takes the form of trace fossils in shallow lagoon environments within a marine carbonate platform/shelf during the Middle Devonian, although these traces have been questioned and an interpretation as fish feeding traces ( Piscichnus ) has been advanced. Many Early Devonian plants did not have true roots or leaves like extant plants, although vascular tissue
8512-421: The males having pelvic claspers and possibly claspers on the head as well. Rhenanida (" Rhine fish") were flattened, ray-like , bottom-dwelling predators with large, upturned mouths that lived in marine environments. The rhenanids were once presumed to be the most primitive, or at least the closest to the ancestral placoderm, as their armour was made of unfused components—a mosaic of tubercles—as opposed to
8624-477: The massive Dunkleosteus . Various groups of placoderms were diverse and abundant during the Devonian, but all placoderms became extinct at the end-Devonian Hangenberg event 358.9 million years ago, leaving the niches open for the osteichthyan and chondrichthyan survivors who subsequently radiated during the Carboniferous . Many placoderms, particularly the Rhenanida , Petalichthyida , Phyllolepida , and Antiarchi , were bottom-dwellers. In particular,
8736-401: The mid-Devonian extinction event. Petalichthyida ("thin-plated fish") were small, flattened placoderms, typified by their splayed fins and numerous tubercles that decorated all of the plates and scales of their armour. They reached a peak in diversity during the Early Devonian and were found throughout the world. The petalichthids Lunaspis and Wijdeaspis are among the best known. There
8848-450: The middle to upper portions of the water column. A study of the arthrodire Compagopiscis published in 2012 concluded that placoderms (at least this particular genus) likely possessed true teeth contrary to some early studies. The teeth had well defined pulp cavities and were made of both bone and dentine . However, the tooth and jaw development were not as closely integrated as in modern gnathostomes. These teeth were likely homologous to
8960-562: The name "the Old Red Continent". For much of the Devonian, the majority of western Laurussia (North America) was covered by subtropical inland seas which hosted a diverse ecosystem of reefs and marine life. Devonian marine deposits are particularly prevalent in the midwestern and northeastern United States. Devonian reefs also extended along the southeast edge of Laurussia, a coastline now corresponding to southern England , Belgium , and other mid-latitude areas of Europe. In
9072-563: The naming of the period. Murchison and Sedgwick won the debate and named it the Devonian System. While the rock beds that define the start and end of the Devonian Period are well identified, the exact dates are uncertain. According to the International Commission on Stratigraphy , the Devonian extends from the end of the Silurian 419.2 Ma, to the beginning of the Carboniferous 358.9 Ma – in North America , at
9184-411: The nearby microcontinent of Amuria (now Manchuria , Mongolia and their vicinities). Though certainly close to Siberia in the Devonian, the precise location of Amuria is uncertain due to contradictory paleomagnetic data. The Rheic Ocean, which separated Laurussia from Gondwana, was wide at the start of the Devonian, having formed after the drift of Avalonia away from Gondwana. It steadily shrunk as
9296-401: The oldest known plants with woody tissue. By the Middle Devonian, shrub-like forests of primitive plants existed: lycophytes , horsetails , ferns , and progymnosperms evolved. Most of these plants had true roots and leaves, and many were quite tall. The earliest-known trees appeared in the Middle Devonian. These included a lineage of lycopods and another arborescent, woody vascular plant,
9408-703: The order Agoniatitida , which in later epochs evolved to new ammonoid orders, for example Goniatitida and Clymeniida . This class of cephalopod molluscs would dominate the marine fauna until the beginning of the Mesozoic Era. The Middle Devonian comprised two subdivisions: first the Eifelian , which then gave way to the Givetian 387.7 Ma. During this time the jawless agnathan fishes began to decline in diversity in freshwater and marine environments partly due to drastic environmental changes and partly due to
9520-425: The other species found at the same locality. According to Philippe Janvier , anatomical similarities in the brain of Brindabellaspis stensioi and the brain of a jawless fish suggest it is a basal placoderm closest to the ancestral placoderm. Various Early to Middle Devonian placoderm incertae sedis have also been inserted in the order. Phyllolepida ("leaf scales") were flattened placoderms found throughout
9632-403: The period continued, as the two major continents approached near the equator in the early stages of the assembly of Pangaea . The closure of the Rheic Ocean began in the Devonian and continued into the Carboniferous. As the ocean narrowed, endemic marine faunas of Gondwana and Laurussia combined into a single tropical fauna. The history of the western Rheic Ocean is a subject of debate, but there
9744-472: The period it moved northwards and began to twist clockwise, though it was not near its modern location. Siberia approached the eastern edge of Laurussia as the Devonian progressed, but it was still separated by a seaway, the Ural Ocean . Although Siberia's margins were generally tectonically stable and ecologically productive, rifting and deep mantle plumes impacted the continent with flood basalts during
9856-529: The period. Older literature on the Anglo-Welsh basin divides it into the Downtonian, Dittonian, Breconian, and Farlovian stages, the latter three of which are placed in the Devonian. The Devonian has also erroneously been characterised as a "greenhouse age", due to sampling bias : most of the early Devonian-age discoveries came from the strata of western Europe and eastern North America , which at
9968-434: The placoderms is both literally and figuratively fragmented. Until the discovery of Silurolepis (and then, the discoveries of Entelognathus and Qilinyu ), Silurian-aged placoderm specimens consisted of fragments. Some of them have been tentatively identified as antiarch or arthrodire due to histological similarities; and many of them have not yet been formally described or even named. The most commonly cited example of
10080-456: The point of literally resembling a box with eyes, with the sometimes scaled, sometimes naked rear portions often becoming sinuous , particularly with later forms. The pair of pectoral fins were modified into a pair of caliper -like, or arthropod -like limbs. In primitive forms, such as Yunnanolepis , the limbs were thick and short, while in advanced forms, such as Bothriolepis , the limbs were long and had elbow-like joints. The function of
10192-476: The process of giving birth to a 6 cm ( 2 + 1 ⁄ 2 in) offspring and was fossilized with the umbilical cord intact. The fossil, named Materpiscis attenboroughi (after scientist David Attenborough ), had eggs which were fertilized internally, the mother providing nourishment to the embryo and giving birth to live young. With this discovery, the placoderm became the oldest vertebrate known to have given birth to live young (" viviparous "), pushing
10304-613: The rise of the Caledonian Mountains of Great Britain and Scandinavia . As the Caledonian Orogeny wound down in the later part of the period, orogenic collapse facilitated a cluster of granite intrusions in Scotland. Most of Laurussia was located south of the equator, but in the Devonian it moved northwards and began to rotate counterclockwise towards its modern position. While the most northern parts of
10416-509: The scarcity of placoderms in the Silurian fossil record is due to placoderms' living in environments unconducive to fossil preservation, rather than a genuine scarcity. This hypothesis helps to explain the placoderms' seemingly instantaneous appearance and diversity at the very beginning of the Devonian . During the Devonian, placoderms went on to inhabit and dominate almost all known aquatic ecosystems, both freshwater and saltwater . But this diversity ultimately suffered many casualties during
10528-533: The second most successful order of placoderms known, after the Arthrodira . The order's name was coined by Edward Drinker Cope , who, after incorrectly identifying the first fossils as being those of an armored tunicate , mistakenly thought the eye-hole was the mouth, and the opening for the anal siphon was on the other side of the body, as opposed to having both oral and anal siphons together at one end. The front portions of their bodies were heavily armoured, to
10640-561: The similarities between these two groups are superficial. The major differences were that holocephalians have shagreen on their skin, while ptyctodontids do not; the armoured plates and scales of holocephalians are made of dentine , while those of ptyctodontids are made of bone; the craniums of holocephalians are similar to sharks, while those of ptyctodontids are similar to those of other placoderms; and, most importantly, that holocephalians have true teeth, while ptyctodonts have beak-like tooth plates. Ptyctodontids were sexually dimorphic , with
10752-604: The sister group of the Arthrodires + Phyllolepida + Antiarchi trichotomy and the Acanthothoraci + Rhenanida dichotomy . Stensioellida ("[Heintz's] little Stensio ") contains another problematic placoderm of uncertain affinity, known only from the Lower Devonian Hunsrück slates of Germany. Stensioella was a thin fish that, when alive, looked vaguely like an elongated ratfish , or
10864-450: The solidified plates of "advanced" placoderms, such as antiarchs and arthrodires . However, through comparisons of skull anatomies, rhenanids are now considered to be the sister group of the antiarchs. When rhenanids die, their "mosaics" come apart, and it has been suggested that the rarity of rhenanids in the fossil record reflects postmortem disassociation, and is not an actual rarity of the species. Acanthothoraci ("spine chests") were
10976-412: The specimens had been completely ground away (and so destroyed), he made enlarged, three-dimensional models of the skulls to examine the anatomical details more thoroughly. Many other placoderm specialists thought that Stensiö was trying to shoehorn placoderms into a relationship with sharks ; however, as more fossils were found, placoderms were accepted as a sister group of chondrichthyans . Much later,
11088-463: The teeth of other gnathostomes. One of the largest known arthrodires, Dunkleosteus terrelli , was 3.5–4.1 metres (11–13 ft) long, and is presumed to have had a large distribution, as its remains have been found in Europe, North America and possibly Morocco. Some paleontologists regard it as the world's first vertebrate "superpredator", preying upon other predators. Other, smaller arthrodires, such as Fallacosteus and Rolfosteus , both of
11200-628: The terminus of the Devonian, Earth rapidly cooled into an icehouse , marking the beginning of the Late Paleozoic icehouse . The Devonian world involved many continents and ocean basins of various sizes. The largest continent, Gondwana , was located entirely within the Southern Hemisphere . It corresponds to modern day South America , Africa , Australia , Antarctica , and India , as well as minor components of North America and Asia . The second-largest continent, Laurussia,
11312-535: The time straddled the Equator as part of the supercontinent of Euramerica where fossil signatures of widespread reefs indicate tropical climates that were warm and moderately humid. In fact the climate in the Devonian differed greatly during its epochs and between geographic regions. For example, during the Early Devonian, arid conditions were prevalent through much of the world including Siberia, Australia, North America, and China, but Africa and South America had
11424-419: The top of their head. The orbits for the eyes were extremely small, suggesting the eyes were vestigial and that the phyllolepids may have been blind. Ptyctodontida ("folded teeth") were lightly armoured placoderms with big heads, big eyes and long bodies. They have a strong but superficial resemblance to modern day chimaeras . Their armour was reduced to a pattern of small plates around the head and neck. Like
11536-455: The vertebrate shoulder girdle evolved from gill arches. It was thought for a time that placoderms became extinct due to competition from the first bony fish and early sharks , given a combination of the supposed inherent superiority of bony fish and the presumed sluggishness of placoderms. With more accurate summaries of prehistoric organisms, it is now thought that they systematically died out as marine and freshwater ecologies suffered from
11648-521: The warmer conditions of the early and late Devonian, while coral-stromatoporoid reefs dominated during the cooler middle Devonian. By the Devonian Period, life was well underway in its colonization of the land. The moss forests and bacterial and algal mats of the Silurian were joined early in the period by primitive rooted plants that created the first stable soils and harbored arthropods like mites , scorpions , trigonotarbids and myriapods (although arthropods appeared on land much earlier than in
11760-555: The world saw the disappearance of an estimated 96% of vertebrates like conodonts and bony fishes , and all of the ostracoderms and placoderms. Land plants as well as freshwater species, such as our tetrapod ancestors, were relatively unaffected by the Late Devonian extinction event (there is a counterargument that the Devonian extinctions nearly wiped out the tetrapods ). The reasons for the Late Devonian extinctions are still unknown, and all explanations remain speculative. Canadian paleontologist Digby McLaren suggested in 1969 that
11872-400: The world. Like other flattened placoderms they were bottom-dwelling predators that ambushed prey. Unlike other flattened placoderms, they were freshwater fish. Their armour was made of whole plates, rather than the numerous tubercles and scales of Petalichthyida. The eyes were on the sides of the head, unlike visual bottom-dwelling predators, such as stargazers or flatfish , which have eyes on
11984-472: Was an independent diversification event that occurred in what is now Southern China, producing a handful of unique genera that were once placed in their own order, "Quasipetalichthyida", named after the first discovered species there, Quasipetalichthys haikouensis . Soon after the petalichthids' diversification, they went into decline. Because they had compressed body forms, it is supposed they were bottom-dwellers that pursued or ambushed smaller fish. Their diet
12096-680: Was by far the largest continent on the planet. It was completely south of the equator, although the northeastern sector (now Australia) did reach tropical latitudes. The southwestern sector (now South America) was located to the far south, with Brazil situated near the South Pole . The northwestern edge of Gondwana was an active margin for much of the Devonian, and saw the accretion of many smaller land masses and island arcs. These include Chilenia , Cuyania , and Chaitenia , which now form much of Chile and Patagonia . These collisions were associated with volcanic activity and plutons , but by
12208-442: Was mostly a passive margin, hosting extensive marine deposits in areas such as northwest Africa and Tibet . The eastern margin, though warmer than the west, was equally active. Numerous mountain building events and granite and kimberlite intrusions affected areas equivalent to modern day eastern Australia , Tasmania , and Antarctica. Several island microcontinents (which would later coalesce into modern day Asia) stretched over
12320-438: Was northwest of Gondwana, and corresponds to much of modern-day North America and Europe . Various smaller continents, microcontinents , and terranes were present east of Laurussia and north of Gondwana, corresponding to parts of Europe and Asia. The Devonian Period was a time of great tectonic activity, as the major continents of Laurussia and Gondwana drew closer together. Sea levels were high worldwide, and much of
12432-440: Was originally considered to be from the late Llandovery , although later study reconsidered its age at Ludfordian . Shimenolepis plates are very similar to the early Devonian yunnanolepid Zhanjilepis , also known from distinctively ornamented plates. In 2022, Xiushanosteus is described from complete fossils from Telychian , late Llandovery of Chongqing , China. Paleontologists and placoderm specialists suspect that
12544-577: Was the newest addition to the Asian microcontinents, it was not the first. North China and the Tarim Block (now northwesternmost China) were located westward and continued to drift northwards, powering over older oceanic crust in the process. Further west was a small ocean (the Turkestan Ocean), followed by the larger microcontinents of Kazakhstania , Siberia , and Amuria . Kazakhstania was
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