The coelacanths constitute a now-rare order of fish that includes two extant species in the genus Latimeria: the West Indian Ocean coelacanth found near the Comoro Islands off the east coast of Africa and the Indonesian coelacanth. They follow the oldest-known living lineage of Sarcopterygii, which means they are more related to lungfish and tetrapods than to ray-finned fishes, they are found along the coastlines of the Indonesia. The West Indian Ocean coelacanth is a critically endangered species. Coelacanths belong to the subclass Actinistia, a group of lobed-finned fish related to lungfish and certain extinct Devonian fish such as osteolepiforms, porolepiforms and Panderichthys. Coelacanths were thought to have become extinct in the Late Cretaceous, around 66 million years ago, but were rediscovered in 1938 off the coast of South Africa; the coelacanth was long considered a "living fossil" because scientists thought it was the sole remaining member of a taxon otherwise known only from fossils, with no close relations alive, that it evolved into its current form 400 million years ago.
However, several recent studies have shown that coelacanth body shapes are much more diverse than thought. The word Coelacanth is an adaptation of the Modern Latin Cœlacanthus, from the Greek κοῖλ-ος, it is a common name for the oldest living line of Sarcopterygii, referring to the hollow caudal fin rays of the first fossil specimen described and named by Louis Agassiz in 1839. The genus name Latimeria commemorates Marjorie Courtenay-Latimer who discovered the first specimen in a fish market; the coelacanth, related to lungfishes and tetrapods, was believed to have been extinct since the end of the Cretaceous period. More related to tetrapods than to the ray-finned fish, coelacanths were considered transitional species between fish and tetrapods. On 23 December 1938, the first Latimeria specimen was found off the east coast of South Africa, off the Chalumna River. Museum curator Marjorie Courtenay-Latimer discovered the fish among the catch of a local angler, Captain Hendrick Goosen. Latimer contacted a Rhodes University ichthyologist, J. L. B.
Smith, sending him drawings of the fish, he confirmed the fish's importance with a famous cable: "MOST IMPORTANT PRESERVE SKELETON AND GILLS = FISH DESCRIBED."Its discovery 66 million years after it was believed to have become extinct makes the coelacanth the best-known example of a Lazarus taxon, an evolutionary line that seems to have disappeared from the fossil record only to reappear much later. Since 1938, West Indian Ocean coelacanth have been found in the Comoros, Tanzania, Madagascar, in iSimangaliso Wetland Park, Kwazulu-Natal in South Africa; the Comoro Islands specimen was discovered in December 1952. Between 1938 and 1975, 84 specimens were recorded; the second extant species, Indonesian coelacanth, was described from Manado, North Sulawesi, Indonesia in 1999 by Pouyaud et al. based on a specimen discovered by Mark V. Erdmann in 1998 and deposited at the Indonesian Institute of Sciences. Erdmann and his wife Arnaz Mehta first encountered a specimen at a local market in September 1997, but took only a few photographs of the first specimen of this species before it was sold.
After confirming that it was a unique discovery, Erdmann returned to Sulawesi in November 1997 to interview fishermen to look for further examples. A second specimen was caught by a fisherman in July 1998 and it was handed to Erdmann; the coelacanth has no real commercial value apart from being coveted by museums and private collectors. As a food fish it is worthless, as its tissues exude oils that give the flesh a foul flavor; the coelacanth's continued survival may be threatened by commercial deep-sea trawling, in which coelacanths are caught as bycatch. Coelacanths are the lobe-finned fishes. Externally, several characteristics distinguish the coelacanth from other lobe-finned fish, they possess a three-lobed caudal fin called a trilobate fin or a diphycercal tail. A secondary tail extending past the primary tail separates the upper and lower halves of the coelacanth. Cosmoid scales act as thick armor to protect the coelacanth's exterior. Several internal traits aid in differentiating coelacanths from other lobe-finned fish.
At the back of the skull, the coelacanth possesses a hinge, the intracranial joint, which allows it to open its mouth wide. Coelacanths retain an oil-filled notochord, a hollow, pressurized tube, replaced by the vertebral column early in embryonic development in most other vertebrates; the coelacanth heart is shaped differently from that of most modern fish, with its chambers arranged in a straight tube. The coelacanth braincase is 98.5% filled with fat. The cheeks of the coelacanth are unique because the opercular bone is small and holds a large soft-tissue opercular flap. A spiracular chamber is present. Coelacanth possess a unique rostral organ within the ethmoid region of the braincase. Unique to extant coelacanths is the presence of a "fatty lung" or a fat-filled single-lobed vestigial lung, homologous to other fishes' swim bladder; the parallel development of a fatty organ for buoyancy control suggest a unique specialization for deep-water habitats. There has been discovered small, hard but flexible plates around the vestigial lung in adult specimen, though not around the fatty organ.
The plates most had a regulation
Anders Erik Vilhelm Jarvik was a Swedish paleontologist who worked extensively on the sarcopterygian fish Eusthenopteron. In a career that spanned some 60 years, Jarvik produced some of the most detailed anatomical work on this fish, making it arguably the best known fossil vertebrate. Jarvik was born at a farm in Utby Parish near Mariestad in northern Västergötland, he studied botany, zoology and paleontology at Uppsala University, where he took his licentiate's degree in 1937. In 1942, he completed his PhD with the dissertation On the structure of the snout of Crossopterygians and lower Gnathostomes in general, he participated in the Greenland expedition of Gunnar Säve-Söderbergh in 1932 and was appointed assistant in the Department of Palaeozoology of the Swedish Museum of Natural History in Stockholm in 1937. Jarvik's research concerned the sarcopterygian fishes, his main interests were in the so-called "rhipidistian" sarcopterygian fishes, which he held to be divided into two groups: the Osteolepiformes and the Porolepiformes.
He published several solidly descriptive works on Devonian sarcopterygians. In particular, he conducted detailed anatomical studies of the cranium of Eusthenopteron foordi using a serial-section technique introduced by William Johnson Sollas and applied to fossil fishes by Erik Stensiö. A fossil of limited external quality was sectioned by grinding off a thin section, photographing the grind-off end and repeat the process until the whole fossil was worked through; the internal structures would show up on long series of photographs. Working in the day before computer simulations, models was made by projecting reversal film on a board, cut thin wax plates to match; the sticky wax plates could be assembled to a three-dimensional scaled-up model of the skull, complete with internal structures such as nerve channels and other internal hollows seen in fossils. Further section to the cranium could be made by cutting the wax model at the desired angle. Due to the sticky nature of the wax used, a sectioned skull was put back together by pressing the two sections back together.
This technique was applied to the cranium of the porolepiform Glyptolepis groenlandica. Jarvik proposed controversial hypotheses about the principal structure of the vertebrate head and the origin of the tetrapods, he thus held, on the basis of detailed analyses of the snout and nasal capsule structures as well as the intermandibular and occipital regions, that Tetrapoda was biphyletic. In his view, the anatomical details of the Caudata bound them to the primitive porolepiform fishes, while all other tetrapods – apodans excepted – were descended from primitive osteolepiforms, thus Amphibia had arisen twice. On the basis of his findings, he argued that Amphibia should be split, with salamanders in one class, the frogs as a separate class, the Batrachomorpha; the Lepospondyli was thought as possible Urodelomorphans, while the other Labyrinthodonts were thought to be Batracomorphs. Jarvik's ideas was never accepted, though Friedrich von Huene did include his system in systematic treatment of tetrapods.
Few other supported his ideas, today it has been abandoned by vertebrate paleontologists. The term "Batrachomorpha" is however sometimes used in a cladistic sense to denote Labyrinthodonts more related to modern amphibians than to amniotes. Jarvik studied the anatomy and relationships of lungfish which he held to be primitive gnathostomes related to holocephalans, of acanthodians, which he considered to be elasmobranchs rather than osteichthyans, he made contributions to a number of classical problems in comparative anatomy, including the origin of the vertebrates the origin of the pectoral and pelvic girdles and paired fins, the homologies of the frontal and parietal bones in fishes and tetrapodsFinally, Jarvik investigated the anatomy of Ichthyostega, resulting in a monograph with an extensive photographic documentation of the material collected in 1929-1955. Some of Jarvik's views did not accord with general opinion in vertebrate paleontology. However, his anatomical studies of Eusthenopteron foordi laid the foundations for modern studies of the transition from fishes to tetrapods.
Jarvik was a member of the Royal Swedish Academy of Sciences and French Academy of Sciences and Knight of the Order of Vasa. The lungfish Jarvikia and the osteolepiform Jarvikina are named after him. Hans C. Bjerring Tor Ørvig Gunnar Säve-Söderbergh Erik Stensiö Théories de l'évolution des vertébrés reconsidérées à la lumière des récentes découvertes sur les vertébrés inférieurs. Masson, Paris. 1960. Basic Structure and Evolution of Vertebrates, 2 Vols. Academic Press, London. 1980 An Obituary to Erik Jarvik "Erik Jarvik: Palaeontologist renowned for his work on the'four-legged fish'", obituary by Philippe Janvier
The Devonian is a geologic period and system of the Paleozoic, spanning 60 million years from the end of the Silurian, 419.2 million years ago, to the beginning of the Carboniferous, 358.9 Mya. It is named after Devon, where rocks from this period were first studied; the first significant adaptive radiation of life on dry land occurred during the Devonian. Free-sporing vascular plants began to spread across dry land, forming extensive forests which covered the continents. By the middle of the Devonian, several groups of plants had evolved leaves and true roots, by the end of the period the first seed-bearing plants appeared. Various terrestrial arthropods became well-established. Fish reached substantial diversity during this time, leading the Devonian to be dubbed the "Age of Fishes." The first ray-finned and lobe-finned bony fish appeared, while the placoderms began dominating every known aquatic environment. The ancestors of all four-limbed vertebrates began adapting to walking on land, as their strong pectoral and pelvic fins evolved into legs.
In the oceans, primitive sharks became more numerous than in the Late Ordovician. The first ammonites, species of molluscs, appeared. Trilobites, the mollusc-like brachiopods and the great coral reefs, were still common; the Late Devonian extinction which started about 375 million years ago affected marine life, killing off all placodermi, all trilobites, save for a few species of the order Proetida. The palaeogeography was dominated by the supercontinent of Gondwana to the south, the continent of Siberia to the north, the early formation of the small continent of Euramerica in between; 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 distributed throughout the county was resolved by the definition of the Devonian period in the geological timescale. The Great Devonian Controversy was a long period of vigorous argument and counter-argument between the main protagonists of Roderick Murchison with Adam Sedgwick against Henry De la Beche supported by George Bellas Greenough.
Murchison and Sedgwick named the period they proposed as 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 Mya, to the beginning of the Carboniferous 358.9 Mya. In nineteenth-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 the period. Older literature on the Anglo-Welsh basin divides it into the Downtonian, Dittonian and Farlovian stages, the latter three of which are placed in the Devonian; the Devonian has 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 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 but in fact the climate in the Devonian differed during its epochs and between geographic regions.
For example, during the Early Devonian, arid conditions were prevalent through much of the world including Siberia, North America, China, but Africa and South America had 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 and Late subdivisions. The rocks corresponding to those epochs are referred to as belonging to the Lower and Upper parts of the Devonian System. Early DevonianThe Early Devonian lasted from 419.2 ± 2.8 to 393.3 ± 2.5 and began with the Lochkovian stage, which lasted until the Pragian. It spanned from 410.8 ± 2.8 to 407.6 ± 2.5, was followed by the Emsian, which lasted until the Middle Devonian began, 393.3± 2.7 million years ago. During this time, the first ammonoids appeared. Ammonoids during this time period differed little from their nautiloid counterparts; these ammonoids belong to the order Agoniatitida, which in 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. Middle DevonianThe Middle Devonian comprised two subdivisions: first the Eifelian, which gave way to the Givetian 387.7± 2.7 million years ago. During this time the jawless agnathan fishes began to decline in diversity in freshwater and marine environments due to drastic environmental changes and due to the increasing competition and diversity of jawed fishes; the shallow, 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, the ability to crawl out of the water and onto the land for short periods of time. Late DevonianFinally, the Late Devonian started with the Frasnian, 382.7 ± 2.8 to 372.2 ± 2.5, during which the first forests took shape on land. The first tetrapods appeared in the fossil record in the ensuing Famennian subdivisi
Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal refers only to non-human animals; the study of non-human animals is known as zoology. Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan; the Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes and molluscs—and the deuterostomes, containing the echinoderms and chordates.
Life forms interpreted. Many modern animal phyla became established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified. Aristotle divided animals into those with those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa. Humans make use of many other animal species for food, including meat and eggs. Dogs have been used in hunting, while many aquatic animals are hunted for sport.
Non-human animals have appeared in art from the earliest times and are featured in mythology and religion. The word "animal" comes from the Latin animalis, having soul or living being; the biological definition includes all members of the kingdom Animalia. In colloquial usage, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals. Animals have several characteristics. Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With few exceptions, animals breathe oxygen and respire aerobically. All animals are motile during at least part of their life cycle, but some animals, such as sponges, corals and barnacles become sessile; the blastula is a stage in embryonic development, unique to most animals, allowing cells to be differentiated into specialised tissues and organs.
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible; this may be calcified, forming structures such as shells and spicules. In contrast, the cells of other multicellular organisms are held in place by cell walls, so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, desmosomes. With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues; these include muscles, which enable locomotion, nerve tissues, which transmit signals and coordinate the body. There is an internal digestive chamber with either one opening or two openings. Nearly all animals make use of some form of sexual reproduction, they produce haploid gametes by meiosis.
These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement, it first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm develops between them; these germ layers differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. In some species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality; some animals are capable of asexual reproduction, which results
Panderichthys is a genus of extinct sarcopterygian from the late Devonian period, about 380 Mya. Panderichthys, recovered from Frasnian deposits in Latvia, is represented by two species. P. stolbovi is known only from an incomplete lower jaw. P. rhombolepis is known from several more complete specimens. Although it belongs to a sister group of the earliest tetrapods, Panderichthys exhibits a range of features transitional between tristichopterid lobe-fin fishes and early tetrapods, it is named after the German-Baltic paleontologist Christian Heinrich Pander. A recent study uncovered tetrapod tracks dating back to before the appearance of Panderichthys in the fossil record, which suggests that Panderichthys is not a transitional fossil, but nonetheless shows the traits that evolved during the fish-tetrapod evolution Panderichthys is represented by two different species: Panderichthys rhombolepis and Panderichthys stobolvi. P. rhombolepis was discovered by Gross in 1930 and P. stobolvi was discovered and figured by Emilia Vorobyeva in 1960.
P. rhombolepis was discovered in Lode, Latvia within Frasnian deposits and according to P. E. Ahlberg can be found in other Frasnian deposits in Latvia. Although fossils of Panderichthys have been known for a long time, but they have only been examined in full; the first time they were recognized as being phylogenetically closer to tetrapods than fish was by Shultze and Arsenault in 1985. Panderichthys is a 90–130 cm long fish with a large tetrapod-like head that's flattened, narrow at the snout and wide in the back; the intracranial joint, characteristic of most lobe-fin fishes, has been lost from the external elements of the skull, but is still present in the braincase. The patterns of external bones in the skull roof and cheeks are more similar to those of early tetrapods than those of other lobe-fins; the transitional qualities of Panderichthys are evident in the rest of the body. It lacks the dorsal and anal fins and its tail is more like those of early tetrapods than the caudal fins of other lobe-fins.
The shoulders exhibits several tetrapod-like features, while the humerus is longer than those found in other lobe-fins. The vertebral column is ossified throughout its length and the vertebrae are comparable to those of early tetrapods. On the other hand, the distal parts of the front fins are unlike those of tetrapods; as would be expected from a fin, there are numerous lepidotrichia. Panderichthys has many features that can be considered an intermediate form during the fish-tetrapod evolution and displays some features that are more derived than its phylogenetic position indicates, while others that are more basal; the body form of Panderichthys and Tiktaalik represents a major step in the transition from fish to tetrapods and they were able to haul out on land. According to Shultze and Trueb, Panderichthys shares ten features with tetrapods: The skull roof is flat compared to fish skulls; the orbits are closer together. The external naris is close to the margin of the upper jaw. Paired frontals.
Lack of external intracranial joint. Parietal located between main portion posterior to orbits. In P. rhombolepis, the squamosal touches the maxilla. Teeth have complex polyplocodont structure. Lack of median fins. In panderichthyids and Ichthyostega ribs are attached to the neural arch and there is an intercentrum. One of the key transitional features of Panderichthys is its humerus. During the transition from fish to tetrapods the limbs began to move and became located at a right angle to the body rather than being oriented toward the posterior end; as a result, the muscles became perpendicular to the body and caused the limbs to move in a more anteroposterior and dorsoventral pattern. This in turn affected the shape of the humerus and as a result early tetrapods have an L-shaped humerus. Due to a recent discovery of a humerus of Panderichthys, not flattened, the specimen could be analyzed in much greater detail; the humerus of Panderichthys displays a variety of features including ones that are both primitive and derived.
Despite being placed as basal to Tiktaalik, the humerus of Panderichthys has features that are more derived, but overall is similar. Both Panderichthys and Tiktaalik have humeri that are dorsoventrally flattened with a blade like entepicondyle curving ventrally, separated epipodial facets, a latissimus dorsi process and ectepicondule process, parallel to the preaxial margin; the humeri of both species are considered transitional forms because they are L-shaped, have a low latissimus dorsi process, a low entepicondyle, an intermediate entepicondylar canal. The humerus of Panderichthys is more derived than that of Tiktaalik because of the presence of a more preaxially oriented radial facet as well as a more slender shaft. One feature, unique to Panderichthys is that the entepicondyle does not project as far as the epipodial facets and the humeral ridge does not go into the entepicondyle; the result of the analysis of the humerus of Panderichthys is that the transition of the humerus from the fish-like organisms to tetrapods occurred much slower than thought and Panderichthys now provides a base to determine many autapomorphies.
Due to the orientation of the fin towards the posterior end, the attitude of the limb is more horizontal than vertical and the operational space in which it acts is level to the shoulder joint, which causes the muscles to pull at a right angle to the body. This resulted in the ability of Panderichthys to prop up its large head most to breathe. Another key feature of Panderichthys is its intermediate form during the
The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago, to the beginning of the Devonian Period, 419.2 Mya. The Silurian is the shortest period of the Paleozoic Era; as with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by several million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when 60% of marine species were wiped out. A significant evolutionary milestone during the Silurian was the diversification of jawed fish and bony fish. Multi-cellular life began to appear on land in the form of small, bryophyte-like and vascular plants that grew beside lakes and coastlines, terrestrial arthropods are first found on land during the Silurian. However, terrestrial life would not diversify and affect the landscape until the Devonian; the Silurian system was first identified by British geologist Roderick Murchison, examining fossil-bearing sedimentary rock strata in south Wales in the early 1830s.
He named the sequences for a Celtic tribe of Wales, the Silures, inspired by his friend Adam Sedgwick, who had named the period of his study the Cambrian, from the Latin name for Wales. This naming does not indicate any correlation between the occurrence of the Silurian rocks and the land inhabited by the Silures. In 1835 the two men presented a joint paper, under the title On the Silurian and Cambrian Systems, Exhibiting the Order in which the Older Sedimentary Strata Succeed each other in England and Wales, the germ of the modern geological time scale; as it was first identified, the "Silurian" series when traced farther afield came to overlap Sedgwick's "Cambrian" sequence, provoking furious disagreements that ended the friendship. Charles Lapworth resolved the conflict by defining a new Ordovician system including the contested beds. An early alternative name for the Silurian was "Gotlandian" after the strata of the Baltic island of Gotland; the French geologist Joachim Barrande, building on Murchison's work, used the term Silurian in a more comprehensive sense than was justified by subsequent knowledge.
He divided the Silurian rocks of Bohemia into eight stages. His interpretation was questioned in 1854 by Edward Forbes, the stages of Barrande, F, G and H, have since been shown to be Devonian. Despite these modifications in the original groupings of the strata, it is recognized that Barrande established Bohemia as a classic ground for the study of the earliest fossils; the Llandovery Epoch lasted from 443.8 ± 1.5 to 433.4 ± 2.8 mya, is subdivided into three stages: the Rhuddanian, lasting until 440.8 million years ago, the Aeronian, lasting to 438.5 million years ago, the Telychian. The epoch is named for the town of Llandovery in Wales; the Wenlock, which lasted from 433.4 ± 1.5 to 427.4 ± 2.8 mya, is subdivided into the Sheinwoodian and Homerian ages. It is named after Wenlock Edge in England. During the Wenlock, the oldest-known tracheophytes of the genus Cooksonia, appear; the complexity of later Gondwana plants like Baragwanathia, which resembled a modern clubmoss, indicates a much longer history for vascular plants, extending into the early Silurian or Ordovician.
The first terrestrial animals appear in the Wenlock, represented by air-breathing millipedes from Scotland. The Ludlow, lasting from 427.4 ± 1.5 to 423 ± 2.8 mya, comprises the Gorstian stage, lasting until 425.6 million years ago, the Ludfordian stage. It is named for the town of Ludlow in England; the Přídolí, lasting from 423 ± 1.5 to 419.2 ± 2.8 mya, is the final and shortest epoch of the Silurian. It is named after one locality at the Homolka a Přídolí nature reserve near the Prague suburb Slivenec in the Czech Republic. Přídolí is the old name of a cadastral field area. In North America a different suite of regional stages is sometimes used: Cayugan Lockportian Tonawandan Ontarian Alexandrian In Estonia the following suite of regional stages is used: Ohessaare stage Kaugatuma stage Kuressaare stage Paadla stage Rootsiküla stage Jaagarahu stage Jaani stage Adavere stage Raikküla stage Juuru stage With the supercontinent Gondwana covering the equator and much of the southern hemisphere, a large ocean occupied most of the northern half of the globe.
The high sea levels of the Silurian and the flat land resulted in a number of island chains, thus a rich diversity of environmental settings. During the Silurian, Gondwana continued a slow southward drift to high southern latitudes, but there is evidence that the Silurian icecaps were less extensive than those of the late-Ordovician glaciation; the southern continents remained united during this period. The melting of icecaps and glaciers contributed to a rise in sea level, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming an unconformity; the continents of Avalonia and Laurentia drifted together near the equator, starting the formation of a second supercontinent known as Euramerica. When the proto-Europe coll
Neoteny called juvenilization, is the delaying or slowing of the physiological development of an organism an animal. Neoteny is found in modern humans. In progenesis, sexual development is accelerated. Both neoteny and progenesis result in paedomorphism, a type of heterochrony; some authors define paedomorphism as the retention of larval traits. Both neoteny and progenesis cause the retention in adults of traits seen only in the young; such retention is important in evolutionary biology and evolutionary developmental biology. The origins of the concept of neoteny have been traced to the Bible and to the poet William Wordsworth's "The Child is the father of the Man"; the term itself was invented in 1885 by Julius Kollmann as he described the axolotl's maturation while remaining in a tadpole-like aquatic stage complete with gills, unlike other adult amphibians like frogs and toads. The word neoteny is borrowed from the German Neotenie, the latter constructed by Kollmann from the Greek νέος and τείνειν.
The adjective is either "neotenic" or "neotenous". For the opposite of "neotenic", different authorities use either "gerontomorphic" or "peramorphic". Bogin points out that Kollmann had intended the meaning to be "retaining youth", but had evidently confused the Greek teínein with the Latin tenere, which had the meaning he wanted, "to retain", so that the new word would mean "the retaining of youth". In 1926 Louis Bolk described neoteny as the major process in humanization. In his 1977 book Ontogeny and Phylogeny, Steven Jay Gould noted that Bolk's account constituted an attempted justification for "scientific" racism and sexism, but acknowledged that Bolk had been right in the core idea that humans differ from other primates in becoming sexually mature in an infantile stage of body development. Neoteny in humans is the slowing or delaying of body development, compared to non-human primates, resulting in features such as a large head, a flat face, short arms and legs; these neotenic changes may have been brought about by sexual selection in human evolution.
In turn, they may have permitted the development of human capacities such as emotional communication. However, humans have large noses and long legs, both peramorphic traits; some evolutionary theorists have proposed. Gould argued that the "evolutionary story" of humans is one where we have been "retaining to adulthood the juvenile features of our ancestors". J. B. S. Haldane mirrors Gould's hypothesis by stating a "major evolutionary trend in human beings" is "greater prolongation of childhood and retardation of maturity." Delbert D. Thiessen said that "neoteny becomes more apparent as early primates evolved into forms" and that primates have been "evolving toward flat face." However, in light of some groups using arguments based around neoteny to support racism, Gould argued "that the whole enterprise of ranking groups by degree of neoteny is fundamentally unjustified". Doug Jones argued that human evolution's trend toward neoteny may have been caused by sexual selection in human evolution for neotenous facial traits in women by men with the resulting neoteny in male faces being a "by-product" of sexual selection for neotenous female faces.
Neoteny is seen in domesticated animals such as mice. This is because there are more resources available, less competition for those resources, with the lowered competition the animals expend less energy obtaining those resources; this allows them to mature and reproduce more than their wild counterparts. The environment that domesticated animals are raised in determines whether or not neoteny is present in those animals. Evolutionary neoteny can arise in a species when those conditions occur, a species becomes sexually mature ahead of its "normal development". Another explanation for the neoteny in domesticated animals can be the selection for certain behavioral characteristics. Behavior is linked to genetics which therefore means that when a behavioral trait is selected for, a physical trait may be selected for due to mechanisms like linkage disequilibrium. Juvenile behaviors are selected for in order to domesticate more a species. If there is no need for competition there is no need for aggression.
Selecting for juvenile behavioral characteristics can lead to neoteny in physical characteristics because, for example, with the reduced need for behaviors like aggression there is no need for developed traits that would help in that area. Traits that may become neotenized due to decreased aggression may be a shorter muzzle and smaller general size among the domesticated individuals; some common neotenous physical traits in domesticated animals include: floppy ears, changes in reproductive cycle, curly tails, piebald coloration, fewer or shortened vertebra, large eyes, rounded forehead, large ears, shortened muzzle. When the role of dogs expanded from just being working dogs to being companions, humans started selective breeding dogs for morphological neoteny, this selective breeding for "neoteny or paedomorphism" had the effect of enhancing the bond between humans and dogs. Humans bred dogs to have more "juvenile physical traits" as adults such as short snouts and wide-set eyes which are associated with puppies, because people consider these traits to be more attractive.
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