{"id":2526,"date":"2017-02-06T20:54:33","date_gmt":"2017-02-06T20:54:33","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/wm-biology2\/?post_type=chapter&#038;p=2526"},"modified":"2017-07-05T17:13:52","modified_gmt":"2017-07-05T17:13:52","slug":"amphibians","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-wmopen-biology2\/chapter\/amphibians\/","title":{"raw":"Amphibians","rendered":"Amphibians"},"content":{"raw":"<h2>Identify characteristics of amphibians<\/h2>\r\nAmphibians are vertebrate tetrapods. <b>Amphibia<\/b> includes frogs, salamanders, and caecilians. The term amphibian loosely translates from the Greek as \u201cdual life,\u201d which is a reference to the metamorphosis that many frogs and salamanders undergo and their mixture of aquatic and terrestrial environments in their life cycle. Amphibians evolved during the Devonian period and were the earliest terrestrial tetrapods.\r\n<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\n<ul>\r\n \t<li>Identify characteristics of amphibians<\/li>\r\n \t<li>Describe the evolutionary history of amphibians<\/li>\r\n \t<li>Describe the important difference between the life cycle of amphibians and the life cycles of other vertebrates<\/li>\r\n \t<li>Distinguish between the characteristics of Urodela, Anura, and Apoda<\/li>\r\n<\/ul>\r\n<\/div>\r\n<h2>Characteristics of Amphibians<\/h2>\r\nAs tetrapods, most amphibians are characterized by four well-developed limbs. Some species of salamanders and all caecilians are functionally limbless; their limbs are vestigial. An important characteristic of extant amphibians is a moist, permeable skin that is achieved via mucus glands that keep the skin moist; thus, exchange of oxygen and carbon dioxide with the environment can take place through it (<b>cutaneous respiration<\/b>). Additional characteristics of amphibians include pedicellate teeth\u2014teeth in which the root and crown are calcified, separated by a zone of noncalcified tissue\u2014and a papilla amphibiorum and papilla basilaris, structures of the inner ear that are sensitive to frequencies below and above 10,00 hertz, respectively. Amphibians also have an auricular operculum, which is an extra bone in the ear that transmits sounds to the inner ear.\r\n\r\nThe fossil record provides evidence of amphibian species, now extinct, that arose over 400 million years ago as the first tetrapods. Amphibia can be divided into three clades: salamanders (Urodela), frogs (Anura), and caecilians (Apoda). The life cycle of frogs, like the majority of amphibians, consists of two distinct stages: the larval stage and metamorphosis to an adult stage. Some species in all orders bypass a free-living larval stage. All extant adult amphibians are carnivorous, and some terrestrial amphibians have a sticky tongue that is used to capture prey.\r\n<h2>Evolution of Amphibians<\/h2>\r\nThe fossil record provides evidence of the first tetrapods: now-extinct amphibian species dating to nearly 400 million years ago. Evolution of tetrapods from fishes represented a significant change in body plan from one suited to organisms that respired and swam in water, to organisms that breathed air and moved onto land; these changes occurred over a span of 50 million years during the Devonian period.\r\n\r\n[caption id=\"attachment_2533\" align=\"alignright\" width=\"350\"]<img class=\" wp-image-2533\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/06205738\/Figure_29_03_01.jpg\" alt=\"The image shows a tetrapod-like fish with fin-like legs.\" width=\"350\" height=\"202\" \/> Figure\u00a01.\u00a0The recent fossil discovery of <em>Tiktaalik roseae<\/em> suggests evidence for an animal intermediate to finned fish and legged tetrapods. (credit: Zina Deretsky, National Science Foundation)[\/caption]\r\n\r\nOne of the earliest known tetrapods is from the genus <b><em>Acanthostega<\/em><\/b>. <em>Acanthostega<\/em> was aquatic; fossils show that it had gills similar to fishes. However, it also had four limbs, with the skeletal structure of limbs found in present-day tetrapods, including amphibians. Therefore, it is thought that <em>Acanthostega<\/em> lived in shallow waters and was an intermediate form between lobe-finned fishes and early, fully terrestrial tetrapods. What preceded <em>Acanthostega<\/em>?\r\n\r\nIn 2006, researchers published news of their discovery of a fossil of a \u201ctetrapod-like fish,\u201d <em>Tiktaalik roseae<\/em>, which seems to be an intermediate form between fishes having fins and tetrapods having limbs (Figure\u00a01). <em>Tiktaalik<\/em> likely lived in a shallow water environment about 375 million years ago.\r\n\r\nThe early tetrapods that moved onto land had access to new nutrient sources and relatively few predators. This led to the widespread distribution of tetrapods during the early Carboniferous period.\r\n<h3>The Paleozoic Era and the Evolution of Vertebrates<\/h3>\r\nThe climate and geography of Earth was vastly different during the Paleozoic Era, when vertebrates arose, as compared to today. The Paleozoic spanned from approximately 542 to 251 million years ago. The landmasses on Earth were very different from those of today. Laurentia and Gondwana were continents located near the equator that subsumed much of the current day landmasses in a different configuration (Figure\u00a02). At this time, sea levels were very high, probably at a level that hasn\u2019t been reached since. As the Paleozoic progressed, glaciations created a cool global climate, but conditions warmed near the end of the first half of the Paleozoic. During the latter half of the Paleozoic, the landmasses began moving together, with the initial formation of a large northern block called Laurasia. This contained parts of what is now North America, along with Greenland, parts of Europe, and Siberia. Eventually, a single supercontinent, called Pangaea, was formed, starting in the latter third of the Paleozoic. Glaciations then began to affect Pangaea\u2019s climate, affecting the distribution of vertebrate life.\r\n\r\n[caption id=\"attachment_2534\" align=\"aligncenter\" width=\"544\"]<img class=\"size-full wp-image-2534\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/06205918\/Figure_29_03_05.jpg\" alt=\"A world map shows two continents, Gondwana and Laurentia, which are shaped very differently from the continents of today. Gondwana was made up of two smaller subcontinents separated by a narrow sea. One continent contained modern Antarctica, and the other contained parts of Africa.\" width=\"544\" height=\"497\" \/> Figure\u00a02.\u00a0During the Paleozoic Era, around 550 million years ago, the continent Gondwana formed. Both Gondwana and the continent Laurentia were located near the equator.[\/caption]\r\n\r\nDuring the early Paleozoic, the amount of carbon dioxide in the atmosphere was much greater than it is today. This may have begun to change later, as land plants became more common. As the roots of land plants began to infiltrate rock and soil began to form, carbon dioxide was drawn out of the atmosphere and became trapped in the rock. This reduced the levels of carbon dioxide and increased the levels of oxygen in the atmosphere, so that by the end of the Paleozoic, atmospheric conditions were similar to those of today.\r\n\r\nAs plants became more common through the latter half of the Paleozoic, microclimates began to emerge and ecosystems began to change. As plants and ecosystems continued to grow and become more complex, vertebrates moved from the water to land. The presence of shoreline vegetation may have contributed to the movement of vertebrates onto land. One hypothesis suggests that the fins of aquatic vertebrates were used to maneuver through this vegetation, providing a precursor to the movement of fins on land and the development of limbs. The late Paleozoic was a time of diversification of vertebrates, as amniotes emerged and became two different lines that gave rise, on one hand, to mammals, and, on the other hand, to reptiles and birds. Many marine vertebrates became extinct near the end of the Devonian period, which ended about 360 million years ago, and both marine and terrestrial vertebrates were decimated by a mass extinction in the early Permian period about 250 million years ago.\r\n<div class=\"textbox\">View <a href=\"http:\/\/wn.com\/earth's_paleogeography\" target=\"_blank\" rel=\"noopener\">Earth\u2019s Paleogeography: Continental Movements Through Time<\/a> to see changes in Earth as life evolved.<\/div>\r\n<h2>The Life Cycle of Amphibians<\/h2>\r\n<strong>Metamorphosis<\/strong> is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation (Figure\u00a03). Metamorphosis is iodothyronine-induced and an ancestral feature of all chordates.[footnote]Robert J. Denver.\u00a0<a href=\"http:\/\/www.cell.com\/current-biology\/pdf\/S0960-9822(08)00661-1.pdf\" target=\"_blank\" rel=\"noopener\">Chordate Metamorphosis: Ancient Control by Iodothyronines.<\/a>\u00a0<em>Current Biology<\/em>, 2008, Vol 18 No 13, R567-9. DOI: 10.1016\/j.cub.2008.05.024[\/footnote] Some insects, fishes, amphibians, mollusks, crustaceans, cnidarians, echinoderms and tunicates undergo metamorphosis, which is often accompanied by a change of nutrition source or behavior. Animals that goes through metamorphosis are called <strong>metamorphoses<\/strong>. Very few vertebrates undergo metamorphosis, but all the amphibians do to some extent.\r\n\r\n[caption id=\"attachment_3629\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-3629\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/03\/01190405\/1024px-Greenfrog_life_stages.svg_-1024x765.png\" alt=\"Throughout a frog's life, it adopts several different forms, changing its body through metamorphosis. It starts as an egg, hatches into a tadpole, becomes a frog.\" width=\"1024\" height=\"765\" \/> Figure\u00a03. The life cycle of a green frog.[\/caption]\r\n\r\nIn typical amphibian development, eggs are laid in water and larvae are adapted to an aquatic lifestyle. Frogs, toads, and newts all hatch from the eggs as larvae with external gills but it will take some time for the amphibians to interact outside with pulmonary respiration. Afterwards, newt larvae start a predatory lifestyle, while tadpoles mostly scrape food off surfaces with their horny tooth ridges.\r\n\r\nMetamorphosis in amphibians is regulated by thyroxin concentration in the blood, which stimulates metamorphosis, and prolactin, which counteracts its effect. Specific events are dependent on threshold values for different tissues. Because most embryonic development is outside the parental body, development is subject to many adaptations due to specific ecological circumstances. For this reason tadpoles can have horny ridges for teeth, whiskers, and fins. They also make use of the lateral line organ. After metamorphosis, these organs become redundant and will be resorbed by controlled cell death, called apoptosis. The amount of adaptation to specific ecological circumstances is remarkable, with many discoveries still being made.\r\n<h3>Frogs and toads<\/h3>\r\nWith frogs and toads, the external gills of the newly hatched tadpole are covered with a gill sac after a few days, and lungs are quickly formed. Front legs are formed under the gill sac, and hindlegs are visible a few days later. Following that there is usually a longer stage during which the tadpole lives off a vegetarian diet. Tadpoles use a relatively long, spiral\u2010shaped gut to digest that diet.\r\n\r\nRapid changes in the body can then be observed as the lifestyle of the frog changes completely. The spiral\u2010shaped mouth with horny tooth ridges is resorbed together with the spiral gut. The animal develops a big jaw, and its gills disappear along with its gill sac. Eyes and legs grow quickly, a tongue is formed, and all this is accompanied by associated changes in the neural networks (development of stereoscopic vision, loss of the lateral line system, etc.) All this can happen in about a day, so it is truly a metamorphosis (Figure\u00a04). It is not until a few days later that the tail is reabsorbed, due to the higher thyroxin concentrations required for tail resorption.\r\n\r\n[caption id=\"attachment_3618\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-3618\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/03\/01000021\/RanaTemporariaLarva2-1024x418.jpg\" alt=\"\" width=\"1024\" height=\"418\" \/> Figure\u00a04. (a) Just before metamorphosis, only 24 hours are needed to reach the stage in part b. (b) Almost functional common frog with some remains of the gill sac and a not fully developed jaw[\/caption]\r\n<h3>Salamanders<\/h3>\r\nSalamander development is highly diverse; some species go through a dramatic reorganization when transitioning from aquatic larvae to terrestrial adults, while others, such as the Axolotl, display paedomorphosis and never develop into terrestrial adults. Within the genus Ambystoma, species have evolved to be paedomorphic several times, and paedomorphosis and complete development can both occur in some species.[footnote]Laudet, Vincent. 2011. \u201cThe Origins and Evolution of Vertebrate Metamorphosis.\u201d\u00a0<em>Current Biology<\/em> 21: R726\u2013R737[\/footnote]\r\n<h3>Newts<\/h3>\r\n[caption id=\"attachment_3619\" align=\"alignright\" width=\"350\"]<img class=\"wp-image-3619\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/03\/01000108\/LarveKamsalamander.jpg\" alt=\"Crested Newt Triturus cristatus larval stage\" width=\"350\" height=\"263\" \/> Figure\u00a05. The large external gills of the crested newt[\/caption]\r\n\r\nIn newts, there is no true metamorphosis because newt larvae already feed as predators and continue doing so as adults. Newts' gills are never covered by a gill sac (Figure\u00a05) and will be resorbed only just before the animal leaves the water. Just as in tadpoles, their lungs are functional early, but newts use them less frequently than tadpoles. Newts often have an aquatic phase in spring and summer, and a land phase in winter. For adaptation to a water phase, prolactin is the required hormone, and for adaptation to the land phase, thyroxin. External gills do not return in subsequent aquatic phases because these are completely absorbed upon leaving the water for the first time.\r\n<h3>Caecilians<\/h3>\r\nBasal caecilians such as Ichthyophis go through a metamorphosis in which aquatic larva transition into fossorial adults, which involves a loss of the lateral line.[footnote]Dunker, Nicole, Marvalee H. Wake, Wendy M. Olson. 2000. \u201cEmbryonic and Larval Development in the Caecilian Ichthyophis kohtaoensis (Amphibia, Gymnophiona): A Staging Table\u201d <em>Journal of Morphology<\/em> 243: 3\u201334[\/footnote] More recently diverged caecilians (the Teresomata) do not undergo an ontogenetic niche shift of this sort and are in general fossorial throughout their lives. Thus, most caecilians do not undergo an anuran-like metamorphosis.[footnote]San Mauro, D., Gower, D. J., Oommen, O. V., Wilkinson, M. &amp; Zardoya, R. 2004. \"Phylogeny of caecilian amphibians (Gymnophiona) based on complete mitochondrial genomes and nuclear RAG1.\"\u00a0<em>Molecular Phylogenetics and Evolution<\/em> 33: 413\u2013427.[\/footnote]\r\n<h2>Clades of Amphibians<\/h2>\r\nAmphibia comprises an estimated 6,770 extant species that inhabit tropical and temperate regions around the world. Amphibians can be divided into three clades: <b>Urodela<\/b> (\u201ctailed-ones\u201d), the salamanders; <b>Anura<\/b> (\u201ctail-less ones\u201d), the frogs; and <b>Apoda<\/b> (\u201clegless ones\u201d), the caecilians.\r\n<h3>Urodela: Salamanders<\/h3>\r\n[caption id=\"attachment_2539\" align=\"alignright\" width=\"350\"]<img class=\"wp-image-2539\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/06210823\/Figure_29_03_02.jpg\" alt=\"The photo shows a black salamander with bright yellow spots.\" width=\"350\" height=\"234\" \/> Figure\u00a06.\u00a0Most salamanders have legs and a tail, but respiration varies among species. (credit: Valentina Storti)[\/caption]\r\n\r\n<b>Salamanders<\/b> are amphibians that belong to the order Urodela. Living salamanders (Figure\u00a06) include approximately 620 species, some of which are aquatic, other terrestrial, and some that live on land only as adults. Adult salamanders usually have a generalized tetrapod body plan with four limbs and a tail. They move by bending their bodies from side to side, called lateral undulation, in a fish-like manner while \u201cwalking\u201d their arms and legs fore and aft. It is thought that their gait is similar to that used by early tetrapods. Respiration differs among different species. The majority of salamanders are lungless, and respiration occurs through the skin or through external gills. Some terrestrial salamanders have primitive lungs; a few species have both gills and lungs.\r\n\r\nUnlike frogs, virtually all salamanders rely on internal fertilization of the eggs. The only male amphibians that possess copulatory structures are the caecilians, so fertilization among salamanders typically involves an elaborate and often prolonged courtship. Such a courtship allows the successful transfer of sperm from male to female via a spermatophore. Development in many of the most highly evolved salamanders, which are fully terrestrial, occurs during a prolonged egg stage, with the eggs guarded by the mother. During this time, the gilled larval stage is found only within the egg capsule, with the gills being resorbed, and metamorphosis being completed, before hatching. Hatchlings thus resemble tiny adults.\r\n<div class=\"textbox\">View <a href=\"http:\/\/www.animalplanet.com\/tv-shows\/river-monsters\/videos\/fish-with-arms-and-hands\/\" target=\"_blank\" rel=\"noopener\">River Monsters: Fish With Arms and Hands?<\/a> to see a video about an unusually large salamander species.<\/div>\r\n<h3>Anura: Frogs<\/h3>\r\n<b>Frogs<\/b> are amphibians that belong to the order Anura (Figure\u00a07a). Anurans are among the most diverse groups of vertebrates, with approximately 5,965 species that occur on all of the continents except Antarctica. Anurans have a body plan that is more specialized for movement. Adult frogs use their hind limbs to jump on land. Frogs have a number of modifications that allow them to avoid predators, including skin that acts as camouflage. Many species of frogs and salamanders also release defensive chemicals from glands in the skin that are poisonous to predators.\r\n\r\n[caption id=\"attachment_2538\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-2538\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/06210751\/Figure_29_03_03ab-1024x394.jpg\" alt=\"Part a shows a big, bright green frog sitting on a branch. Part b shows a frog with a long tail from the tadpole stage.\" width=\"1024\" height=\"394\" \/> Figure\u00a07.\u00a0(a) The Australian green tree frog is a nocturnal predator that lives in the canopies of trees near a water source. (b) A juvenile frog metamorphoses into a frog. Here, the frog has started to develop limbs, but its tadpole tail is still evident.[\/caption]\r\n\r\nFrog eggs are fertilized externally, and like other amphibians, frogs generally lay their eggs in moist environments. A moist environment is required as eggs lack a shell and thus dehydrate quickly in dry environments. Frogs demonstrate a great diversity of parental behaviors, with some species laying many eggs and exhibiting little parental care, to species that carry eggs and tadpoles on their hind legs or backs. The life cycle of frogs, as other amphibians, consists of two distinct stages: the larval stage followed by metamorphosis to an adult stage. The larval stage of a frog, the <b>tadpole<\/b>, is often a filter-feeding herbivore. Tadpoles usually have gills, a lateral line system, long-finned tails, and lack limbs. At the end of the tadpole stage, frogs undergo metamorphosis into the adult form (Figure\u00a07b). During this stage, the gills, tail, and lateral line system disappear, and four limbs develop. The jaws become larger and are suited for carnivorous feeding, and the digestive system transforms into the typical short gut of a predator. An eardrum and air-breathing lungs also develop. These changes during metamorphosis allow the larvae to move onto land in the adult stage.\r\n<h3>Apoda: Caecilians<\/h3>\r\n[caption id=\"attachment_2537\" align=\"alignright\" width=\"350\"]<img class=\"wp-image-2537\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/06210325\/Siphonops_paulensis02-e1486415464662.jpg\" alt=\"This photo shows a limbless, serpentine amphibian, which appears very similar to a worm\" width=\"350\" height=\"193\" \/> Figure\u00a08. The Boettger's caecilian, <em>Siphonops paulensis<\/em>, is a species of caecilian.[\/caption]\r\n\r\nAn estimated 185 species comprise <b>caecilians\u00a0<\/b>(Figure\u00a08), a group of amphibians that belong to the order Apoda. Although they are vertebrates, a complete lack of limbs leads to their resemblance to earthworms in appearance.\r\n\r\nThey are adapted for a soil-burrowing or aquatic lifestyle, and they are nearly blind. These animals are found in the tropics of South America, Africa, and Southern Asia. They have vestigial limbs, evidence that they evolved from a legged ancestor.\r\n<h2><strong>Check Your Understanding<\/strong><\/h2>\r\nAnswer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does\u00a0<strong>not<\/strong>\u00a0count toward your grade in the class, and you can retake it an unlimited number of times.\r\n\r\nUse this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.\r\n\r\nhttps:\/\/assessments.lumenlearning.com\/assessments\/4985","rendered":"<h2>Identify characteristics of amphibians<\/h2>\n<p>Amphibians are vertebrate tetrapods. <b>Amphibia<\/b> includes frogs, salamanders, and caecilians. The term amphibian loosely translates from the Greek as \u201cdual life,\u201d which is a reference to the metamorphosis that many frogs and salamanders undergo and their mixture of aquatic and terrestrial environments in their life cycle. Amphibians evolved during the Devonian period and were the earliest terrestrial tetrapods.<\/p>\n<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<ul>\n<li>Identify characteristics of amphibians<\/li>\n<li>Describe the evolutionary history of amphibians<\/li>\n<li>Describe the important difference between the life cycle of amphibians and the life cycles of other vertebrates<\/li>\n<li>Distinguish between the characteristics of Urodela, Anura, and Apoda<\/li>\n<\/ul>\n<\/div>\n<h2>Characteristics of Amphibians<\/h2>\n<p>As tetrapods, most amphibians are characterized by four well-developed limbs. Some species of salamanders and all caecilians are functionally limbless; their limbs are vestigial. An important characteristic of extant amphibians is a moist, permeable skin that is achieved via mucus glands that keep the skin moist; thus, exchange of oxygen and carbon dioxide with the environment can take place through it (<b>cutaneous respiration<\/b>). Additional characteristics of amphibians include pedicellate teeth\u2014teeth in which the root and crown are calcified, separated by a zone of noncalcified tissue\u2014and a papilla amphibiorum and papilla basilaris, structures of the inner ear that are sensitive to frequencies below and above 10,00 hertz, respectively. Amphibians also have an auricular operculum, which is an extra bone in the ear that transmits sounds to the inner ear.<\/p>\n<p>The fossil record provides evidence of amphibian species, now extinct, that arose over 400 million years ago as the first tetrapods. Amphibia can be divided into three clades: salamanders (Urodela), frogs (Anura), and caecilians (Apoda). The life cycle of frogs, like the majority of amphibians, consists of two distinct stages: the larval stage and metamorphosis to an adult stage. Some species in all orders bypass a free-living larval stage. All extant adult amphibians are carnivorous, and some terrestrial amphibians have a sticky tongue that is used to capture prey.<\/p>\n<h2>Evolution of Amphibians<\/h2>\n<p>The fossil record provides evidence of the first tetrapods: now-extinct amphibian species dating to nearly 400 million years ago. Evolution of tetrapods from fishes represented a significant change in body plan from one suited to organisms that respired and swam in water, to organisms that breathed air and moved onto land; these changes occurred over a span of 50 million years during the Devonian period.<\/p>\n<div id=\"attachment_2533\" style=\"width: 360px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2533\" class=\"wp-image-2533\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/06205738\/Figure_29_03_01.jpg\" alt=\"The image shows a tetrapod-like fish with fin-like legs.\" width=\"350\" height=\"202\" \/><\/p>\n<p id=\"caption-attachment-2533\" class=\"wp-caption-text\">Figure\u00a01.\u00a0The recent fossil discovery of <em>Tiktaalik roseae<\/em> suggests evidence for an animal intermediate to finned fish and legged tetrapods. (credit: Zina Deretsky, National Science Foundation)<\/p>\n<\/div>\n<p>One of the earliest known tetrapods is from the genus <b><em>Acanthostega<\/em><\/b>. <em>Acanthostega<\/em> was aquatic; fossils show that it had gills similar to fishes. However, it also had four limbs, with the skeletal structure of limbs found in present-day tetrapods, including amphibians. Therefore, it is thought that <em>Acanthostega<\/em> lived in shallow waters and was an intermediate form between lobe-finned fishes and early, fully terrestrial tetrapods. What preceded <em>Acanthostega<\/em>?<\/p>\n<p>In 2006, researchers published news of their discovery of a fossil of a \u201ctetrapod-like fish,\u201d <em>Tiktaalik roseae<\/em>, which seems to be an intermediate form between fishes having fins and tetrapods having limbs (Figure\u00a01). <em>Tiktaalik<\/em> likely lived in a shallow water environment about 375 million years ago.<\/p>\n<p>The early tetrapods that moved onto land had access to new nutrient sources and relatively few predators. This led to the widespread distribution of tetrapods during the early Carboniferous period.<\/p>\n<h3>The Paleozoic Era and the Evolution of Vertebrates<\/h3>\n<p>The climate and geography of Earth was vastly different during the Paleozoic Era, when vertebrates arose, as compared to today. The Paleozoic spanned from approximately 542 to 251 million years ago. The landmasses on Earth were very different from those of today. Laurentia and Gondwana were continents located near the equator that subsumed much of the current day landmasses in a different configuration (Figure\u00a02). At this time, sea levels were very high, probably at a level that hasn\u2019t been reached since. As the Paleozoic progressed, glaciations created a cool global climate, but conditions warmed near the end of the first half of the Paleozoic. During the latter half of the Paleozoic, the landmasses began moving together, with the initial formation of a large northern block called Laurasia. This contained parts of what is now North America, along with Greenland, parts of Europe, and Siberia. Eventually, a single supercontinent, called Pangaea, was formed, starting in the latter third of the Paleozoic. Glaciations then began to affect Pangaea\u2019s climate, affecting the distribution of vertebrate life.<\/p>\n<div id=\"attachment_2534\" style=\"width: 554px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2534\" class=\"size-full wp-image-2534\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/06205918\/Figure_29_03_05.jpg\" alt=\"A world map shows two continents, Gondwana and Laurentia, which are shaped very differently from the continents of today. Gondwana was made up of two smaller subcontinents separated by a narrow sea. One continent contained modern Antarctica, and the other contained parts of Africa.\" width=\"544\" height=\"497\" \/><\/p>\n<p id=\"caption-attachment-2534\" class=\"wp-caption-text\">Figure\u00a02.\u00a0During the Paleozoic Era, around 550 million years ago, the continent Gondwana formed. Both Gondwana and the continent Laurentia were located near the equator.<\/p>\n<\/div>\n<p>During the early Paleozoic, the amount of carbon dioxide in the atmosphere was much greater than it is today. This may have begun to change later, as land plants became more common. As the roots of land plants began to infiltrate rock and soil began to form, carbon dioxide was drawn out of the atmosphere and became trapped in the rock. This reduced the levels of carbon dioxide and increased the levels of oxygen in the atmosphere, so that by the end of the Paleozoic, atmospheric conditions were similar to those of today.<\/p>\n<p>As plants became more common through the latter half of the Paleozoic, microclimates began to emerge and ecosystems began to change. As plants and ecosystems continued to grow and become more complex, vertebrates moved from the water to land. The presence of shoreline vegetation may have contributed to the movement of vertebrates onto land. One hypothesis suggests that the fins of aquatic vertebrates were used to maneuver through this vegetation, providing a precursor to the movement of fins on land and the development of limbs. The late Paleozoic was a time of diversification of vertebrates, as amniotes emerged and became two different lines that gave rise, on one hand, to mammals, and, on the other hand, to reptiles and birds. Many marine vertebrates became extinct near the end of the Devonian period, which ended about 360 million years ago, and both marine and terrestrial vertebrates were decimated by a mass extinction in the early Permian period about 250 million years ago.<\/p>\n<div class=\"textbox\">View <a href=\"http:\/\/wn.com\/earth's_paleogeography\" target=\"_blank\" rel=\"noopener\">Earth\u2019s Paleogeography: Continental Movements Through Time<\/a> to see changes in Earth as life evolved.<\/div>\n<h2>The Life Cycle of Amphibians<\/h2>\n<p><strong>Metamorphosis<\/strong> is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal&#8217;s body structure through cell growth and differentiation (Figure\u00a03). Metamorphosis is iodothyronine-induced and an ancestral feature of all chordates.<a class=\"footnote\" title=\"Robert J. Denver.\u00a0Chordate Metamorphosis: Ancient Control by Iodothyronines.\u00a0Current Biology, 2008, Vol 18 No 13, R567-9. DOI: 10.1016\/j.cub.2008.05.024\" id=\"return-footnote-2526-1\" href=\"#footnote-2526-1\" aria-label=\"Footnote 1\"><sup class=\"footnote\">[1]<\/sup><\/a> Some insects, fishes, amphibians, mollusks, crustaceans, cnidarians, echinoderms and tunicates undergo metamorphosis, which is often accompanied by a change of nutrition source or behavior. Animals that goes through metamorphosis are called <strong>metamorphoses<\/strong>. Very few vertebrates undergo metamorphosis, but all the amphibians do to some extent.<\/p>\n<div id=\"attachment_3629\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-3629\" class=\"size-large wp-image-3629\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/03\/01190405\/1024px-Greenfrog_life_stages.svg_-1024x765.png\" alt=\"Throughout a frog's life, it adopts several different forms, changing its body through metamorphosis. It starts as an egg, hatches into a tadpole, becomes a frog.\" width=\"1024\" height=\"765\" \/><\/p>\n<p id=\"caption-attachment-3629\" class=\"wp-caption-text\">Figure\u00a03. The life cycle of a green frog.<\/p>\n<\/div>\n<p>In typical amphibian development, eggs are laid in water and larvae are adapted to an aquatic lifestyle. Frogs, toads, and newts all hatch from the eggs as larvae with external gills but it will take some time for the amphibians to interact outside with pulmonary respiration. Afterwards, newt larvae start a predatory lifestyle, while tadpoles mostly scrape food off surfaces with their horny tooth ridges.<\/p>\n<p>Metamorphosis in amphibians is regulated by thyroxin concentration in the blood, which stimulates metamorphosis, and prolactin, which counteracts its effect. Specific events are dependent on threshold values for different tissues. Because most embryonic development is outside the parental body, development is subject to many adaptations due to specific ecological circumstances. For this reason tadpoles can have horny ridges for teeth, whiskers, and fins. They also make use of the lateral line organ. After metamorphosis, these organs become redundant and will be resorbed by controlled cell death, called apoptosis. The amount of adaptation to specific ecological circumstances is remarkable, with many discoveries still being made.<\/p>\n<h3>Frogs and toads<\/h3>\n<p>With frogs and toads, the external gills of the newly hatched tadpole are covered with a gill sac after a few days, and lungs are quickly formed. Front legs are formed under the gill sac, and hindlegs are visible a few days later. Following that there is usually a longer stage during which the tadpole lives off a vegetarian diet. Tadpoles use a relatively long, spiral\u2010shaped gut to digest that diet.<\/p>\n<p>Rapid changes in the body can then be observed as the lifestyle of the frog changes completely. The spiral\u2010shaped mouth with horny tooth ridges is resorbed together with the spiral gut. The animal develops a big jaw, and its gills disappear along with its gill sac. Eyes and legs grow quickly, a tongue is formed, and all this is accompanied by associated changes in the neural networks (development of stereoscopic vision, loss of the lateral line system, etc.) All this can happen in about a day, so it is truly a metamorphosis (Figure\u00a04). It is not until a few days later that the tail is reabsorbed, due to the higher thyroxin concentrations required for tail resorption.<\/p>\n<div id=\"attachment_3618\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-3618\" class=\"size-large wp-image-3618\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/03\/01000021\/RanaTemporariaLarva2-1024x418.jpg\" alt=\"\" width=\"1024\" height=\"418\" \/><\/p>\n<p id=\"caption-attachment-3618\" class=\"wp-caption-text\">Figure\u00a04. (a) Just before metamorphosis, only 24 hours are needed to reach the stage in part b. (b) Almost functional common frog with some remains of the gill sac and a not fully developed jaw<\/p>\n<\/div>\n<h3>Salamanders<\/h3>\n<p>Salamander development is highly diverse; some species go through a dramatic reorganization when transitioning from aquatic larvae to terrestrial adults, while others, such as the Axolotl, display paedomorphosis and never develop into terrestrial adults. Within the genus Ambystoma, species have evolved to be paedomorphic several times, and paedomorphosis and complete development can both occur in some species.<a class=\"footnote\" title=\"Laudet, Vincent. 2011. \u201cThe Origins and Evolution of Vertebrate Metamorphosis.\u201d\u00a0Current Biology 21: R726\u2013R737\" id=\"return-footnote-2526-2\" href=\"#footnote-2526-2\" aria-label=\"Footnote 2\"><sup class=\"footnote\">[2]<\/sup><\/a><\/p>\n<h3>Newts<\/h3>\n<div id=\"attachment_3619\" style=\"width: 360px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-3619\" class=\"wp-image-3619\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/03\/01000108\/LarveKamsalamander.jpg\" alt=\"Crested Newt Triturus cristatus larval stage\" width=\"350\" height=\"263\" \/><\/p>\n<p id=\"caption-attachment-3619\" class=\"wp-caption-text\">Figure\u00a05. The large external gills of the crested newt<\/p>\n<\/div>\n<p>In newts, there is no true metamorphosis because newt larvae already feed as predators and continue doing so as adults. Newts&#8217; gills are never covered by a gill sac (Figure\u00a05) and will be resorbed only just before the animal leaves the water. Just as in tadpoles, their lungs are functional early, but newts use them less frequently than tadpoles. Newts often have an aquatic phase in spring and summer, and a land phase in winter. For adaptation to a water phase, prolactin is the required hormone, and for adaptation to the land phase, thyroxin. External gills do not return in subsequent aquatic phases because these are completely absorbed upon leaving the water for the first time.<\/p>\n<h3>Caecilians<\/h3>\n<p>Basal caecilians such as Ichthyophis go through a metamorphosis in which aquatic larva transition into fossorial adults, which involves a loss of the lateral line.<a class=\"footnote\" title=\"Dunker, Nicole, Marvalee H. Wake, Wendy M. Olson. 2000. \u201cEmbryonic and Larval Development in the Caecilian Ichthyophis kohtaoensis (Amphibia, Gymnophiona): A Staging Table\u201d Journal of Morphology 243: 3\u201334\" id=\"return-footnote-2526-3\" href=\"#footnote-2526-3\" aria-label=\"Footnote 3\"><sup class=\"footnote\">[3]<\/sup><\/a> More recently diverged caecilians (the Teresomata) do not undergo an ontogenetic niche shift of this sort and are in general fossorial throughout their lives. Thus, most caecilians do not undergo an anuran-like metamorphosis.<a class=\"footnote\" title=\"San Mauro, D., Gower, D. J., Oommen, O. V., Wilkinson, M. &amp; Zardoya, R. 2004. &quot;Phylogeny of caecilian amphibians (Gymnophiona) based on complete mitochondrial genomes and nuclear RAG1.&quot;\u00a0Molecular Phylogenetics and Evolution 33: 413\u2013427.\" id=\"return-footnote-2526-4\" href=\"#footnote-2526-4\" aria-label=\"Footnote 4\"><sup class=\"footnote\">[4]<\/sup><\/a><\/p>\n<h2>Clades of Amphibians<\/h2>\n<p>Amphibia comprises an estimated 6,770 extant species that inhabit tropical and temperate regions around the world. Amphibians can be divided into three clades: <b>Urodela<\/b> (\u201ctailed-ones\u201d), the salamanders; <b>Anura<\/b> (\u201ctail-less ones\u201d), the frogs; and <b>Apoda<\/b> (\u201clegless ones\u201d), the caecilians.<\/p>\n<h3>Urodela: Salamanders<\/h3>\n<div id=\"attachment_2539\" style=\"width: 360px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2539\" class=\"wp-image-2539\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/06210823\/Figure_29_03_02.jpg\" alt=\"The photo shows a black salamander with bright yellow spots.\" width=\"350\" height=\"234\" \/><\/p>\n<p id=\"caption-attachment-2539\" class=\"wp-caption-text\">Figure\u00a06.\u00a0Most salamanders have legs and a tail, but respiration varies among species. (credit: Valentina Storti)<\/p>\n<\/div>\n<p><b>Salamanders<\/b> are amphibians that belong to the order Urodela. Living salamanders (Figure\u00a06) include approximately 620 species, some of which are aquatic, other terrestrial, and some that live on land only as adults. Adult salamanders usually have a generalized tetrapod body plan with four limbs and a tail. They move by bending their bodies from side to side, called lateral undulation, in a fish-like manner while \u201cwalking\u201d their arms and legs fore and aft. It is thought that their gait is similar to that used by early tetrapods. Respiration differs among different species. The majority of salamanders are lungless, and respiration occurs through the skin or through external gills. Some terrestrial salamanders have primitive lungs; a few species have both gills and lungs.<\/p>\n<p>Unlike frogs, virtually all salamanders rely on internal fertilization of the eggs. The only male amphibians that possess copulatory structures are the caecilians, so fertilization among salamanders typically involves an elaborate and often prolonged courtship. Such a courtship allows the successful transfer of sperm from male to female via a spermatophore. Development in many of the most highly evolved salamanders, which are fully terrestrial, occurs during a prolonged egg stage, with the eggs guarded by the mother. During this time, the gilled larval stage is found only within the egg capsule, with the gills being resorbed, and metamorphosis being completed, before hatching. Hatchlings thus resemble tiny adults.<\/p>\n<div class=\"textbox\">View <a href=\"http:\/\/www.animalplanet.com\/tv-shows\/river-monsters\/videos\/fish-with-arms-and-hands\/\" target=\"_blank\" rel=\"noopener\">River Monsters: Fish With Arms and Hands?<\/a> to see a video about an unusually large salamander species.<\/div>\n<h3>Anura: Frogs<\/h3>\n<p><b>Frogs<\/b> are amphibians that belong to the order Anura (Figure\u00a07a). Anurans are among the most diverse groups of vertebrates, with approximately 5,965 species that occur on all of the continents except Antarctica. Anurans have a body plan that is more specialized for movement. Adult frogs use their hind limbs to jump on land. Frogs have a number of modifications that allow them to avoid predators, including skin that acts as camouflage. Many species of frogs and salamanders also release defensive chemicals from glands in the skin that are poisonous to predators.<\/p>\n<div id=\"attachment_2538\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2538\" class=\"size-large wp-image-2538\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/06210751\/Figure_29_03_03ab-1024x394.jpg\" alt=\"Part a shows a big, bright green frog sitting on a branch. Part b shows a frog with a long tail from the tadpole stage.\" width=\"1024\" height=\"394\" \/><\/p>\n<p id=\"caption-attachment-2538\" class=\"wp-caption-text\">Figure\u00a07.\u00a0(a) The Australian green tree frog is a nocturnal predator that lives in the canopies of trees near a water source. (b) A juvenile frog metamorphoses into a frog. Here, the frog has started to develop limbs, but its tadpole tail is still evident.<\/p>\n<\/div>\n<p>Frog eggs are fertilized externally, and like other amphibians, frogs generally lay their eggs in moist environments. A moist environment is required as eggs lack a shell and thus dehydrate quickly in dry environments. Frogs demonstrate a great diversity of parental behaviors, with some species laying many eggs and exhibiting little parental care, to species that carry eggs and tadpoles on their hind legs or backs. The life cycle of frogs, as other amphibians, consists of two distinct stages: the larval stage followed by metamorphosis to an adult stage. The larval stage of a frog, the <b>tadpole<\/b>, is often a filter-feeding herbivore. Tadpoles usually have gills, a lateral line system, long-finned tails, and lack limbs. At the end of the tadpole stage, frogs undergo metamorphosis into the adult form (Figure\u00a07b). During this stage, the gills, tail, and lateral line system disappear, and four limbs develop. The jaws become larger and are suited for carnivorous feeding, and the digestive system transforms into the typical short gut of a predator. An eardrum and air-breathing lungs also develop. These changes during metamorphosis allow the larvae to move onto land in the adult stage.<\/p>\n<h3>Apoda: Caecilians<\/h3>\n<div id=\"attachment_2537\" style=\"width: 360px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2537\" class=\"wp-image-2537\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/06210325\/Siphonops_paulensis02-e1486415464662.jpg\" alt=\"This photo shows a limbless, serpentine amphibian, which appears very similar to a worm\" width=\"350\" height=\"193\" \/><\/p>\n<p id=\"caption-attachment-2537\" class=\"wp-caption-text\">Figure\u00a08. The Boettger&#8217;s caecilian, <em>Siphonops paulensis<\/em>, is a species of caecilian.<\/p>\n<\/div>\n<p>An estimated 185 species comprise <b>caecilians\u00a0<\/b>(Figure\u00a08), a group of amphibians that belong to the order Apoda. Although they are vertebrates, a complete lack of limbs leads to their resemblance to earthworms in appearance.<\/p>\n<p>They are adapted for a soil-burrowing or aquatic lifestyle, and they are nearly blind. These animals are found in the tropics of South America, Africa, and Southern Asia. They have vestigial limbs, evidence that they evolved from a legged ancestor.<\/p>\n<h2><strong>Check Your Understanding<\/strong><\/h2>\n<p>Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does\u00a0<strong>not<\/strong>\u00a0count toward your grade in the class, and you can retake it an unlimited number of times.<\/p>\n<p>Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.<\/p>\n<p>\t<iframe id=\"lumen_assessment_4985\" class=\"resizable\" src=\"https:\/\/assessments.lumenlearning.com\/assessments\/load?assessment_id=4985&#38;embed=1&#38;external_user_id=&#38;external_context_id=&#38;iframe_resize_id=lumen_assessment_4985\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:400px;\"><br \/>\n\t<\/iframe><\/p>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-2526\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Original<\/div><ul class=\"citation-list\"><li>Introduction to Amphibians. <strong>Authored by<\/strong>: Shelli Carter and Lumen Learning. <strong>Provided by<\/strong>: Lumen Learning. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em><\/li><li>Adaptation and Revision of Metamorphosis. <strong>Authored by<\/strong>: Shelli Carter and Lumen Learning. <strong>Provided by<\/strong>: Lumen Learning. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/\">CC BY-SA: Attribution-ShareAlike<\/a><\/em><\/li><\/ul><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Biology. <strong>Provided by<\/strong>: OpenStax CNX. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\">http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/li><li>Metamorphosis. <strong>Provided by<\/strong>: Metamorphosis. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/en.wikipedia.org\/wiki\/Metamorphosis\">https:\/\/en.wikipedia.org\/wiki\/Metamorphosis<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/\">CC BY-SA: Attribution-ShareAlike<\/a><\/em><\/li><li>Green frog life stages. <strong>Authored by<\/strong>: LadyofHats. <strong>Provided by<\/strong>: Wikipedia. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Greenfrog_life_stages.svg\">https:\/\/commons.wikimedia.org\/wiki\/File:Greenfrog_life_stages.svg<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/about\/cc0\">CC0: No Rights Reserved<\/a><\/em><\/li><li>Siphonops paulensis. <strong>Authored by<\/strong>: Ariovaldo Giaretta. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Siphonops_paulensis02.jpg\">https:\/\/commons.wikimedia.org\/wiki\/File:Siphonops_paulensis02.jpg<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/\">CC BY-SA: Attribution-ShareAlike<\/a><\/em><\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section><hr class=\"before-footnotes clear\" \/><div class=\"footnotes\"><ol><li id=\"footnote-2526-1\">Robert J. Denver.\u00a0<a href=\"http:\/\/www.cell.com\/current-biology\/pdf\/S0960-9822(08)00661-1.pdf\" target=\"_blank\" rel=\"noopener\">Chordate Metamorphosis: Ancient Control by Iodothyronines.<\/a>\u00a0<em>Current Biology<\/em>, 2008, Vol 18 No 13, R567-9. DOI: 10.1016\/j.cub.2008.05.024 <a href=\"#return-footnote-2526-1\" class=\"return-footnote\" aria-label=\"Return to footnote 1\">&crarr;<\/a><\/li><li id=\"footnote-2526-2\">Laudet, Vincent. 2011. \u201cThe Origins and Evolution of Vertebrate Metamorphosis.\u201d\u00a0<em>Current Biology<\/em> 21: R726\u2013R737 <a href=\"#return-footnote-2526-2\" class=\"return-footnote\" aria-label=\"Return to footnote 2\">&crarr;<\/a><\/li><li id=\"footnote-2526-3\">Dunker, Nicole, Marvalee H. Wake, Wendy M. Olson. 2000. \u201cEmbryonic and Larval Development in the Caecilian Ichthyophis kohtaoensis (Amphibia, Gymnophiona): A Staging Table\u201d <em>Journal of Morphology<\/em> 243: 3\u201334 <a href=\"#return-footnote-2526-3\" class=\"return-footnote\" aria-label=\"Return to footnote 3\">&crarr;<\/a><\/li><li id=\"footnote-2526-4\">San Mauro, D., Gower, D. J., Oommen, O. V., Wilkinson, M. &amp; Zardoya, R. 2004. \"Phylogeny of caecilian amphibians (Gymnophiona) based on complete mitochondrial genomes and nuclear RAG1.\"\u00a0<em>Molecular Phylogenetics and Evolution<\/em> 33: 413\u2013427. <a href=\"#return-footnote-2526-4\" class=\"return-footnote\" aria-label=\"Return to footnote 4\">&crarr;<\/a><\/li><\/ol><\/div>","protected":false},"author":17,"menu_order":4,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology\",\"author\":\"\",\"organization\":\"OpenStax CNX\",\"url\":\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\"},{\"type\":\"original\",\"description\":\"Introduction to Amphibians\",\"author\":\"Shelli Carter and Lumen Learning\",\"organization\":\"Lumen Learning\",\"url\":\"\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"\"},{\"type\":\"original\",\"description\":\"Adaptation and Revision of Metamorphosis\",\"author\":\"Shelli Carter and Lumen Learning\",\"organization\":\"Lumen Learning\",\"url\":\"\",\"project\":\"\",\"license\":\"cc-by-sa\",\"license_terms\":\"\"},{\"type\":\"cc\",\"description\":\"Metamorphosis\",\"author\":\"\",\"organization\":\"Metamorphosis\",\"url\":\"https:\/\/en.wikipedia.org\/wiki\/Metamorphosis\",\"project\":\"\",\"license\":\"cc-by-sa\",\"license_terms\":\"\"},{\"type\":\"cc\",\"description\":\"Green frog life stages\",\"author\":\"LadyofHats\",\"organization\":\"Wikipedia\",\"url\":\"https:\/\/commons.wikimedia.org\/wiki\/File:Greenfrog_life_stages.svg\",\"project\":\"\",\"license\":\"cc0\",\"license_terms\":\"\"},{\"type\":\"cc\",\"description\":\"Siphonops paulensis\",\"author\":\"Ariovaldo Giaretta\",\"organization\":\"\",\"url\":\"https:\/\/commons.wikimedia.org\/wiki\/File:Siphonops_paulensis02.jpg\",\"project\":\"\",\"license\":\"cc-by-sa\",\"license_terms\":\"\"}]","CANDELA_OUTCOMES_GUID":"48be4a60-0050-487e-b563-581a2c71e181, 602d2064-d56d-44d9-af0a-16c0637aba18, a01e30be-4dbc-4d45-b7cd-2718025181ae, d31ceab2-b124-40b3-ac71-28f30ad4be29, cb7f803f-3526-4fea-8ad9-e91701dd4867","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-2526","chapter","type-chapter","status-publish","hentry"],"part":148,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-wmopen-biology2\/wp-json\/pressbooks\/v2\/chapters\/2526","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/suny-wmopen-biology2\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/suny-wmopen-biology2\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-wmopen-biology2\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":6,"href":"https:\/\/courses.lumenlearning.com\/suny-wmopen-biology2\/wp-json\/pressbooks\/v2\/chapters\/2526\/revisions"}],"predecessor-version":[{"id":5916,"href":"https:\/\/courses.lumenlearning.com\/suny-wmopen-biology2\/wp-json\/pressbooks\/v2\/chapters\/2526\/revisions\/5916"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-wmopen-biology2\/wp-json\/pressbooks\/v2\/parts\/148"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-wmopen-biology2\/wp-json\/pressbooks\/v2\/chapters\/2526\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-wmopen-biology2\/wp-json\/wp\/v2\/media?parent=2526"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-wmopen-biology2\/wp-json\/pressbooks\/v2\/chapter-type?post=2526"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-wmopen-biology2\/wp-json\/wp\/v2\/contributor?post=2526"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-wmopen-biology2\/wp-json\/wp\/v2\/license?post=2526"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}