Amphibians are ectothermic, tetrapod vertebrates of the class Amphibia. Modern amphibians are all Lissamphibia, they inhabit a wide variety of habitats, with most species living within terrestrial, arboreal or freshwater aquatic ecosystems. Thus amphibians start out as larvae living in water, but some species have developed behavioural adaptations to bypass this; the young undergo metamorphosis from larva with gills to an adult air-breathing form with lungs. Amphibians use their skin as a secondary respiratory surface and some small terrestrial salamanders and frogs lack lungs and rely on their skin, they are superficially similar to lizards but, along with mammals and birds, reptiles are amniotes and do not require water bodies in which to breed. With their complex reproductive needs and permeable skins, amphibians are ecological indicators; the earliest amphibians evolved in the Devonian period from sarcopterygian fish with lungs and bony-limbed fins, features that were helpful in adapting to dry land.
They diversified and became dominant during the Carboniferous and Permian periods, but were displaced by reptiles and other vertebrates. Over time, amphibians shrank in size and decreased in diversity, leaving only the modern subclass Lissamphibia; the three modern orders of amphibians are Anura and Apoda. The number of known amphibian species is 8,000, of which nearly 90% are frogs; the smallest amphibian in the world is a frog from New Guinea with a length of just 7.7 mm. The largest living amphibian is the 1.8 m Chinese giant salamander, but this is dwarfed by the extinct 9 m Prionosuchus from the middle Permian of Brazil. The study of amphibians is called batrachology, while the study of both reptiles and amphibians is called herpetology; the word "amphibian" is derived from the Ancient Greek term ἀμφίβιος, which means "both kinds of life", ἀμφί meaning "of both kinds" and βιος meaning "life". The term was used as a general adjective for animals that could live on land or in water, including seals and otters.
Traditionally, the class Amphibia includes all tetrapod vertebrates. Amphibia in its widest sense was divided into three subclasses, two of which are extinct: Subclass Lepospondyli† Subclass Temnospondyli† Subclass Lissamphibia Salientia: Jurassic to present—6,200 current species in 53 families Caudata: Jurassic to present—652 current species in 9 families Gymnophiona: Jurassic to present—192 current species in 10 families The actual number of species in each group depends on the taxonomic classification followed; the two most common systems are the classification adopted by the website AmphibiaWeb, University of California and the classification by herpetologist Darrel Frost and the American Museum of Natural History, available as the online reference database "Amphibian Species of the World". The numbers of species cited above follows Frost and the total number of known amphibian species as of March 31, 2019 is 8,000, of which nearly 90% are frogs. With the phylogenetic classification, the taxon Labyrinthodontia has been discarded as it is a polyparaphyletic group without unique defining features apart from shared primitive characteristics.
Classification varies according to the preferred phylogeny of the author and whether they use a stem-based or a node-based classification. Traditionally, amphibians as a class are defined as all tetrapods with a larval stage, while the group that includes the common ancestors of all living amphibians and all their descendants is called Lissamphibia; the phylogeny of Paleozoic amphibians is uncertain, Lissamphibia may fall within extinct groups, like the Temnospondyli or the Lepospondyli, in some analyses in the amniotes. This means that advocates of phylogenetic nomenclature have removed a large number of basal Devonian and Carboniferous amphibian-type tetrapod groups that were placed in Amphibia in Linnaean taxonomy, included them elsewhere under cladistic taxonomy. If the common ancestor of amphibians and amniotes is included in Amphibia, it becomes a paraphyletic group. All modern amphibians are included in the subclass Lissamphibia, considered a clade, a group of species that have evolved from a common ancestor.
The three modern orders are Anura and Gymnophiona. It has been suggested that salamanders arose separately from a Temnospondyl-like ancestor, that caecilians are the sister group of the advanced reptiliomorph amphibians, thus of amniotes. Although the fossils of several older proto-frogs with primitive characteristics are known, the oldest "true frog" is Prosalirus bitis, from the Early Jurassic Kayenta Formation of Arizona, it is anatomically similar to modern frogs. The oldest known caecilian is another Early Jurassic species, Eocaecilia micropodia from Arizona; the earliest salamander is Beiyanerpeton jianpingensis from the Late Jurassic of northeastern China. Authorities disagree as to whether Salientia is a superorder that includes the order Anura, or whether
The oval window is a membrane-covered opening that leads from the middle ear to the vestibule of the inner ear. Vibrations that contact the tympanic membrane travel through the three ossicles and into the inner ear; the oval window is the intersection of the middle ear with the inner ear and is directly contacted by the stapes. It is a reniform opening leading from the tympanic cavity into the vestibule of the internal ear, it is occupied by the base of the stapes, the circumference of, fixed by the annular ligament to the margin of the foramen. Round window This article incorporates text in the public domain from page 1040 of the 20th edition of Gray's Anatomy Diagram at Washington University The Anatomy Wiz. Oval Window
Anatomical terms of bone
Many anatomical terms descriptive of bone are defined in anatomical terminology, are derived from Greek and Latin. A long bone is one, cylindrical in shape, being longer than it is wide. However, the term describes the shape of a bone, not its size, relative. Long bones are found in the legs, as well as in the fingers and toes. Long bones function as levers, they are responsible for the body's height. A short bone is one, cube-like in shape, being equal in length and thickness; the only short bones in the human skeleton are in the carpals of the wrists and the tarsals of the ankles. Short bones provide support as well as some limited motion; the term “flat bone” is something of a misnomer because, although a flat bone is thin, it is often curved. Examples include the cranial bones, the scapulae, the sternum, the ribs. Flat bones serve as points of attachment for muscles and protect internal organs. Flat bones do not have a medullary cavity. An irregular bone is one that does not have an classified shape and defies description.
These bones tend to have more complex shapes, like the vertebrae that support the spinal cord and protect it from compressive forces. Many facial bones the ones containing sinuses, are classified as irregular bones. A sesamoid bone is a round bone that, as the name suggests, is shaped like a sesame seed; these bones form in tendons. The sesamoid bones protect tendons by helping them overcome compressive forces. Sesamoid bones vary in number and placement from person to person but are found in tendons associated with the feet and knees; the only type of sesamoid bone, common to everybody is the kneecap, the largest of the sesamoid bones. A condyle is the round prominence at the end of a bone, most part of a joint – an articulation with another bone; the epicondyle refers to a projection near a condyle the medial epicondyle of the humerus. These terms derive from Greek. An eminence refers to a small projection or bump of bone, such as the medial eminence. A process refers to a large projection or prominent bump, as does a promontory such as the sacral promontory.
Both tubercle and tuberosity refer to a projection or bump with a roughened surface, with a "tubercle" smaller than a "tuberosity". These terms are derived from Tuber. A ramus refers to an extension of bone, such as the ramus of the mandible in the jaw or Superior pubic ramus. Ramus may be used to refer to nerves, such as the ramus communicans. A facet refers to a flattened articular surface. A line refers to a long, thin projection with a rough surface. Ridge and crest refer to a narrow line. Unlike many words used to describe anatomical terms, the word ridge is derived from Old English. A spine, as well as referring to the spinal cord, may be used to describe a long, thin projection or bump; these terms are used to describe bony protuberances in specific parts of the body. The Malleolus is the bony prominence on each side of the ankle; these are known as the lateral malleolus. Each leg is supported by two bones, the tibia on the inner side of the leg and the fibula on the outer side of the leg; the medial malleolus is the prominence on the inner side of the ankle, formed by the lower end of the tibia.
The lateral malleolus is the prominence on the outer side of the ankle, formed by the lower end of the fibula. The trochanters are parts of the femur, it may refer to the greater, lesser, or third trochanter The following terms are used to describe cavities that connect to other areas: A foramen is any opening referring to those in bone. Foramina inside the body of humans and other animals allow muscles, arteries, veins, or other structures to connect one part of the body with another. A canal is a long, tunnel-like foramen a passage for notable nerves or blood vessels; the following terms are used to describe cavities that do not connect to other areas: A fossa is a depression or hollow in a bone, such as the hypophyseal fossa, the depression in the sphenoid bone. A meatus is a short canal. A fovea is a small pit on the head of a bone. An example of a fovea is the fovea capitis of the head of the femur; the following terms are used to describe the walls of a cavity: A labyrinth refers to the bony labyrinth and membranous labyrinth, components of the inner ear, due to their fine and complex structure.
A sinus refers to a bony cavity within the skull. A joint, or articulation is the region where adjacent bones contact each other, for example the elbow, shoulder, or costovertebral joint. Terms that refer to joints include: articular process, referring to a projection that contacts an adjacent bone. Suture, referring to an articulation between cranial bones. Bones are described with the terms head, shaft and base The head of a bone refers to the proximal end of the bone; the shaft refers to the elongated sections of long bone, the neck the segment between the head and shaft. The end of the long bone opposite to the head is known as the base; the cortex of a bone is used to refer to its outer layers, medulla used to
The hyomandibula referred to as hyomandibular is a set of bones, found in the hyoid region in most fishes. It plays a role in suspending the jaws and/or operculum, it is suggested that in tetrapods, the hyomandibula evolved into the columella. In jawless fishes a series of gills opened behind the mouth, these gills became supported by cartilaginous elements; the first set of these elements surrounded the mouth to form the jaw. There are ample evidences; the upper portion of the second embryonic arch supporting the gill became the hyomandibular bone of jawed fishes, which supports the skull and therefore links the jaw to the cranium. When vertebrates found their way onto land, the hyomandibula, with its location near the ear, began to function as a sound amplifier beside its function to support the skull; as evolution attached the cranium of terrestrial vertebrates to the rest of the skull, the hyomandibula lost its supportive function and became an interior organ, the stapes, thus its secondary function had become its primary function.
Fish anatomy Palatoquadrate Kardong, Kenneth V. Vertebrates Comparative Anatomy, Evolution. Pp. 227, 693. "Hyomandibula". ZipcodeZoo.com. Archived from the original on 14 March 2012. Retrieved 30 January 2010. Gilbert, Scott F.. "The anatomical tradition: Evolutionary Embryology: Embryonic homologies". Developmental Biology. Sunderland: Sinauer Associates, Inc.. Retrieved 2018-04-09. Gilbert. Figure 1.14. Jaw structure in the fish and mammal
Evolution of mammalian auditory ossicles
The evolution of mammalian auditory ossicles was an evolutionary event in which bones in the jaw of reptiles were co-opted to form part of the hearing apparatus in mammals. The event is well-documented and important as a demonstration of transitional forms and exaptation, the re-purposing of existing structures during evolution. In reptiles, the eardrum is connected to the inner ear via a single bone, the columella, while the upper and lower jaws contain several bones not found in mammals. Over the course of the evolution of mammals, one bone from the lower and one from the upper jaw lost their purpose in the jaw joint and were put to new use in the middle ear, connecting to the existing stapes bone and forming a chain of three bones, the ossicles, which transmit sounds more efficiently and allow more acute hearing. In mammals, these three bones are known as the malleus and stapes. Mammals and birds differ from other vertebrates by having evolved a cochlea; the evidence that the malleus and incus are homologous to the reptilian articular and quadrate was embryological, since this discovery an abundance of transitional fossils has both supported the conclusion and given a detailed history of the transition.
The evolution of the stapes was an distinct event. Following on the ideas of Étienne Geoffroy Saint-Hilaire, studies by Johann Friedrich Meckel the Younger, Carl Gustav Carus, Martin Rathke, Karl Ernst von Baer, the relationship between the reptilian jaw bones and mammalian middle-ear bones was first established on the basis of embryology and comparative anatomy by Karl Bogislaus Reichert and advanced by Ernst Gaupp and this is known as the Reichert–Gaupp theory. In the course of the development of the embryo, the incus and malleus arise from the same first pharyngeal arch as the mandible and maxilla, are served by mandibular and maxillary division of the trigeminal nerve....the discovery that the mammalian malleus and incus were homologues of visceral elements of the "reptilian" jaw articulation... ranks as one of the milestones in the history of comparative biology.... It is one of the triumphs of the long series of researches on the extinct Theromorph reptiles, begun by Owen, continued by Seeley and Watson, to have revealed the intermediate steps by which the change may have occurred from an inner quadrate to an outer squamosal articulation...
Yet the transition between the "reptilian" jaw and the "mammalian" middle ear was not bridged in the fossil record until the 1950s with the elaboration of such fossils as the now-famous Morganucodon. There are more recent studies in the genetic basis for the development of the ossicles from the embryonic arch, relating this to evolutionary history."Bapx1 known as Nkx3.2, is the vertebrate homologue of the Drosophila gene Bagpipe. A member of the NK2 class of homeobox genes...", this gene is implicated in the change from the jaw bones of non-mammals to the ossicles of mammals. Others are Dlx genes, Prx genes, Wnt genes; the earliest mammals were small animals nocturnal insectivores. This suggests a plausible evolutionary mechanism driving the change. Natural selection would account for the success of this feature. There is still one more connection with another part of biology: genetics suggests a mechanism for this transition, the kind of major change of function seen elsewhere in the world of life being studied by evolutionary developmental biology.
The mammalian middle ear contains three tiny bones known as the ossicles: malleus and stapes. The ossicles are a complex system of levers whose functions include: reducing the amplitude of the vibrations; the ossicles act as the mechanical analog of an electrical transformer, matching the mechanical impedance of vibrations in air to vibrations in the liquid of the cochlea. The net effect of this impedance matching is to increase the overall sensitivity and upper frequency limits of mammalian hearing, as compared to reptilian hearing; the details of these structures and their effects vary noticeably between different mammal species when the species are as related as humans and chimpanzees. Living mammal species can be identified by the presence in females of mammary glands which produce milk. Other features are required when classifying fossils, since mammary glands and other soft-tissue features are not visible in fossils. Paleontologists therefore use a distinguishing feature, shared by all living mammals, but is not present in any of the early Triassic therapsids: mammals use two bones for hearing that all other amniotes use for eating.
The earliest amniotes had a jaw joint composed of the quadrate. All non-mammalian amniotes use this system including lizards, crocodilians and therapsids, but mammals have a different jaw joint, composed only of the squamosal. In mammals, the quadrate and articular bones have evolved into the incus and malleus bones in the middle ear. Here is a ver
The utricle, along with the saccule, is one of the two otolith organs located in the vertebrate inner ear. The utricle and the saccule are parts of the balancing apparatus located within the vestibule of the bony labyrinth; these use a viscous fluid to stimulate hair cells to detect motion and orientation. The utricle detects linear head-tilts in the horizontal plane; the word utricle comes from Latin uter, meaning'leather bag'. The utricle is larger than the saccule and is of an oblong form, compressed transversely, occupies the upper and back part of the vestibule, lying in contact with the recessus ellipticus and the part below it; the macula of utricle is a thickening in the wall of the utricle where the epithelium contains vestibular hair cells that allows a person to perceive changes in longitudinal acceleration as well as effects of gravity. The gelatinous layer and the statoconia together are referred to as the otolithic membrane, where the tips of the stereocilia and kinocilium are embedded.
When the head is tilted such that gravity pulls on the statoconia the gelatinous layer is pulled in the same direction causing the sensory hairs to bend. Within the utricle is a small 2 by 3 mm patch of hair cells called the macula of utricle; the macula of utricle, which lies horizontally on the floor of the utricle, contains the hair cells. These hair cells are mechanoreceptors which consist of 40 to 70 stereocilia and only one true cilium called a kinocilium; the kinocilium is the only sensory aspect of the hair cell and is what causes hair cell polarization. The tips of these stereocilia and kinocilium are embedded in a gelatinous otolithic membrane; this membrane is weighted with calcium carbonate-protein granules called otoliths. The otolithic membrane increases their inertia; the addition in weight and inertia is vital to the utricle's ability to detect linear acceleration, as described below, to determine the orientation of the head. The macula consists of three layers; the bottom layer is made of sensory hair cells which are embedded in the bottom of a gelatinous layer.
Each hair cells consists of 40 to 70 stereocilia and a kinocilium, which lies in the middle of the stereocilia and is the most important receptor. On top of this layer lie calcium carbonate crystals called otoconia; the otoliths are heavy, providing weight to the membrane as well as inertia. This allows for a greater sense of motion. Labyrinthine activity responsible for the nystagmus induced by off-vertical axis rotation arises in the otolith organs and couples to the oculomotor system through the velocity storage mechanism; that portion, lodged in the recess forms a pouch or cul-de-sac, the floor and anterior wall of which are thickened and form the macula acustica utriculi, which receives the utricular filaments of the acoustic nerve. The cavity of the utricle communicates behind with the semicircular ducts by five orifices; the ductus utriculosaccularis comes off of the anterior wall of the utricle and opens into the ductus endolymphaticus. The utricle contains mechanoreceptors called hair cells that distinguish between degrees of tilting of the head, thanks to their apical stereocilia set-up.
These are covered by otoliths which, due to gravity, tilt them. Depending on whether the tilt is in the direction of the kinocilium or not, the resulting hair cell polarisation is excitatory or inhibitory, respectively. Any orientation of the head causes a combination of stimulation to the utricles and saccules of the two ears; the brain interprets head orientation by comparing these inputs to each other and to other input from the eyes and stretch receptors in the neck, thereby detecting whether only the head is tilted or the entire body is tipping. The inertia of the otolithic membranes is important in detecting linear acceleration. Suppose you are sitting in a car at a stoplight and begin to move; the otolithic membrane of the macula utriculi lags behind the rest of the tissues, bends the stereocilia backward, stimulates the cells. When you stop at the next light, the macula stops but the otolithic membrane keeps going for a moment, bending the stereocilia forward; the hair cells convert this pattern of stimulation to nerve signals, the brain is thus advised of changes in your linear velocity.
This signal to the vestibular nerve does not adapt with time. The effect of this is that, for example, an individual lying down to sleep will continue to detect that they are lying down hours when they awaken; this article incorporates text in the public domain from page 1051 of the 20th edition of Gray's Anatomy Diagram at ipfw.edu
Mammals are vertebrate animals constituting the class Mammalia, characterized by the presence of mammary glands which in females produce milk for feeding their young, a neocortex, fur or hair, three middle ear bones. These characteristics distinguish them from reptiles and birds, from which they diverged in the late Triassic, 201–227 million years ago. There are around 5,450 species of mammals; the largest orders are the rodents and Soricomorpha. The next three are the Primates, the Cetartiodactyla, the Carnivora. In cladistics, which reflect evolution, mammals are classified as endothermic amniotes, they are the only living Synapsida. The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period around 300 million years ago, this group diverged from the sauropsid line that led to today's reptiles and birds; the line following the stem group Sphenacodontia split off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals in the early Mesozoic era.
The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, have been among the dominant terrestrial animal groups from 66 million years ago to the present. The basic body type is quadruped, most mammals use their four extremities for terrestrial locomotion. Mammals range in size from the 30–40 mm bumblebee bat to the 30-meter blue whale—the largest animal on the planet. Maximum lifespan varies from two years for the shrew to 211 years for the bowhead whale. All modern mammals give birth to live young, except the five species of monotremes, which are egg-laying mammals; the most species-rich group of mammals, the cohort called placentals, have a placenta, which enables the feeding of the fetus during gestation. Most mammals are intelligent, with some possessing large brains, self-awareness, tool use. Mammals can communicate and vocalize in several different ways, including the production of ultrasound, scent-marking, alarm signals and echolocation.
Mammals can organize themselves into fission-fusion societies and hierarchies—but can be solitary and territorial. Most mammals are polygynous. Domestication of many types of mammals by humans played a major role in the Neolithic revolution, resulted in farming replacing hunting and gathering as the primary source of food for humans; this led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, the development of the first civilizations. Domesticated mammals provided, continue to provide, power for transport and agriculture, as well as food and leather. Mammals are hunted and raced for sport, are used as model organisms in science. Mammals have been depicted in art since Palaeolithic times, appear in literature, film and religion. Decline in numbers and extinction of many mammals is driven by human poaching and habitat destruction deforestation. Mammal classification has been through several iterations since Carl Linnaeus defined the class.
No classification system is universally accepted. George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" provides systematics of mammal origins and relationships that were universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself through the new concept of cladistics. Though field work made Simpson's classification outdated, it remains the closest thing to an official classification of mammals. Most mammals, including the six most species-rich orders, belong to the placental group; the three largest orders in numbers of species are Rodentia: mice, porcupines, beavers and other gnawing mammals. The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes and lemurs. According to Mammal Species of the World, 5,416 species were identified in 2006.
These were grouped into 153 families and 29 orders. In 2008, the International Union for Conservation of Nature completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. According to a research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495 species included 96 extinct; the word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma. In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes and therian m