The Sarcopterygii or lobe-finned fish —sometimes considered synonymous with Crossopterygii —constitute a clade of the bony fish, though a strict cladistic view includes the terrestrial vertebrates. The living sarcopterygians include six species of lungfish. Early lobe-finned fishes are bony fish with fleshy, paired fins, which are joined to the body by a single bone; the fins of lobe-finned fishes differ from those of all other fish in that each is borne on a fleshy, scaly stalk extending from the body. The scales of sarcopterygians are true scaloids, consisting of lamellar bone surrounded by layers of vascular bone, dentine-like cosmine, external keratin; the morphology of tetrapodomorphs, fish that are similar-looking to tetrapods, give indications of the transition from water to terrestrial life. Pectoral and pelvic fins have articulations resembling those of tetrapod limbs; these fins evolved into the legs of the first tetrapod land vertebrates, amphibians. They possess two dorsal fins with separate bases, as opposed to the single dorsal fin of actinopterygians.
The braincase of sarcopterygians primitively has a hinge line, but this is lost in tetrapods and lungfish. Many early sarcopterygians have a symmetrical tail. All sarcopterygians possess teeth covered with true enamel. Most species of lobe-finned fishes are extinct; the largest known lobe-finned fish was Rhizodus hibberti from the Carboniferous period of Scotland which may have exceeded 7 meters in length. Among the two groups of extant species, the coelacanths and the lungfishes, the largest species is the West Indian Ocean coelacanth, reaching 2 m in length and weighing up 110 kg; the largest lungfish is the African lungfish which can weigh up to 50 kg. Taxonomists who subscribe to the cladistic approach include the grouping Tetrapoda within this group, which in turn consists of all species of four-limbed vertebrates; the fin-limbs of lobe-finned fishes such as the coelacanths show a strong similarity to the expected ancestral form of tetrapod limbs. The lobe-finned fishes followed two different lines of development and are accordingly separated into two subclasses, the Rhipidistia and the Actinistia.
The classification below follows Benton 2004, uses a synthesis of rank-based Linnaean taxonomy and reflects evolutionary relationships. Benton included the Superclass Tetrapoda in the Subclass Sarcopterygii in order to reflect the direct descent of tetrapods from lobe-finned fish, despite the former being assigned a higher taxonomic rank. Subclass Sarcopterygii †Order Onychodontida Order Actinistia Infraclass Dipnomorpha †Order Porolepiformes Subclass Dipnoi Order Ceratodontiformes Order Lepidosireniformes Infraclass Tetrapodomorpha †Order Rhizodontida Superorder Osteolepidida †Order Osteolepiformes †Family Tristichopteridae †Order Panderichthyida Superclass Tetrapoda The cladogram presented below is based on studies compiled by Philippe Janvier and others for the Tree of Life Web Project, Mikko's Phylogeny Archive and Swartz 2012. Sarcopterygii incertae sedis †Guiyu oneiros Zhu et al. 2009 †Diabolepis speratus †Langdenia campylognatha Janvier & Phuong, 1999 †Ligulalepis Schultze, 1968 †Meemannia eos Zhu, Yu, Zhao & Jia, 2006 †Psarolepis romeri Yu 1998 sensu Zhu, Yu, Zhao & Jia, 2006 †Megamastax ambylodus Choo, Zhao, Jia, & Zhu, 2014 †Sparalepis tingi Choo,Zhu,Qu,Yu,Jia & Zhaoh, 2017 Paraphyletic Osteolepida incertae sedis: †Bogdanovia orientalis Obrucheva 1955 †Canningius groenlandicus Säve-Söderbergh, 1937 †Chrysolepis †Geiserolepis †Latvius †L. grewingki †L. porosus Jarvik, 1948 †L. obrutus Vorobyeva, 1977 †Lohsania utahensis Vaughn, 1962 †Megadonichthys kurikae Vorobyeva, 1962 †Platyethmoidia antarctica Young, Long & Ritchie, 1992 †Shirolepis ananjevi Vorobeva, 1977 †Sterropterygion brandei Thomson, 1972 †Thaumatolepis edelsteini Obruchev, 1941 †Thysanolepis micans Vorobyeva, 1977 †Vorobjevaia dolonodon Young, Long & Ritchie, 1992 Paraphyletic Elpistostegalia/Panderichthyida incertae sedis †Parapanderichthys stolbovi Vorobyeva, 1992 †Howittichthys warrenae Long & Holland, 2008 †Livoniana multidentata Ahlberg, Luksevic & Mark-Kurik, 2000 Stegocephalia incertae sedis †Antlerpeton clarkii Thomson, Shubin & Poole, 1998 †Austrobrachyops jenseni Colbert & Cosgriff, 1974 †Broilisaurus raniceps Kuhn, 1938 †Densignathus rowei Daeschler, 2000 †Doragnathus woodi Smithson, 1980 †Jakubsonia livnensis Lebedev, 2004 †Limnerpeton dubium Fritsch, 1901 †Limnosceloides Romer, 1952 †L. dunkardensis Romer, 1952 †L. brahycoles Langston, 1966 †Occidens portlocki Clack & Ahlberg, 2004 †Ossinodus puerorum emend Warren & Turner, 2004 †Romeriscus periallus Baird & Carroll, 1968 †Sigournea multidentata Bolt & Lombard, 2006 †Sinostega pani Zhu et al. 2002 †Ymeria denticulata Clack et al. 2012 Lobe-finned fishes and their relatives the ray-finned fishes comprise the superclass of bony fishes characterized by their bony skeleton rather than cartilage.
There are otherwise vast differences in fin and circulatory structures between the Sarcopterygii and the Actinopterygii, such as the presence of cosmoid layers in the scales of sarcopterygians. The earliest fossils of sarcopterygians, found in the uppermost Silurian resembled the
Golden moles are small, insectivorous burrowing mammals endemic to Southern Africa, where their Afrikaans names are gouemolle or kruipmolle. They comprise the family Chrysochloridae and as such they are taxonomically distinct from the true moles, family Talpidae, other mole-like families, all of which, to various degrees, they resemble as a result of evolutionary convergence. Like most burrowing mammals with similar habits, the Chrysochloridae have short legs with powerful digging claws dense fur that repels dirt and moisture, toughened skin on the head, their eyes are covered with furred skin. The external ears are just tiny openings. In particular, golden moles bear a remarkable resemblance to the marsupial moles of Australia, family Notoryctidae, which they resemble so suggestively that at one time, the marsupial/placental divide not withstanding, some argued that they were related. Considerations that influenced the debate might have included the view that the Chrysochloridae are primitive placentals and the fact that they have many mole-like specializations similar to specializations in marsupial moles.
The rhinarium is a enlarged, dry leathery pad that protects their nostrils while the animal digs. In this respect too, they resemble the marsupial moles; some authors claim their primary sense is of touch, they are sensitive to vibrations which may indicate approaching danger. Note below however, the observations on the malleus in the middle ear; the species range in size from about 8 to about 20 cm. They have muscular shoulders and the forelimbs are radically adapted for digging; the fifth digit is absent and the first and fourth digits are vestigial. The adaptations of the hind feet are less dramatic, they retain all five toes and are webbed as an adaptation to efficient backward shovelling of soil loosened by the front claws. At one time the Chrysochloridae were regarded as primitive. Supporting arguments included: that they were thought to have originated in Gondwana, that they had a low resting metabolic rate, they could switch off thermoregulation when inactive. Like the tenrecs, they possess a cloaca, males lack a scrotum.
However, such points are no longer regarded as suggestive of golden moles as undeveloped "reptilian mammals". Going into a torpor when resting or during cold weather enables them to conserve energy and reduce urgent requirements for food, they have developed efficient kidneys and most species do not need to drink water at all. Most species of Chrysochloridae live exclusively underground in their preferred environments, beneath either grassveld, swamps, deserts, or mountainous terrain. However, Chrysospalax species tend to forage above ground in meadows. Eremitalpa species such as Grant's golden mole live in the sandy Namib desert, where they cannot form tunnels because the sand collapses. Instead during the day, when they must seek shelter, they swim through the loose sand, using their broad claws to paddle, dive down some 50 cm to where it is bearably cool. There they enter a state of torpor, thus conserving energy. At night they emerge to forage on the surface rather than wasting energy shifting sand.
Their main prey are termites that live under isolated grass clumps, they might travel for 6 kilometres a night in search of food. They seek promising clumps by listening for wind-rustled grass-root stresses and termites' head-banging alarm signals, neither of which can be heard above ground, so they stop periodically and dip their heads under the sand to listen. Most other species construct both foraging superficial burrows and deeper permanent burrows for residence. Residential burrows are complex in form, may penetrate as far as a metre below ground and include deep chambers for use as bolt-holes, other chambers as latrines, they compact it into the tunnel walls. They feed on small vertebrates such as lizards or burrowing snakes, they depend on their sense of hearing to locate much of their prey, the cochleas of a number of golden mole species have been found to be long and coiled, which may indicate a greater ecological dependence on low frequency auditory cues than we see in Talpid moles.
Some species have hypertrophied middle ear ossicles, in particular the malleus, adapted towards the detection of seismic vibrations. In this respect there is some apparent convergent evolution to burrowing reptiles in the family Amphisbaenidae. Females give birth to one to three hairless young in a grass-lined nest within the burrow system. Breeding occurs throughout the year; the adults are solitary, their burrowing territory may be aggressively defended from intruders where resources are scarce. Of the 21 species of golden mole, no fewer than 11 are threatened with extinction; the primary causes are sand mining, poor agricultural practices, increasing urbanisation, predation by domestic cats and dogs. The taxonomy of the Chrysochloridae is undergoing a review in the light of new genetic information, they have traditionally been listed with the shrews, hedgehogs and a grab-bag of small, difficult-to-place creatures as part of the order Insectivora. Some authorities retain this classification, at least for the time being.
Others group the golden moles with the tenrecs in a new order, sometimes known as Tenrecomorpha, while others cal
Bird anatomy, or the physiological structure of birds' bodies, shows many unique adaptations aiding flight. Birds have a light skeletal system and light but powerful musculature which, along with circulatory and respiratory systems capable of high metabolic rates and oxygen supply, permit the bird to fly; the development of a beak has led to evolution of a specially adapted digestive system. These anatomical specializations have earned birds their own class in the vertebrate phylum. Birds have many bones that are hollow with criss-crossing trusses for structural strength; the number of hollow bones varies among species, though large gliding and soaring birds tend to have the most. Respiratory air sacs form air pockets within the semi-hollow bones of the bird's skeleton; the bones of diving birds are less hollow than those of non-diving species. Penguins and puffins are without pneumatized bones entirely. Flightless birds, such as ostriches and emus, have pneumatized femurs and, in the case of the emu, pneumatized cervical vertebrae.
The bird skeleton is adapted for flight. It is lightweight but strong enough to withstand the stresses of taking off and landing. One key adaptation is the fusing of bones such as the pygostyle; because of this, birds have a smaller number of bones than other terrestrial vertebrates. Birds lack teeth or a true jaw, instead have a beak, far more lightweight; the beaks of many baby birds have a projection called an egg tooth, which facilitates their exit from the amniotic egg, which falls off once it has done its job. The vertebral column is divided into five sections of vertebrae: Cervical Trunk vertebrae fused in the notarium. Synsacrum; this region is similar to the sacrum in mammals and is unique in the pigeon because it is a fusion of the sacral and caudal vertebra. It supports terrestrial locomotion of the pigeon's legs. Caudal: This region is similar to the coccyx in mammals and helps control the movement of feathers during flight. Pygostyle: This region is made up of 4 to 7 fused vertebrae and is the point of feather attachment.
The neck of a bird is composed of 13–25 cervical vertebrae enabling birds to have increased flexibility. A flexible neck allows many birds with immobile eyes to move their head more productively and center their sight on objects that are close or far in distance. Most birds have about three times as many neck vertebrae than humans, which allows for increased stability during fast movements such as flying and taking-off; the neck plays a role in head-bobbing, present in at least 8 out of 27 orders of birds, including Columbiformes and Gruiformes. Head-bobbing is an optokinetic response which stabilizes a birds surroundings as they alternate between a thrust phase and a hold phase. Head-bobbing is synchronous with the feet. Data from various studies suggest that the main reason for head-bobbing in some birds is for the stabilization of their surroundings, although it is uncertain why some but not all bird orders show head-bob. Birds are the only vertebrates to have fused a keeled breastbone; the keeled sternum serves as an attachment site for the muscles used in swimming.
Flightless birds, such as ostriches, lack a keeled sternum and have denser and heavier bones compared to birds that fly. Swimming birds have a wide sternum, walking birds have a long sternum, flying birds have a sternum, nearly equal in width and height; the chest consists of the furcula and coracoid, together with the scapula, form the pectoral girdle. The side of the chest is formed by the ribs. Birds have uncinate processes on the ribs; these are hooked extensions of bone which help to strengthen the rib cage by overlapping with the rib behind them. This feature is found in the tuatara; the skull consists of five major bones: the frontal, parietal and nasal, the mandible. The skull of a normal bird weighs about 1% of the bird's total body weight; the eye occupies a considerable amount of the skull and is surrounded by a sclerotic eye-ring, a ring of tiny bones. This characteristic is seen in reptiles. Broadly speaking, avian skulls consist of many non-overlapping bones. Paedomorphosis, maintenance of the ancestral state in adults, is thought to have facilitated the evolution of the avian skull.
In essence, adult bird skulls will resemble the juvenile form of their theropod dinosaur ancestors. As the avian lineage has progressed and has paedomorphosis has occurred, they have lost the postorbital bone behind the eye, the ectopterygoid at the back of the palate, teeth; the palate structures have become altered with changes reductions, seen in the ptyergoid and jugal bones. A reduction in the adductor chambers has occurred These are all conditions seen in the juvenile form of their ancestors; the premaxillary bone has hypertrophied to form the beak while the maxilla has become diminished, as suggested by both developmental and paleontological studies. This expansion into the beak has occurred in tandem with the loss of a functional hand and the developmental of a point at the front of the beak that resembles a "finger"; the premaxilla is known to play a large role in feeding behaviours in fish. The structure of the avian skull has important implications for their feeding behaviours. Birds show independen
Kiwi or kiwis are flightless birds native to New Zealand, in the genus Apteryx and family Apterygidae. The size of a domestic chicken, kiwi are by far the smallest living ratites. DNA sequence comparisons have yielded the surprising conclusion that kiwi are much more related to the extinct Malagasy elephant birds than to the moa with which they shared New Zealand. There are five recognised species, four of which are listed as vulnerable, one of, near-threatened. All species have been negatively affected by historic deforestation but the remaining large areas of their forest habitat are well protected in reserves and national parks. At present, the greatest threat to their survival is predation by invasive mammalian predators; the kiwi's egg is one of the largest in proportion to body size of any species of bird in the world. Other unique adaptations of kiwi, such as their hairlike feathers and stout legs, using their nostrils at the end of their long beak to detect prey before they see it, have helped the bird to become internationally well-known.
The kiwi is recognised as an icon of New Zealand, the association is so strong that the term Kiwi is used internationally as the colloquial demonym for New Zealanders. The Māori language word kiwi is accepted to be "of imitative origin" from the call. However, some linguists derive the word from Proto-Nuclear Polynesian *kiwi, which refers to Numenius tahitiensis, the bristle-thighed curlew, a migratory bird that winters in the tropical Pacific islands. With its long decurved bill and brown body, the curlew resembles the kiwi. So when the first Polynesian settlers arrived, they may have applied the word kiwi to the new-found bird; the genus name Apteryx is derived from Ancient Greek "without wing": a-, "without" or "not". The name is uncapitalised, with the plural either the anglicised "kiwis" or, consistent with the Māori language, appearing as "kiwi" without an "-s". Although it was long presumed that the kiwi was related to the other New Zealand ratites, the moa, recent DNA studies have identified its closest relative as the extinct elephant bird of Madagascar, among extant ratites, the kiwi is more related to the emu and the cassowaries than to the moa.
Research published in 2013 on an extinct genus, known from the Miocene deposits of the Saint Bathans Fauna, found that it was smaller and capable of flight, supporting the hypothesis that the ancestor of the kiwi reached New Zealand independently from moas, which were large and flightless by the time kiwi appeared. There are five known species of kiwi, as well as a number of subspecies. Relationships in the genus Apteryx Their adaptation to a terrestrial life is extensive: like all the other ratites, they have no keel on the sternum to anchor wing muscles; the vestigial wings are so small that they are invisible under the bristly, hair-like, two-branched feathers. While most adult birds have bones with hollow insides to minimise weight and make flight practicable, kiwi have marrow, like mammals and the young of other birds. With no constraints on weight due to flight requirements, brown kiwi females carry and lay a single egg that may weigh as much as 450 g. Like most other ratites, they have no uropygial gland.
Their bill is long and sensitive to touch, their eyes have a reduced pecten. Their feathers lack barbules and aftershafts, they have large vibrissae around the gape, they have no tail and a small pygostyle. Their gizzard is weak and their caecum is long and narrow; the eye of the kiwi is the smallest relative to body mass in all avian species resulting in the smallest visual field as well. The eye has small specialisations for a nocturnal lifestyle, but kiwi rely more on their other senses; the sight of the kiwi is so underdeveloped that blind specimens have been observed in nature, showing how little they rely on sight for survival and foraging. In an experiment, it was observed that one-third of a population of A. rowi in New Zealand under no environmental stress had ocular lesions in one or both eyes. The same experiment examined three specific specimens that showed complete blindness and found them to be in good physical standing outside of ocular abnormalities. A 2018 study revealed that the kiwi's closest relatives, the extinct elephant birds shared this trait despite their massive size.
Unlike every other palaeognath, which are small-brained by bird standards, kiwi have proportionally large encephalisation quotients. Hemisphere proportions are similar to those of parrots and songbirds, though there is no evidence of complex behaviour. Before the arrival of humans in the 13th century or earlier, New Zealand's only endemic mammals were three species of bat, the ecological niches that in other parts of the world were filled by creatures as diverse as horses and mice were taken up by birds; the kiwi's nocturnal habits may be a result of habitat intrusion by predators, including humans. In areas of New Zealand where introduced predators have been removed, such as sanctuaries, kiwi are seen in daylight, they prefer subtropical and temperate podocarp and beech forests, but they are being forced to adapt to different habitat, such as sub-alpine scrub, tussock grassland, the mountains. Kiwi have a developed sense of smell, unusual in a bird, are the only birds with nostrils at the end of their long beaks.
Kiwi eat small invertebrates, seeds
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
Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia and the Americas. A distinctive characteristic common to these species is that most of the young are carried in a pouch. Well-known marsupials include kangaroos, koalas, opossums and Tasmanian devils; some lesser-known marsupials are the dunnarts and cuscuses. Marsupials represent the clade originating from the last common ancestor of extant metatherians. Like other mammals in the Metatheria, they give birth to undeveloped young that reside in a pouch located on their mothers’ abdomen for a certain amount of time. Close to 70% of the 334 extant species occur on the Australian continent; the remaining 100 are found in the Americas — in South America, but thirteen in Central America, one in North America, north of Mexico. The word marsupial comes from the technical term for the abdominal pouch. It, in turn, is borrowed from Latin and from the ancient Greek μάρσιππος mársippos, meaning "pouch".
Marsupials are taxonomically identified as members of the mammalian infraclass Marsupialia, first described as a family under the order Pollicata by German zoologist Johann Karl Wilhelm Illiger in his 1811 work Prodromus Systematis Mammalium et Avium. However, James Rennie, author of The Natural History of Monkeys and Lemurs, pointed out that the placement of five different groups of mammals - monkeys, tarsiers, aye-ayes and marsupials - under a single order did not appear to have a strong justification. In 1816, French zoologist George Cuvier classified all marsupials under the order Marsupialia. In 1997, researcher J. A. W. Kirsch and others accorded infraclass rank to Marsupialia. There are two primary divisions: Australian marsupials. Marsupialia is further divided as follows:† - Extinct Superorder Ameridelphia Order Didelphimorphia Family Didelphidae: opossums Order Paucituberculata Family Caenolestidae: shrew opossums Superorder Australidelphia Order Microbiotheria Family Microbiotheriidae: monito del monte Order †Yalkaparidontia Order Dasyuromorphia Family †Thylacinidae: thylacine Family Dasyuridae: antechinuses, dunnarts, Tasmanian devil, relatives Family Myrmecobiidae: numbat Order Notoryctemorphia Family Notoryctidae: marsupial moles Order Peramelemorphia Family Thylacomyidae: bilbies Family †Chaeropodidae: pig-footed bandicoots Family Peramelidae: bandicoots and allies Order Diprotodontia Suborder Vombatiformes Family Vombatidae: wombats Family Phascolarctidae: koalas Family †Diprotodontidae: Giant wombats Family †Palorchestidae: Marsupial tapirs Family †Thylacoleonidae: marsupial lions Suborder Phalangeriformes Family Acrobatidae: feathertail glider and feather-tailed possum Family Burramyidae: pygmy possums Family †Ektopodontidae: sprite possums Family Petauridae: striped possum, Leadbeater's possum, yellow-bellied glider, sugar glider, mahogany glider, squirrel glider Family Phalangeridae: brushtail possums and cuscuses Family Pseudocheiridae: ringtailed possums and relatives Family Tarsipedidae: honey possum Suborder Macropodiformes Family Macropodidae: kangaroos and relatives Family Potoroidae: potoroos, rat kangaroos, bettongs Family Hypsiprymnodontidae: musky rat-kangaroo Comprising over 300 extant species, several attempts have been made to interpret the phylogenetic relationships among the different marsupial orders.
Studies differ on whether Didelphimorphia or Paucituberculata is the sister group to all other marsupials. Though the order Microbiotheria is found in South America, morphological similarities suggest it is related to Australian marsupials. Molecular analyses in 2010 and 2011 identified Microbiotheria as the sister group to all Australian marsupials. However, the relations among the four Australidelphid orders are not as well understood; the cladogram below, depicting the relationships among the various marsupial orders, is based on a 2015 phylogenetic study. DNA evidence supports a South American origin for marsupials, with Australian marsupials arising from a single Gondwanan migration of marsupials from South America to Australia. There are many small arboreal species in each group; the term "opossum" is used to refer to American species, while similar Australian species are properly called "possums". Marsupials have the typical characteristics of mammals—e.g. Mammary glands, three middle ear bones, true hair.
There are, striking differences as well as a number of anatomical features that separate them from Eutherians. In addition to the front pouch, which contains multiple nipples for the protection and sustenance of their young, marsupials have other common structural features. Ossified patellae are absent in most modern marsupials and epipubic bones are present. Marsupials lack a gross communication between the right and left brain hemispheres; the skull has peculiarities in comparison to placental mammals. In general, the skull is small and tight. Holes are located in the front of the orbit; the cheekbone extends further to the rear. The angular extension of the lower jaw is bent toward the center. Another feature is the hard palate which, in contrast to the placental mammals' foramina, always have more openings. The
Placentalia is one of the three extant subdivisions of the class of animals Mammalia. The Placentals are distinguishable from other mammals in that the fetus is carried in the uterus of its mother to a late stage of development, it is somewhat of a misnomer since marsupials nourish their fetuses via a placenta. Placental mammals are anatomically distinguished from other mammals by: a sufficiently wide opening at the bottom of the pelvis to allow the birth of a large baby relative to the size of the mother; the absence of epipubic bones extending forward from the pelvis, which are found in all other mammals. The rearmost bones of the foot fit into a socket formed by the ends of the tibia and fibula, forming a complete mortise and tenon upper ankle joint; the presence of a malleolus at the bottom of the fibula. Analysis of retroposon presence/absence patterns has provided a rapid, unequivocal means for revealing the evolutionary history of organisms: this has resulted in a revision in the classification of placentals.
There are now thought to be three major subdivisions or lineages of placental mammals: Boreoeutheria and Afrotheria, all of which diverged from common ancestors. The orders of placental mammals in the three groups are: Magnorder Afrotheria Superorder Afroinsectiphilia Order Afrosoricida Order Macroscelidea Order Tubulidentata Superorder Paenungulata Order Hyracoidea Mirorder Tethytheria Order Proboscidea Order Sirenia Magnorder Boreoeutheria Superorder Euarchontoglires Grandorder Gliriformes Mirorder Glires Order Lagomorpha Order Rodentia Grandorder Euarchonta Order Scandentia Mirorder Primatomorpha Order Dermoptera Order Primates Superorder Laurasiatheria Order Eulipotyphla Order Chiroptera Order Cetartiodactyla Order Perissodactyla Mirorder Ferae Order Pholidota Order Carnivora Magnorder Xenarthra Order Cingulata Order Pilosa The exact relationships among these three lineages is a subject of debate, three different hypotheses have been proposed with respect to which group is basal or diverged first from other placentals.
These hypotheses are Atlantogenata and Exafroplacentalia. Estimates for the divergence times among these three placental groups range from 105 to 120 million years ago, depending on the type of DNA and varying interpretations of paleogeographic data. Cladogram based on Amrine-Madsen, H. et al. and Asher, R. J. et al. True placental mammals arose from stem-group members of the clade Eutheria, which had existed since at least the Middle Jurassic period, about 170 MYA); these early eutherians were nocturnal insect eaters, with adaptations for life in trees. True placentals may have originated in the Late Cretaceous around 90 MYA, but the earliest undisputed fossils are from the early Paleocene, 66 MYA, following the Cretaceous–Paleogene extinction event; the species Protungulatum donnae was thought to be a stem-ungulate known 1 meter above the Cretaceous-Paleogene boundary in the geological stratum that marks the Cretaceous–Paleogene extinction event and Purgatorius considered a stem-primate, appears no more than 300,000 years after the K-Pg boundary.
The rapid appearance of placentals after the mass extinction at the end of the Cretaceous suggests that the group had originated and undergone an initial diversification in the Late Cretaceous, as suggested by molecular clocks. The lineages leading to Xenarthra and Afrotheria originated around 90 MYA, Boreoeutheria underwent an initial diversification around 70-80 MYA, producing the lineages that would lead to modern primates, insectivores and carnivorans. However, modern members of the placental orders originated in the Paleogene around 66 to 23 MYA, following the Cretaceous–Paleogene extinction event; the evolution of crown orders such modern primates and carnivores appears to be part of an adaptive radiation that took place as mammals evolved to take advantage of ecological niches that were left open when most dinosaurs and other animals disappeared following the Chicxulub asteroid impact. As they occupied new niches, mammals increased in body size, began to take over the large herbivore and large carnivore niches, left open by the decimation of the dinosaurs.
Mammals exploited niches that the dinosaurs had never touched: for example, bats evolved flight and echolocation, allowing them to be effective nocturnal, aerial insectivores.