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60-530: The cloacal region is also often associated with a secretory organ, the cloacal gland, which has been implicated in the scent-marking behavior of some reptiles, marsupials, amphibians, and monotremes . The word is from the Latin verb cluo , "(I) cleanse", thus the noun cloaca , " sewer , drain". Birds reproduce using their cloaca; this occurs during a cloacal kiss in most birds. Birds that mate using this method touch their cloacae together, in some species for only

120-488: A clade termed Australosphenida , a group of mammals from the Jurassic and Cretaceous of Madagascar, South America and Australia, that share tribosphenic molars . However, in a 2022 review of monotreme evolution, it was noted that Teinolophos , the oldest ( Barremian ~ 125 million years ago) and the most primitive monotreme differed substantially from non-monotreme australosphenidans in having five molars as opposed to

180-423: A separate genital tract , whereas most placental mammalian females have separate openings for reproduction (the vagina ), urination (the urethra ), and defecation (the anus ). In monotremes, only semen passes through the penis while urine is excreted through the male's cloaca. The monotreme penis is similar to that of turtles and is covered by a preputial sac. Monotreme eggs are retained for some time within

240-506: A "tribosphenic" form of molars (with the occlusal surface formed by three cusps arranged in a triangle), which is one of the hallmarks of extant mammals. Some recent work suggests that monotremes acquired this form of molar independently of placental mammals and marsupials, although this hypothesis remains disputed. Tooth loss in modern monotremes might be related to their development of electrolocation . Monotreme jaws are constructed somewhat differently from those of other mammals, and

300-518: A constant body temperature in a variety of circumstances, such as the platypus in icy mountain streams. Early researchers were misled by two factors: firstly, monotremes maintain a lower average temperature than most mammals; secondly, the short-beaked echidna , much easier to study than the reclusive platypus, maintains normal temperature only when active; during cold weather, it conserves energy by "switching off" its temperature regulation. Understanding of this mechanism came when reduced thermal regulation

360-501: A few seconds, sufficient time for sperm to be transferred from the male to the female. For palaeognaths and waterfowl , the males do not use the cloaca for reproduction, but have a phallus . One study has looked into birds that use their cloaca for cooling. Among falconers , the word vent is also a verb meaning "to defecate". Among fish, a true cloaca is present only in elasmobranchs (sharks and rays) and lobe-finned fishes . In lampreys and in some ray-finned fishes , part of

420-473: A reptile-like gait, with legs on the sides of, rather than underneath, their bodies. The monotreme leg bears a spur in the ankle region; the spur is not functional in echidnas, but contains a powerful venom in the male platypus. This venom is derived from β-defensins , proteins that are present in mammals that create holes in viral and bacterial pathogens. Some reptile venom is also composed of different types of β-defensins, another trait shared with reptiles. It

480-475: A substantial reduction at the elevated temperature of 28 °C (82 °F). Monotreme milk contains a highly expressed antibacterial protein not found in other mammals, perhaps to compensate for the more septic manner of milk intake associated with the absence of teats. During the course of evolution the monotremes have lost the gastric glands normally found in mammalian stomachs as an adaptation to their diet. Monotremes synthesize L- ascorbic acid only in

540-512: A true cloaca. In marsupials , the genital tract is separate from the anus, but a trace of the original cloaca does remain externally. This is one of the features of marsupials (and monotremes) that suggest their basal nature, as the amniotes from which mammals evolved had a cloaca, and probably so did the earliest mammals . Unlike other marsupials, marsupial moles have a true cloaca. This fact has been used to argue that they are not marsupials. Most adult placental mammals have no cloaca. In

600-479: Is a similar pattern of tooth replacement seen in monotremes and marsupials, which originally provided the basis for the competing " Marsupionta " hypothesis in which the divergence between monotremes and marsupials happened later than the divergence between these lineages and the placental mammals. Van Rheede (2005) concluded that the genetic evidence favors the Theria hypothesis, and this hypothesis continues to be

660-645: Is disputed and their taxonomy is under debate. All extant species of monotremes are indigenous to Australia and New Guinea , although they were also present during the Late Cretaceous and Paleocene epochs in southern South America, implying that they were also present in Antarctica, though remains have not yet been found there. The name monotreme derives from the Greek words μονός ( monós 'single') and τρῆμα ( trêma 'hole'), referring to

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720-508: Is disputed. Nonetheless, findings on the extinct species Teinolophos confirm that suspended ear bones evolved independently among monotremes and therians. The external opening of the ear still lies at the base of the jaw. The sequencing of the platypus genome has also provided insight into the evolution of a number of monotreme traits, such as venom and electroreception , as well as showing some new unique features, such as monotremes possessing 5 pairs of sex chromosomes and that one of

780-572: Is primarily driven by human poaching and habitat destruction , primarily deforestation . Over 70% of mammal species are in the orders Rodentia , Chiroptera , and Eulipotyphla . Mammal classification has been through several revisions since Carl Linnaeus initially defined the class, and at present , no classification system is universally accepted. McKenna & Bell (1997) and Wilson & Reeder (2005) provide useful recent compendiums. Simpson (1945) provides systematics of mammal origins and relationships that had been taught universally until

840-426: Is the viviparous placental mammals , so named for the temporary organ ( placenta ) used by offspring to draw nutrition from the mother during gestation . Most mammals are intelligent , with some possessing large brains, self-awareness , and tool use . Mammals can communicate and vocalize in several ways, including the production of ultrasound , scent marking , alarm signals , singing , echolocation ; and, in

900-539: Is the oldest known platypus-like fossil. The durophagous Kollikodon , the pseudotribosphenic Steropodon , and Stirtodon , Dharragarra , Opalios , and Parvopalus occur in the same Cenomanian deposits. Oligo-Miocene fossils of the toothed platypus Obdurodon have also been recovered from Australia, and fossils of a 63 million-year old platypus relative occur in southern Argentina ( Monotrematum ), see fossil monotremes below. The extant platypus genus Ornithorhynchus in also known from Pliocene deposits, and

960-609: Is the opposite of that hypothesized for Australia's other dominant mammal group, the marsupials , which likely migrated across Antarctica to Australia from South America. In 2024, a prominent assemblage of early monotremes was described from the Cenomanian deposits (100–96.6 Ma) of the Griman Creek Formation in Lightning Ridge, New South Wales. One of these, the fossil jaw fragment of Dharragarra ,

1020-468: Is the primary communication route between the right and left brain hemispheres. The anterior commissure does provide an alternate communication route between the two hemispheres, though, and in monotremes and marsupials it carries all the commissural fibers arising from the neocortex , whereas in placental mammals the anterior commissure carries only some of these fibers. Extant monotremes lack teeth as adults. Fossil forms and modern platypus young have

1080-431: Is thought to be an ancient mammalian characteristic, as many non-monotreme archaic mammal groups also possess venomous spurs . The key anatomical difference between monotremes and other mammals gives them their name; monotreme means "single opening" in Greek, referring to the single duct (the cloaca ) for their urinary, defecatory, and reproductive systems. Like reptiles, monotremes have a single cloaca. Marsupials have

1140-499: Is thought to provide some insight into the most recent common ancestor of the synapsid lineage leading to mammals and the sauropsid lineage leading to birds and modern reptiles, which are believed to have split about 315 million years ago during the Carboniferous . The presence of vitellogenin genes (a protein necessary for egg yolk formation) is shared with birds; the presence of this symplesiomorphy suggests that

1200-496: Is where reproductive activity occurs. Some turtles , especially those specialized in diving, are highly reliant on cloacal respiration during dives. They accomplish this by having a pair of accessory air bladders connected to the cloaca, which can absorb oxygen from the water. Sea cucumbers use cloacal respiration. The constant flow of water through it has allowed various fish , polychaete worms and even crabs to specialize to take advantage of it while living protected inside

1260-620: The Cretaceous , indicating that monotremes were diversifiying by the early Late Cretaceous. Monotremes have been found in the latest Cretaceous and Paleocene of southern South America, so one hypothesis is that monotremes arose in Australia in the Late Jurassic or Early Cretaceous , and that some migrated across Antarctica to South America , both of which were still united with Australia at that time. This direction of migration

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1320-415: The Cretaceous . The relationships between these three lineages is contentious, and all three possible hypotheses have been proposed with respect to which group is basal . These hypotheses are Atlantogenata (basal Boreoeutheria), Epitheria (basal Xenarthra) and Exafroplacentalia (basal Afrotheria). Boreoeutheria in turn contains two major lineages— Euarchontoglires and Laurasiatheria . Estimates for

1380-564: The International Union for Conservation of Nature (IUCN) completed a five-year Global Mammal Assessment for its IUCN Red List , which counted 5,488 species. According to research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495, including 96 recently extinct. The word " mammal " is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from

1440-505: The Latin mamma ("teat, pap"). 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 ( echidnas and platypuses ) and therian mammals ( marsupials and placentals ) and all descendants of that ancestor. Since this ancestor lived in the Jurassic period, Rowe's definition excludes all animals from

1500-606: The biological classification scheme used, are the primates : apes , monkeys , and lemurs ; the Cetartiodactyla : whales and even-toed ungulates ; and the Carnivora which includes cats , dogs , weasels , bears , seals , and allies. According to Mammal Species of the World , 5,416 species were identified in 2006. These were grouped into 1,229  genera , 153  families and 29 orders. In 2008,

1560-434: The cloaca . Like other mammals, monotremes are endothermic with a high metabolic rate (though not as high as other mammals; see below); have hair on their bodies; produce milk through mammary glands to feed their young; have a single bone in their lower jaw; and have three middle-ear bones. In common with reptiles and marsupials , monotremes lack the connective structure ( corpus callosum ) which in placental mammals

1620-420: The development of the urinary and reproductive organs . However, a few human congenital disorders result in persons being born with a cloaca, including persistent cloaca and sirenomelia (mermaid syndrome). In reptiles, the cloaca consists of the urodeum , proctodeum , and coprodeum . Some species have modified cloacae for increased gas exchange (see reptile respiration and reptile reproduction ). This

1680-406: The dominant terrestrial animal group from 66 million years ago to the present. The basic mammalian body type is quadrupedal , with most mammals using four limbs for terrestrial locomotion ; but in some, the limbs are adapted for life at sea , in the air , in trees or underground . The bipeds have adapted to move using only the two lower limbs, while the rear limbs of cetaceans and

1740-428: The rodents , bats , and Eulipotyphla (including hedgehogs , moles and shrews ). The next three are the primates (including humans , monkeys and lemurs ), the even-toed ungulates (including pigs , camels , and whales ), and the Carnivora (including cats , dogs , and seals ). Mammals are the only living members of Synapsida ; this clade , together with Sauropsida (reptiles and birds), constitutes

1800-439: The sea cows are mere internal vestiges . Mammals range in size from the 30–40 millimetres (1.2–1.6 in) bumblebee bat to the 30 metres (98 ft) blue whale —possibly the largest animal to have ever lived. 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 lay eggs. The most species-rich group

1860-531: The X ;chromosomes resembles the Z ;chromosome of birds, suggesting that the two sex chromosomes of marsupial and placental mammals evolved after the split from the monotreme lineage. Additional reconstruction through shared genes in sex chromosomes supports this hypothesis of independent evolution. This feature, along with some other genetic similarities with birds, such as shared genes related to egg-laying,

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1920-515: The already eutherian Juramaia is dated to 161–160 million years ago). Teinolophos like modern monotremes displays adaptations to elongation and increased sensory perception in the jaws, related to mechanoreception or electroreception . Molecular clock and fossil dating give a wide range of dates for the split between echidnas and platypuses, with one survey putting the split at 19–48 million years ago, but another putting it at 17–89 million years ago. It has been suggested that both

1980-465: The averages of 35 °C (95 °F) for marsupials and 37 °C (99 °F) for placental mammals . Research suggests this has been a gradual adaptation to the harsh, marginal environmental niches in which the few extant monotreme species have managed to survive, rather than a general characteristic of extinct monotremes. Monotremes may have less developed thermoregulation than other mammals, but recent research shows that they easily maintain

2040-492: The case of humans, complex language . Mammals can organize themselves into fission–fusion societies , harems , and hierarchies —but can also be solitary and territorial . Most mammals are polygynous , but some can be monogamous or polyandrous . Domestication of many types of mammals by humans played a major role in the Neolithic Revolution , and resulted in farming replacing hunting and gathering as

2100-812: The clade originating with the last common ancestor of Sinoconodon and living mammals. The earliest-known synapsid satisfying Kemp's definitions is Tikitherium , dated 225 Ma , so the appearance of mammals in this broader sense can be given this Late Triassic date. However, this animal may have actually evolved during the Neogene. As of the early 21st century, molecular studies based on DNA analysis have suggested new relationships among mammal families. Most of these findings have been independently validated by retrotransposon presence/absence data . Classification systems based on molecular studies reveal three major groups or lineages of placental mammals— Afrotheria , Xenarthra and Boreoeutheria —which diverged in

2160-443: The cloaca remains in the adult to receive the urinary and reproductive ducts, although the anus always opens separately. In chimaeras and most teleosts , however, all three openings are entirely separated. With a few exceptions noted below, mammals have no cloaca. Even in the marsupials that have one, the cloaca is partially subdivided into separate regions for the anus and urethra . The monotremes (egg-laying mammals) possess

2220-444: The closest thing to an official classification of mammals, despite its known issues. 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 , rats , porcupines , beavers , capybaras , and other gnawing mammals; Chiroptera : bats; and Eulipotyphla : shrews , moles , and solenodons . The next three biggest orders, depending on

2280-520: The common ancestor of monotremes, marsupials, and placental mammals was oviparous , and that this trait was retained in monotremes but lost in all other extant mammal groups. DNA analyses suggest that although this trait is shared and is synapomorphic with birds, platypuses are still mammals and that the common ancestor of extant mammals lactated. The monotremes also have extra bones in the shoulder girdle , including an interclavicle and coracoid , which are not found in other mammals. Monotremes retain

2340-549: The cucumber. At night, many of these species emerge through the anus of the sea cucumber in search of food. Monotreme Monotremes ( / ˈ m ɒ n ə t r iː m z / ) are mammals of the order Monotremata . They are the only group of living mammals that lay eggs , rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas . Monotremes are typified by structural differences in their brains, jaws, digestive tract, reproductive tract, and other body parts, compared to

2400-503: The divergence of the monotreme lineage from the Metatheria ( marsupial ) and Eutheria ( placental ) lineages happened prior to the divergence between marsupials and placental mammals, and this explains why monotremes retain a number of primitive traits presumed to have been present in the synapsid ancestors of later mammals, such as egg-laying. Most morphological evidence supports the Theria hypothesis, but one possible exception

2460-585: The earlier Triassic , despite the fact that Triassic fossils in the Haramiyida have been referred to the Mammalia since the mid-19th century. If Mammalia is considered as the crown group, its origin can be roughly dated as the first known appearance of animals more closely related to some extant mammals than to others. Ambondro is more closely related to monotremes than to therian mammals while Amphilestes and Amphitherium are more closely related to

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2520-570: The embryo, the embryonic cloaca divides into a posterior region that becomes part of the anus, and an anterior region that develops depending on sex: in males, it forms the penile urethra , while in females, it develops into the vestibule or urogenital sinus that receives the urethra and vagina. However, some placental mammals retain a cloaca as adults: those are members of the order Afrosoricida (small mammals native to Africa) as well as some shrews . Being placental animals, humans have an embryonic cloaca which divides into separate tracts during

2580-451: The end of the 20th century. However, since 1945, a large amount of new and more detailed information has gradually been found: The paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics . Though fieldwork and lab work progressively outdated Simpson's classification, it remains

2640-424: The jaw opening muscle is different. As in all true mammals, the tiny bones that conduct sound to the inner ear are fully incorporated into the skull, rather than lying in the jaw as in non-mammal cynodonts and other premammalian synapsids ; this feature, too, is now claimed to have evolved independently in monotremes and therians , although, as with the analogous evolution of the tribosphenic molar, this hypothesis

2700-461: The kidneys. Both the platypus and echidna species have spurs on their hind limbs. The echidna spurs are vestigial and have no known function, while the platypus spurs contain venom. Molecular data show that the main component of platypus venom emerged before the divergence of platypus and echidnas, suggesting that the most recent common ancestor of these taxa was also possibly a venomous monotreme. The traditional " Theria hypothesis" states that

2760-668: The larger Amniota clade. Early synapsids are referred to as " pelycosaurs ." The more advanced therapsids became dominant during the Guadalupian . Mammals originated from cynodonts , an advanced group of therapsids, during the Late Triassic to Early Jurassic . Mammals achieved their modern diversity in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs , and have been

2820-452: The monotremes diverged from the mammalian lineage before the marsupials and placental mammals arose. The only Mesozoic monotremes are Teinolophos (Barremian, 126 Ma), Sundrius and Kryoryctes (Albian, 113–108 Ma), and Dharragarra , Kollikodon , Opalios , Parvopalus , Steropodon , and Stirtodon (all Cenomanian, 100.2–96.6 Ma) from Australian deposits, and Patagorhynchus (Maastrichtian) from Patagonian deposits in

2880-411: The more common mammalian types. Although they are different from almost all mammals in that they lay eggs, like all mammals, the female monotremes nurse their young with milk . Monotremes have been considered by some authors to be members of Australosphenida , a clade that contains extinct mammals from the Jurassic and Cretaceous of Madagascar, South America, and Australia, but this categorization

2940-433: The more widely accepted one. Monotremes are conventionally treated as comprising a single order Monotremata. The entire grouping is also traditionally placed into a subclass Prototheria , which was extended to include several fossil orders, but these are no longer seen as constituting a group allied to monotreme ancestry. A controversial hypothesis now relates the monotremes to a different assemblage of fossil mammals in

3000-533: The mother and receive nutrients directly from her, generally hatching within 10 days after being laid – much shorter than the incubation period of sauropsid eggs. Much like newborn marsupials (and perhaps all non-placental mammals ), newborn monotremes, called "puggles", are larval- and fetus-like and have relatively well-developed forelimbs that enable them to crawl around. Monotremes lack nipples , so puggles crawl about more frequently than marsupial joeys in search of milk. This difference raises questions about

3060-465: The oldest fossil tachyglossids are Pleistocene (1.7 Ma) in age. Excepting Ornithorhynchus anatinus , all the animals listed in this section are known only from fossils. Some family designations are hesitant, given the fragmentary nature of the specimens. Mammal A mammal (from Latin mamma  'breast') is a vertebrate animal of the class Mammalia ( / m ə ˈ m eɪ l i . ə / ). Mammals are characterized by

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3120-514: The ovum splits into multiple, divisible daughter cells. In contrast, monotreme zygotes, like those of birds and reptiles, undergo meroblastic (partial) division. This means that the cells at the yolk's edge have cytoplasm continuous with that of the egg, allowing the yolk and embryo to exchange waste and nutrients with the surrounding cytoplasm. Monotremes' metabolic rate is remarkably low by mammalian standards. The platypus has an average body temperature of about 31 °C (88 °F) rather than

3180-555: The presence of milk -producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair , and three middle ear bones . These characteristics distinguish them from reptiles and birds , from which their ancestors diverged in the Carboniferous Period over 300 million years ago. Around 6,400 extant species of mammals have been described and divided into 27 orders . The largest orders of mammals, by number of species , are

3240-716: 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, and ultimately the development of the first civilizations . Domesticated mammals provided, and continue to provide, power for transport and agriculture, as well as food ( meat and dairy products ), fur , and leather . Mammals are also hunted and raced for sport, kept as pets and working animals of various types, and are used as model organisms in science. Mammals have been depicted in art since Paleolithic times, and appear in literature, film, mythology, and religion. Decline in numbers and extinction of many mammals

3300-478: The short-beaked and long-beaked echidna species are derived from a platypus-like ancestor. The precise relationships among extinct groups of mammals and modern groups such as monotremes are uncertain, but cladistic analyses usually put the last common ancestor (LCA) of placentals and monotremes close to the LCA of placentals and multituberculates , whereas some suggest that the LCA of placentals and multituberculates

3360-432: The supposed developmental restrictions on marsupial forelimbs. Rather than through nipples, monotremes lactate from their mammary glands via openings in their skin. All five extant species show prolonged parental care of their young, with low rates of reproduction and relatively long life-spans. Monotremes are also noteworthy in their zygotic development: Most mammalian zygotes go through holoblastic cleavage , where

3420-548: The therians; as fossils of all three genera are dated about 167 million years ago in the Middle Jurassic , this is a reasonable estimate for the appearance of the crown group. T. S. Kemp has provided a more traditional definition: " Synapsids that possess a dentary – squamosal jaw articulation and occlusion between upper and lower molars with a transverse component to the movement" or, equivalently in Kemp's view,

3480-511: The three present in non-monotreme australosphenidians. Aptian and Cenomanian monotremes of the family Kollikodontidae (113–96.6 ma) have four molars. This suggests that the monotremes are likely to be unrelated to the australosphenidan tribosphenids. The time when the monotreme line diverged from other mammalian lines is uncertain, but one survey of genetic studies gives an estimate of about 220 million years ago, while others have posited younger estimates of 163 to 186 million years ago (though

3540-517: Was more recent than the LCA of placentals and monotremes. Ornithorhynchus anatinus Tachyglossus aculeatus Z. bartoni Z. attenboroughi Z. bruijnii Ornithorhynchus anatinus Tachyglossus aculeatus Zaglossus bruijnii The first Mesozoic monotreme to be discovered was the Cenomanian (100–96.6 Ma) Steropodon galmani from Lightning Ridge, New South Wales. Biochemical and anatomical evidence suggests that

3600-487: Was observed in the hyraxes , which are placental mammals . The echidna was originally thought to experience no rapid eye movement sleep . However, a more recent study showed that REM sleep accounted for about 15% of sleep time observed on subjects at an environmental temperature of 25 °C (77 °F). Surveying a range of environmental temperatures, the study observed very little REM at reduced temperatures of 15 °C (59 °F) and 20 °C (68 °F), and also

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