Monotremes (from the Greek Word,
monos 'single' +
trema 'hole', referring to the
cloacaIn zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive and urinary tracts of certain animal species. The word comes from Latin, and means sewer...
) are
mammalMammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.Mammals are divided into three main...
s that lay eggs (
PrototheriaPrototheria is a taxonomic group, or taxon, to which the order Monotremata belongs. It is conventionally ranked as a subclass within the mammals.Most of the animals in this group are extinct...
) instead of giving birth to live young like marsupials (
MetatheriaMetatheria is a grouping within the animal class Mammalia. First proposed by Thomas Henry Huxley in 1880, it is nearly synonymous with the earlier taxon Marsupialia though it is slightly wider since it also contains the nearest fossil relatives of marsupial mammals.The earliest known...
) and placental mammals (
EutheriaEutheria are a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from non-eutherians by various features of the feet, ankles, jaws and teeth...
).
They are conventionally treated as comprising a single order
Monotremata, though a recent classification proposes to divide them into the orders
PlatypodaPlatypoda is a suborder of the monotremes; it includes three families and a single living species, the Platypus....
(the
PlatypusThe Platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...
along with its fossil relatives) and Tachyglossa (the
echidnaEchidnas , also known as spiny anteaters, are four extant mammal species belonging to the Tachyglossidae family of the monotremes, an order of egg laying mammals. Together with the Platypus, they are the only surviving members of that order comprising the only extant mammals that lay eggs...
s, or spiny anteaters). The entire grouping is also traditionally placed into a subclass
PrototheriaPrototheria is a taxonomic group, or taxon, to which the order Monotremata belongs. It is conventionally ranked as a subclass within the mammals.Most of the animals in this group are extinct...
, which was extended to include several fossil orders but these are no longer seen as constituting a natural group allied to monotreme ancestry. A controversial hypothesis now relates the monotremes to a different assemblage of fossil mammals in a
cladeA clade is a term used in modern alpha taxonomy, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.[The term "monophyletic group" is used in this article...]
termed
AustralosphenidaThe Australosphenida are a clade of mammals. Today, living specimens exist only in Australia and New Guinea with only five surviving species, but fossils have been found in Madagascar and Argentina...
.
General characteristics
Like other mammals, monotremes are warm-blooded with a high metabolic rate (though not as high as other mammals, see below); have
hairHair is a protein filament that grows through the epidermis from follicles deep within the dermis. The fine, soft hair found on many nonhuman mammals is typically called fur; wool is the characteristically curly hair found on sheep and goats. Found exclusively in mammals, hair is one of the...
on their bodies; produce
milkMilk is an opaque white liquid produced by the mammary glands of mammals. It provides the primary source of nutrition for young mammals before they are able to digest other types of food. The early lactation milk is known as colostrum, and carries the mother's antibodies to the baby. It can reduce...
through mammary glands to feed their young; have a single bone in their lower jaw; and have three
middle earThe middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has...
bones.
Monotremes were very poorly understood for many years, and to this day some of the 19th century myths that grew up around them endure. It is still sometimes thought, for example, that the monotremes are "inferior" or quasi-reptilian, and that they are a distant ancestor of the "superior" placental mammals. It now seems clear that modern monotremes are the survivors of an early branching of the mammal tree; a later branching is thought to have led to the
marsupialMarsupials are an infraclass of mammals, characterized by a distinctive pouch , in which females carry their young through early infancy.- History :...
and placental groups.
A feature of monotremes (and also
marsupialMarsupials are an infraclass of mammals, characterized by a distinctive pouch , in which females carry their young through early infancy.- History :...
s) is the claim they don't have a gross communication (
corpus callosumThe corpus callosum is a structure of the mammalian brain in the longitudinal fissure that connects the left and right cerebral hemispheres. It facilitates communication between the two hemispheres. It is the largest white matter structure in the brain, consisting of 200-250 million contralateral...
) between the right and left brain hemisphere.
The key anatomical difference between monotremes and other mammals is the one that gave them their name;
Monotreme means 'single opening' in Greek, and comes from the fact that their urinary, defecatory, and reproductive systems all open into a single duct, the
cloacaIn zoological anatomy, a cloaca is the posterior opening that serves as the only such opening for the intestinal, reproductive and urinary tracts of certain animal species. The word comes from Latin, and means sewer...
. This structure is very similar to the one found in reptiles. Monotremes and marsupials have a single cloaca (though marsupials also have a separate genital tract) while placental mammal females have separate openings for reproduction, urination and defecation: the
vaginaThe vagina is a fibromuscular tubular tract leading from the uterus to the exterior of the body in female placental mammals and marsupials, or to the cloaca in female birds, monotremes, and some reptiles. Female insects and other invertebrates also have a vagina, which is the terminal part of the...
, the
urethraIn anatomy, the urethra is a tube which connects the urinary bladder to the outside of the body. The urethra has an excretory function in both sexes to pass urine to the outside, and also a reproductive function in the male, as a passage for semen.The external urethral sphincter is a striated...
, and the
anusThe anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to expel feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest, such as bones; food...
.
Monotremes lay
eggIn most birds and reptiles, an egg is the zygote, resulting from fertilization of the ovum. To enable incubation the egg is usually kept within a favourable temperature range as it nourishes and protects the growing embryo. When the embryo is adequately developed it breaks out of the egg in the...
s. However, the egg is retained for some time within the mother, who actively provides the egg with nutrients. Monotremes also lactate, but have no defined
nippleIn its most general form, a nipple is a structure from which a fluid emanates. More specifically, it is the projection on the breasts of a mammal by which breast milk is delivered to a mother's young.-Anatomy:...
s, excreting the milk from their
mammary glandMammary glands are the organs that, in mammals, produce milk for the sustenance of the young. These exocrine glands are enlarged and modified sweat glands and give mammals their name...
s via openings in their skin. All species are long-lived, with low rates of reproduction and relatively prolonged parental care of infants. Infant echidnas are sometimes known as
puggles, referencing their similarity in appearance to the Australian children's toy designed by Tony Barber. The same term, though not generally accepted, is popularly applied to young platypus as well.
Extant monotremes lack teeth as adults. Fossil forms and modern platypus young have the "tribosphenic"
molarMolars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
s (with the
occlusalThere are numerous commonly used terms of relationship and comparison that refer to different aspects of teeth and are frequently utilized in articles about dentistry...
surface formed by three
cuspA cusp is an occlusal or incisal eminence on a tooth.Canine teeth, otherwise known as cuspids, each possess a single cusp, while premolars, otherwise known as bicuspids, possess two each. Molars normally possess either four or five cusps...
s arranged in a triangle), which are 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 is not well established. The jaw of monotremes is constructed somewhat differently from those of other mammals, and 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
cynodontCynodontia or Cynodonts are a taxon of Therapsids endemic to all seven continents beginning during the Early Triassic 256 Ma. This taxon includes modern mammals and their extinct close relatives. They were one of the most diverse groups of therapsids. They are named after their dog-like...
s and other pre-mammalian synapsids; this feature, too, is now claimed to have evolved independently in monotremes and
theriaTheria is a subclass of mammals that give birth to live young without using a shelled egg, including both eutherians and metatherians .- Extent :...
ns, although, as with the analogous evolution of the tribosphenic molar, this is disputed. The external opening of the ear still lies at the base of the jaw. The imminent sequencing of the platypus genome should shed light on this and many other questions regarding the evolutionary history of the monotremes.
The monotremes also have extra bones in the shoulder girdle, including an
interclavicleAn interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals are the only tetrapods which never have an interclavicle, although some members of other groups also lack one...
and
coracoidThe coracoid process is a small hook-like structure on the lateral edge of the superior anterior portion of the scapula. Pointing laterally forward, it, together with the acromion, serves to stabilize the shoulder joint...
, which are not found in other mammals. Monotremes retain a reptile-like gait, with legs that are on the sides of rather than underneath the body. The monotreme leg bears a spur in the ankle region; the spur is non-functional in echidnas, but contains a powerful
venomThe platypus is one of the few mammals to produce venom. Both male and female have a pair of spurs on their hind limbs. The males pair of spurs delivers a cocktail of poisons that, while excruciatingly painful, is not lethal to most animals.-Spur and crural gland:...
in the male platypus.
Physiology
It is still sometimes said that monotremes have less developed
internal temperature controlThermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different...
mechanisms than other mammals, but recent research shows that monotremes maintain a constant body temperature in a wide variety of circumstances without difficulty (for example, the Platypus while living in an icy mountain stream). Early researchers were misled by two factors: monotremes maintain a lower average temperature than most mammals (around , compared to about for marsupials, and for most placentals); secondly, the
Short-beaked EchidnaThe Short-beaked Echidna , also known as the Spiny Anteater because of its diet of ants and termites, is one of four living species of echidna and the only member of the genus Tachyglossus...
(which is much easier to study than the reclusive Platypus) only maintains normal temperature when it is active: during cold weather, it conserves energy by "switching off" its temperature regulation. Finally, poor thermal regulation has also been observed in the
hyraxA hyrax is any of four species of fairly small, thickset, herbivorous mammals in the order Hyracoidea. They live in Africa and the Middle East....
es, which are placental mammals.
Their metabolic rate is remarkably low by mammalian standards. The Platypus has an average body temperature of about rather than the typical of placental mammals. Research suggests this has been a gradual adaptation to harsh environmental conditions on the part of the small number of surviving monotreme species rather than a historical characteristic of monotremes.
Contrary to previous research, the Echidna does enter REM sleep, albeit only when the ambient temperature of its environment is around . At the temperatures of and , REMS is suppressed.
Taxonomy
The only surviving examples of monotremes are all indigenous to
AustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the continental mainland , the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans...
and
New GuineaNew Guinea, located north of Australia, is the world's second largest island. It became separated from the Australian mainland when the area now known as the Torres Strait flooded after the last glacial period. The name Papua has long been associated with the island...
, although there is evidence that they were once more widespread. Fossil and genetic evidence shows that the monotreme line diverged from other mammalian lines about 150 million years ago and that both the short-beaked and long-beaked echidna species are derived from a
platypusThe Platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...
-like ancestor. Fossils of a jaw fragment 110 million years old were found at Lightning Ridge,
New South WalesNew South Wales is Australia's most populous state, located in the south-east of the country, north of Victoria, south of Queensland and east of South Australia...
. These fragments, from species
Steropodon galmani, are the oldest known fossils of monotremes. Fossils from the genera
KollikodonKollikodon ritchiei is a fossil monotreme species. It is known only from an opalised dentary fragment, with one premolar and two molars in situ...
,
TeinolophosTeinolophos trusleri was a prehistoric species of monotreme, or egg-laying mammal. It is known from a lower jawbone found in Flat Rocks, Victoria, Australia. It lived during the Aptian age of the Lower Cretaceous. It is the earliest known relative of the Platypus.The species name honours the...
, and
ObdurodonObdurodon is an extinct monotreme genus containing three species. Obdurodon differed from modern Platypuses in that it had molar teeth .-Obdurodon dicksoni:...
have also been discovered. In 1991, a fossil tooth of a 61-million-year-old platypus was found in southern
ArgentinaArgentina, officially the Argentine Republic , is the second largest country in South America, constituted as a federation of 23 provinces and an autonomous city, Buenos Aires. It is the eighth largest country in the world by land area and the largest among Spanish-speaking nations, though Mexico,...
(since named
Monotrematum, though it is now considered to be an
Obdurodon species). (See fossil monotremes below.)
Molecular clockThe molecular clock is a technique in molecular evolution which uses fossil constraints and rates of molecular change to deduce the time in geologic history when two species or other taxa diverged. It is used to estimate the time of occurrence of events called speciation or radiation...
and fossil dating suggest echidna split from platypuses 19–48 million years ago.
- ORDER MONOTREMATA
- Family Ornithorhynchidae
Ornithorhynchidae is one of the two extant families in the order Monotremata, and contains the Platypus and its extinct relatives. The other family is the Tachyglossidae, or echidnas...
: platypus
- Genus Ornithorhynchus
- Platypus
The Platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...
, Ornithorhynchus anatinus
- Family Tachyglossidae: echidnas
- Genus Tachyglossus
- Short-beaked Echidna
The Short-beaked Echidna , also known as the Spiny Anteater because of its diet of ants and termites, is one of four living species of echidna and the only member of the genus Tachyglossus...
, Tachyglossus aculeatus
- Tachyglossus aculeatus aculeatus
- Tachyglossus aculeatus acanthion
- Tachyglossus aculeatus lawesii
- Tachyglossus aculeatus multiaculeatus
- Tachyglossus aculeatus setosus
- Genus Zaglossus
- Sir David's Long-beaked Echidna
Sir David's Long-beaked Echidna , also known as the Attenborough's Long-beaked Echidna or Cyclops Long-beaked Echidna, is one of three species from the genus Zaglossus to occur in New Guinea. It is named in honour of Sir David Attenborough, the legendary naturalist...
, Zaglossus attenboroughi
- Eastern Long-beaked Echidna
The Eastern Long-beaked Echidna , also known as Barton's Long-beaked Echidna, is one of three species from the genus Zaglossus to occur in New Guinea...
, Zaglossus bartoni
- Zaglossus bartoni bartoni
- Zaglossus bartoni clunius
- Zaglossus bartoni diamondi
- Zaglossus bartoni smeenki
- Western Long-beaked Echidna
The Western Long-beaked Echidna is one of the four extant echidnas and one of three species of Zaglossus that occur in New Guinea. Fossils of this species also occur in Australia...
, Zaglossus brujinii
Fossil monotremes
The fossil record of monotremes is relatively sparse. Although biochemical and anatomical evidence suggests that monotremes diverged from the mammalian lineage before the marsupials and placental mammals arose, only a handful of monotreme fossils are known from before the
MioceneThe Miocene is a geological epoch of the Neogene period and extends from about 23.03 to 5.33 million years before the present . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the...
epoch. The few
MesozoicThe Mesozoic Era is one of three geologic eras of the Phanerozoic eon. The division of time into eras dates back to Giovanni Arduino, in the 18th century, although his original name for the era now called the "Mesozoic" was "Secondary" The Mesozoic Era is one of three geologic eras of the...
fossils that do exist, such as that of
SteropodonSteropodon galmani was a prehistoric species of monotreme, or egg-laying mammal, that lived during the middle Albian stage, in the Lower Cretaceous period...
, seem to indicate that the monotremes first evolved in Australia, during the
Late JurassicThe Late Jurassic epoch of the Jurassic Period is the unit of geologic time from 161.2 ± 4.0 to 145.5 ± 4.0 million years ago, which is preserved in Upper Jurassic strata. In European lithostratigraphy, the name Malm indicates rocks of Late Jurassic age...
or
Early CretaceousThe Early Cretaceous or the Lower Cretaceous , is the earlier or lower of the two major divisions of the Cretaceous...
. They subsequently spread to both
South AmericaSouth America is the southern continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere...
and
Antarctica| style="border-top:solid 1px #ccd2d9; padding:0.4em 1em 0.4em 0; vertical-align:top;" | 14,000,000 km
2 280,000 km
2 13,720,000 km
2 |-! style="border-top: solid 1px #ccd2d9; padding: 0.4em 1em 0.4em 0; vertical-align: top;...
, which were still united with Australia at that time, but may not have survived on either continent for long.
Fossil species
Excepting
Ornithorhynchus anatinus, all the animals listed in this section are only known from fossils.
- Family Kollikodontidae
- Genus Kollikodon
Kollikodon ritchiei is a fossil monotreme species. It is known only from an opalised dentary fragment, with one premolar and two molars in situ...
- Species Kollikodon ritchiei. Ancient monotreme, 100-105 million years old.
- Family Ornithorhynchidae
Ornithorhynchidae is one of the two extant families in the order Monotremata, and contains the Platypus and its extinct relatives. The other family is the Tachyglossidae, or echidnas...
- Genus Ornithorhynchus. Oldest Ornithorhynchus specimen 9 million years old.
- Species Ornithorhynchus anatinus (Platypus
The Platypus is a semi-aquatic mammal endemic to eastern Australia, including Tasmania. Together with the four species of echidna, it is one of the five extant species of monotremes, the only mammals that lay eggs instead of giving birth to live young...
). Oldest specimen 10,000 years old.
- Genus Obdurodon
Obdurodon is an extinct monotreme genus containing three species. Obdurodon differed from modern Platypuses in that it had molar teeth .-Obdurodon dicksoni:...
. Includes a number of MioceneThe Miocene is a geological epoch of the Neogene period and extends from about 23.03 to 5.33 million years before the present . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the...
(5-24 million years ago) Platypuses.
- Species Obdurodon dicksoni (Riversleigh Platypus
The Riversleigh Platypus is an ancient, semi-aquatic Monotreme from Australia during the lower and middle Miocene. Native to Queensland, the Riversleigh Platypus was discovered by Michael Archer, F. A. Jenkins, S. J. Hand, P. Murray, and H...
)
- Species Obdurodon insignis
- Species Monotrematum sudamericanum. 61 million years old. (Originally placed in separate genus, now thought an Obdurodon)
- Family Tachyglossidae
- Genus Zaglossus. Upper Pleistocene
The Pleistocene is the epoch from 2.588 million to 12 000 years BP covering the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
(.1-1.8 million years ago).
- Species Zaglossus hacketti
Zaglossus hacketti is an extinct species of long-beaked echidna from the Pleistocene of Western Australia. It is known only from a few bones found in Western Australia. It was the size of a sheep, weighing probably up to 100 kg .-References:...
- Species Zaglossus robustus
Zaglossus robustus is an extinct species long-beaked echidna from the Pleistocene of Tasmania.-References:* Australia's Lost World: Prehistoric Animals of Riversleigh by Michael Archer, Suzanne J. Hand, and Henk Godthelp...
- Genus Megalibgwilia
Megalibgwilia is a genus of echidna known only from Australian fossils that incorporates the oldest known echidna species. It lived during the Pleistocene, becoming extinct about 50,000 years ago....
- Megalibgwiilia ramsayi Late Pleistocene
The Late Pleistocene is a stage of the Pleistocene Epoch. The beginning of the stage is defined by the base of Eemian interglacial phase before final glacial episode of Pleistocene 126,000 ± 5,000 years ago. The end of the stage is defined exactly at 10,000 Carbon-14 years BP...
- Megalibgwiilia robusta Miocene
The Miocene is a geological epoch of the Neogene period and extends from about 23.03 to 5.33 million years before the present . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the...
- Family Steropodontidae
The Steropodontidae was a family of monotremes that are known from fossils from the Early Cretaceous in Australia.There are two genera placed in this family; Steropodon, and Teinolophos which has been tentatively placed in the family due to the similarity of the lower molars in these two...
. May be part of Ornithorhynchidae; closely related to modern platypus.
- Genus Steropodon
Steropodon galmani was a prehistoric species of monotreme, or egg-laying mammal, that lived during the middle Albian stage, in the Lower Cretaceous period...
- Species Steropodon galmani.
- Genus Teinolophos
Teinolophos trusleri was a prehistoric species of monotreme, or egg-laying mammal. It is known from a lower jawbone found in Flat Rocks, Victoria, Australia. It lived during the Aptian age of the Lower Cretaceous. It is the earliest known relative of the Platypus.The species name honours the...
- Species Teinolophos trusleri. 123 million years old — oldest monotreme specimen.
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