Monotreme

Monotremes[1] Temporal range: Early Cretaceous (Barremian) – Present PreꞒ Ꞓ O S D C P T J K Pg N
Four of the five extant monotreme species: platypus (top-left), short-beaked echidna (top-right), western long-beaked echidna (bottom-left), and replica eastern long-beaked echidna (bottom-right)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Monotremata C.L. Bonaparte, 1837[2]
Subgroups
†Kryoryctes? †Patagorhynchus †Sundrius? Steropodontidae †Teinolophidae Kollikodontidae Ornithorhynchidae Tachyglossidae

Monotremes (/ˈmɒnətriːmz/) are mammals of the order Monotremata. They are the only known 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 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 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 in the Late Cretaceous and Paleocene of southern South America, implying that they were also present in Antarctica, though remains have not been found there.

The name monotreme derives from the Greek words μονός (monós 'single') and τρῆμα (trêma 'hole'), referring to the cloaca.

General characteristics

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 is the primary communication route between the right and left brain hemispheres.^[3] 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.^[4]

Extant monotremes lack teeth as adults. Fossil forms and modern platypus young have a "tribosphenic" form of

Monotreme jaws are 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 non-mammal cynodonts and other premammalian synapsids; this feature, too, is now claimed to have evolved independently in monotremes and therians,^[8] although, as with the analogous evolution of the tribosphenic molar, this hypothesis is disputed.^[9][10] Nonetheless, findings on the extinct species Teinolophos confirm that suspended ear bones evolved independently among monotremes and therians.^[11] 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

The monotremes also have extra bones in the shoulder girdle, including an interclavicle and coracoid, which are not found in other mammals. Monotremes retain 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.^[15] It is thought to be an ancient mammalian characteristic, as many non-monotreme archaic mammal groups also possess venomous spurs.^[18]

Reproductive system

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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 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.^[19] The monotreme penis is similar to that of turtles and is covered by a preputial sac.^[20][21]

Monotreme

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

Monotremes may have less developed thermoregulation than other mammals, but recent research shows that they easily maintain 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 was observed in the hyraxes, which are placental mammals.

The echidna was originally thought to experience no rapid eye movement sleep.^[31] 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 a substantial reduction at the elevated temperature of 28 °C (82 °F).^[32]

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 nipples.^[33]

During the course of evolution the monotremes have lost the

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.^[36] 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.^[37]

Taxonomy

The traditional "

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,^[46] but another putting it at 17–89 million years ago.^[47] It has been suggested that both the short-beaked and long-beaked echidna species are derived from a platypus-like ancestor.^[44]

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 was more recent than the LCA of placentals and monotremes.^[48][49]

• ORDER MONOTREMATA
  • Family Ornithorhynchidae: platypus
    • Genus
      Ornithorhynchus
      • Platypus, O. anatinus
  • Family
    Tachyglossidae
    : echidnas
    • Genus
      Tachyglossus
      • Short-beaked echidna, T. aculeatus
        • T. a. aculeatus (Common short-beaked echidna)
        • T. a. acanthion (Northern short-beaked echidna)
        • T. a. lawesii (New Guinea short-beaked echidna)
        • T. a. multiaculeatus (
          Kangaroo Island short-beaked echidna
          )
        • T. a. setosus (Tasmanian short-beaked echidna)
    • Genus
      Zaglossus
      • Sir David's long-beaked echidna
        , Z. attenboroughi
      • Eastern long-beaked echidna, Z. bartoni
        • Z. b. bartoni
        • Z. b. clunius
        • Z. b. diamondi
        • Z. b. smeenki
      • Western long-beaked echidna, Z. bruijni

Fossil monotremes

The first Mesozoic monotreme to be discovered was the Cenomanian (100–96.6 Ma) Steropodon galmani from Lightning Ridge, New South Wales.^[54] Biochemical and anatomical evidence suggests that 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), Steropodon, Stirtodon, Kollikodon, and an unnamed ornithorhynchid (all Cenomanian) from Australian deposits in the Cretaceous, indicating that monotremes were diversifiying by the early Late Cretaceous.^[55] 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.^[56][57]

A fossil jaw fragment attributed to a platypus from Cenomanian deposits (100–96.6 Ma) from the Griman Creek Formation in Lightning Ridge, New South Wales, is the oldest platypus-like fossil.^[44] The durophagous Kollikodon, the pseudotribosphenic Steropodon, and Stirtodon 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 occur in southern Argentina (Monotrematum), see fossil monotremes below. The platypus genus Ornithorhynchus in known from Pliocene deposits, and the oldest fossil tachyglossids are Pleistocene (1.7 Ma) in age.^[44]

Fossil species

Excepting Ornithorhynchus anatinus, all the animals listed in this section are known only from fossils.

• Family Incertae sedis
  • Genus Kryoryctes
    • Species Kryoryctes cadburyi
  • Genus Patagorhynchus
    • Species Patagorhynchus pascuali - Maastrichtian, earliest known South American monotreme^[57]
• Family
  Steropodontidae
  – paraphyletic assemblage
  • Genus Steropodon
    • Species
      Steropodon galmani
  • Genus Teinolophos
    • Species
      Teinolophos trusleri
      – 123 Ma, oldest monotreme specimen
• Family Ornithorhynchidae
  • Genus
    Ornithorhynchus
    – oldest Ornithorhynchus specimen 9 Ma
    • Species
      Ornithorhynchus anatinus
      (platypus) – oldest specimen 10,000 years old
  • Genus Obdurodon – includes a number of Miocene (24–5 Ma) Riversleigh platypuses)
    • Species
      Obdurodon dicksoni
    • Species
      Obdurodon insignis
    • Species
      Upper Miocene
      (15–5 Ma)
  • Genus Monotrematum
    • Species Monotrematum sudamericanum – 61& Ma, southern South America
• Family
  Tachyglossidae
  • Genus
    Upper Pleistocene
    (1.8–0.1 Ma)
    • Species
      Zaglossus robustus
  • Genus Murrayglossus
    • Species Murrayglossus hacketti
  • Genus Megalibgwilia
    • Species Megalibgwilia ramsayi – Late Pleistocene
    • Species Megalibgwilia robusta – Miocene

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Further reading

• Nowak, Ronald M. (1999). Walker's Mammals of the World (6th ed.). Baltimore, MD: Johns Hopkins University Press.
  LCCN 98023686
  .

External links

Wikimedia Commons has media related to Monotremata.

The Wikibook Dichotomous Key has a page on the topic of: Monotremata

Wikispecies has information related to Monotremata.

• "Introduction to Monotremes". U.C. Museum of Peleontology. University of California – Berkeley.