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Monotreme

Monotremes are the lowest mammals. They lay eggs. The intestine and urogenital duct open into the cloaca and pass through a common hole to the outside of the body, hence the name "monotreme". The mammary duct opens into the mammary gland area on both sides of the abdomen without nipples; the body is covered with hair; the body temperature is relatively low but has the ability to regulate body temperature. Such as the platypus and echidna, distributed in Australia and New Guinea.

I. Introduction to Monotremata

1. Key Characteristics

  • Egg-Laying Mammals: Monotremes are the only living mammals that lay eggs rather than giving birth to live young. They include the platypus and echidnas.

  • Cloaca: Like reptiles and birds, monotremes have a single opening (cloaca) for excretion and reproduction, giving rise to the name “monotreme” (“one hole”).

  • Lower, Less Stable Body Temperature: Compared to marsupial and placental mammals, monotremes generally maintain a slightly lower and more variable body temperature.

  • Special Sensory Organs: Platypuses possess electroreceptors on their bills that detect faint electric fields generated by prey; echidnas also have some degree of electroreception in their snouts.

  • Distribution and Ecology: Monotremes are found exclusively in Australia, Tasmania, and parts of New Guinea, inhabiting diverse environments such as rivers, forests, and alpine regions.

2. Diversity and Status

  • There are very few monotreme species—only two families, three genera, and around five or six recognized living species (depending on taxonomic interpretation).

  • They play unique roles within their ecosystems, retaining a combination of ancient mammalian traits alongside specialized adaptations.

II. Evolutionary History of Monotremes

  1. Relationship with Other Mammals

    • Mammals are traditionally divided into three major groups: Monotremata (monotremes), Marsupialia (marsupials), and Placentalia (placentals).

    • Molecular studies confirm that monotremes diverged early in mammalian evolution. They are not closely related to marsupials and placentals but represent a surviving lineage from a basal branch.

  2. Fossils and Origins

    • The earliest confirmed monotreme fossils date to the Early Cretaceous (about 120 million years ago) in Australia, including extinct genera like Steropodon.

    • Other fossil finds (e.g., Teinolophos, Obdurodon) further indicate that monotremes split off from the ancestors of marsupials and placentals very early, evolving their unique features in relative isolation in Gondwanan Australia.

  3. Geography and Paleoenvironment

    • Shifting continental plates separated Australia (and Antarctica, South America) from other landmasses, allowing monotremes to persist without direct competition from most placental mammals.

    • Climatic changes, including glaciations and habitat shifts, also influenced monotreme distribution. Today, extant species remain restricted to Australia, Tasmania, and parts of New Guinea.

III. Classification Overview

Modern monotremes are typically divided into two families: the Ornithorhynchidae (platypus) and the Tachyglossidae (echidnas). The table below lists the main families, genera, and representative species, along with key notes.

Order Monotremata

FamilyGenusLiving SpeciesDistribution & Notes
Ornithorhynchidae (Platypus)OrnithorhynchusPlatypus (Ornithorhynchus anatinus)The only extant species in this family, found in eastern Australia and Tasmania. Uses electroreception to detect aquatic prey; males have venomous spurs on their ankles.
Tachyglossidae (Echidnas)TachyglossusShort-beaked echidna (Tachyglossus aculeatus)Found across mainland Australia, Tasmania, and New Guinea; covered in spines, uses a long, sticky tongue to feed on ants and termites.

ZaglossusLong-beaked echidnas (Zaglossus bruijnii, Z. bartoni, Z. attenboroughi)Restricted to New Guinea; larger than short-beaked echidnas, with longer snouts. Most species are endangered and feed primarily on earthworms and other soil invertebrates.

Note: The exact species count of long-beaked echidnas (genus Zaglossus) can vary in different taxonomic references (some recognize two species, others three or more).

Extinct Genera and Fossil Highlights

  • Steropodon, Teinolophos, Obdurodon, among others, are extinct monotreme genera, showing that both platypus-like and echidna-like lines once had greater morphological diversity. Obdurodon species retained functional adult dentition (modern platypuses lose functional teeth in adulthood).

IV. Summary

Monotremes hold a pivotal place in mammalian evolution. They combine certain “reptilian/bird-like” features (egg laying, cloaca) with classic mammalian traits (hair, lactation). Over tens of millions of years, they have survived in the Australian region with limited competition from placental mammals.

Today, monotremes face ecological threats such as habitat destruction, invasive predators (e.g., foxes, cats), and pollution. Understanding their classification, ancient origins, and biological characteristics emphasizes the need to protect these “living fossils,” safeguarding their role in Australia’s rich and unique ecosystems.

Carnivora Primates Rodents Chiroptera Eulipotyphla Ungulata Cetacea Lagoiformes Pangolins Proboscidea Monotremata
Tachyglossidae

Tachyglossidae

Spiny Anteater

Monotremata LC

Features：

Echidna (Tachyglossidae), one of four peculiar egg-laying mammals found in Australia, Tasmania and New Guinea. They eat and breathe through a bald tube-like beak extending from their dome-shaped body. Covered with spines. Echidnas have round eyes and slits for ears, two small nostrils (or nostrils) at the end of their beak, and a small mouth. Electroreceptors in the skin of the beak sense electrical signals produced by the muscles of invertebrate prey. Echidnas are active day and night....

platypus

platypus

Ornithorhynchus anatinus

Monotremata LC

Features：

The platypus (Ornithorhynchus anatinus) is a small Australian amphibian mammal known for its strange combination of primitive features and special adaptations, particularly its flat, almost comical beak, which early observers believed was sewn onto the mammal's body duck's beak. There is noticeable white fur under the eyes, adding to its unique appearance. The hair on the rest of the body is dark brown to light brown above and lighter underneath. Platypus in Australia...

Zaglossus bartoni

Zaglossus bartoni

Zaglossus bartoni

Monotremata LC

Features：It is the largest of the monotremes

The Great long-robed echidna (Zaglossus bartoni) is a species of proto-echidna found in New Guinea, with four subspecies. They curl their bodies to defend themselves when attacked. The hard black hair on the back is covered with thorns like "steel nails", no tail, slow action, although the...

Zaglossus attenboroughi

Zaglossus attenboroughi

Zaglossus attenboroughi

Monotremata LC

Features：It is the tiniest member of the protoechidna genus, closely related to the Australian echidna

Attenborough's scientific name is Zaglossus attenboroughi, after David Attenborough. Like to dig holes, harm crops, harmful to agriculture, so it is a pest. But that's just from a human point of view, from a biological chain point of view, where moles play an important role. The hair is brow...

Western Long-beaked Echidna

Western Long-beaked Echidna

Western Long-beaked Echidna，Long-nosed Spiny Anteater，New Guinea Long-nosed Echidna，Long-beaked Echidna，Long-nosed Echidna，Zaglossus bruijnii

Monotremata LC

Features：It is the largest member of the monotreme

Echidna long-rostris Zaglossus bruijnii) Western Long-beaked Echidna, Long-nosed Spiny Anteater, New Guinea Long-nosed Echidna, Long-beaked Echidna, Long-nosed Echidna, almost twice the size of the short-rossed echidna, is the largest member of the monotreme order, with no subspecies.Although they d...

Tachyglossus aculeatus

Tachyglossus aculeatus

Tachyglossus aculeatus

Monotremata LC

Features：It is the most widely distributed and common monotreme in existence

The Australian echidna Tachyglossus aculeatus is adapted to an ant-eating lifestyle and lives in rocky, sandy and scrub areas, living in crevasses and self-dug burrows. Come out at dusk and night. Powerful claws for digging. Usually on the ground activities, encounter danger, can quickly dig into th...

Tachyglossidae

Tachyglossidae

Spiny Anteater,echidna

Monotremata EN

Features：One of the most primitive mammals and one of only two monotremes in the world.

Echidna, or Tachyglossidae, is commonly known as the spiny anteater. It is a surviving monotreme along with the platypus. There are three genera and six species in this family, of which the giant echidna is extinct, and the other two genera live in Australia and New Guinea.Echidna is a warm-blooded...

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