Lexi Webster
Australia's monotremes are the world's only living, egg-laying mammals. The platypus and the echidna are the only two surviving monotreme species, and they are indigenous to small areas of Australia and New Guinea. However, monotremes were also present in South America during the Cretaceous and Paleocene epochs, and fossils of a 63-million-year-old platypus relative have been found in Argentina.
| Characteristics | Values |
|---|---|
| What are monotremes? | Mammals of the order Monotremata |
| What makes them unique? | Monotremes are the only group of living mammals that lay eggs, rather than bearing live young. |
| Extant monotreme species | Platypus and four species of echidnas |
| Platypus habitat | Freshwater systems on the eastern coast of Australia |
| Echidnas habitat | Across the Australian continent |
| Other names | Prototheria |
| Monotreme fossils | Found in Australia, South America, and Madagascar |
| Monotreme shoulder girdle | More advanced than other mammalian groups |
| Countries with monotremes | Australia and New Guinea |
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What You'll Learn
Monotremes are mammals of the order Monotremata
Monotremes are an ancient group of mammals. They diverged from the mammalian lineage before marsupials and placentals arose. The oldest monotreme fossil is the jaw fragment of Dharragarra, dating to 100-96.6 million years ago. Monotremes have been found in Cretaceous deposits in Australia and South America, indicating that monotremes were diversifying by the Late Cretaceous. One hypothesis suggests that monotremes arose in Australia in the Late Jurassic or Early Cretaceous and migrated to South America via an Antarctic land bridge.
The monotreme order includes four families, two of which are known from the Cretaceous Period: the platypus-like Steropodontidae and the Kollikodontidae. The Steropodontidae are represented by a single species, Steropodon galmani, while the Kollikodontidae are represented by Kollikodon ritchiei. These species are known only from opalised jaw fragments. The discovery of Kollikodon ritchiei suggested that Cretaceous monotremes may have been more diverse and widespread than previously thought and may have been the dominant mammals in the Australian sector of the ancient supercontinent of Gondwanaland.
Today, monotremes are only found in Australia and New Guinea. They include the platypus and the four species of echidnas. The Short-beaked Echidna is the only mammal found across the entire Australian continent, adapting to a range of environments from snowy conditions to arid deserts. Platypuses are found in freshwater systems on Australia's eastern coast and are known for their bills, which are full of electroreceptors used to find prey.
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They are the only living mammals that lay eggs
Monotremes are an anomaly within the animal kingdom. They are the only living mammals that lay eggs, making them distinct from other mammals that bear live young. This unique group of mammals, known as Monotremata, includes the platypus and four species of echidnas.
The platypus, a semi-aquatic carnivore-insectivore, is one of the two known monotremes. Platypuses are known for their bills, which are equipped with electroreceptors that aid in finding food such as molluscs, crustaceans, and other invertebrates. The other monotreme species are the echidnas, which have a specialised diet of small invertebrates like ants, termites, and worms. Echidnas are famous for their sharp spines, but they also have fur, with the Short-beaked Echidna in Tasmania having longer fur that blends in with its spines.
Monotremes are indigenous to Australia and New Guinea, and they have a unique evolutionary history. They are believed to have diverged from the mammalian lineage before marsupials and placentals arose. Monotreme fossils have been discovered in Australia, dating back to the Cretaceous Period, indicating that monotremes may have been dominant mammals in the Australian sector of the ancient supercontinent of Gondwana.
While monotremes lay eggs, female monotremes share a common trait with other mammals: they nurse their young with milk. This sets them apart from other egg-laying animals, as they exhibit a combination of mammalian and reptilian characteristics.
The classification of monotremes is still a subject of debate. Some authors consider them part of Australosphenida, a clade that includes extinct mammals from the Jurassic and Cretaceous periods found in Madagascar, South America, and Australia. However, this hypothesis is controversial, and the evolutionary relationships of monotremes continue to intrigue scientists studying mammalian evolution.
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All extant monotremes are indigenous to Australia and New Guinea
Monotremes are a highly specialised group of mammals that lay eggs and have no teats. They are the only group of living mammals with these characteristics. The extant monotreme species are the platypus and the four species of echidnas. All extant monotremes are indigenous to Australia and New Guinea.
The platypus is a semi-aquatic carnivore-insectivore, native to the eastern coast of Australia. Platypuses are famous for their bills, which are full of electroreceptors that help them find food. The Short-beaked Echidna is the only mammal found across the entire Australian continent. It is well adapted to a variety of habitats, from snowy conditions to arid deserts. Echidnas are known for their sharp spines and fur.
Monotremes are considered to have diverged from the mammalian lineage before marsupials and placentals. They are thought to have arisen in Australia during the Late Jurassic or Early Cretaceous. During this time, Australia was part of the supercontinent Gondwana, which also included Antarctica and South America. Monotremes may have migrated to South America via an Antarctic land bridge.
Monotreme fossils have been found in Australia, dating back to the Cretaceous Period. The oldest platypus-like fossil was discovered in Lightning Ridge, New South Wales, and is estimated to be around 100 million years old. Other fossils indicate that platypuses were once distributed across Australia and South America.
While monotremes are now only found in Australia and New Guinea, their evolutionary history is closely tied to the ancient supercontinent of Gondwana. The presence of monotreme fossils in both Australia and South America provides valuable insights into their diversification and migration patterns during the Cretaceous Period.
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Monotremes diverged from the mammalian lineage before marsupials and placentals
Monotremes are mammals of the order Monotremata. They are the only living mammals that lay eggs instead of giving birth to their young. The extant monotreme species are the platypus and four species of echidnas. They are indigenous to Australia and New Guinea. Monotremes are characterised by structural differences in their brains, jaws, digestive tracts, and reproductive tracts, compared to other mammals.
The Theria hypothesis, supported by most morphological evidence, states that monotremes diverged from the Metatheria (marsupial) and Eutheria (placental) lineages before the divergence between marsupials and placentals. This hypothesis is also supported by genetic evidence and is the more widely accepted one. According to this hypothesis, monotremes share a common ancestor with marsupials and placentals, which was likely oviparous. This is supported by genetic similarities with birds, such as genes related to egg-laying and egg-yolk formation.
Biochemical and anatomical evidence also supports the idea that monotremes diverged before marsupials and placentals. Monotremes have unique jaw structures, with the tiny bones that conduct sound to the inner ear attached to the jaw, and the jaw-opening muscles attached differently from other mammals. They also lack the corpus callosum, a connective structure in the brain present in placentals. Instead, they have an anterior commissure, which provides an alternative communication route between the right and left brain hemispheres.
The discovery of fossil mammal skeletons and dentitions has shed light on the anatomy and lifestyle of early mammals. Monotremes have been found in the latest Cretaceous and Paleocene deposits in Australia and South America, indicating that monotremes were diversifying by the early Late Cretaceous. The first Mesozoic monotreme discovered was Steropodon galmani, dating back to the Cenomanian Age (100 to 96.6 million years ago). Other fossils, such as Teinolophos, Sundrius, and Kollikodon, suggest that monotremes were present during the Barremian (126 million years ago) and Albian (113-108 million years ago) periods.
While the Theria hypothesis is widely accepted, there are competing hypotheses, such as the Marsupionta hypothesis, which suggests that the divergence between monotremes and marsupials occurred after the split between marsupials and placentals. However, this hypothesis is not as strongly supported by the genetic evidence. The exact relationship between extinct mammalian groups and modern ones, like monotremes, remains a subject of ongoing research and discussion.
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They are highly modified for their ecological niches
Monotremes are mammals of the order Monotremata. They are the only living mammals that lay eggs instead of giving birth to their young. The platypus and four species of echidnas are the only extant monotreme species. All extant monotremes are indigenous to Australia and New Guinea.
Monotremes are highly modified for their ecological niches. The platypus, for example, is a semi-aquatic carnivore-insectivore. Its bill is full of electroreceptors that help it find molluscs, crustaceans, and other invertebrates in the freshwater systems on the eastern coast of Australia. The echidnas, on the other hand, have a specialised diet of small invertebrates like ants, termites, and worms. They are the only mammal found across the entire Australian continent, adapting to both snowy conditions and arid deserts. Their tongues can flick up to 100 times per minute, allowing them to skillfully lap up ants and termites.
The name Monotremata means "one-holed," referring to the fact that both male and female monotremes have a single opening at the rear of their bodies used for both reproduction and excretion. Monotremes are also structurally different from other mammals in their brains, jaws, digestive tracts, and reproductive tracts. For instance, they lack the connective structure (corpus callosum) that serves as the primary communication route between the right and left brain hemispheres in placentals. Instead, the anterior commissure provides an alternate communication route in monotremes and marsupials, carrying all the commissural fibres arising from the neocortex.
The evolutionary history of monotremes is a long and complex story. Monotremes diverged from the mammalian lineage before marsupials and placentals arose. They have been found in Cretaceous deposits in Australia and South America, suggesting that monotremes may have arisen in Australia during the Jurassic or Early Cretaceous and migrated to South America via an Antarctic land bridge when the continents were still connected. The oldest known platypus-like fossil was discovered in Lightning Ridge, New South Wales, dating back to the Cretaceous Period.
While monotremes are now only found in Australia and New Guinea, they were present during the Late Cretaceous and Paleocene epochs, indicating a wider distribution in the past. The absence of monotremes outside of Australia and New Guinea today remains a subject of interest for scientists studying mammalian evolution.
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Frequently asked questions
No, monotremes are also found in New Guinea.
There are currently only five monotreme species remaining. These include the platypus and four species of echidna.
Monotremes were also present in South America during the Late Cretaceous and Paleocene epochs. However, they are now only found in Australia and New Guinea.
Monotremes may have been outcompeted by therian mammals during the Cretaceous. Therian mammals were able to take advantage of the angiosperm evolution and diversification.
