Monotremes are the only mammals that lay eggs to reproduce (oviparous), a form that diversified from the traditional live-bearing ones, which includes marsupials and eutherians. However, like other mammals, they possess body hair and mammary glands and are endothermic or warm-blooded. Found primarily in Australia and New Guinea, they exhibit a mixture of reptilian and mammalian traits.
They belong to the order Monotremata, containing only four extant species of terrestrial echidnas and one extant species of amphibious platypus.
These mammals derive their name from the Greek word monotreme, which means ‘single opening.’ This term refers to the single duct or cloaca that serves as the common exit point for their urinary, defecatory, and reproductive systems.
Being mammals, monotremes possess body hair, mammary glands, three ear middle-ear ossicles, and a single lower jaw bone. They are endothermic (warm-blooded) and maintain a constant internal body temperature. However, unlike most mammals, their adults lack teeth, once present in their ancestral forms (tribosphenic molars with three cusps).
Apart from being oviparous, they also possess some other unique features that set them apart from other mammals:
According to the most widely accepted ‘Theria Hypothesis,’ the monotremes have diverged from the marsupial (Metatheria) and placental (Eutheria) lineages much earlier than the divergence of placentals and marsupials themselves. This hypothesis explains why monotremes retain primitive traits like egg-laying, similar to those found in synapsid ancestors of mammals. However, a competing hypothesis, the Marsupionta, states that the divergence between monotremes and marsupials happened after placentals and marsupials diverged.
Monotremata was traditionally treated as an order under the subclass Prototheria, but this classification is no longer accepted. Based on another hypothesis, monotremes are related to fossil mammals belonging to the clade Australosphenida, which survived in the Jurassic and Cretaceous Periods. However, recent studies show that Teinolophos, the oldest monotreme, differs from non-monotreme australosphenidans in having five molars, and thus, these two groups may be completely unrelated phylogenetically.
Although the exact relationship between extinct mammalian groups and modern ones, like monotremes, remains unclear, cladistic analysis places the last common ancestor (LCA) of placentals and monotremes close to that of placentals and multituberculates.
Currently, the five extant species of monotremes are listed under two families, Ornithorhynchidae and Tachyglossidae, and three genera.
Studies on the platypus genome have revealed surprising insights into the evolutionary relationships between monotremes and other vertebrate groups. For example, monotremes possess five pairs of sex chromosomes, one of which resembles the Z chromosome of birds. This suggests that the sex chromosomes of marsupials and placentals evolved after the split from the monotreme lineage.
Other genetic similarities with birds, such as the similarity between genes related to egg-laying and egg-yolk formation (vitellogenin gene), suggest that the common ancestor of monotremes, marsupials, and placental mammals was oviparous, and through evolution, this trait was only retained in monotremes.
Steropodon galmani, dating back to the Cenomanian Age (100 to 96.6 million years ago), was the first Mesozoic monotreme to be discovered. Fossils like those of Teinolophos, Sundrius, Kryoryctes, Steropodon, Stirtodon, Kollikodon, and an unnamed ornithorhynchid from Australian deposits suggest that monotremes had started diversifying by the early Late Cretaceous. It is also believed that monotremes had diverged from the mammalian lineage before the marsupials and placental mammals did.
Since fossils of monotremes from the Cretaceous and Paleocene Periods have also been unearthed from South America, a hypothesis suggests that they probably originated in Australia during the Late Jurassic or Early Cretaceous, following which they migrated across Antarctica to South America, since both the continents merged with Australia many years ago.
The oldest platypus-like fossil (Cenomanian) was discovered in the Griman Creek Formation in Lightning Ridge, New South Wales. Similarly, fossils of the platypus genus Ornithorhynchus and the oldest fossil tachyglossids have been unearthed from deposits of the Pleistocene Epoch.
Platypus is mainly found in Eastern Australia, inhabiting freshwater creeks, slow-moving rivers, and lakes. In contrast, echidnas are found in New Guinea and mainland Australia, as well as in Tasmania, King Island, Flinders Island, and Kangaroo Island.
While echidnas prefer ants and termites as their food, the platypus feeds on insect larvae, yabbies, small fish, and worms.
Like reptiles, all monotremes possess a single duct called the cloaca, which drains their urinary, defecatory, and reproductive systems. The males possess a penis covered by a preputial sac for releasing semen, while the females have ovaries that produce eggs, which are fertilized by sperm.
After fertilization, the zygote goes through meroblastic cleavage (partial division), unlike other mammalian zygotes that undergo holoblastic cleavage (complete division). In this type of cleavage, the cytoplasm of the cells is continuous with the egg yolk, allowing easy exchange of waste and nutrients.
The eggs are then retained in the mother’s body and continue to receive nutrition until they are released. The laid eggs are incubated for around 12 days (much less than the reptile incubation period), after which they hatch. The newborn monotremes (puggles) have relatively well-developed forelimbs.
Echidnas protect their young in a temporary pouch, which is absent in platypi. Young monotremes crawl up their mother’s body to access milk, which feeds through their mammary glands that secrete milk directly onto the skin, as they lack nipples.
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