Monotreme

Monotremes are special kinds of mammals that lay eggs instead of having babies the way most mammals do. Today, there are only five types of monotremes alive: the platypus and four kinds of echidnas. Even though they lay eggs, female monotremes care for their babies by making milk to feed them, just like other mammals.

Monotremes have many different body parts compared to other mammals. Their brains, jaws, digestion, and reproduction systems all look a bit different. Scientists sometimes group monotremes with ancient mammals from long ago, but this idea is still being discussed and is not completely settled.

All of the monotremes living today are found in Australia and New Guinea. In the very distant past, during the time when dinosaurs lived, monotremes were also in southern South America. This suggests they might have lived in Antarctica too, but scientists have not found any remains there yet.

The word "monotreme" comes from ancient Greek words meaning "single hole." This name refers to a special opening in their bodies called a cloaca, which is used for both digestion and reproduction.

General characteristics

Monotremes are special kinds of mammals that lay eggs instead of having babies. They have hair, produce milk to feed their young, and have a single bone in their lower jaw. Unlike other mammals, monotremes do not have teeth as adults.

Monotremes also have unique traits, such as extra bones in their shoulder area and a special spur on their leg that can produce a strong toxin in male platypuses. They have a single opening called a cloaca for their bodily functions, similar to birds and reptiles. Their babies, called puggles, develop inside eggs that hatch after about ten days and then crawl to find milk from their mother’s skin.

Physiology

Monotremes have a slower metabolism than most mammals. For example, the platypus has a body temperature of about 31 °C (88 °F), which is lower than the 35 °C (95 °F) for marsupials and 37 °C (99 °F) for placentals. This change likely happened over time as these animals adapted to tough environments.

Monotremes might not control their temperature as well as other mammals, but they can still keep a steady body temperature in different situations. Early scientists thought they didn’t sleep deeply, but newer research shows they do sleep deeply about 15% of the time when conditions are right.

Monotreme milk has special proteins that fight germs, maybe because they don’t have teats and drink milk in a different way.

Over time, monotremes lost certain stomach parts that most mammals have. Because of this, some say they don’t have stomachs, though scientists still use the term when studying them.

Both platypuses and echidnas have small spurs on their back legs. For echidnas, these spurs don’t do anything, but platypus spurs have venom. Studies suggest that the ancestor of platypuses and echidnas might have also had venom.

Taxonomy

Monotremes are special mammals because they lay eggs instead of having live babies. There are five kinds of monotremes today: the platypus and four types of echidnas.

Scientists have different ideas about how monotremes are related to other mammals. Some think monotremes split off from other mammal groups very early, before marsupials and placental mammals diverged. Others think they may be related to some ancient fossil mammals found in places like Madagascar and Australia. The exact time when monotremes separated from other mammals is still unclear, but it might have been around 220 million years ago.

The platypus and echidnas likely share a common ancestor that looked more like a platypus. The family tree of these animals is still being studied, and scientists are learning more about how they are connected to each other and to other mammals.

Summary of extant species

Monotremes are special kinds of mammals that lay eggs instead of having babies the usual way. There are five kinds that are still alive today. These include the platypus and four different types of echidnas.

Fossil monotremes

See also: Evolution of mammals

The first Mesozoic monotreme found was Steropodon galmani from Lightning Ridge, New South Wales, dating back to the Cenomanian period (100–96.6 million years ago). Evidence shows that monotremes split from other mammals before marsupials and placentals evolved. Fossils of monotremes from the Mesozoic period include Teinolophos, Sundrius, Kryoryctes, and several others from Australia, as well as Patagorhynchus from Patagonian deposits in the Cretaceous period. This suggests monotremes were diversifying by the early Late Cretaceous.

In 2024, an important group of early monotreme fossils was found in the Griman Creek Formation in Lightning Ridge, New South Wales, dating to the Cenomanian period. These include the oldest known platypus-like fossil, a jaw fragment named Dharragarra. Other fossils from this time include Kollikodon, Steropodon, Stirtodon, Dharragarra, Opalios, and Parvopalus. Fossils of a toothed platypus called Obdurodon have also been found in Australia, along with a 63 million-year-old platypus relative in southern Argentina (Monotrematum). The current platypus genus, Ornithorhynchus, is known from Pliocene deposits, and the oldest fossils of echidna relatives are from the Pleistocene period (1.7 million years ago).

Fossil species

Except for Ornithorhynchus anatinus, all the animals listed here are known only from fossils. Some family names are uncertain because the specimens are incomplete.

• Family †Kollikodontidae
  • Genus †Kollikodon
    • Species †Kollikodon ritchiei
  • Genus †Kryoryctes
    • Species †Kryoryctes cadburyi
  • Genus †Sundrius
    • Species †Sundrius ziegleri
• Family †Steropodontidae
  • Genus †Parvopalus
    • Species †Parvopalus clytiei
  • Genus †Steropodon
    • Species †Steropodon galmani
• Family †Teinolophidae
  • Genus †Stirtodon
    • Species †Stirtodon elizabethae
  • Genus †Teinolophos
    • Species †Teinolophos trusleri – 123 million years ago, oldest monotreme specimen
• Superfamily Ornithorhynchoidea
  • Family †Opalionidae
    • Genus †Opalios
      • Species †Opalios splendens
  • Family Ornithorhynchidae
    • Genus †Dharragarra
      • Species †Dharragarra aurora
    • Genus †Monotrematum
      • Species †Monotrematum sudamericanum – 61 million years ago, southern South America
    • Genus Ornithorhynchus – oldest Ornithorhynchus specimen 9 million years ago
      • Species Ornithorhynchus anatinus (platypus) – oldest specimen 10,000 years old
    • Genus †Obdurodon – includes several Miocene (24–5 million years ago) Riversleigh platypuses
      • Species †Obdurodon dicksoni
      • Species †Obdurodon insignis
      • Species †Obdurodon tharalkooschild – Middle Miocene and Upper Miocene (15–5 million years ago)
    • Genus †Patagorhynchus
      • Species †Patagorhynchus pascuali - Maastrichtian, earliest known South American monotreme
  • Family Tachyglossidae
    • Genus Zaglossus – Upper Pleistocene (1.8–0.1 million years ago)
      • Species†Zaglossus robustus
    • Genus †Murrayglossus
      • Species †Murrayglossus hacketti
    • Genus †Megalibgwilia
      • Species †Megalibgwilia ramsayi – Late Pleistocene
      • Species †Megalibgwilia robusta – Miocene