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.

All of the monotremes living today are found in Australia and New Guinea.

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 and produce milk to feed their young. They 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. 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 other animals. This change happened over time as they adapted to tough places.

Monotremes may not control their temperature as well as other mammals, but they can keep a steady body temperature. Early scientists thought they didn’t sleep deeply, but newer research shows they do sleep deeply sometimes.

Monotreme milk has special proteins that fight germs, maybe because they don’t have teats.

Over time, monotremes lost some 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. Others think they may be related to some ancient fossil mammals. The family tree of these animals is still being studied.

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, from the Cenomanian period (100–96.6 million years ago). Monotremes split from other mammals before marsupials and placentals evolved. Fossils from the Mesozoic period include Teinolophos, Sundrius, Kryoryctes, and others from Australia, and Patagorhynchus from Patagonian deposits in the Cretaceous period. This shows monotremes were diversifying by the early Late Cretaceous.

In 2024, important early monotreme fossils were found in the Griman Creek Formation in Lightning Ridge, New South Wales, from 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 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.

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