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| Monotremata
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Kollikodontidae (extinct) Ornithorhynchidae - Platypus Tachyglossidae - Echidnas Steropodontidae (extinct)
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Monotremes are mammals that are best known for laying eggs, instead of giving birth to live young like marsupials and placental mammals ( Eutheria). The subclass comprises a single order, Monotremata (though sometimes the subclass Prototheria is used).
Like other mammals, monotremes are warm-blooded with high metabolic rates (though not as high as other mammals, see below); have hair on their bodies; produce milk to feed their young; have a single bone in their lower jaw; and have three inner ear bones.
Monotremes were very poorly understood for many years, and to this day some of the 19th centuryAlternative meaning: Nineteenth Century (periodical ( 18th century — 19th century — 20th century — more centuries) As a means of recording the passage of time, the 19th century was that century which lasted from 1801- 1900. Events The Little Ice Age ended myths that grew up around them endure, particularly in the northern hemisphere. It is still sometimes thought, for example, that the monotremes are "inferior" or quasi- reptilianCrocodilia Crocodilians Rhynchocephalia Tuataras Squamata Suborder Sauria Lizards Suborder Serpentes Snakes Testudines Turtles Superorder Dinosauria Saurischia Ornithischia The reptiles are a group of vertebrate animals. Most reptiles are tetrapods, and t, and that they are a distant ancestor of the "superior" placental mammals. It now seems plain that modern monotremes are the survivors of an early branching of the mammal tree; a later branching is thought to have led to the marsupialSuperorder Ameridelphia Didelphimorphia Paucituberculata Superorder Australidelphia Microbiotheria Dasyuromorphia Peramelemorphia Notoryctemorphia Diprotodontia Marsupials are mammals in which the female typically has a pouch (called the marsupium from wh and placental groups.
Similarly, it is still sometimes said that monotremes have less developed internal temperature control mechanisms than other mammals, but more recent research shows that monotremes maintain a constant body temperature in a wide variety of circumstances without difficulty. (Consider the case of a Platypus living in an icy mountain stream.) Early researchers were misled by two factors. Monotremes maintain a lower average temperature than most placentals (around 32°C, as compared with about 35° for marsupials, 38° for most placentals, or 41° for typical birds). Secondly, the Short-beaked Echidna (which is much easier to study than the reclusive Platypus) only maintains normal temperature when it is active: during cold weather, it conserves energy by "switching off" its temperature regulation.