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They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified by structural differences in their brains, jaws, digestive tract, reproductive tract, and other body parts, compared to the more common mammalian types.
Monotremes are the only mammals (apart from the Guiana dolphin) [50] known to have a sense of electroreception, and the platypus's electroreception is the most sensitive of any monotreme. [51] [49] Feeding by neither sight nor smell, [52] the platypus closes its eyes, ears, and nose when it dives. [53]
A mammal (from Latin mamma 'breast') [1] is a vertebrate animal of the class Mammalia (/ m ə ˈ m eɪ l i. ə /).Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle ear bones.
Figure 1:In mammals, the quadrate and articular bones are small and part of the middle ear; the lower jaw consists only of dentary bone.. While living mammal species can be identified by the presence of milk-producing mammary glands in the females, other features are required when classifying fossils, because mammary glands and other soft-tissue features are not visible in fossils.
The Old World species are divided into apes and monkeys depending on the number of cusps on their molars: monkeys have four, apes have five [72] - although humans may have four or five. [78] The main hominid molar cusp evolved in early primate history, while the cusp of the corresponding primitive lower molar (paraconid) was lost. Prosimians ...
The subclass Prototheria (Monotremes) comprises the five species of egg-laying mammals: platypus and four echidna species. The infraclasses Metatheria (Marsupials) and Eutheria (Placentals) together form the subclass Theria. [4] In the 2000s understanding of the relationships among eutherian mammals has experienced a virtual revolution.
Most mammals are viviparous, giving birth to live young. [1] However, the five species of monotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals. [2] In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian ...
Some have active camouflage that changes with need. [132] Distraction eye, many fish have spot on the tail to fool potential predators and prey; both are not sure which is the front, the direction of travel. [133] Gills appear in unrelated fish, some amphibians, some crustacean, aquatic insects and some mollusk, like freshwater snails, squid ...