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The traditional "Theria hypothesis" states that the divergence of the monotreme lineage from the Metatheria and Eutheria lineages happened prior to the divergence between marsupials and placentals, and this explains why monotremes retain a number of primitive traits presumed to have been present in the synapsid ancestors of later mammals, such ...
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.
Monotremes are the only mammals (apart from the Guiana dolphin) [55] known to have a sense of electroreception, and the platypus's electroreception is the most sensitive of any monotreme. [56] [54] Feeding by neither sight nor smell, [57] the platypus closes its eyes, ears, and nose when it dives. [58]
Jared Diamond in his book Guns, Germs, and Steel enquired as to why, among the world's 148 large wild terrestrial herbivorous mammals, only 14 were domesticated, and proposed that their wild ancestors must have possessed six characteristics before they could be considered for domestication: [3]: p168-174
Marsupials have the typical characteristics of mammals—e.g., mammary glands, three middle ear bones, (and ears that usually have tragi, [3] varying in hearing thresholds [4]) and true hair. [5] There are, however, striking differences as well as a number of anatomical features that separate them from eutherians.
The bipeds have adapted to move using only the two lower limbs, while the rear limbs of cetaceans and the sea cows are mere internal vestiges. Mammals range in size from the 30–40 millimetres (1.2–1.6 in) bumblebee bat to the 30 metres (98 ft) blue whale—possibly the largest animal to have ever
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.
Theria (/ ˈ θ ɪər i ə / or / ˈ θ ɛr i ə /; from Ancient Greek θηρίον (thēríon) 'wild beast') is a subclass of mammals [2] amongst the Theriiformes.Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes and various extinct mammals evolving prior to the common ancestor of placentals ...