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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.
Occipital condyles – Two knobs at the base of the skull fit into the topmost neck vertebra; most other tetrapods, in contrast, have only one such knob. [78] For the most part, these characteristics were not present in the Triassic ancestors of the mammals. [79] Nearly all mammaliaforms possess an epipubic bone, the exception being modern ...
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 ...
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]
The basic mammalian body type is quadrupedal, with most mammals using four limbs for terrestrial locomotion; but in some, the limbs are adapted for life at sea, in the air, in trees or underground. 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.
Eutheria (from Greek εὐ-, eú-'good, right' and θηρίον, thēríon 'beast'; lit. ' true beasts '), also called Pan-Placentalia, is the clade consisting of placentals and all therian mammals that are more closely related to placentals than to marsupials.
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 .
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.