The typical features of mammals are: a body (more or less) covered with hair; the presence of the diaphragm muscle (that separates the thorax from the abdomen); mammary glands that produce milk (in females); non-nucleated blood red cells; and a middle ear with three ossicles.
The three groups into which mammals are divided are: monotremes (or Prototheria, such as the platypus), marsupials (or Metatherian, such as kangaroos) and placentals (or Eutheria, such as humans).
Mammals of the monotreme group (platypus, echidnas) are oviparous, or egg-laying, and do not have a placenta. Mammals of the marsupial group (kangaroos, koalas, opossums) do not have a placenta either; females of this group give birth to embryonic young that then continue development within the mother’s pouch. The placenta only forms in female placental mammals.
Mammal Review - Image Diversity: placenta
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The orders into which placental mammals are divided are the following:
Artiodactyls are mammals with an even number of fingers in claws or paws like, such as cows, sheep and giraffes. Carnivorous mammals are predators with canine teeth such as dogs, lions and tigers. Cetaceans are aquatic animals without posterior limbs, such as whales and dolphins; they are similar to fish. Edentates are mammals with rare or absent teeth, such as sloths, armadillos and anteaters. Lagomorphs are small-sized mammals with three pairs of continuously growing incisor teeth specialized in gnawing, such as rabbits and hares. Perissodactyls, also known as ungulates (hooved), are large animals with an odd number of fingers on each paw, such as horses and rhinos. Primates are characterized by their large cranium and well-developed brain, such as humans and apes. Proboscideans are large animals whose nose and upper lip form a trunk (snout), such as elephants. Chiropterans are flying nocturnal mammals (bats). Rodents are animals with two pairs of continuously growing incisor teeth, such as mice, rats, beavers and squirrels. Sirenians are freshwater aquatic mammals lacking of posterior limbs, such as dugongs and manatees.
Mammal Review - Image Diversity: artiodactyls carnivorous cetaceans edentates lagomorphs perissodactyls primates proboscideans chiropterans rodents sirenians
Mammals breathe through lungs. Their respiration is pulmonary.
Mammals have a closed and complete circulatory system. Their heart has four chambers and arterial blood does not mix with venous blood.
Mammal Review - Image Diversity: mammal heart
Like chondrichthyan fish and adult amphibians, mammals are ureotelic, meaning that they excrete urea.
Placental mammals reproduce sexually. They have internal fertilization and are viviparous, meaning that their embryo develops within the mother’s body and obtain nutrients from her via the placenta.
Fertilization in mammals is internal, with copulation. In the contemporary world, human technology is able to carry out the artificial external fertilization of human gametes and gametes of other animals.
In mammals, embryonic development is direct, without a larval stage.
Cetaceans (whales, dolphins) and sirenians (dugongs, manatees) are aquatic mammals. Chiropterans (bats) are flying mammals.
Bat and bird wings have the same function and the same origin (they are modified limbs) so they are analogous and homologous organs. Whale fins are a modification of posterior limbs while fish fins, although they have the same function, are not descended from modified limbs. Therefore, they are analogous but not homologous structures.
Evolutionary homology suggests common ancestry. Biological analogy relates to the concept of evolutionary convergence, that is, the appearance of similar features in evolutionarily distant species that live in the same type of environment (in the case mentioned, the aquatic habitat).
Examples of representative species: dogs, cats, horses, giraffes, elephants, apes, humans, bats, whales, dolphins, opossums, kangaroos, platypuses. Basic morphology: hair, diaphragm muscle, mammary glands, enucleated red blood cells. Skin: impermeable, hairy. Respiration: pulmonary. Circulation: closed and complete, heart with four chambers. Nitrogen waste: urea. Thermal control: homeothermic. Types of reproduction: sexual, internal fertilization, oviparous monotremes (prototherians), marsupials (metatherians), placental (eutherians).
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