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In the appendicular skeleton, the shoulder girdle of therian mammals is modified from that of other vertebrates in that it does not possess a procoracoid bone or an interclavicle, and the scapula is the dominant bone.

Mammals evolved from therapsids in the late Triassic period, as the earliest known mammal fossils are from the early Jurassic period, some 205 million years ago. Early mammals were small, about the size of a small rodent. Mammals first began to diversify in the Mesozoic Era, from the Jurassic to the Cretaceous periods, although most of these mammals were extinct by the end of the Mesozoic. During the Cretaceous period, another radiation of mammals began and continued through the Cenozoic Era, about 65 million years ago.

Living mammals

The eutherians, or placental mammals, and the marsupials together comprise the clade of therian mammals. Monotremes, or metatherians, form their sister clade.

There are three living species of monotremes : the platypus and two species of echidnas, or spiny anteaters. The leathery-beaked platypus belongs to the family Ornithorhynchidae    (“bird beak”), whereas echidnas belong to the family Tachyglossidae    (“sticky tongue”) ( [link] ). The platypus and one species of echidna are found in Australia, and the other species of echidna is found in New Guinea. Monotremes are unique among mammals as they lay eggs, rather than giving birth to live young. The shells of their eggs are not like the hard shells of birds, but are a leathery shell, similar to the shells of reptile eggs. Monotremes have no teeth.

These illustrations show two short-haired mammals (platypus and echidna) with webbed feet, flat tails and a flat snout.
(a) The platypus, a monotreme, possesses a leathery beak and lays eggs rather than giving birth to live young. (b) The echidna is another monotreme. (credit b: modification of work by Barry Thomas)

Marsupials are found primarily in Australia, though the opossum is found in North America. Australian marsupials include the kangaroo, koala, bandicoot, Tasmanian devil ( [link] ), and several other species. Most species of marsupials possess a pouch in which the very premature young reside after birth, receiving milk and continuing to develop. Marsupials differ from eutherians in that there is a less complex placental connection: The young are born at an extremely early age and latch onto the nipple within the pouch.

The illustration shows an animal resembling a small bear lying in the grass.
The Tasmanian devil is one of several marsupials native to Australia. (credit: Wayne McLean)

Eutherians are the most widespread of the mammals, occurring throughout the world. There are 18 to 20 orders of placental mammals. Some examples are Insectivora, the insect eaters; Edentata, the toothless anteaters; Rodentia, the rodents; Cetacea, the aquatic mammals including whales; Carnivora, carnivorous mammals including dogs, cats, and bears; and Primates, which includes humans. Eutherian mammals are sometimes called placental mammals because all species possess a complex placenta that connects a fetus to the mother, allowing for gas, fluid, and nutrient exchange. While other mammals possess a less complex placenta or briefly have a placenta, all eutherians possess a complex placenta during gestation.

Section summary

Mammals in general are vertebrates that possess hair and mammary glands. The mammalian integument includes various secretory glands, including sebaceous glands, eccrine glands, apocrine glands, and mammary glands. Mammals are synapsids, meaning that they have a single opening in the skull. A key characteristic of synapsids is endothermy rather than the ectothermy seen in other vertebrates. Mammals probably evolved from therapsids in the late Triassic period, as the earliest known mammal fossils are from the early Jurassic period. There are three groups of mammals living today: monotremes, marsupials, and eutherians. Monotremes are unique among mammals as they lay eggs, rather than giving birth to young. Eutherian mammals are sometimes called placental mammals, because all species possess a complex placenta that connects a fetus to the mother, allowing for gas, fluid, and nutrient exchange.

Questions & Answers

what is dormancy?
Aliyu Reply
hello guys what's the difference between prokaryotes and eukaryotes
Nwachukwu Reply
hlo what are the applications of biology?
Namawejje
deviation from mendelian
Ogali Reply
what is lethal allele
Ogali
Explain how chemical , bioligical and physical interaction between themselves and the non living components ?
Beyan Reply
what is Tissues
Faith Reply
Group of similar cells performing some related functions We have some type of tissues Connective Muscle Epithelial Nervous tissues
peter
group of cells that is called tissue.
Nikita
what is abiotic?
Williams Reply
state 2 abiotic factors that affect the rate of transpiration in plants?
Benenge
non living things
Chris
what is a cell
Esther Reply
structural and functional group is called cells
Nikita
the structural and functional unit of life is called cell. the cell was discovered by Robert hook in 1665Ad.
ROHIT
The structural functional of unit is called cell
Anas
The structural and functional unit of cell
Anas
what is contained in a smooth endoplasmic reticulum?
Bleyd Reply
sperm
Chris
cool
Jo
sorry i meant the structure within the smooth endoplasmic reticulum
Bleyd
ribosomes
Ogali
what is cell made of
Jo Reply
cytoplasm,cell wall and nucleus
Ogali
what is sponge
Joseph Reply
sponge are the multicellular that have bodies full of pore. and do not have nervous, digestive and circulatry system. the studies of sponge is known as spongiology.
ROHIT
what is a photometer
Abigail Reply
photometer is an electrical device which use to measure the strength of electromagnetic rediation.
ROHIT
what is respiration
Alao Reply
Is the release of energy from food substance in a living organism
innocent
What is cytoplasm
Nitta Reply
it is a gell like structure found in cell it contains the mitochondria, nucleus, lysosome etc
Ogali
in other words it's the material that contains the organelles of an organism
Nwachukwu
Parasitic adaptation of tapeworm
Angela Reply
Describe osmoregulator or osmoconformers and how these tools allow animals to adapt to different environment.
Alick Reply
applications of biology
Namawejje Reply

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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