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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 chromosomes?
Mabiya Reply
it is a cell structure that contains DNA histones protein and other structural proteins
Ekechi
what is liver
Hassan Reply
liver is an organ in the body that meterbolise nutrients and produce bile.
Ekechi
what is the function of granum?
Gulfam
What is biology
Eric Reply
Biology is you
muayad
Biology is the natural science that studies life and living organisms, including their physical structure, chemical processes, molecular interactions, physiological mechanisms, development and. evolution
Suzette
biology is the study of living things and none living things
Ekechi
The mamalian endoskeleton has 206 bones
kaazim Reply
hii
Hannah
hi
Suzette
how you
Hannah
good thanks
Suzette
have you done the actual course?
Suzette
How many bones do human body have?
moses Reply
what are these geneti materials
ONELIA Reply
Name five parts of female reproductive system and state the function of each part
Angel Reply
What is biology
Angel
What is biology
Angel
biology is the study of all living organisms
ketchem
Ok
Angel
Name five parts of female reproductive system and state the function of each part
Angel
What is the meaning of respiration
Idriss Reply
the act or process of breathing .
Suzette
the process is very long as I know
Idriss
do you want more details ?
Suzette
yes offcourse
Smiso
what does the term diploid and haploid mean?
ONELIA
what is virgina
Shebas Reply
A female reproductive organs
Julius
fuvu LA binadam linamifupa mingapi?
Hamis Reply
explain dicot n monocot plant
Henry Reply
Di= 2cotiledon mono=cotiledon
muayad
Mono = 1 cotiledon
muayad
what is virginia
Shebas
A Virginia is a part of a female ,the passage of a child during child birth, receiver of the penis and the passage of menstral flow
Idriss
what is biology
Hassan Reply
what is liver
Hassan
Biology is the study of living organisms
Gifty
is the study of life of living organisms
Chansa
study all about organisms like their reproduction, evolution , behaviour Physiology, etc
Ubaid
this is what we call "life"@
Chansa
is the study of living organisms on earth
okubasu
It is the study of living organism
Idriss
Biology is the study of all living organisms
ketchem
what is copulation
ketchem
metabolism is the anabolic and catabolic process of how food are broken from large molecules into smaller soluble molecules.
Prince Reply
what is Metabolism
Nakagolo Reply
Metabolism are all chemical reaction that takes place inside cells including those that takes energy and realise energy
Idriss
Describe digestion in a cat
Nakagolo Reply
what is hoemostasis
Nakagolo
Way of animals to keep internal conditions
muayad
balancing of water in the body
Dennis

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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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