Mammal Subclasses and Infraclasses

Among the mammals, there are three major variations in reproductive systems.  This is the basis for dividing them into subclasses and infraclasses.

class: Mammalia
subclass: Prototheria Theria
infraclass:   Metatheria  Eutheria

Members of the subclass Prototheria click this icon to hear the preceding term pronounced lay eggs like most non-mammalian vertebrates.  However, they feed their newborn with mammary gland secretions like all other mammals.  They lack nipples, but the skin over their mammary glands exude milk for their babies.  The Prototheria are also referred to as monotremes click this icon to hear the preceding term pronounced, which literally means that they have one opening for excretion and reproduction.  This is similar to birds and reptiles.  The Prototheria are also similar to reptiles in some aspects of their skeletons.  Notably, their legs are on the sides of their bodies rather than underneath them.  This results in a reptile-like gait.  There are only three surviving rare species groups of Prototheria.  These are the Australian platypus and 2 echidna (spiny anteater) species of Australia and New Guinea.

drawing of two  platypuses (or platypi)   drawing of an echidna

Platypus (subclass Prototheria)


Echidna (subclass Prototheria)

click this icon in order to see the following video Genome of the Platypus--video clip from
       (length = 7 mins. 30 secs.)

To find out a little more about the strange lives of monotremes, select the "Echidna Reproduction" button below:

  Echidna Reproduction  

All other living mammalian species, including humans, are in the subclass Theria click this icon to hear the preceding term pronounced. They have in common the fact that they give birth to live young.  Therian mammals apparently did not evolve from the Prototheria. The relatively primitive prototherian reproductive system evidently evolved after their evolutionary line separated from the other early mammals.

The oldest infraclass of therian mammals is the Metatheria click this icon to hear the preceding term pronounced, or the marsupials click this icon to hear the preceding term pronounced.  Their young are born very immature and cannot live without further development in the mother's pouch.  The word marsupial comes from marsupium, the Latin word for purse.  Marsupials include kangaroos, koalas, opossums, and many other similar animals.  Most of them are native only to Australia and New Guinea.

photo of a kangaroo mother with a baby in her pouch   photo of an adult koala in a tree  
Kangaroo, koala, and opossum (infraclass Metatheria)
  Koala Reproduction  
  drawing of a human fetus in utero with the placenta and umbilical cord highlighted
Human fetus in utero

Most mammal species, including humans, are in the infraclass Eutheria click this icon to hear the preceding term pronounced.  They are also referred to as placental click this icon to hear the preceding term pronounced mammals.  Eutherian mothers carry their unborn children within the uterus where they are nourished and protected until an advanced stage is reached.  This is made possible by the umbilical cord and placenta which connects the fetus to the uterus wall and enables nutrients and oxygen to get to the offspring as well as provides a means of eliminating its waste.  At the same time, the placenta functions as a barrier to keep the blood cells and other components of the immune systems of the mother and her fetuses separate to prevent their destruction. 

Giant pandas are an exception among the placental mammals.  Their babies are born at only 1/4 the size predicted for the general placental mammal pattern.  Marsupial babies are born at an even more immature stage because their rudimentary placentas are comparatively inefficient in nurturing fetuses.

Placental mammals have been extremely successful in out-competing monotremes and marsupials for ecological niches.  This is mostly due to the fact that their babies are born more mature, which increases their chances of survival.  This is particularly true of herbivores that are predated on by carnivores.  Marsupials give birth to early stage fetuses.  Placental mammals give birth after fetuses are much more developed.  The downside is that pregnant placental mammals must consume significantly more calories to nurture their fetuses and themselves, especially during the second half of their pregnancies.  Like monotremes and marsupials, placental mammals feed their babies with milk from their mammary glands.  Species that have multiple births at the same time generally have more mammary glands.  The number ranges from 2 in primates, goats, sheep, and horses to 18 in pigs.

Placental mammals are found on all continents, in the air, and in the seas.  Primates, cats, dogs, bears, hoofed animals, rodents, bats, seals, dolphins, and whales are among the dominant placental mammal groups today.  Nearly 94% of all mammal species now are placental mammals (5,080 species out of 5,416).

3 photos--a whale and a dolphin swimming together, a monkey, and a zebra
Whale, dolphin, monkey, and zebra (infraclass Eutheria)

The next tutorial in this series, The Primates, investigates all of the Linnaean classification categories below the infraclass level for humans, apes, monkeys, and some other closely related animals.  This will take us from the "order" level down to "species."

NEWS:  A team of researchers led by Wesley Warren at Washington University School of Medicine reported their completion of a draft of the platypus genome sequence in the May 8, 2008 issue of the journal Nature.  This showed that the platypus has about 18,500 genes (about 2/3 as many as humans) and that 82% are shared with humans, mice, dogs, opossums, and chickens.  Other platypus genes show links to reptiles, including those related to egg-laying, vision, and venom production.  Adult male platypuses can inject their poison with a spur just above the heel of each hind foot.  Apparently, they use this as a weapon against other males during the mating season.  Platypuses are also unusual in having sensors in their bills that are used to detect faint electrical fields from their prey when they hunt them under water.

NEWS:  The results of a 5 year global project sponsored by the Union for Conservation of Nature to survey all living mammals has been completed.  The researchers concluded in October 2008 that one half of the 5487 mammal species are declining in numbers and at least 1/4 are now threatened with extinction due primarily to habitat destruction, hunting by humans, and climate change (Jan Schipper et al., Science 1165115, 2008).


  Previous Topic  

  Return to Menu  

  Practice Quiz 

Copyright 1998-2012 by Dennis O'Neil. All rights reserved.
illustration credits