The Class Mammalia includes some 4000 species of living mammals
assigned to 26 orders. All mammals are endotherms and share three unique features,
not found in other animals.
- mammary glands under the skin on the ventral side of the body that produce milk for suckling the young
- three bones in the middle ear
Mammals also have a unique jaw articulation - a single bone in the
lower jaw - and their teeth are specialised to fulfil different functions (heterodonts).
The classification of mammals is still under debate.
In the scheme used here, there are two major groupings:
- the Subclass Prototheria - the monotremes (egg-laying mammals);
- the Subclass Theria - the live-bearing mammals comprising
- the Infra-class Metatheria - marsupials;
- the Infra-class Eutherians - the placental mammals.
Evolution of mammals
The great diversity of mammalian families and
species represents a number of different evolutionary experiments
that appear to have begun with minute, less than 5cm
long, and most probably nocturnal, creatures in the Triassic period (195-225 mya).
- The egg-laying monotremes (Prototherians) diverged from the main stock in the early
Jurassic period ( some 200 mya).
- Therian mamals split to form marsupials (Metatherians) and placental
mammals (Eutherians) in the Cretaceous period (around 90 mya).
- Marsupials and placental mammals evolved for the main part
in geographical isolation leading to many examples of parallel
evolution and convergence as they adapted to similar ecological niches.
Skulls and dentition
As mammals radiated, they adapted to feed on all kinds of vegetable material
and vertebrates, as well as the invertebrates thought to have been the
principal diet of their earliest ancestors. Mammalian success may be attributed therefore, at least in part, to
the capacity of their teeth and jaws to adapt:
- to manipulating, ingesting and processing different foodstuffs;
- to defending them against predators and rivals.
The refinement of their senses - sight, hearing, touch
and scent - has been instrumental in enabling mammals to adapt:
- to locating and capturing a wide range of food;
- to finding mates and evading predators.
The different ways that the senses and brains evolved in different species
- the relative dominance of their eyes, ears, whiskers, noses;
- the size of their brain cases;
- the shape of their skulls;
- the outward appearance of the head.
As a result the skull and dentition, as well as the limbs,
may be said to reveal more about a mammal's
life style that any other skeletal feature. The Natural History Collections
has therefore acquired an extensive collection of skulls and limbs, as well as skeletons of small mammals, to
illustrate courses in mammalian biology.
The following web-pages use these skeletons, skulls and limbs:
- to illustrate the diversity of the Class Mammalia;
- to show how the composition and structure of teeth help mammals
- obtain, manipulate and process their individual diets in their
- defend themselves against rivals and predators.
Photos of our stuffed specimens and living animals
show the outward features that characterize the different families
of mammals and identify individual species.
Monotremes are egg-laying mammals.
They are called monotremes because they have
only one posterior opening, the cloaca into which the rectum and urinogenital sinus both open and
through which gametes, urine and faeces all pass to the outside. The eggs are fertilised internally
and laid in a nest. The young are suckled from mammary glands, which do not have nipples,
but open individually over a large area of the ventral surface of the female.
monotremes lack external ears. The two families of extanct monotremes - the platypuses and
echidnas - have decidely different appearances and life styles. They occur only in Australasia.
Marsupials and placental mammals
Marsupials and placental mammals give birth to live young.
Marsupials like opossums give birth to very immature
young which develop attached to nipples in an abdominal pouch or 'marsupium'.
The rectum and urinogenital sinus open together at a common, short cloaca.
The 266 species of living marsupials are confined to the Americas and Australiasia.
Placental mammals also give birth to live young
but their young are nurtured before birth and develop to a relatively mature
condition within a uterus. The foetuses are attached to the mother by an allantoic placenta. There is no cloaca and the anus opens separately from the urinogenital tract. There are about
3800 species of placental mammals. They occur in terrestrial and aquatic habitats throughout the world.