Mammals stand at the pinnacle of evolutionary success amongst the living organisms. The question of mammalian ancestry is of special interest to man, because their history includes ours in a broader sense, as man is included under this great class, Mammalia.
Mammals constitute a very unique group and separate themselves from others by having many morphological and physiological characteristics which help them to overcome in hospital environmental hurdles.
A survey of the past geological records reveals that in Mesozoic era, there was a storm of evolution. This era is regarded as the age of reptiles. The majestic and dominant forms were the Dinosaurs, the then giant rulers of the horizon.
When the Dinosaurs ruled the earth, a group of small and insignificant creatures evolved under their shadows. Though lowly in origin, but unique in their endowments, they constituted the mammals.
The cause of extinction of Dinosaurs is quite obvious. Of other factors, hugeness in size, lowly developed nervous system, inability to retain constant body temperature, can be regarded as the prime causes of extinction.
Mammals, with efficient mechanism of regulating body temperature, welldeveloped nervous system, efficient power of swift movement confronted the challenges of evolution. From such a lowly start arose the mammals—which became diversified and ruled the earth.
Definition of Mammal
Mammals are warm-blooded vertebrates. The body is covered more or less with hairs. Young mammals are born alive (excepting the egg-laying monotremes) and are nourished by mothers with milk in infancy secreted by the mammary glands.
Separate opening of the anus and the urino-genital system (excepting monotremes) is characteristic.
Biological Organisation of Mammals
For a better understanding of the mammalian ancestry, a generalised survey of the mammalian organisation of existing as well as of the extinct forms is desirable.
- Heterodont dentition, with differentiation into incisors, canines, premolars and molars.
- Thecodont teeth, i.e., each tooth is embedded in an alveolar pocket on the jaw bone.
- Diphyodont teeth, i.e., milk set is replaced by permanent set of teeth.
- Normal dental battery is:
- Presence of two Aoccipital condyles.
- Lower jaw is made up of one pair of bones—the dentaries.
- Articular and quadrate bones are transformed into malleus and incus respectively.
- Secondary palate is complete and functional.
- Cervical vertebrae are seven in number.
- Ribs are double headed with capitulum and tuberculum.
- Vertebrae are gastrocentrous.
- Coracoid of the pectoral girdle is absent as a separate bone and is represented as the coracoid process of the scapula.
- Heart is fully four- chambered with only left aortic arch.
- Mature erythrocytes are enucleated, biconcave and circular.
- A muscular diaphragm separates the thoracic and abdominal cavities.
- Presence of sweat and sebaceous glands.
- Highest development of brain, great development of neopallium and presence of corpus callosum (absent in monotremes, rudimentary in marsupials).
- Pinna is present excepting some aquatic mammals.
- Complete separation of oxygenated and deoxygenated blood with a free scheme of circulation.
- Homoiothermous condition is a new attribute which keeps the body temperature constant and is independent of environmental fluctuation.
- Viviparous excepting monotremes and young’s are nourished in the uterine cavity by placenta in embryonic condition.