Mammals are the highest group of vertebrates occupying the top position in the evolutionary history. They include approximately 5,000 living species and numerous fossil forms. Mammals have been evolved from cotylosauri on reptiles. They originated in the Upper Carboniferous period and still continuing. Tertiary period is called as the age of mammals during which they attained the peak in the diversity.

Class mammalian is classified in to three subclasses. These are:

1. Subclass – I – Prototheria

2. Subclass – II – Allotheria


3. Subclass -III – Theria

Subclass – PROTOTHERIA: (Gr. Protos, – first + therios, beasts)

(i) There are primitive reptiles like, oviparous or egg-lying mammals.

(ii) Aquatic or terrestrials, nocturnal and insectivorous.


(iii) Mammary glands present without nipples or teats. Placenta absent.

(iv) Egg provided with large amount of yolk.

(v) Coracoids and precoracoid are well developed in pectoral girdle.

(vi) Ribs are single headed.


(vii) Corpus callosum is absent.

(viii) Cloaca is present.

(ix) The testis and abdominal, not outside.

(x) Uterus and vagina not formed.


(xi) Not fully homoeotheramous.

This subclass includes a single order – monotremata.

Example: Echidna, ornithorhynchus.

Subclass – II – ALLOTHERIA


(i) These are extinct mammals which were found during Jurassic to Ecocene period.

(ii) Canines were absent.

(iii) One lower and upper single order – multituberalata.

Example: plagianlax,ctenacoden etc.


Subclass – III – THERIA

(i) It includes few extinct forms and all other modern mammals.

(ii) Viviparous mammals Give birth to young ones.

(iii) Ears with external pinna.

(iv) Teeth formed in both adult and young.

(v) Mammary glands with nipples and teats.

(vi) Each oviduct in female is differentiated into upper fallopian tube and lower uterine part.

(vii) Ribes double headed.

(viii) Corpus callosum is well developed.

(ix) Cloaca is present or absent.

(x) Single vagina in female and single penis in male is the corpulatory organs.

(xi) Testes are extra abdominal, confined inside the scrotal sac.

(xii) Heart is completely four chambered.

This subclass is further sub-divided into two living infraclasses and one extinct infraclass.

Infraclass – I – Patotheria (All extinct)

It includes two orders.

Example: Docedon, Auphitherlary.

Infraclass – II -methatheria

(i) They are primitive type of mammals. Their young are born in immature condition. Their lather development takes place in the marsupial pouch found in the abdomen of female.

(ii) The mammary glands posses teats which open into the marsupium.

(iii) Without or with a rudimentary yolksac placenta.

(iv) Confined to Australia region and includes a single order marsupialia

Example: Marcropus (kangaroo)

Didelphis (opossum)

Infraclass – III – Eutheria

(i) Higher viviparous placenta mammals without marsupium.

(ii) Young born in a relatively advanced stage.

(iii) Dentition never exceeds 3.14.3/3.14.3 = 44

(iv) It includes a vast majority of living mammals arranged in 16 orders.

Example: Bat, Tiger, Rabbit, Elephant, Lion, Whale, Rhinos, Man etc.