By morphological, anatomical and physiological modifications of characters mammals have occupied all habitable spaces of the earth. Due to perfect adaptive radiation this group has become so vast that a satisfactory classification becomes a voluminous one. So, in this text we have restricted to classify this class up to Cohort mentioning the names of the orders with examples. Mammals have originated from mammal like reptile in Triassic age.
In this text Young’s (1981) classification has followed.
General characters of the class Mammalia:
- The body of all mammals is covered with hair.
- Body distinctly divided into head, neck, trunk and tail.
- Well developed pinnae present except monotremata.
- Various skin glands such as sweat gland, cebaceous gland etc. are present.
- Mammary glands are present in both sexes but only functional in females. The secretion of this gland gives nourishment to the young.
- Skull with two occipital condyles.
- Lower jaw is composed of a single bone dentary only.
- Vertebrae acoelous type (If both the anterior and posterior surfaces of the vertebra are flat).
- Cervical vertebrae generally 7 in number.
- Double headed ribs resent.
- Foot posture is plantigrade, digitigrade or unguiligrade type.
[A plantigrde is an animal walking with the entire sole of the foot on the ground, as humans.
A digitigrade is an animal that stands or walks on its digits, or toes.
A ungulate, walking on the nail or nails of the toes (the hoof).]
- Thoracic cavity is separated from abdominal cavity by a diaphragm.
- Heart completely four chambered, only left aortic arch present.
- RBC circular, non nucleated.
- Kidneys metanephric. Ureters open into a muscular urinary bladder.
- Male with a penis for copulation.
- Separate urinary and genital apertures (except monotremata).
- Brain well developed with cerebellum and large cerebrum.
- Fertilization internal, embryo develops within uterus.
- Embryo remains connected with uterus through placenta through which the developing embryo gets its nourishment.
- Except monotremata which lay eggs, all mammals give birth to young.
- Thecodont, heterodont and mostly diphyodont dentition present.
(Thecodont– having teeth that grow in sockets,
Heterodont– having teeth of different types,
Diphyodont– Having two successive sets of teeth, deciduous and permanent
Homodont– having teeth that are all of the same type
Polyphyodont– having many successive sets of teeth, as fishes and other lower vertebrates)
Sub Class I: Prototheria
Primitive, reptile-like, oviparous or egg-laying mammals.
- They lay eggs.
- Mammary gland without nipple.
- Well developed cloaca.
- Urinary and genital apertures are not separate.
- No well developed pinnae.
Order (1) Docodonta- This order includes controversial fossils of uncertain affinities, probably close to Triconnodont. All are fossils. Ex.- Docodon.
Order (2) Trichodont- All are fossils. Ex. Tricodont.
Order (3) Multituberculata. All are fossils. Ex.- Morganucodont.
- Body covered with hairs.
- Dorsal hairs have been modified into spines.
- Digits with sharp claws, webbed feet.
- Lips have been modified into beak.
- Teeth present in young, absent in adults.
- Though there is no nipple in mammary gland still milk is secreted.
- Nictitating membrane present but tear glands absent.
- Corpus callosum absent.
- Sutures absent in the skull.
- Coracoid and precoracoid present.
- T-shaped sternum.
- Presence of epi-pubic bone.
- Egg hatches to immature embryo which is reared in special pouch.
Distribution: Australia, New-guinea, Tasmania.
Examples- Ornithorhynchus, Tachyglossus, Echidna.
Sub-Class (II) Theria:
Modern or typical viviparous mammals that give birth to living young.
- Uterine gestation.
- Placenta developed.
Order I- Eupantotheria. All are fossils. Ex.- Eupantotheria
- Immature youngs are born and are reared in a special pouch known as marsupium.
- They do not possess any placenta.
- Body covered with soft fur.
- All females possess marsupium.
- Tail well developed and powerful.
- Forelimbs are shorter than hindlimbs.
- Presence of epi-pubic bone.
- Placenta generally absent.
- Presence of bifid penis in male and double vagina in females.
- The 4th digit of hind limb is the tallest, 2nd and 3rd digit of this limb are smaller and covered by skin-syndactylous (An animal, especially a bird or mammal, that has two or more fused digits).
Examples- Macropus, Didelphis.
- Single vagina.
- No marsupium.
- Snout present.
- No epi-pubic bone.
- Anus and reproductive openings are separate.
- Ribs double headed.
- Tympanic bone circular.
- Placenta well developed.
Order (1)- Insectivora
1. Small mammals with long pointed snout.
2. Usually with 5 toe and claws.
3. Molars with pointed peg-like cusps for insect feeding.
Examples- Talpa, Solenodon, Erinaceous.
Order (2)- Chiroptera
1. Flying mammals in which forelimbs are modified into wings.
2. Hind legs short and included in wing membrane.
3. Teeth small, sharp, peg like
4. Nocturnal, capable of true flight and known as bats.
It has two sub-orders namely Megachiroptera and Microchiroptera.
Examples- Pteropus, Vespertitio.
Order (3)- Dermoptera
1. Limbs are equal.
2. Limbs and tail are included in a lateral furry skin fold- the Petagium.
3. Arboreal and nocturnal, capable of flight, known as flying lemur.
Order (4)- Edentata
1. Teeth absent or reduced to molars without enamel.
2. Toes with large strong, curved claws.
3. Mostly insectivorous.
Examples- Myrmecophaga, Dasypus.
Order (5)- Pholidota
1. Body covered with horny overlapping scale with sparse hair between them.
2. No teeth.
3. Tongue long and protrusible, used to capture insects.
Example- Manis (Pangolin)
Order (6)- Tubulidentata
1. No incisor or canines.
2. Each jaw with 4-5 teeth.
3. Each tooth is perforated by numerous fine tubules and hence the name tubulidentata.
Order (7)- Primates
1. Brain is highly developed.
2. Flat nails on fingers and toes.
3. First digit usually opposable- an adaptation for grasping.
4. Eyes totally large and turned forward.
5. Mostly arboreal.
Examples- Lemurs, Loris, Tarsius, Anthropus (apes), Homo (Man)
Order (8)- Rodentia
1. Small mammals.
2. Each jaw with one pair of long chisel-like incisors growing throughout life.
3. No canine, so diastema present.
Examples- Rattus (Rat), Mus (Mouse), Funambulus (Squirrel), Cavia (Guineapig)
Order (9)- Lagomorpha
1. Incisors two pairs.
2. Size larger than rodents.
Example- Oryctolagus (Rabbit), Lepus (Hare)
Cohort- II- Mutica
Order (10)- Cetacea
1. Marine, fish-like mammals, large sized mammals well adapted for aquatic life.
2. Pectoral limbs modified into broad paddle like flippers.
3. Tail is divided into two horizontal fleshy flukes divided by a notch.
4. Claws, hind limbs and external ears are absent.
Examples-Physeter (Sperm whale), Orcinus (Killer whale), Platanista gangetica (Ganges Dolphin) etc.
Order (11)- Sirenia
1. Large herbivorous aquatic mammals.
2. Paddle-like forelimbs, hind limb absent.
3. Flattened tail with horizontal lateral fleshy fluke with or without notch.
4. No external ears, muzzle blunt.
Examples- Trichechus (Manatee), Dugong etc.
Order (12)- Carnivora
1. Small to large predatory flesh eating mammals.
2. Digit with claws well developed.
3. Incisor small, canines large, molars cutting types.
Examples-Canis familiaris (Dog), Canis lupus (Wolf), Hyaena, Phoca (Seal)
Order (13)- Hyracocoidea
1. Small guineapig like mammals.
2. Snout, ears and legs short.
3. No canine, molar lophodont (Having molar teeth whose grinding surfaces have transverse ridges.)
Order (14)- Proboscidea
1. Largest living land mammals.
2. Hairless skin (Pachyderm).
3. Large pillar like, 3-5 toes with small nail like hoops.
4. Nose and upper lip together modified to form a long flexible organ known as proboscis or trunk.
5. Upper incisor is elongated as an ivory tusk.
6. Cheek teeth lophodont type.
Examples- Indian elephant-Elephas maximus.
African elephant- Loxodonta africana
Order (15)- Perissodactyla
1. They are odd toed hoofed mammals (toes 1 or 3).
2. Functional axis of the foot passes through 3rd toe.
3. Incisor present in both jaws.
Examples- Equus cabalus (Horse), Rhinoceros (Rhino)
Order (16)- Artiodactyla:
1. They are even toed hoofed mammals (toes 2 or 4).
2. Axis of the foot passes through 3rd and 4th digit.
3. All are ruminant except pigs and peccaries.
Examples- Sus (Pig), Capra (Goat), Bos indicus (Cow), Bubalus bubalis (Buffalo), Ovis (Sheep), Camelus (Camel), Cervus (Deer), Giraffa (Giraffe) etc.