Class Mammalia – Mammals

Class Mammalia

The term mammal was provided by Linnaeus to the group of animals that are nurtured by milk from the mother. The group is considered to be the highest in the animal kingdom.

  • Their improvement over other groups is quite pronounced.
  • The most essential advancement is the evolution and development of their brain (nerve system) over the other vertebrates.
  • It is widely accepted by biologists that mammals have evolved from reptilian forefathers, the cotylosaurs.
  • This has been determined based on the fossil record which is easily readily available because of the hard bones that were protected as fossils, unlike the birds which have soft bones and primarily have not been preserved.
  • The ancestors of mammals lived concurrently together with reptiles during the Jurassic times and have been called mammal-like reptiles.
  • Some were only the size of mice and resided in trees. One of these early reptiles was varanope which was found as a fossil in Texas.
  • Most likely a minimum of five groups of such mammal-like reptiles developed mammalian characters and were 50% mammals.
  • Mammals ended up being dominant in the Cenozoic period.
General Characters of Mammals:
  1. The body of mammals is covered by skin hair.
  2. Integumentary glands are– sweat (sudoriferous), sebaceous (oil), odor secreting (odoriferous) glands.
  3. Teeth are of various types (heterodont) and are embedded in the sockets of jaws (the codont). These are developed two times during the life-time of the animal (diphyodont), milk, and permanent teeth.
  4. Other than a couple of, mammals have 7 cervical (neck) vertebrae.
  5. The skull is dicondylic i.e., with two occipital condyles.
  6. Mammary glands exist to provide milk for the nourishment of the suckling young.
  1. External fleshy pinna exists in mammals.
  2. Eyes are with upper and lower eyelids and often with eyelashes.
  3. The nictitating membrane is translucent and hairless; it is vestigial in higher mammals.
  4. The four-chambered heart is highly powerful.
  5. Just a left aortic arch exists in the arterial system.
  6. Cerebral hemispheres are large and highly complicated.
  7. The cerebellum is large, complex, and solid in mammals.
  8. There is a single urinary bladder in mammals.
  9. Except for egg-laying mammals they are viviparous. There exist 4 embryonic membranes: chorion, amnion, allantois, and yolk sac. Except for egg-laying mammals, a well-established placenta is present.
  10. Mammals are present in almost all sorts of environments. They are dominant animals and are capable to find out because of their better-developed brain.
  11. 17.Endo-thermal homeotherm animals.
  1. RBCs are non-nucleated, biconcave, and generally circular in form.
  2. Cranial nerves are twelve in pairs.
  3. A bony palate is formed by the union of premaxillae, maxillae, and palatines that separates the nasal passage from the buccal cavity.
Classification of Living Mammals

Mammals are classified into 3 sub-classes:

  1. Prototheria – egg-laying mammals
  2. Metatheria – pouched mammals
  3. Eutheria – Placental mammals including Humans
1.Sub-Class Prototheria

The Prototheria is that group that has attributes of both reptiles and mammals and for that reason form a connecting link between the two. They also supply evidence of the evolution/origin of mammals from reptilian stock. Particular members of this sub-class are adjusted for water life as the duckbill which has a bill comparable to that of a duck and has webbed toes. It has thick fur on its body.

The female has mammary glands to feed the young. Both these are mammalian characters. At the same time, these animals have cloaca and cloacal opening instead of different openings for the gastrointestinal system and urinogenital system. Both of these characters are reptilian characters. These animals are found in Australia, e.g., Duckbill Platypus and Echidna (Spiny anteater).

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2.Sub-Class Metatheria


Besides Prototheria, the Metatheria are the most primitive mammals. They are identified by a stomach pouch the marsupium where they rear their young. The young when born are immature and are brought by the mother in the marsupium till they develop to their optimum.

Throughout this period, they are fed on the milk produced by the milk glands of the mother, the nipples of which remain in the marsupium. For this reason, these animals are likewise called marsupials or pouched mammals, e.g., Opossum, Kangaroo, and Tasmanian wolf found in Australia and America.


3.Sub-Class Eutheria

This sub-class includes placental mammals. In the body of the mother development of the young is maximum and the young, when born, are fully developed. In these mammals throughout development, a structure called the placenta is formed through which the fetus is nurtured. Likewise, the placenta has an endocrine function i.e., it produces certain hormonal agents, for this reason, these mammals are also called placental mammals. Placental mammals have maximum mammalian characters but in some, the hair has become modified into scales (pangolin) and spines (porcupine).



Principal Orders of Placental Mammals

Some of the principal orders of placental mammals are briefly explained here:

(1) Insectivora (L. insectum- insect, vorare- to consume)

They eat insects. Testes are abdominal. The water shrew is the tiniest mammal which is as big as a human thumb e.g., shrews, moles, and hedgehogs.

(2) Dermoptera (Gr. derm- skin, pteron- wing)

A hairy skin fold called patagium extends like a parachute from neck to tail for sliding, e.g., flying lemurs. Actually, flying lemurs are neither real lemurs nor do they fly.

(3) Chiroptera (Gk. Cheiros- hand pteron- wing)

They are flying mammals. The forelimbs are modified into wings, e.g., bats and flying foxes.


(4) Edentata (L edentatus- toothless)

They are toothless. This order consists of the armadillos and sloths of South America.


(5) Primates (L. primus- of the first rank)

Primates have a highly developed brain. The living primates include prosimians (significance prior to monkeys) and simians. The prosimians consist of lemurs, lorises, and tarsiers the simians include monkeys, apes, and humans.


(6) Rodentia (L. rodo- gnaw)

They have one set of sharp chisel-like incisors in each jaw. The canines are missing, leaving a toothless area, the diastema in the jaw no canines, e.g., rats, mice, squirrels, guinea-pigs, and porcupines.


(7) Cetacea (L. cetus- whale)

They have a fish-like body, well adapted for marine life. They have fin-like forelimbs, however no hind limbs. Testes are abdominal. The skin has a thick layer of fat called blubber working as reserve food, an insulator for reducing the particular gravity. Pinnae are reduced or absent. Hair is just on lips. They do not have sweat and oil glands, e.g., whales, dolphins, and porpoises. The blue whale is the biggest living animal. Whales typically do not have pelvic girdle and hind limbs.

The Green land whales, however, possess vestiges of pelvic girdles and bones of hind limbs inside the body.

(8) Carnivora (L. Caro- flesh, vorare- to eat)

They are flesh consuming mammals. These animals have sharply pointed canines, strong jaws, and well-established claws, e.g., dogs, cat, wolf, jackal, fox, cheetah, lion, tiger, hyaena, mongoose, bear, panda, otter, seal, walrus, sea lion.

Multiple Choice Questions (MCQs) – Class Mammalia – Mammals

  1. What is the term “mammal” derived from?
    • a) Mammalia
    • b) Mammary glands
    • c) Mammal-like reptiles
    • d) Linnaeus
  2. What distinguishes mammals from other vertebrates in terms of brain development?
    • a) Larger kidneys
    • b) Highly developed brain
    • c) Longer tails
    • d) Enhanced sense of smell
  3. Which sub-class of mammals lays eggs?
    • a) Eutheria
    • b) Prototheria
    • c) Metatheria
    • d) Amphibia
  4. What is the main characteristic of the Sub-Class Eutheria?
    • a) Pouched mammals
    • b) Egg-laying mammals
    • c) Placental mammals
    • d) Mammal-like reptiles
  5. How are teeth in mammals classified?
    • a) Homodont
    • b) Diphyodont
    • c) Heterodont
    • d) Isodont
  6. Which order includes flying mammals with modified forelimbs as wings?
    • a) Insectivora
    • b) Dermoptera
    • c) Chiroptera
    • d) Edentata
  7. What is the characteristic feature of Primates?
    • a) Highly developed brain
    • b) Toothless
    • c) Flying ability
    • d) Gnawing teeth
  8. Which order of mammals is known for gnawing with chisel-like incisors?
    • a) Rodentia
    • b) Carnivora
    • c) Cetacea
    • d) Edentata
  9. Which mammals are adapted for marine life and lack hind limbs?
    • a) Rodents
    • b) Cetaceans
    • c) Carnivores
    • d) Primates
  10. What is the primary function of the blubber in Cetaceans?
  • a) Flying
  • b) Thermal insulation
  • c) Gnawing
  • d) Nourishing young
  1. Which order includes mammals that consume insects?
  • a) Edentata
  • b) Insectivora
  • c) Carnivora
  • d) Primates
  1. Which Sub-Class of mammals has a pouch for rearing young?
  • a) Eutheria
  • b) Metatheria
  • c) Prototheria
  • d) Amphibia
  1. How many cervical (neck) vertebrae do most mammals have?
  • a) 5
  • b) 7
  • c) 12
  • d) 15
  1. What is the characteristic feature of the skin in Dermoptera?
  • a) Scales
  • b) Blubber
  • c) Patagium
  • d) Feathers
  1. Which mammalian order is characterized by toothless members?
  • a) Carnivora
  • b) Edentata
  • c) Rodentia
  • d) Cetacea
  1. How many pairs of cranial nerves do mammals typically have?
  • a) 5
  • b) 8
  • c) 10
  • d) 12
  1. What is the significance of the term “Eutheria”?
  • a) Egg-laying mammals
  • b) Toothless mammals
  • c) True placental mammals
  • d) Flying mammals
  1. Which order of mammals is known for its flying members with modified forelimbs as wings?
  • a) Chiroptera
  • b) Dermoptera
  • c) Primates
  • d) Cetacea
  1. What is the primary diet of animals in the Insectivora order?
  • a) Fruits
  • b) Insects
  • c) Fish
  • d) Leaves
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  1. b, 2. b, 3. b, 4. c, 5. c, 6. c, 7. a, 8. a, 9. b, 10. b, 11. b, 12. b, 13. b, 14. c, 15. b, 16. d, 17. c, 18. a, 19. b


Frequently Asked Questions (FAQs) – Class Mammalia – Mammals

  1. What is the significance of the term “mammal”?
    • Answer: The term “mammal” was coined by Linnaeus and refers to animals that are nourished by milk from the mother.
  2. How did mammals evolve, and from which ancestors?
    • Answer: Mammals are believed to have evolved from reptilian ancestors known as cotylosaurs, with evidence from the fossil record.
  3. What characterized the early reptilian ancestors of mammals during the Jurassic period?
    • Answer: These early reptiles, termed mammal-like reptiles, coexisted with reptiles during the Jurassic period, with some living on trees and resembling mice in size.
  4. How are the three sub-classes of mammals classified?
    • Answer: Mammals are classified into three sub-classes: Prototheria (egg-laying mammals), Metatheria (pouched mammals), and Eutheria (placental mammals, including humans).
  5. What is the distinguishing feature of Prototheria?
    • Answer: Prototheria serve as a connecting link between reptiles and mammals, exhibiting characteristics of both. They include water-adapted members like the duckbill and echidna.
  6. What defines Metatheria, and give an example?
    • Answer: Metatheria are pouched mammals with a stomach pouch (marsupium) where they rear their immature young. Examples include Opossum, Kangaroo, and Tasmanian wolf.
  7. Describe a key feature of Eutheria.
    • Answer: Eutheria represents placental mammals, where young are fully developed at birth. The placenta, facilitating fetal nourishment, is a defining feature.
  8. Can you name a few orders of placental mammals and their characteristics?
    • Answer: Orders include Insectivora (insect-eating), Dermoptera (with a patagium for sliding), Chiroptera (flying mammals with winged forelimbs), Edentata (toothless mammals like armadillos and sloths), and more.
  9. How are mammals generally classified based on their teeth?
    • Answer: Mammals typically have various types of teeth (heterodont) embedded in sockets (codont). They undergo two sets of teeth development (diphyodont) – milk and permanent teeth.
  10. What is the characteristic feature of the skin in Dermoptera?
    • Answer: Dermoptera, such as flying lemurs, have a hairy skin fold called a patagium that extends like a parachute from neck to tail for sliding.
  11. How many pairs of cranial nerves do mammals typically have?
    • Answer: Mammals generally have twelve pairs of cranial nerves.
  12. What are the characteristics of Cetacea, the order including whales and dolphins?
    • Answer: Cetaceans have a fish-like body adapted for marine life, lack hind limbs, and possess a thick layer of fat (blubber) for insulation. They have fin-like forelimbs and reduced or absent pinnae.
  13. Why are members of the order Carnivora named so?
    • Answer: Carnivores are flesh-consuming mammals characterized by sharply pointed canines, strong jaws, and well-established claws.
  14. Are mammals present in diverse environments, and why?
    • Answer: Yes, mammals are present in various environments due to their dominant status and better-developed brains, enabling them to adapt and learn effectively.
  15. Do all mammals have a placenta, and what is its role?
    • Answer: No, only Eutherian mammals have a well-established placenta. The placenta serves as a structure for fetal nourishment and has an endocrine function, producing certain hormones.
  16. What is the significance of the term “Eutheria”?
    • Answer: “Eutheria” refers to placental mammals, where the young undergo maximum development inside the mother and are born fully developed.
  17. How did mammals become dominant in the Cenozoic period?
    • Answer: Mammals became dominant in the Cenozoic period, likely due to the development of their brain and the evolution of mammalian characters from reptilian ancestors.
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Wrap up – Class Mammalia Tutorial

The tutorial on Class Mammalia provides a comprehensive overview of mammals, highlighting their evolutionary history, general characteristics, and classification. Key points include:

  1. Evolutionary Background:
    • Mammals, named by Linnaeus, are distinguished by nurturing their young with milk.
    • They evolved from reptilian ancestors, cotylosaurs, as evidenced by the fossil record.
  2. General Characteristics:
    • Mammals exhibit diverse features, such as skin covering, sweat and sebaceous glands, heterodont teeth, and unique skull structures.
    • Essential structures include mammary glands, external fleshy pinna, and a four-chambered heart.
  3. Classification of Living Mammals:
    • Mammals are classified into three sub-classes: Prototheria (egg-laying), Metatheria (pouched mammals), and Eutheria (placental mammals, including humans).
  4. Sub-Classes Overview:
    • Prototheria, representing a link between reptiles and mammals, includes water-adapted members like the duckbill.
    • Metatheria, or marsupials, rear their immature young in a pouch, e.g., Kangaroo and Opossum.
    • Eutheria, the placental mammals, have fully developed young at birth and possess a placenta for nourishment.
  5. Principal Orders of Placental Mammals:
    • Various orders, such as Insectivora, Dermoptera, Chiroptera, Edentata, Primates, Rodentia, Cetacea, and Carnivora, showcase diverse adaptations and characteristics.
    • Each order is unique in terms of diet, locomotion, and anatomical features.
  6. Adaptations for Environment:
    • Mammals are versatile, present in diverse environments due to their dominant status and well-developed brains.
  7. Notable Characteristics:
    • Endo-thermal homeotherm animals with specific characteristics like non-nucleated, biconcave RBCs and twelve pairs of cranial nerves.
    • Reproductive aspects, including viviparity, embryonic membranes, and well-established placentas.
  8. Conclusion:
    • Mammals became dominant during the Cenozoic period, marking a significant evolutionary milestone.
    • The tutorial provides a holistic understanding of mammalian diversity and adaptations.