In this article we will discuss about the classification of mammalia.
1. The skin is clothed with hairs and provided with sweat and sebaceous glands. Sweat evaporation keeps the body cool and sebum keeps the hair and skin soft and pliable.
2. Dermal milk gland or mammary gland present.
3. The teeth are heterodont with differentiation into incisors, canines and molars; thecodont, each tooth lying embedded in an alveolar pocket of jaw bones; and diphyodont, having two sets, a milk sot replaced by a permanent set.
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4. Fleshy cheeks and lips cover the edges of jaws and teeth.
5. A muscular diaphragm separates the abdominal and thoracic cavities.
6. The skull articulates with the atlas by two exoccipital condyles.
7. The lower jaw is composed of one pair of bones, the dentaries. Each dentary articulates with squamosal bone of the skull.
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8. The vertebrae are gastrocentrous. Each vertebra consists of a. centrum, a neural arch and two epiphyses. A cartilaginous intervertebral disc separates the centra of adjacent vertebrae.
9. The ribs articulate with vertebrae by two heads, capitulum and tuberculum.
10. Cervical vertebrae are seven.
11. The coracoid bone of pectoral girdle is reduced as a coracoid process on the scapula.
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12. The heart four-chambered, A single left aortic arch; enucleate erythrocytes.
13. The opening of the trachea is guarded by an epiglottis.
14. The cerebral hemispheres, especially the cerebral cortex or neopallium, highly developed; corpus callosum, a massive bundle of fibre tracts, connect the two cerebral hemispheres (absent in monotremes) cerebellum solid, large and complex.
15. Pinna present (except some aquatic mammals); tympanic membrane greatly depressed at the bottom of the external passage or auditory meatus; cochlea, the auditory sense organ, is spirally coiled -(not coiled in monotremes).
16. The middle ear contains three minute auditory ossicles or bonelets—malleus, incus and stapes—which serve to transmit sound vibrations from tympanum to perilymph. The bonelets are modified articulate, quadrate and hyomandibular bones, respectively, of reptiles.
17. The nasal passages-are provided with turbinal bones, which serve to .warm the inbreathed air.
18. Part of oviducts are specialised to form uterus; uterine gestation (except monotremes) occur; eggs microscopic (except monotremes, laying large eggs); placenta allantoic (not in monotremes and marsupials);.viviparous (except monotremes).
The mammals are placed in three subclasses:
A. Subclass Prototheria:
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Primitive mammals with both reptilian and mammalian characters, the former predominating. The living prototherians are included in the order Monotremata.
Order Monotremata:
In many respects monotremes are transitional between reptiles and mammals and sometimes are referred to as “living fossil.”
1. Pinna absent.
2. Cloaca present.
3. Skull bones in many respect transitional or reptilian; lacrimal bone absent; jugal reduced or absent; auditory bulla absent; traces of reptilian prevomer present.
4. Vertebrae for most part are without epiphyses.
5. The ribs, except the cervicals, are one- headed, the tuberculum being absent; cervical ribs present (absent in higher mammals).
6. Pectoral girdle possesses large, separate coracoid bones and an interclavicle.
7. The pelvic girdle possesses epipubic or marsupial bones, extending from the front of pelvis.
8. Corpus callosum absent.
9. Cochlea is a simple process, not coiled as in higher- mammals.
10. Paired oviducts open separately into the shallow cloaca.
11. Lays 1 to 3 eggs, measuring about 12 x 18 mm.
12. Embryonic development reptilian.
13. The typical mammalian characters are (i) milk glands, (ii) hair, (iii) a complete diaphragm.
Examples: Orinthorhynchus, Tachyglossus.
B. Subclass Allotheria:
Extinct forms. Flourished between Upper Jurassic and Eocene.
Example: Ctenacodon.
C. Subclass Theria:
Marsupials and Placental mammals.
1. Pinna present.
2. Cloaca absent.
3. Ureters open into the base of urinary bladder.
4. Testes of males usually in a scrotal sac; vasa deferentia and bladder opening through a common urethra running through the penis.
5. Oviduct differentiated into upper fallopian and lower uterine region opening into a distinct vagina.
b. Mammary glands with nipples.
7. Viviparous (produce living young).
The existing therians are placed in two infraclasses:
a. Infraclass METATHERIA (Pouched mammals):
Order Marsupialia or Didelphia:
1. Epipubic bone usually present.
2. Incisor teeth not equal in 2 jaws. Dental formula is 5.1.3.4./4.1.3.4.
3. Female usually with marsupium (ventral pouch) or marsupial folds surrounding nipples or abdomen.
4. Uterus and vagina double.
5. Usually no placenta.
6. Fertilization internal; development begins in uterus, after a few days premature baby leaves and crawls to marsupium and attaches closely to a mammary nipple by its mouth and remains as “mammary foetus” until fully formed.
7. All existing marsupials, except Didelphidae and Caenolestidae, are restricted and dominant mammals in Australia.
Examples: Didelphys, Myrmecobius, Dasyurns, Thylacines, Peragole, Notoryctes, Petaurus, Phalanger, Macropus, PJtascolomys, Caenotestes, Phascolarctos, etc.
b. Infraclass Eutheria or Placentalia or Monodelphia:
The placental mammals include about 95 per cent of all living, mammalian species.
1. Marsupial bones and pouch absent.
2. Dental formula 3.1.4.3/3.1.3.4.
3. Internal carotid artery enters the skull through the foramen lacrum medius or through the bulla.
4. Corpus callosum connects the large cerebral hemispheres; brain highly organised by remarkable development of neo-pallial region of the hemispheres.
5. Urinary ducts pass to the bladder, directly.
6. Vagina single.
7. Single uterus (uterus simplex), placenta develops for providing nourishment throughout long gestatation period till young are born; placenta allantoic and highly organised.
Classification of Eutheria proposed by Simpson.
Cohort Unguiculata:
Includes extinct and living orders in which archaic mammalian characters are retained.
Order 1. Lipotyphla (Upper Cretacoeus- Recent):
1. Small mammals with plantigrade manus and pes; all five digits present, except in a few.
2. The snout prolonged to form a mobile proboscis.
3. The mammary glands are usually many, 3-6 pairs (only 2 pairs in Chrysochloroidea).
4. The frontal part of orbital wall is formed by extension of the maxilla and palatine does not reach the lacrimal.
5. The tympanic bone is wing-shaped.
6. The zygomatic arch is incomplete, when present, it is usually slender and the jugal greatly reduced.
7. Nodular structures, the remains of chevron bones are present in the vertebral column between the vertebrae on the ventral side.
8. The public symphysis is reduced to a short contact between the ischia (completely absent in Soricoidea).
9. The incisors are frequently enlarged to function as forceps (in Soricidae) and the canines are small or absent.
10. Predominantly insectivorous with sharp molar cusps and interlocking teeth.
11. The tympanic bone usually ring- shaped.
12. No caecum.
13. The cerebral hemispheres are small and smooth; olfactory lobes large.
14. Testes are frequently abdominal fully developed scrotum absent.
15. Uterus bicornuate.
16. The number of young given birth usually large, even up to 21 in a litter.
Examples: Sorex (shrews), Suncus, Talpa (moles), Erinaceus (hedgehog), Solenodon, Blarina, Chrysochloris, etc.
Order 2. Monotyphla (Upper Cretaceous- Recent):
Small mammals with two distinct existing families the elephant shrews of Africa and tree shrews of Orient.
1. The eyes are relatively large.
2. One to three pairs of mammary glands.
3. Caecum present.
4. The cranial cavity larger and extends forward between the large orbits.
5. A vertical plate of palatine extends up to the anterior medial wall of the orbit to meet the lacrimal.
6. Jugal well-developed.
7. Pubic symphysis not reduced.
8. The number of young ones in a litter never more than three.
Examples: Rhynchocyon, Elephatitubus, Tupaia, Ptilocercus, etc.
Order 3. Primates (Palaeocene-Recent)
(Lemurs, Monkeys, Apes, Man):
1. Primarily arboreal with prehensile digits.
2. Plantigrade, walking position of the feet.
3. Hair covers entire body except palms, soles and part of the face.
4. Presence of thumb, the greater toe shorter than other digits and more or less opposable to the latter.
5. Terminal flattened nail instead of claws.
6. A single pair of usually pectoral mammae.
7. Eyes directed anteriorly instead of laterally.
8. The orbit completely surrounded with bone.
9. Clavicle always present.
10. The stomach simple.
11. The cerebrum usually large and well- convoluted.
Suborder i. Prosomii:
More primitive primates; low brain case, uncovered cerebellum, non-deciduous placenta.
Examples: Lemur, Chiromys (Aye-aye), Galago, Perodicticus, Loris, Tarsius, etc.
Suborder ii. Anthropoidea:
1. Pinnae reduced and generally pressed against the head.
2. The upper lip lacks a ligament and freely protrusible.
3. The eyes are forwardly directed and vision is binocular. The retina is provided with cones and macula lutea is generally present.
4. The cranium is expanded as the cerebral hemispheres reach their highest development and overhang the cerebellum and the medulla as well and the neopallium is generally markedly convoluted with the development of the Sylvian fissure, and central and other sulci. The occipital visual areas well-developed.
5. The nostrils are completely ringed round by naked skin.
6. The enhancement, of visual sense is correlated with a corresponding reduction in olfactory reception.
Examples: Cebid monkey (Cebus), spider monkey (Ateles); howler monkey (Alouatta), woolly monkey (Lagotlirix), rhesus monkey (Mucaca), mandrill (Mandrillus), langur (Semnopithecus), baboon (Papio), gorilla, Gibbon (Hylobates), orang-utan (Pongo), [Homo crectus [Upright Man (extinct)], Homo sapiens (Wise Man), etc.
Order 4. Dermoptera (Palaeocene-Recent):
1. Bat-like, nearly as large as cat, with hairy, muscular parachuting membrane, patagium. The membranes stretched between the head and the hand, hand and feet and feet and tail.
2. The brain is primitive, macrosmatic and the corpora quadrigemina uncovered by the cerebrum.
3. The bulla is formed by the tympanic ring, which also form the lower border of the spout-like external auditory meatus.
4. The lower incisors are procumbent and comb-like. The molars multicuspidase.
Example: Cynocephalus (Galeopithecus). The only genus named flying lemur.
Order 5. Chiroptera (Eocene-Recent):
The Chiropterans or bats are capable of true flight.
1. The forelimbs have greatly elongated segments, especially the forearm and the fore-ulnar digits. These support a thin membranous fold of the integument stretching up to the hind limbs to form the wing.
2. The sternum is keeled for attachment of the large pectoral flight muscles.
3. A cartilaginous rod, calcar, is often attached to the inner side of the ankle, which helps to support a second membranous fold, the inter-femoral membrane, extending between hind limbs and sometimes involving tail.
4. The ulna is vestigeal.
5. The pollex much shorter than other digits, directed forwards and terminates in a well-developed curved claw.
The chiropterans are arranged into two suborders:
Suborder i. Megachiroptera:
1. Frugivorous bats with large eyes, long snout, without foliaceous nasal or auricular appendages.
2. The muzzle is nearly always elongated and- pinna of the ear simple.
3. The second digit of manus, usually end in claw.
4. The tail, when present, is not enclosed in the inter-femoral membrane, but lies below it.
5. The crowns of the molar teeth are devoid of sharp cusps.
Examples: Pteropus, Xantharpyia, etc.
Suborder ii. Microchiropera:
1. Insectivorous bats; the hind limbs rotate outward, the knee is directed backward instead of forward.
2. The five pedal digits are all provided with claws and the limbs are so adapted for flight and suspension that bats can only shuffle with difficulty on ground.
3. The second digits of the manus are always clawless.
4. Usually with short snout and frequently foliaceous nasal and auricular appendages.
5. The muzzle is generally short, the pinna is usually complicated by the presence of an inner lobe or tragus, and often produced into arborescent appendages.
6. The tail, when present, is at least partially enclosed in the inter-femoral membrane.
7. The body is covered with soft fur except in a group of nucrochiropteran in which integument is practically naked.
8. The crowns of molar teeth have typically sharp cusps. The dentition complete, heterodont, diphiodont.
9. The cerebral hemispheres are smooth and do not overlap the cerebellum.
10. The olfactory centres are reduced.
11. The penis is large and pendent.
12. The testes abdominal, descend seasonally to a prominent situation in the groin.
13. The uterus is simple or bicornuate.
14. The placenta is deciduous and discoidal.
Examples: Vespertilio, Rhinolophus, Desmodus, Diphylla, etc.
Order 6. Edentata (Paleocene-Recent):
1. The name of the order implies a total lack of teeth (except ant bears).
2. The dentition of toothed edentates is peculiar in that incisors or canines are absent and the teeth in the definitive condition are without enamel.
3. The clavicle is always present.
4. An additional pair of zygapophyses on the posterior dorsal and lumbar vertebrae.
5. The testes are abdominal.
6. Distribution restricted to America since their origin.
Suborder i. Palaeanodonta:
1. The North American species appears to represent a primitive Palaeocene offshoot from the ancestry.
2. Reduction and degeneration of teeth and enamel-covered canines.
3. No extra-zygapophyses and ischio-cau- dal symphysis.
Examples: Palaeanodcm, Metacheiromys, Epoicotherjum, etc.
Suborder ii. Xenarthra:
The armoured armadillos, ant-eaters, sloth, etc. of almost entirely South American forms constitute this suborder.
1. Feet heavily clawed.
2. Enamel is entirely absent from teeth in living groups.
3. The teeth are transformed into pillar like persistently growing stumps (the armadillos) or reduced in number (the sloths) or may be altogether absent (ant-eaters).
4. An extra pair of zygapophyses upon the posterior dorsal and lumbar vertebrae present (like snakes or Iguana).
5. The frequent reduction or even absence of the zygomatic arch.
6. Fusion of coracoid process to the front border of the scapula so as to enclose a large foramen and articulation of several of the anterior caudal vertebrae to the ischium, producing an unusually long ischiosacral region.
7. Brain simple type.
8. Persistently abdominal testes.
9. Heavily clawed feet.
Examples: Myrmecophaga, Tamandua, Cyclopes, Bradypus, Chaelopus, Tolypeutes, Dasypus, Glyptodon, Mylodon, etc.
Order 7. Pholidota (Oligocene-Recent):
1. The scaly ant-eater or pangolin, with body covered by imbricating horny scales over the head, body and tail and few scattered hairs. Lower surface is hair-covered.
2. The head is produced into a short pointed muzzle.
3. The tongue is long and teeth are entirely absent.
4. The skull is long and cylindrical.
5. No bony separation of orbit and temporal fossa.
6. Jugal arch and clavicle absent.
7. About five feet long from head to tail end.
Example: Manis.
Cohort Glires:
Includes rodents, rabbits, pikas, etc. which show early departures from the primitive eutherian insectivoran stem and have many adaptive parallel morphology, physiology and behaviour.
The cohort has two orders—Rodentia and Lagomorpha:
Order I. Rodentia (Palaeocene-Recent):
1. The pre-maxillae and maxillae enlarged to provide greater surface for attachment to enlarged masseter jaw muscle.
2. Antorbital vacuity enormously enlarged.
3. A single pair of Chisel-shaped incisors in the upper and lower jaw with persistently open roots-; posterior surface without enamel- grow throughout life and are always of great size.
4. No symphysis between the mandibles, special modification of the mylohyoids and masseters permit the separation and return of the prominent lower incisors causing scissor-action in feeding.
5. Canines absent and diastema between the incisors and cheek-teeth present.
6. The glenoid cavities for the mandibular condyles are elongated in a fore-and-aft direction, enabling the jaws to move forward.
7. The jaw muscles are highly enlarged.
8. A clavicle is usually present.
9. The caecum tends to be large.
10. The brain is little convoluted and the cerebellum is not covered by the cerebral lobes.
11. 3,000 living species, the largest of all the mammalian groups showing adaptive character for gnawing.
Suborder i. Sciuromorpha:
1. Primitive, two upper and one lower premolars retained.
2. The antorbital foramen not enlarged.
Examples: Funambulus squirrels, gophers, chipmunks, (Petaurista flying squirrels), etc.
Suborder ii. Myomorpha:
1. The infraorbital opening is large.
Examples: Dipus, Rattus (rats), Mus (mice), jarboas, hamsters, etc.
Suborder iii. Hystricomorpha:
1. The tibia and fibula remain separated.
2. The infraorbital canal is very enlarged.
Examples: Cavia (guinea pigs), Hystrix, (true porcupines), Lagostomus, etc.
Order 2. Lagomorpha (Palaceocene-Re- cent):
1. Maxillae laterally fenestrated.
2. The orbital foramina small
3. A diastema present.
4. Incisors with persistent pulps and with additional small pair of upper incisors.
5. Three upper, two lower premolars and three molars are present in each jaw.
6. Cheek pouch present.
7. Masseter muscle enormous and temporal muscle weak.
8. Tail is invariably reduced and the hind limbs are highly specialised for a saltatorial mode of progression.
Examples: Lepus (hares), Oryctolagus (rabbits), pika, etc.
Cohort Mutica:
Constitutes highly specialised, widely divergent mammals like whales. They are perfectly adapted to aquatic life.
Order Cetacea (Eocene-Recent):
Assemblage of highly specialised aquatic mammals profoundly modified for marine life.
1. The large torpedo-shaped body with naked smooth, skin.
2. The forelimbs or flippers and dorsal fin, when present, act as radders and stabilizers.
3. The dorsal fin and tail flukes are specialised skin folds, devoid of any bony skeleton and supported by underlying fibrous tissue.
4. Extra digits (hyperdactyly) and extra phalanges (hyperphalangy) serve to enlarge the surface of the flipper.
5. A thick insulating layer of dermal fat, the blubber in the hypodermis, constitute as much as 27 per cent of total weight.
6. The proximal elements of forelimbs are enclosed inside the body and the skeleton is secondarily specialised by the addition of extra joints in the digits.
7. The skull has greatly elongated facial and dorsoventrally compressed cranial portions. Nasal openings often asymmetrical, placed far back on the upper surface of head; tympanic bulla is loosely attached; the cranial bones, in general, are much modified in shape.
8. The cervical vertebrae shortened, compressed and fused together to a greater or lesser extent (complete fusion in Balaena).
9. The sternum is reduced.
10. Scapula with extremely reduced prescapular fossa, forwardly directed acromion process and absence of spine.
11. Bilobed liver, relatively simple, and non-lobulated lungs.
12. The stomach is subdivided into several digestive regions.
The features which have been lost are:
i. Hair, except for a very few facial bristles.
ii. Finger nails, except for traces in foetus.
iii. All skin glands.
iv. Lacrimal glands.
v. Nictitating membrane in the eye.
vi. The heads of ribs in some.
vii. The odontoid process in most.
viii. Hind limbs, except traces in some.
Modern Cetaceans are divided into two suborders—Odontoceti and Mysticeti.
Suborder i. Odontoceti:
1. Skull with a varying degree of asymmetry; the external nostril single and usually crescentic in outline.
2. Teeth present and whale bone absent.
3. The upper jaw bones widen posteriorly.
4. The mandibular rami are wide at their articular end, gradually narrowed to the symphysis. It may be of considerable length (dolphins and sperm whale) or greatly shortened (porpoise and pilot whale).
5. The anterior ribs usually have capitulum and tuberculum.
6. The sternum is formed of two or more Sterne brae.
Examples: Physeter (sperm whale), bottle- nosed whale, beaked whale, Phocaena (porpoises), Dolphinus (dolphin), white whale, Orca (killer whale), etc.
Suborder ii. Mysticeti:
1. Baleen whales having numerous horny baleen (or whale been) plates arranged in two series, one on each side of the upper jaw. The baleen is a horny material developed from the epithelial lining at the upper jaws acting as a strainer.
2. The teeth are rudimentary, function- less, present in young but in adult replaced by baleen.
3. Skull symmetrical.
4. Nostrils paired.
5. Sternum single.
6. The ribs are one-headed, articulating only with transverse processes of the vertebrae.
Examples: Balaena, Capered, Eschrichtius, Balaenoptera, Megaptera, etc.
Cohort Ferungulata:
A large group, divided into five super roders.
Superorder Ferae
The sole representative of the superorder is order Camivora.
Order Carnivora (Palaeocene-Recent):
1. Universal retention of the full number of incisors and well-developed canines in these flesh-eaters.
2. The post-canine dentition always with a certain amount of reduction and a rather wide range of adaptive variation.
3. The pedal digits are usually five and never less than four.
4. The toes are armed with sharp claws which are often retractile.
5. The brain is well-developed.
The living carnivores are arranged in two suborders:
Suborder i. Fissipedia:
1. Terrestrial carnivores with six incisors of small size in each jaw; canines are large and strong, the last premolar and first molar “camassial” or cutting teeth and the last two molars are crushing teeth.
2. The carnivores are distributed in all the continents except Australia.
Examples: Vulpes (foxes), Viverricula (civet), Felis (cat), Panther a (lion, tiger), Herpestes (mongooses), Hyaena, Canis (dogs), Procyon (racoons), weasels and Ursus (bears).
Suborder ii. Pinnipedia:
1. Marine forms in which there has been a secondary adaptation of the whole body for aquatic life.
2. The greater part of limbs are enclosed within the body skin.
3. The claws, are reduced and the digits increased in number,
4. The milk dentition is feeble and shed early.
5. The cranial cavity is large as compared with face.
6. No carnassial teeth; never more than five pairs of lower post-canines, four premolars and one molar. The teeth are basically conical, with a single crown.
Examples: Otter (sea lion), Odobenus, (walrus), Phoca (seals).
Superorder Protungulata:
1. Hoofed grazing animals with specialised grinding teeth of various kinds as well as numerous muscular, articular and alimentary adaptations and with long necks and legs.
Order Tubulidentata:
1. The body is thickset and covered with thick skin bearing scattered hairs; the head produced into a long muzzle.
2. The forelimbs are short and stout with four clawed digits, the palmar surfaces are placed on the ground for walking.
3. The hind limb has five clawed digits.
4. The tongue long and mouth small.
5. 4-5 peg-like teeth of peculiar structure on each jaw present. Teeth lack enamel but possess a coating of cement and the body of vasodentine is perforated by large number of fine tubules (hence the order name).
Example: Orycteropus.
Superorder Paenungulata (Sub-ungulata):
A widely diversified “near” or “sub-ungulate” group forming a loose assembly of related animals like hyrax, elephants, dugongs and other extinct forms.
Order 1. Hyracoidea (Oligocene-Recent):
Small, hooved, rabbit-like animals of Africa and South-West Asia, included in a single family, Procaviidae (= Hyracidae) with genera Procavia (= Hyrax) and Dendrohyrax.
1. Manus and pes are plantigrade. The former bears four digits and a vestige of a pollex; the latter bears three digits of which the third is the longest.
2. All digits terminate in small flat hooves except the second pedal digit with a curved claw.
3. The centrale persists in the carpus.
4. The radius and ulna partially crossed.
5. The astragalus differs greatly in shape from that of any other mammal in that the malleolus of the tibia has a large flat bearing on it.
6. Clavicle absent.
7. The postorbital process arises largely from the parietal.
8. The jugal is a very stout bone extending backwards to take a large share of glenoid cavity.
9. The periotic and tympanic are ankylosed together, but not to the squamosal.
10. The tympanic bulla and a spout-like auditory meatus present.
11. The scapula triangular and lacks an acromion.
12. Dorsolumbar vertebrae as many as twenty two.
13. A single pair of large curved upper incisors grow from persistent roots like those of rodents; canine absent; four premolars and three molars which are lophodont.
14. Lower jaw carries two pairs of spatulate, comb-like incisors.
15. Coronoid foramen just behind the last molar present.
16. No gall bladder.
17. Unlike other mammals, the caecum has a pair of caecal pouches.
18. Testes abdominal.
19. A dorsal gland on the back present.
Examples: Procavia, Dendrohyrax, etc.
Order 2. Proboscidea (Pliocene-Recent):
Comprises the largest and in many respects the most highly specialised terrestrial mammals.
1. The elongation of nose and upper lip form a very long trunk.
2. The facial region shortened; the nasal opening recedes towards the tip of the head and the nasal bones reduced in size.
3. Both fore-and hind feet possess five functional digits.
4. In forelimb the ulna and radius are permanently crossed in pronation and the ulna is unique in having its distal articulation with the carpus larger than that of the radius.
5. The skull is immensely big and its bones contain large air cavities.
6. The incisor teeth of upper jaw are specialised into great tusks. The ivory of the tusks is solid, specialised dentine. Enamel absent.
7. Clavicle absent
8. The pelvis has broad ilia and is vertical in direction.
9. Paired precaval veins.
10. The cerebral hemispheres are much convoluted but do not cover the cerebellum.
11. The testes are permanently abdominal.
Examples: Loxodonta africanus (African elephant), Elephas maximus (Indian elephant).
Order 3. Sirenia, (Eocene-Recent):
1. Fully aquatic and present a number of structural features convergent with those of the Cetacea.
2. Thick, almost hairless, skin with an underlying layer of blubber.
3. External hind limbs, external ear, and sacrum absent.
4. The forelimbs are paddle-like.
5. The tail is flattened and either rounded or rhomboidal.
6. The diaphragm, like in the cetacean, is oblique and very muscular;
7. The skull and dentition are specialised.
8. The tympanic bone is ring-shaped.
9. The cervical vertebrae are not ankylosed; the Whole skeleton is strong and made of very strong bones.
10. The scapula is long and narrow.
Examples: Trichechus (manatee), Dugong (sea cow).
Super-order Mesaxonia:
1. “Odd-toed” hooved animals including horses, tapirs and rhinoceroses and certain extinct forms.
2. In the mesaxonic foot, exemplified by the horse, the skeletal axis passes down the third digit and other digits are reduced and do not support any weight.
Order Perissodactyla (Eocene-Recent):
1. The second and fourth digits are represented in horses by the splint bone (metacarpals and metatarsals).
2. Odontoid process peg-like.
3. Dorsolumber vertebrae twenty-two to twenty-three.
4. Almost complete molarisation of the premolar teeth.
5. Presence of a third trochanter
6. No ruminant stomach and true horns with a bony horn-core.
Suborder i. Hippomorpha:
1. The second and fourth toes are represented by vestigeal remnants called “splint bones”.
2. The molar teeth are highly complex and wear down through most of the life of individual.
Examples: Equus cabalus (horses), E. hemionus (asses), and E. zebra (zebras).
Suborder ii. Ceratomorpha:
Least specialised perissodactyls; feet have stout digits, four in the manus and three in the pes, the third digit in each case being somewhat enlarged.
The anterior nasal openings are enlarged and recessed. The nasal bones are small and pointed upwards in correlation with the proboscis.
The teeth are persistently brachydont and without cement. The molar teeth are bilophodont.
Examples: Tapirus indicus (Tapirs) and T. terrestris.
Suborder iii. Ruminentia:
1. Total loss of upper incisors. The upper canine is also usually lost but, in rare cases where it persists, it is much enlarged.
2. Flat spatulate lower incisors. The lower canine has the same shape as the incisors and is closely pressed against them.
3. The feet have always a cannon bone. The lateral toes are usually lost and the feet absolutely didactyle. In some cases the second and fifth digits remain, but are never complete.
4. The ruminating stomach attains its highest development with four complete chambers.
5. Horns of several types are usually present.
Examples: Deers, giraffes, okapi, antelopse, gazelles, lmpala, eland, yak, Bos (cow), Capra (goat), Ovis (sheep), etc..