1. Metatherians, or marsupial mammals, evolved from reptiles during the Triassic period and give birth to underdeveloped young that develop further in the mother's pouch. 2. They are found primarily in Australia and have hair-covered bodies, tails, and females with an abdominal pouch. 3. While showing both primitive and advanced traits compared to other mammals, metatherians are considered a transitional form between egg-laying prototherians and placental eutherians in mammalian evolution.

2. INTRODUCTION
Mammals evolved from reptiles during Triassic period
and occupied a different nitch.
GENERAL CHARACTERS :
Following characters were established during evolution of mammals are:
1. Hair-clad mostly terrestrial ,air-breathing, warm-
blooded , viviparous ,tetrapod vertebrates.
2. Limbs 2 pair , pentadactyle.
3. Exoskeleton include lifeless ,epidermal hairs,
spines ,scales ,claws , nails etc.
4. Skin contain sebaceous and sometime scent gland .
Female have mammary gland.
5. Endoskeleton ossified . Skull dicondylic , having
two occipital condyles formed by exoccipitals.
6. Alimentary canal terminates by anus , there being
no cloaca.

3. METATHERIA
Class Metatheria includes pouched mammals without a true placenta. The
young are born in an immature condition .
General organisation
Distribution : A most entirely confined to the Australian region with
the exception of the American Opossum.
Habit and Habitat: Terrestrial , burrowing
or arboreal , herbivorous or carnivorous,
nocturnal or diurnal, air breathing ,
warm-blooded, and pouched mammals.
External feature:
1) Body covered with soft hairs, external
ear lobe or pinnae are well developed.
2) Tail are generally long and prehensile.
3)Female usually with a ventral abdominal
pouch called marsupium.
Ref . A textbook of zoology by Parker and Haswell

4. Skeleton:
The exoskeleton include :
1)The presence of epidermal
hair and claws.
2) Endoskeleton well
developed skull dicondylic
skull sutures present.
3) Orbit and temporal fossa
become fused due to absence
of post Orbital path.
4) Nasal bone large and expanded posteriorly.
Ref. A textbook of zoology by Parker and Haswell
Body cavity : Typical muscular diaphragm present.
Respiratory system : It is of typically mammalian type that is
respiration by lungs.

5. Vertebral column:
1) Vertebrae with epiphysis.
2) Cervical vertebrae are 7.
3)Thoracic vertebrae are 13 in
number whereas lumber
vertebrae are 7 in number.
4) Thoracic ribs are bicephalic.
A pair of epipubic bone present in front of pubis symphysis for the support
of marsupium.
Digestive system :
1) Teeth are present in large number . These are monophydont ,
heterodont or thecodont.
2) Anus and urinogenital aperture open into a shallow cloaca
surrounded by a common sphincter .
3) Shape and size of stomach variable.
Ref.
Man
and
Vertebrates
by
Romer

6. Circulatory and excretory system : Both system are well
developed and typically mammalian type.
Nervous system:
• Brain relatively small and less convoluted .
• Olfactory lobe large but cerebral hemisphere small .
• Cerebellum small and exposed .
• Corpus callosum, feebly developed .
• Cochlea of internal ear are spirally coiled.
Reproductive system:
• In male, scrotal sac containing testis lie in front of
penis.
• In females, two oviducts open separately into
urinogenital
sinus , so that there are two uteri and two vagina .

7. Development :
 Female are viviparous .
 Gestation period for uterine development is
small ,2 weeks in
Opossum to 5 weeks in Kangaroo.
 Young are born exceedingly small , naked and
blind.
 They are kept in marsupium and nourished on
milk until they
are fully form.
Ref . A textbook of zoology Parker and Haswell

8. Phylogenetic relationship):
Metatherian show a mixture of primitive and advance characters. They show
following affinities:
Affinities with Prototheria ( egg laying mammals
Similarities :
1. Presence of cloaca.
2. Presence of clavicle , epipubic bone and ring like tympanic.
3. Absence of tympanic bulla ,absence of true allantoic placenta.
4. Brain relatively simple with large olfactory lobe and anterior commissure ,but
without corpus callosum.
However, metatherian differ from prototherian
mainly in being viviparous, having permanent
marsupial pouch, well developed external ear,
vertebrae with epiphysis , ribs bicephalous, no
interclavicle and separate coracoids, testis in scrotal
sac, uterine gestation and viviparity.

9. Affinity with Eutheria ( placental mammals) :
Similarities:
• Mammary glands sebaceous and with teats.
• Brain with 4 optic lobes, cochlea spirally coiled.
• Coracoid reduced. Interclavicle absent. Rib bi
cephalous.
• Teeth heterodont.
• Male with penis . Testis in scrotal sac.
• Presence of uterus and vagina . Female
viviparous.
• Presence of hair and external ears.

10. However , metatherian differ from
eutherian in restricted distribution , having
shallow cloaca., marsupial pouch , No tympanic
but alisphenoid bulla ,Epipubic bone , jugal
extended back ,Palate perforated back , more
incisor in both jaws ,Corpus callosum absent in
brain ,Scrotal sac in front of penis,
• Gestation period small, no true allantoic
placenta .

11. Systematic position :
It is obvious that the Metatherian are more advanced than the primitive
reptiles-like, oviparous Prototheria.
They are more closely related with Eutherian , but do not belong to the
same grade of evolution.
Therefore , they are put under a separate infraclass Metatheria , while
the rest of the higher and truly placental mammals are placed in infraclass
Eutheria, and are both combined in sub class Theria.
Phylogenetic consideration :
• Comparative study of organization of marsupial and placental mammals
reveals that marsupials are “IInd grade mammals”.
• These are usually regarded as transitional step in evolution of mammals
by many workers .
• It is believed that placental mammals and marsupials evolved
independently from some common Panthotherian ancestor in the late
Jurassic period.

12. Thank you..