The subclass Theria constitutes the “modern” mammals. This group includes three infraclasses, viz., the probably ancestral Jurassic Pantotheria (= Trituberculata), marsupial Metatheria and placental Eutheria, the most highly organised and advanced mammals. Metatheria and Eutheria are presumed to have arisen from some untraced branch of Jurassic pantotheres and both diverged along their separate lines of evolution during early Cretaceous period.
In the upper Cretaceous period, the marsupials were more numerous than the eutherians. It was possibly during Cretaceous that marsupials entered New Guinea, Australia and adjacent islands, which were isolated from Asia probably in the late Cretaceous. Here they were able to survive free from competition with eutherians (except for bats and later rodent invaders).
It is probable that the basic stock was arboreal, though many have become terrestrial. Tree-kangaroos (Dendrolagus) have become secondarily arboreal from a fully terrestrial offshoot. Metatheria were widely distributed over many parts of the world as recently as the Miocene period. But they are now confined to the Australian region (except New Zealand), to South America and a few species to North America.
Distinctive Characters:
1. Distribution:
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Almost entirely continued to the Australian region with the exception of the American opossums.
2. Habits and Habitat:
Marsupials are terrestrial and carnivorous (e.g., native-cats, tasmanian Devil, marsupial wolf), terrestrial and herbivorous kangaroos; arboreal and insectivorous Didelphys, Marmosa, Chironectes, etc.; arboreal phalangers or opossums and semi-arboreal Phascogale and pouched rats and mice, and pouchless Jerboa-like marsupials and banded ant-eaters; burrowing desert marsupial mole and bandicoots; and flying and climbing phalangers.
Marsupials are herbivorous (e.g., kangaroos), insectivorous, carnivorous (thylacines and dasyures), and omnivorous, diurnal or nocturnal, warm-blooded, air-breathing, viviparous pouched animals.
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3. External Features:
Body is covered over with hairs. Pinna (external ear) is well developed, but absent in moles. Tail is generally long and prehensile, and an important organ of balance in kangaroo rats and jumping mice. Mammary glands are modified sebaceous glands and have elevated nipples.
Females usually have a marsupial pouch or marsupium but it is absent in Didelphys and the Dasyuridae (e.g., Dasyurus, Thylacinus, etc.). Marsupium encloses the nipples. The number of teats bears a relation to the number of young produced at a birth.
4. Exoskeleton:
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It includes ectodermal structures, the hairs derived from the Malpighian layer of epidermis and claws. Skin also contains sweat gland, sebaceous glands and scent glands.
5. Body Cavity:
Typical muscular diaphragm divides the body cavity into thorax and abdominal cavities.
6. Endoskeleton:
(i) Skull:
Skull is dicondylic. Brain case small. Orbit and temporal fossa fully confluent and no postorbital bar. Bony palate is incomplete posteriorly. Jugal bone reaches back so as to participate in the formation of glenoid cavity for articulation of lower jaw. Lacrymal bone extends outside the orbit.
Tympanic bulla is absent. Tympanic, periotic and squamosal remain separate. Alisphenoid assists in forming the bulla. In kangaroos alisphenolid extends backward to join the greatly elongated paroccipital process. The lower jaw, consisting of a single dentary, has inflected or inturned inner angle to which large lateral pterygoid muscle is attached.
(ii) Vertebral Column:
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The odontoid process fuses early with the axis. Cervical ribs with their respective vertebrae. Seventh cervical vertebra is pierced by a foramen for the vertebral artery. Presence of 13 rib-bearing thoracic vertebrae, as in placentals, and 7 lumbars as usual.
Sacrum has two vertebrae, but it may be reinforced by caudal vertebrae. Caudal region is short in koala and wombats, long in opossums, dasyures, phalangers and kangaroos. Chevron bones are generally present in caudal vertebrae, except in koala and wombats.
(iii) Girdles:
In pectoral girdle, coracoid is reduced as in placentals, and scapula enlarged and provided with a spine. Clavicle is present except in bandicoots. There is no interclavicle. In pelvic girdle, epipubic bones project forward from the pubes in all. In Thylacinus these are represented by small cartilages.
(iv) Limbs:
Forearm bones are separate and generally adapted for pronation and supination. Thumb is not opposable, but the two inner digits of the hand can frequently be opposed to the three outer in grasping. Hand with five clawed digits except in Chaeropus and the carpus is without centrale.
In hindlimb five digits, fibula is free and can be rotated on the tibia. In some cases first digit can be used as a thumb. Hallux is frequently absent. Second and third digits are in most cases slender and united by the skin (syndactyly).
7. Digestive System:
There is only one set of teeth functional throughout life with the exception of a milk molar which is replaced by the last premolar. Incisors are more numerous than in placentals. 3 premolars and 4 molars as against 4 P and 3 M in placentals. Dentition in opossum is I 3/4, C 1/1, P 3/3, M 4/4. Stomach is usually simple, but in kangaroos it is much elongated and sacculated. Caccum is usually present, absent in dasyures.
8. Respiratory, Circulatory and Excretory Systems:
Respiratory, circulatory and excretory systems are typically mammalian. Respiration by lungs. Heart is without fossa ovalis. Auriculo-ventricular valves are membranous and attached to the papillary muscles by chordae tendineae.
9. Nervous System:
Brain is relatively smaller. Olfactory bulbs large. Corpus callosum is absent and anterior commissure connects the two corpora striata (nerve band present in the lateral walls and floor of paracoels, lateral ventricle of cerebrum) is large. Cerebral hemisphere surface is convoluted. Cerebellum is small and simple. Cochlea of internal ear is spirally coiled.
10. Reproductive System:
In male, there are no vesiculae seminales. Testes descend into scrotal sacs placed in front of the penis. Glans penis (tip of the penis) is bifid in several species.
In females, the Mullerian ducts are separate posteriorly and open separately into the long urinogenital sinus. They are differentiated into oviduct, uterus and vagina on each side (e.e., Didelphis). In other forms the anterior part of vagina gives of backwardly directed diverticula, the two meet in the midline as a sinus vaginalis and extend up to the front end of urinogenital sinus. This ends blindly until the young is about to be born. In some forms (e.g., Macropodidae) its hind end acquires an opening into the urinogenital sinus at parturition. Clitoris is also double.
The rectum and urinogenital sinus open together in a common cloaca. Cloaca is longer in female than the male.
11. Development:
Females are viviparous. The egg is yolky and covered with albumen and a shell membrane. Cleavage is unequal. Placenta is of yolk sac type, vascular wall of yolk sac is in contact with the somewhat hypertrophied uterine wall. In Perameles (bandicoot) allantois develop a nutritive function to some extent.
Embryos are born very young as little as 8 days from conception in opossum. They are developed in the pouch attached to the teats. The young stays in the pouch for 8-9 months and the mother remains in lactationally induced anoestras.
Population Control:
The males of marsupial mouse (Antechinus) die one week after mating. It is due to increased adrenocortical activity in the last weeks of life due to which they become more aggressive in nature. Females survive for two or three seasons.
Distribution of Metatheria:
Metatheria were widely distributed over many parts of the world as recently as the Miocene period. They are now confined to the Australian region (with the exception of New Zealand), to South America and (a few species) to North America. During Tertiary period they were widely distributed in South America.
The Australasian region, excluding New Zealand, contains more than four times as many marsupials as occur in the Americas, where their congregation is chiefly in the southern continent. Two families, Didelphyidae and Epanorthidae (Caenolestes) are found in the Neotropical region and the genus Phalanger extends into the island of Celebes, where it is represented by two peculiar species.
Geological history of the group is interesting. In Australia, which is now its chief home, no fossil remains are found before Pleistocene. In America, remains of both polyprotodonts and diprotodonts are found as far back as Eocene, and in the case of Epanorthidae (Genus Cimolestes, Telacodon and Batodon) in the upper Cretaceous.
Didelphyds are found in the Tertiaries of Europe, and there is a number of fossil forms in the Jurassic Formation of Europe and N. America which belong to the group polyprotodont. These continents were probably connected about 100 million years ago. By the time placentals appeared Australia and South America were no longer connected with the other continents and the marsupials became extinct in Europe and North America which were still connected.
South America remained isolated until the formation of the Panama bridge in the Pliocene and most of its marsupial fauna became extinct after this time. In North America also there were no marsupials between the middle Miocene and the Pleistocene, when opossums (Didelphis) appeared, probably from South America. Australian marsupials only became threatened by the advent of Man from Europe and the other placentals he brought with him.
However, large Macropus have thrived with the spread of agriculture. Marsupials diversified in Australia due to isolation, and differentiated structurally into numerous types, arboreal, fruit-eating, grazing, gnawing, digging, burrowing, ant-eating, insectivorous or carnivorous.
Affinities of Marsupials:
Marsupials show a mixture of primitive and advanced features.
1. Affinity with Prototheria:
i. Presence of cloaca. The rectum and urinogenital sinus open together in a common cloaca.
ii. Presence of clavicles, epipubic bones in pelvic girdle and tympanic bone is ring-like.
iii. Tympanic bulla is absent. Tympanic is not united with the periotic.
iv. Brain is relatively smaller with large olfactory bulbs and anterior commissure. Corpus callosum is absent.
v. True allantoic placenta is absent.
vi. Vesicula seminales are absent in males.
Differences with the Prototherians:
a. Metatherians have well develope pinna, absent in prototherians.
b. Metatherians have well-developed marsupium.
c. Vertebrae with epiphysis, Ribs bicephalous (monocephalous in prototherians). No interclavicle. Coracoids reduced as in placentals.
d. Teeth present in the adults (only one set of teeth).
e. Cochlea is spirally coiled, whereas it is partly coiled in Prototheria.
f. Vivparous.
g. Uterine gestation (No gestation in monotremes).
h. Testes in the scrotal sacs, whereas in prototherians these are abdominal. Glans penis is bifid.
2. Affinity with Eutheria:
Metatherians have many advanced features similar to eutherians or higher placental mammals:
i. Presence of hairs over the body.
ii. Presence of external ears (pinnae).
iii. Sebaceous mammary glands with teats.
iv. Coracoids reduced, interclavicle absent and ribs bicephalous.
v. Teeth heterodont: incisors, canines, premolars and molars.
vi. Brain with 4 optic lobes. Cochlea spirally coiled.
vii. Testes in scrotal sacs. Presence of erectile penis in male.
viii. In female uterus and vagina are present in the oviduct.
ix. Viviparous. Ova microlecithal. Uterine gestation and placenta present.
Differences with the Eutherians:
a. Metatherians have restricted distribution, found only in Australia and Americas (North and South America).
b. Presence of marsupial pouch.
c. Number of incisors is more in both the jaws.
d. Tympanic bone absent. Alisphenoid assists in forming the tympanic bulla. Penotic and squamosal remain separate. Jugal extending back and participates in forming the glenoid cavity for the articulation of lower jaw. Well-developed epipubic bones. Ventral posterior border of lower jaw is inwardly inflected.
e. Corpus callosum is absent in the brain.
f. In perameles, a true chorio-allantoic placenta is present but it is absorbed after the birth of the young. Others have yolk sac placenta.
g. A pair of uteri and vaginae are present in females. Each vagina has a separate opening into the urinogenital canal.
h. Ureters pass between the genital ducts, whereas in eutherians they pass outside the genital ducts.
i. In male penis is sometimes bifid and behind the scrotal sacs.
j. Gestation period is brief.
Systematic Position:
It is obvious that the Metatheria are more advanced than the primitive, reptile-like, oviparous Prototheria. They are more closely related with the Eutheria, 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 the infraclass Eutheria and both combined in the subclass Theria.
Phylogenetic Consideration of Metatheria (Marsupials):
Comparative study of biological organisation of marsupials and placentals reveals that the marsupials appear to be ‘second grade of mammals.’ For this reason, it was the common practice for the zoologists to regard the marsupials as the transitional step in the evolution of mammals between the ancestral Jurassic mammals and the Coenozoic placentals.
But now it is believed that placental mammals and marsupials evolved independently from some common pantotherian ancestor in the late Jurassic period and both of them evolved side by side Fig 32.4 will give an idea about the phylogenetic relationship of marsupials with the monotremes and placentals.
