Classification of Mammals

Classification of Mammals

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  Naveed Akhtar Assistant Professorof Zoology Punjab Higher Education Department
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  General Characteristics of Mammals
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  Classification of Mammals
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  Classification of Mammals Many earlier ideas have been completely abandoned by Linnaeus and modern taxonomists, among these are the idea that bats are related to birds or that humans represent a group outside of other living things.  Mammalian classification has been through several iterations since Carl Linnaeus initially defined the class.  Most significantly in recent years, cladistic thinking has led to an effort to ensure that all taxonomic designations represent monophyletic groups.  The field has also seen a recent surge in interest and modification due to the results of molecular phylogenetics.
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  Conti…  Mammals areactually a class organized into 5420 species; which are further sub-classified into 135 families, 1,000 genera, 29 orders, and 2 subclasses.  While studying these subclasses, we come to know several mammals that were parted some 200-million years ago. These primitive mammals include egg-laying Prototheria (platypus and echidnas being the only survivors) along with live-bearing theria.  George Gaylord Simpson's[1945] "Principles of Classification’’ laid out a systematics of mammalian origins and relationships that was universally taught until the end of the 20th century.
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   Though fieldwork gradually made Simpson's classification outdated, it remained the closest thing to an official classification of mammals. Various trials have been made to classify mammals  Molecular classification  Standardized classification  McKenna/Bell classification  Luo, Kielan-Jaworowska, and Cifelli classification  Simplified classification  No classification system is universally accepted; McKenna & Bell (1997) and Wilson & Reader
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  Standardized classification
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  Cladistics Analysis
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  1. Subclass Prototheria
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   Subclass Prototheria(proto-thir ee-a) (Gr. pr|tos, first, thƒr, wild animal).  Cretaceous and early Cenozoic mammals.  Extinct  Some zoologist classify mammals into two majjor groups;  1. Prototheria: Egg laying mammals that have cloaca throughout life  2. Theria: Give birth to young  Infraclass Ornithodelphia  (orni-tho-delfee-a) (Gr. ornis, bird, delphys, womb). Monotreme mammals  Order Monotremata  (monotrema-tah) (Gr. monos, single, trƒma, hole): egg-laying (oviparous) mammals  Living prototherians are placed in the monotreams  Duckbilled platypus, spiny anteater
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   Three speciesin this order are from Australia, Tasmania, and New Guinea.  The most noted member of the order is the duck-billed platypus (Ornithorhynchus anatinus).  The spiny anteater, or echidna (Tachyglossus), has a long, narrow snout adapted for feeding on ants, its chief food  Recent fossil evidence showing monotreme dentition that is characteristic of the subclass Theria has resulted in this group being reassigned to the latter subclass. The Prototheria, therefore, contains only extinct forms
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  Characters of SubclassPrototheria  Shelled Yolky Eggs:  Mammals that lay eggs with leathery shells and nourish the young ones with milk from primitive(open) mammary glands.  Single Cloacal Opening:  They possess a cloaca like reptiles, have no urinary bladder  Lack of Teeth:  Modern monotremes lack teeth as adults; sutures are hard to see; the rostrum is elongate, beak-like, and covered by a leathery sheath; and lacrimal bones are absent.  Testes:  Testes are confined to the abdominal cavity  Absence of The Corpus Collosum)  Brain lacks thre transverse fiber track in brain (The Corpus Collosum) that connect two cerebral hemispehere in all other mammals
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   Monotremes haveseveral important mammalian characters, however, including fur (but they lack vibrissae), a four chambered heart, a single dentary bone, three middle ear bones with larger Incus and Malleus, and the ability to lactate.  Lack of Nipples:  They have no nipples, milky fluid exudes from modified sweat glands onto tufts of hairs in shallow pits of abdomen, from which young lick it up  Endothermy:  Monotreams are endothermic but their body temperature is less stable than other mammals  5 species in 2 families in Order Monotremata Earliest monotreme: 125 mya Teinolophos  Geographic Range
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   Duckbills livein pairs in burrows up to 18 m long, in the banks of the creeks and rivers and spend their most life in water.  Prototherians are either terrestrial (Tachyglossidae) or primarily aquatic (Ornithorhynchidae  Their terrestrial habitats include deserts, sandy plains, rocky areas, and forests in both lowlands and mountains.  Platypuses inhabit lakes, ponds and streams; they shelter in burrows along the banks and spend much of their time foraging in the water.  They possess web feet to move in water and beaklike muzzle fro detection of the prey especially mollusks  During breeding season, female construt nest near water where it lays eggs of 2 cm diameter and incubate them for 10 to 14 days  Echidna:  The spiny anteater, or echidna (Tachyglossus), has a long, narrow snout adapted for feeding on ants, its chief food
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   Geographic Range:Monotremes are restricted to Australia and New Guinea. Their fossil record is very poor; the earliest fossil attributed to this group is from the early Cretaceous.  A fossil from Argentina suggests that the monotremes were more widely distributed early in their history
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  Classification of Monotreams
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  A duck-billed platypus(Ornithorhychus anatinus).
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  An echidna orspiny anteater (Tachyglossus aculeatus)
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  2. SubClass Theria SubClass Theria(Gr. thƒr, wild animal)  Theria is a subclass of mammals amongst the Theriiformes.  Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials)
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  2.1 Infraclass Metatheria Order Marsupialia  viviparous pouched mammals: opossums, kangaroos, koalas, Tasmanian wolves, wombats, bandicoots, numbats, and others  These mammals are characterized by an abdominal pouch, the marsupium, in which they rear their young  The young are nourished in the uterus for a short time by way of a yolk-sac placenta.  Only the opossum is found in the Americas, but the order is the dominant group of mammals in Australia; 260 species
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  The koala (Phascolarctoscinereus) feeds on Eucalyptus leaves in Australia. An opossum (Didelphis marsupialis) with young
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  General Characters  YolkyPlacenta:  Marsupials possess fetal yolk sac (in contact with chorion) which serve as placenta  Marsupium  They are viviparous but have very short gestation periods.  The young crawl into the marsupium after birth, where they feed and complete development  In marsupials, the mammary glands, which vary in number, are located within the pouch and terminate in a teat.  The mother may nurse two offspring, differing widely in age, simultaneously from different mammary glands that secrete milk of very different composition, designed to meet the different specific requirements of each offspring
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   The wallof the marsupium are supported by two bones(epipubic) that project from the pelvic girdle  live in the Australian region and the Americas  Marsupials (and monotremes) also lack a gross communication (corpus callosum) between the right and left brain hemispheres
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   Geographic Rangeand Habitat:  The largest and most-varied assortment of marsupials—some 200 species—is found in Australia, New Guinea, and neighbouring islands, where they make up most of the native mammals found there.  In addition to larger species such as kangaroos, wallabies, wombats, and the koala (Phascolarctos cinereus), there are numerous smaller forms, many of which are carnivorous, with the Tasmanian devil (Sarcophilus harrisii) being the largest of this group (family Dasyuridae).  About 70 species live in the Americas, mainly in South and Central America, but one, the Virginia opossum (Didelphis virginiana), ranges through the United States into Canada.
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   The nichesthat marsupials fill are closely associated with structure. The burrowing species, such as the marsupial moles (Notoryctes typhlops and N. caurinus) and the wombats, have powerful foreclaws with which they can tunnel into the ground for food and shelter.  Terrestrial forms, such as kangaroos and wallabies, possess well- developed hind limbs that serve both as formidable weapons and as catapults by which they can bound over the plains.  The gliders have a membrane along either flank, attached to the forelegs and hind legs, that enables these arboreal animals to glide down from a high perch.  A few marsupials—such as tree kangaroos, koalas, and some cuscuses—spend most of their lives in trees. The water opossum, or yapok (Chironectes minimus), of Central and South America is semiaquati
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   Morphology:  helargest living marsupial is the red kangaroo (Macropus rufus), males of which can grow to about 2 metres (6.6 feet) in height, 3 metres (10 feet) from muzzle to tail tip, and a weight of up to 90 kg (about 200 pounds).  The smallest are the planigales, especially the long- tailed planigale (Planigale ingrami), measuring barely 12 cm (4.7 inches) in total length. Most marsupials range from the size of a squirrel to that of a medium- size dog.
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  Smallest Marsupial (Planigaleingrami)
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   The dietsof marsupials are as varied as the niches they occupy. Many dasyurids live chiefly on insects and other small animals. Dunnarts (Sminthopsis) are so hyperactive— like shrews—that, in order to supply their high energy needs, they must devour their own weight in food (chiefly insects) each day.  The numbat uses its remarkable wormlike tongue to lap up termites and ants. Many Australian possums, bandicoots, and American opossums have a mixed diet of plants and insects. Wombats and many other marsupials are strictly vegetarian.  The small honey possum (Tarsipes rostratus) is specialized to feed on the nectar of flowers, and other marsupials also may serve as important pollinators in that way. Few large carnivores have ever evolved in Australia, because of the low productivity of its
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   Reproduction andDevelopment:  The life cycle of marsupials exhibits peculiarities that have long been considered primitive compared with those of placental mammals but are more likely adaptations to low- productivity environments.  The uterine cycle of the female marsupial has no secretory phase, and the uterine wall is not specialized for embryo implantation, although a transitory placenta does exist in the bandicoots.  The period of intrauterine development in marsupials ranges from about 12 days in the bilby (Macrotis lagotis) to 38 days in the swamp wallaby (Wallabia bicolor).
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   The young,born in a vulnerable embryonic condition, make their own way to the shelter, warmth, and nourishment of the pouch; in pouchless marsupials the young simply cling to the teats.  Those fortunate enough to survive that hard journey may succeed in attaching themselves to the mother’s nipples, which then swell and become firmly fastened—almost physically fused—to the mouth tissues of the young.  In that condition the young continue their development for weeks or months, after which they are weaned and begin to look after themselves.  Frequently, the partially developed young outnumber the available teats, and the excess individuals perish.