Monotremes

"Monotreme" means "one opening" and refers to the single rear orifice, or opening, that these animals have for getting rid of wastes, laying eggs, and mating. The lower intestine, excretory system (system that gets rid of wastes), and reproductive system all end at this opening, called the cloaca (kloh-AY-kah). This feature is common in reptiles and birds but extremely rare among mammals
Trying to describe a "typical" monotreme (MAHN-ah-treem) is difficult, since the only two living types, the platypus and the echidna (ih-KID-nah), do not look much alike at first glance. The platypus is built in a streamlined manner, like an otter, has soft fur, and its snout resembles a duck's bill, while the echidna looks like a pudgy, waddling watermelon covered with fur and sharp spines, with a narrow, hornlike snout. Although echidnas may look overweight, most of the soft tissue mass that might be mistaken for blubber is muscle, lots of it. The platypus is semiaquatic, hunting animal food underwater but sheltering in a dry burrow, but the echidnas are land animals that forage, or search, in the soil for insects and worms

There are only five living monotreme species: the duck-billed platypus and four species of echidna (also known as spiny anteaters). All of them are found only in Australia and New Guinea. Monotremes are not a very diverse group today, and there has not been much fossil information known until rather recently.

In some ways, monotremes are very primitive for mammals because, like reptiles and birds, they lay eggs rather than having live birth. In a number of other respects, monotremes are rather derived, having highly modified snouts or beaks, and modern adult monotremes have no teeth. Like other mammals, however, monotremes have a single bone in their lower jaw, three middle ear bones, high metabolic rates, hair, and they produce milk to nourish the young.

    Adult platypus are about the size of house cats, while echidnas range from twice to three times as large as a house cat. An adult platypus weighs from 3 to 5 pounds (1.4 to 2.3 kilograms), and its adult head and body length runs 12 to 18 inches (30 to 46 centimeters), the tail adding another 4 to 6 inches (10 to 15 centimeters). The short-beaked, or short-nosed, echidna can grow up to 14 pounds (6.4 kilograms), with a head and body length of up to 21 inches (53 centimeters), the stubby tail adding another 3 or 4 inches (7.6 to 10 centimeters). The long-beaked, or long-nosed, echidna weighs up to twenty pounds, with a head and body length ranging from 18 to 31 inches (45 to 77.5 centimeters), while the tail, like that of the short-nosed echidna, is a mere stubby shoot. Male platypus and male echidnas are larger than females.

Platypus and echidnas are often called "primitive" because they carry a number of reptilian, or reptile-like, characteristics along with typically mammalian features. Ever since the first discovery of monotremes by Europeans in the late 1700s, zoologists, scientists who study animals, have been busy studying this mix of details in order to place the monotremes properly in the framework of mammalian evolution. Even more confusing is that the living monotremes have a number of modified, or changed, features all their own, examples being the snouts of platypus and echidnas.

 The most well-known and special feature of the monotremes, and the one that seems most       reptilian, is that the females lay eggs rather than giving live birth. Monotremes are the only living, egg-laying mammals. Other characteristics that platypus and echidnas have in common are similar skeletons and highly modified snouts equipped with nerves whose endings are sensitive to pressure and to natural electricity. Monotremes have fur, but not whiskers, while the echidnas, in addition to fur, have sharp, defensive spines, which are modified hairs, scattered over their backs and sides.
  
    Monotremes walk in a reptilian manner, like alligators and crocodiles. Like the arms of someone in the middle of doing a pushup, the upper bones of monotreme forelimbs and hindlimbs go straight out from the body, horizontal to the ground, and the lower limb bones go straight down. Other lines of mammal evolution have abandoned this clumsy sort of movement and now carry their entire legs vertically beneath their bodies. Zoologists are not yet sure if the push-up style of legs and walking in monotremes is something left over from their reptilian ancestors or if they are more recent changes to fit their lifestyles.
Another odd monotreme characteristic is that male and female platypus, and male echidnas, have short, sharp, hollow, defensive spurs on the inner sides of the ankles of their rear limbs. The spurs of the male platypus connect with poison glands and are fully functional as stingers.

Monotremes: Monotremata - Physical Characteristics

Class Mammalia

Mammals are divided into 3 sub-classes based on their mode of reproduction:
Subclass Prototheria
Prototheria is composed of egg-laying mammals. There are only 6 species of these and they belong to one order:

•  Monotremata (platypus and echidna)

Subclass Metatheria
Metatheria numbers to 250 species and is composed of seven orders (previously 1 - Marsupialia). Their young are born tiny and immature and must climb into the mother's pouch, where they grab hold of a teat and do not leave until they are mature.

•  Didelphimorphia (New World opossums)
•  Paucituberculata (South American rat opossums)
•  Microbiotheria (colocolo)
•  Dasyuromorphia (dasyurids, thylacines) 
•  Peramelemorphia (bandicoots)
•  Notoryctemorphia (marsupial moles)
•  Diprotodontia (kangaroos, koalas, wombats, possums)

Subclass Eutheria
The sub-class Eutheria is composed of all the placental mammals, whose young form as embryos in the mother's stomach. Humans are an example of the sub-class Eutheria. Below is a list of all 19 orders in this sub-class:

•  Insectivora (moles, shrews)
•  Dermoptera (flying lemurs)
•  Chiroptera (bats)
•  Cetacea (whales)
•  Carnivora (cats, bears, dogs, otters, seals, sea lions)
•  Tubulidentata (aardvarks)
•  Proboscidea (elephants)
•  Hyracoidea (hyraxes)
•  Primates (monkeys, lemurs, bushbabies, aye-ayes)
•  Xenarthra or Edentata (armadillos, anteaters, sloths)
•  Pholidota (pangolins)
•  Lagomorpha (rabbits, hares, pikas)
•  Rodentia (mice, rats, squirrels, porcupines, beavers, voles, hamsters)
•  Sirenia (manatees, dugongs)
•  Perissodactyla (horses, donkeys, zebras, rhinoceroses,tapirs)
•  Artiodactyla (pronghorns, deer, camels, gnus, goats, giraffes, hippopotami, pigs, peccaries, chevrotains, musk-deer, cows)
•  Scandentia (tree shrews)
•  Macroscelidea (Elephant Shrews)

mammalia

Mammals  are a clade of warm-bloodedamniotes. Among the features that distinguish them from the other amniotes, thereptiles and the birds, are hair, three middle ear bones, mammary glands in females, and a neocortex (a region of the brain). The mammalian brain regulates body temperature and the circulatory system, including the four-chambered heart. The mammals include the largest animals on the planet, the rorqual whales, as well as some of the most intelligent such as elephants as well as some primates andcetaceans. The basic body type is a four-legged land-borne animal, but some mammals are adapted for life at sea, in the air, in the trees, or on two legs. The largest group of mammals, the placentals, have a placenta which feeds the offspring during pregnancy. Mammals range in size from the 30–40 millimeter (1- to 1.5-inch)bumblebee bat to the 33-meter (108-foot) blue whale.

The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma ("teat, pap"). All female mammalsnurse their young with milk, which is secreted from special glands, the mammary glands. According to Mammal Species of the World, 5,702 species were known in 2006. These were grouped in 1,229 genera, 153 families and 29 orders. In 2008 theIUCN completed a five-year, 17,000-scientist Global Mammal Assessment for its IUCN Red List, which counted 5,488 accepted species at the end of that period. In some classifications, the mammals are divided into two subclasses (not counting fossils): the Prototheria  and the Theria, the latter composed of the infraclasses Metatheria and Eutheria. The marsupials comprise the crown group of the Metatheria and therefore include all living metatherians as well as many extinct ones; the placentals likewise constitute the crown group of the Eutheria.

Except for the five species of monotremes (which lay eggs), all living mammals give birth to live young. Most mammals, including the six most species-rich orders, belong to the placental group. The three largest orders, in descending order, are Rodentia, Chiroptera(bats), and Soricomorpha (shrews, moles and solenodons). The next three largest orders, depending on the classification scheme used, are the primates, to which thehuman species belongs, the Cetartiodactyla (including the even-toed hoofed mammalsand the whales), and the Carnivora (cats, dogs, weasels, bears, seals, and their relatives). While the classification of mammals at the family level has been relatively stable, different treatments at higher levels—subclass, infraclass, and order—appear in contemporaneous literature, especially for the marsupials. Much recent change has reflected the results of cladistic analysis and molecular genetics. Results from molecular genetics, for example, have led to the adoption of new groups such as theAfrotheria and the abandonment of traditional groups such as the Insectivora.

The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that also included Dimetrodon. At the end of the Carboniferous period, this group diverged from the sauropsid line that led to today's reptiles and birds. Preceded by many diverse groups of non-mammalian synapsids (sometimes referred to as mammal-like reptiles), the first mammals appeared in the early Mesozoic era. The modern mammalian orders arose in the Paleogene and Neogene periods of theCenozoic era.

ANIMALIA

Animals are a major group of multicellular, eukaryotic organisms of the kingdomAnimalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals aremotile, meaning they can move spontaneously and independently. All animals must ingest other organisms or their products for sustenance.

Most known animal phyla appeared in the fossil record as marine species during theCambrian explosion, about 542 million years ago. Animals are divided into various sub-groups, including birds, mammals, reptiles, fish and insects.Animals are all the creatures belonging to the kingdom Animalia, ranging in complexity from simple organisms like sponges to highly developed human beings. (Some of the other kingdoms are Plantae, which encompasses things like grass, trees, and flowers, and Fungi, which are things like mold and mushrooms.) Animals make up at least three-quarters of all the species on Earth, and they are distinguished from plants and other organisms by their ability to move. Even tiny animals have muscles and therefore can get around-to find food or a mate, or to get away from enemies.

While humans are animals, often when people use the word "animal" they are referring to all animals except humans. Sometimes people are referring specifically to mammals-warm-blooded creatures like dogs, cows, or lions-as opposed to birds, reptiles, or fish.

Experts estimate that over one million different kinds of animals have been identified in the world. There may be millions more, particularly insect species, that have not yet been identified or discovered by scientists. Hundreds of years ago scientists began dividing the animal kingdom into categories based on certain characteristics like body type, ways of reproducing, and what the animals can do (fly, swim, walk on two legs, and so on). The animal kingdom (and every other kingdom as well) is divided and subdivided into numerous other categories. If animal classification categories were viewed as an upside-down pyramid, kingdom-the largest and broadest classification-would be at the top. The animal kingdom is divided into several different parts, called phyla (the singular form of that word is "phylum"); each phylum is further divided into classes. The other levels of division are order, family, genus, and species, with species being the tip of that upside-down pyramid, or the most specific way to categorize. When scientists give the official name for a type of animal, they use the genus and species names. Human beings are part of the genus Homo and the species sapiens; therefore, our scientific name is Homo sapiens.

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The first animals to evolve were invertebrates. Fossil evidence of invertebrates dates back to the late Precambrian, 600 million years ago. Invertebrates evolved from single-celled microorganisms. Since then, invertebrates have diversified into countless forms. An estimated 97 percent of all species are alive today are invertebrates.

Invertebrates are united more by what they lack (a backbone) than by shared characteristics. Invertebrates include animal groups such as sponges, cnidarians, flatworms, molluscs, arthropods, insects, segmented worms, and echinoderms as well as many other lesser-known groups of animals.

Fishes were among the first vertebrates to evolve. The earliest known fishes were the ostracoderms, a now-extinct group of jawless fishes that appeared in the Cambrian Period, about 510 million years ago. Other early fish include the conodonts and the agnanthans (the hagfish and the lamprey). Fishes later evolved jaws and diversified into a number of lineages including cartilaginous fishes, ray-finned fishes and lobe-finned fishes.

The ray-finned fishes are the most diverse of all vertebrate groups, with some 24,000 species. There are about 810 species of cartilaginous fishes and 8 species of lobe-finned fishes.

Amphibians were the first vertebrates to make the move from life in water to life on land. Despite their early colonization of terrestrial habitats, most lineages of amphibians have never fully severed their ties with aquatic habitats. The first amphibians evolved from lobe-finned fishes approximately 370 million years ago during the Devonian Period.

Amphibians include newts and salamanders, frogs and toads, and caecilians. There are between 5000 and 6000 species of known amphibians alive today. Amphibian species are in decline around the world due to a variety of threats including of invasive species, habitat destruction, disease, climate change and toxins.

Reptiles are cold-blooded vertebrates that diverged from ancestral amphibians about 340 million years ago during the Carboniferous Period. Early reptiles included organisms such as Hylonomus, Petrolacosaurus, Archaeothyris and Paleothyris. The oldest evidence of reptiles is a set of footprints found in Nova Scotia. Two characteristics distinguish early reptiles from amphibians: scales and the ability to lay hard-shelled amniotic eggs.

Reptiles include turtles, squamates, crocodiles and tuataras. There are about 8,000 species of reptiles alive today. Of the four reptile groups, the squamates (amphisbaenians, lizards and snakes) are the most diverse with nearly 7,600 species.

Birds evolved from reptiles during the Mesazoic Era about 150 million years ago. Today, more than 9,000 species inhabit virtually every terrestrial habitat on the planet. Birds have a number of characteristics that sets them apart from other vertebrates such as feathers, bills and a furcula.

Birds, best known for their ability to fly, are unmatched in their command of the skies. Albatrosses glide over the vast open sea, hummingbirds hover motionless in mid-air, and birds of prey capture prey with pinpoint accuracy. But not all birds are aerobatic experts. Some species such as ostriches, kiwis and penguins, lost their ability to fly long ago in favor of lifestyles suited more for land or water.

Mammals are vertebrates that evolved from therapsid reptiles during the Jurassic Period about 200 million years ago. The first mammals, known as morganucodontids, were nocturnal insect eaters that resembled modern-day shrews. For the better part of 130 million years, mammals remained small and lived in a world dominated by the dinosaurs. But around 65 million years ago, a drastic shift in climate caused the extinction of more than two-thirds of the animal species on the planet, including the dinosaurs. Mammals survived this bout of climate change and in its wake, diversified and colonized the many newly-available habitats. Today, mammals are remarkably varied, with some 5,400 species occupying every continent on the globe.

Mammals display a remarkable array of adaptations that enable them to inhabit a wide range of habitats. Mammals range in size from the minute bumblebee bat which measures a mere three centimeters in length, to the magnificent blue whale, which can measure 33 meters head-to-tail. Some of the better-known mammal groups include carnivores, rodents, elephants, marsupials, rabbits, bats, primates, seals, anteaters, cetaceans, odd-toed ungulates and even-toed ungulates.