Hi Everybody! Time Travel Tonight (Backwards)

So, just when did mammals arrive on Earth? The short answer would be: about 210 million years ago. Human civilizations, about 3,000 years ago. You could say the mammals have been around a very long time. Who were they and what did they look like?

Megazostrodon is the poster critter for the transition between the therapsids ("mammal-like reptiles") of the Triassic period and the small, mouselike early mammals of the Jurassic and Cretaceous.

Fossils of the earliest mammals are more than 200 million years old. These small, shrew-like animals probably lived in caves or burrows and hunted insects and small reptiles at night.

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The Evolution of Mammals
http://www.earthlife.net/mammals/evolution.html

Mammals are vertebrates. They have a backbone which encloses a sheath of nerves which leads in turn to a brain in a box or skull. They also have four limbs and special pentadactyl ends to these limbs (i.e. 5 fingers and toes). Reptiles, amphibians and even birds, as well as the awe inspiring dinosaurs, are built around this same simple plan. Before this there were fish which had the backbone and the skull, but not the pentadactyl limbs.

Fish are pretty ancient, they first appeared around 510 million years ago. Nobody lived on land then, only in the seas. By about 350 million years ago the insects and plants had been dominating the land for around 50 million years so there was plenty of food for adventurous fish to eat, if only they could live and move around on the dry, horrible land.

Eventually some fish transformed their fins into legs and in time they also developed lungs - legs for moving and lungs for breathing. The invasion of the land had begun.

About 10 million years later reptiles started to differentiate (become different)from amphibians and diversity was well underway. Various groups evolved out of these early reptiles including crocodiles, dinosaurs and birds, but most importantly to us a strange group of animals called the Mammal-like Reptiles.

The Mammal-like Reptiles, or Therapsids first appeared about 285 million years ago near the begiining of the Permian which is well before the dinosaurs. They evolved quickly and many different groups arose. They were very successful until about the end of the Permian, about 245 million years ago, when something catastrophic affected the earth and nearly all of the species then living died out. New species evolved rapidly to fill this empty habitat, among them the first dinosaurs and a few million years later the first mammals.

The first mammal may never be known, but the Genus Morganucodon and in particular Morganucodon watsoni, a 2-3 cm (1 inch) long weasel-like animal whose fossils were first found in caves in Wales and around Bristol (UK), but later unearthed in China, India , North America, South Africa and Western Europe is a possible contender. It is believed to have lived between 200 MYA and 210 MYA. However Gondwanadon tapani reported from India on the basis of a single tooth in 1994 may be an earlier contender for the title, with a claimed date of 225 MYA.

These early mammals were small, insectivorous, nocturnal, hairy and warm-blooded. Warm-bloodedness is believed to have first evolved among the cynodonts, a late but successful group of mammal-like reptiles from which the mammals evolved. The cynodonts were the only mammal-like reptiles to survive to the Jurassic, in fact they nearly made it into the Cretaceous, and definitely coexisted with many of the major dinosaurs.

During the Jurassic the mammals remained small and mainly nocturnal, living beneath the 'metaphorical' feet of the great dinosaurs. These early mammals were more like small monotremes and probably laid eggs still. Marsupials and placental mammals (cats, dogs, you and me) did not evolve for another 70 million years.

About 8 main lineages of mammals are known from the late Jurassic, many known only from their jaws. Some more complete specimens include Crusafontia, a 20 cm (8 inch) long insectivore which probably looked like a tree shrew - its limbs show definite evidence of an arboreal existence.

Towards the end of the Jurassic a group of mammals known as 'multituberculates' appeared. These were to prove to be the most successful of the primitive mammal groups with species still alive only 30 million years ago (MYA). Meaning that they had survived as a group for 130 million years. Some of the later multituberculates possessed marsupial-like bone structures which indicate that they had pouches like marsupials, suggesting a similar life cycle involving live birth of very premature young. The fossil record gets a little better from the Cretaceous onwards, with a few complete skeletons such as Kamptobrator from Mongolia, however as far as we know mammals were still playing a fairly insignificant role in the ecology of the time.

By the end of the Cretaceous 15 mammal families were in existence that we know about. The end of the Cretaceous however saw another mass extinction. Though scientists are still unsure as to its causes, it is known that this K-T event, as it is known, resulted in the complete extinction of the dinosaurs. It also saw the death of all the Pterosaurs, the flying reptiles. All these species dying out left huge niche vacancies in the habitat. Following this disaster it was the mammals alone of the remaining groups of animals who diversified to take advantage of this new situation. Over the next 15 million years the remaining 10 mammal families (five became extinct with the dinosaurs during the K-T event) expanded to become 78 families by the early Eocene. The number of genera increased from about 40 to over 200 during the same time. This sudden massive increase in species from a single stem group is an example of what is called 'adaptive radiation'.

By the middle of the Eocene (45 MYA) all the major groups of mammals alive today had come into existence, though not necessarily as we know them now. Primates for instance have been around since the beginning of the Paleocene, 65 MYA, but the distant bipedal ancestors of man only occurred for the first time 5 MYA. The same applies to all the other major groups. The Jurassic and Cretaceous (150 million years) was the age of the Dinosaurs, a long reign as a dominant animal group. The Tertiary, however, has been and still is the age of mammals. During the 2nd half of the Eocene (12 million years) the Oligocene (23 million years) and the Miocene (18 million years) the mammals have been dominant.

Though they are still the dominant group of animals on the planet it is worth noting that over the last 10 million years, 6 of the 24 major mammal groups to come out of the Eocene have died out. This is 25% of the existing mammals then and is important because for the previous 20 million years before that no major groups died out at all. We do not know why these groups died out.

Huge numbers of fossils have been found from the Oligocene and Miocene, including the giant terrestrial mammals like Indricotherium which was 5.4 m (18 feet) tall at the shoulder. Brontotherium an elephant-sized oddity with two blunt horns, and the giant sabre-toothed cat, Smilodon. There is far too much information available for the extant (still living) groups for me to add it in here. Instead I will say something about the evolution of each group in the sections on their current biology, i.e. Mammoths will be discussed under elephants as part of their history.

Finally, it is worth knowing that besides fossils found in rocks in bits and pieces as is often the case a lot of what is known from the fossil record come from Logerstatten. Logerstatten are fossil deposits of exceptional richness which include numerous species preserved in their entirety, sometimes even including hair and gut contents. One example of this is the shale deposits at Messel in Germany. Another source of remarkably well-preserved specimens are the tar pits of Texas and southern USA.

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Entertaining and Informative about appearance of first Mammals on Earth. To Enjoy, Just Push Play

http://www.enchantedlearning.com/subjects/Geologictime.html
Geologic Time Scale
Long Version
(mya = million years ago)

The first geologic time scale was proposed in 1913 by the British geologist Arthur Holmes (1890 - 1965). This was soon after the discovery of radioactivity, and using it, Holmes estimated that the Earth was about 4 billion years old - this was much greater than previously believed.

For more information on
continental drift and plate tectonics, click here.

EON	ERA	PERIOD		EPOCH	PIVOTAL EVENTS
P h a n e r o z o i c E o n "Visible Life" Organisms with skeletons or hard shells. 540 mya through today. P h a n e r o z o i c E o n "Visible Life" Organisms with skeletons or hard shells. 540 mya through today. P h a n e r o z o i c E o n "Visible Life" Organisms with skeletons or hard shells. 540 mya through today. P h a n e r o z o i c E o n "Visible Life" Organisms with skeletons or hard shells. 540 mya through today.	Cenozoic Era "The Age of Mammals" 65 mya through today	Quaternary Period "The Age of Man" 1.8 mya to today		Holocene 11,000 ya to today	Human civilization
				Pleistocene The Last Ice Age 1.8-.011 mya	The first humans (Homo sapiens) evolve. Mammoths, mastodons, saber-toothed cats, giant ground sloths, and other Pleistocene megafauna. A mass extinction of large mammals and many birds happened about 10,000 years ago, probably caused by the end of the last ice age.
		Tertiary Period 65 to 1.8 mya	Neogene 24-1.8 mya	Pliocene 5-1.8 mya	First hominids (australopithecines). Modern forms of whales.Megalodon swam the seas
				Miocene 24-5 mya	More mammals, including the horses, dogs and bears. Modern birds. South American monkeys, apes in southern Europe, Ramapithecus.
			Paleogene 65-24 mya	Oligocene 38-24 mya	Starts with a minor extinction (36 mya). Many new mammals (pigs, deer,cats, rhinos, tapirs appear). Grasses common.
				Eocene 54-38 mya	Mammals abound. Rodents appear. Primitive whales appear.
				Paleocene 65-54 mya	First large mammals and primitive primates, plesiadapiforms.
	Mesozoic Era "The Age of Reptiles" 248 to 65 mya	Cretaceous Period 146 to 65 mya		Upper 98-65 mya	High tectonic and volcanic activity. Primitive marsupials develop. Continents have a modern-day look. Minor extinction 82 mya. Ended with large extinction (the K-T extinction) of dinosaurs, pterosaurs, ammonites, about 50 percent of marine invertebrate species, etc., probably caused by asteroid impact or volcanism.
				Lower 146-98 mya	The heyday of the dinosaurs. The first crocodilians, and feathered dinosaurs appear. The earliest-known butterflies appear (about 130 million years ago) as well as the earliest-known snakes, ants, and bees. Minor extinctions at 144 and 120 mya.
		Jurassic Period 208 to 146 mya			Many dinosaurs, including the giant Sauropods. The first birds appear (Archaeopteryx). The first flowering plants evolve. Many ferns, cycads, gingkos, rushes, conifers, ammonites, and pterosaurs. Minor extinctions at 190 and 160 mya.
		Triassic Period 248 to 208 mya			The first dinosaurs, mammals, and crocodyloformes appear. Mollusks are the dominant invertebrate. Many reptiles, for example, turtles, ichthyosaurs. True flies appear. Triassic period ends with a minor extinction 213 mya (35% of all animal families die out, including labyrinthodont amphibians, conodonts, and all marine reptiles except ichthyosaurs). This allowed the dinosaurs to expand into many niches.
	Paleozoic Era 540 to 248 mya Paleozoic Era 540 to 248 mya	Permian Period "The Age of Amphibians" 280 to 248 mya			"The Age of Amphibians" - Amphibians and reptiles dominant. Gymnosperms dominant plant life.The continents merge into a single super-continent, Pangaea. Phytoplankton and plants oxygenate the Earth's atmosphere to close to modern levels. The first stoneflies, true bugs, beetles, and caddisflies, The Permian ended with largest mass extinction. Trilobites go extinct, as do 50% of all animal families, 95% of all marine species, and many trees, perhaps caused by glaciation or volcanism.
		Carboniferous Wide-spread coal swamps, foraminiferans, corals, bryozoans, brachiopods, blastoids, seed ferns, lycopsids, and other plants. Amphibians become more common. 360 to 280 mya	Pennsylvanian Period 325 to 280 mya		First reptiles. Many ferns. The first mayflies and cockroaches appear.
			Mississippian Period 360 to 325 mya		First winged insects.
		Devonian Period "The Age of Fishes" 408 to 360 mya			Fish and land plants become abundant and diverse. First tetrapods appear toward the end of the period. First amphibians appear. First sharks, bony fish, and ammonoids. Many coral reefs, brachiopods, crinoids. New insects, like springtails, appeared. Mass extinction (345 mya) wiped out 30% of all animal families) probably due to glaciation or meteorite impact.
		Silurian Period 438 to 408 mya			The first jawed fishes and uniramians (like insects, centipedes and millipedes) appeared during the Silurian (over 400 million years ago). First vascular plants (plants with water-conducting tissue as compared with non-vascular plants like mosses) appear on land (Cooksonia is the first known). High seas worldwide. Brachiopods, crinoids, corals.
		Ordovician Period 505 to 438 mya			Primitive plants appear on land. First corals. Primitive fishes, seaweed and fungi. Graptolites, bryozoans, gastropods, bivalves, and echinoids. High sea levels at first, global cooling and glaciation, and much volcanism. North America under shallow seas. Ends in huge extinction, due to glaciation.
		Cambrian Period "The Age of Trilobites" 540 to 500 mya			"Age of Trilobites" -The Cambrian Explosion of life occurs; all existent phyla develop. Many marine invertebrates (marine animals with mineralized shells: shell-fish, echinoderms, trilobites, brachiopods, mollusks, primitive graptolites). First vertebrates. Earliest primitive fish. Mild climate. The supercontinent Rodinia began to break into smaller continents (no correspondence to modern-day land masses). Mass extinction of trilobites and nautiloids at end of Cambrian (50% of all animal families went extinct), probably due to glaciation.
Proterozoic Eon 2.5 billion years ago to 540 mya	-	Vendian/Ediacaran Period 600 to 540 Million Years Ago			Vendian biota (Ediacaran fauna) multi-celled animals appear, includingsponges. A mass extinction occurred. The continents had merged into a single supercontinent called Rodinia.
		-			First multicellular life: colonial algae and soft-bodied invertebrates appear.Oxygen build-up in the Mid-Proterozoic.
Archeozoic Eon (Archean) 3.9 to 2.5 billion years ago	-	-			"Ancient Life" - The first life forms evolve - one celled organisms. Blue-green algae, archaeans, and bacteria appear in the sea. This begins to free oxygen into the atmosphere.
Hadean Eon 4.6 to 3.9 billion years ago	-	-			"Rockless Eon" - The solidifying of the Earth's continental and oceanic crusts.

http://en.wikipedia.org/wiki/Mammal

Mammal

From Wikipedia, the free encyclopedia

For other uses, see Mammal (disambiguation).

Mammals Temporal range: See text PreЄ Є O S D C P T J K Pg N

Examples of various mammalian orders, click the image and scroll down for individual descriptions
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Infraphylum:	Gnathostomata
clade:	Eugnathostomata
clade:	Teleostomi
Superclass:	Tetrapoda
clade:	Amniota
clade:	Synapsida
Class:	Mammalia Linnaeus, 1758
Subgroups
Order Monotremata Clade Theriiformes Order †Triconodonta Order †Multituberculata Subclass Theria Infraclass Marsupialia Infraclass Placentalia

Mammals are members of class Mammalia ( /məˈmeɪli.ə/), air-breathingvertebrate animals characterised by the possession of endothermy, hair, threemiddle ear bones, and mammary glands functional in mothers with young. Most mammals also possess sweat glands and specialised teeth. The largest group of mammals, the placentals, have a placenta which feeds the offspring during gestation. The mammalian brain, with its characteristic neocortex, regulatesendothermic and circulatory systems, the latter featuring red blood cells lackingnuclei and a four-chambered heart. 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 mammals nurse their young with milk, which is secreted from special glands, the mammary glands. According to Mammal Species of the World, which is updated through periodic editions, 5,676 species were known in 2005. These were distributed in 1,229 genera, 153 families and 29 orders.^[1] In 2008 the IUCN completed a five-year, 17,000-scientist Global Mammal Assessment for its IUCN Red List, which counted 5488 accepted species at the end of that period.^[2] In some classifications, the class is divided into two subclasses (not counting fossils): the Prototheria (order ofMonotremata) and the Theria, the latter composed of the infraclasses Metatheriaand Eutheria. The marsupials are the crown group of the Metatheria and therefore include all living metatherians as well as many extinct ones; the placentals are likewise the crown group of the Eutheria.

The classification of mammals between the relatively stable class and family levels has changed often; different treatments of subclass, infraclass and order appear in contemporaneous literature, especially for Marsupialia. 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.

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, areRodentia (mice, rats, porcupines, beavers, capybaras, and other gnawing mammals), Chiroptera (bats), and Soricomorpha (shrews, moles and solenodons). The next three largest orders, depending on the classification scheme used, are theprimates, to which the human species belongs, the Cetartiodactyla (including theeven-toed hoofed mammals and the whales), and the Carnivora (dogs, cats,weasels, bears, seals, and their relatives).^[1]

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.

Varying definitions, varying dates

In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group mammals, the clade consisting of themost recent common ancestor of living monotremes (echidnas and platypuses) and therian mammals (marsupials and placentals) and all descendants of that ancestor.^[3] A broader phylogenetic definition was provided in a 2004 book by Kielan-Jaworowska, Cifelli, and Luo, who defined Mammalia as the clade originating with the most recent common ancestor, not only of the monotremes and the therians, but also of Sinoconodon, the morganucodonts, and the docodonts.^[4] The morganucodonts and the docodonts, included by Rowe in the unranked clade Mammaliaformes, had a widespread distribution in the northern continents and had many of the characteristics that traditionally would have classified them as mammals.^[5] In particular, some docodonts were furry.

Mammalia, considered as the crown group, appeared in the Pliensbachian age of the early Jurassic period.^[6] In the broader sense given to the term by Kielan-Jaworowska et al., the group arose in the Norian age in the middle of the Late Triassic.^[4] Finally, some writers consider Adelobasileus to be a mammal; as this animal lived in the Carnian age at the beginning of the Late Triassic, this would mean that mammals appeared even earlier.^[7] In any case, the temporal range of the group extends to the present day.

[edit]Distinguishing features

Living mammal species can be identified by the presence of sweat glands, including those that are specialized to produce milk. In classifying fossils, however, other features must be used, since soft tissue glands and many other features are not visible in fossils.

Among the many traits shared by all living mammals, but not present in any of the early Triassic synapsids, are:

Jaw joint - The dentary (the lower jaw bone which carries the teeth) and the squamosal (another small skull bone) meet to form the joint. In most gnathostomes, including early therapsids, the joint consists of the articular (a small bone at the back of the lower jaw) and the quadrate (a small bone at the back of the upper jaw).

Middle ear - Sound is carried from the eardrum by a chain of three bones, the malleus, the incus, and the stapes. Ancestrally, the malleus and the incus are derived from the articular and the quadrate bones that constituted the jaw joint of early therapsids.

Tooth replacement - Teeth are replaced once or (as in toothed whales and murid rodents) not at all, rather than being replaced continually throughout life.^[8]

Prismatic enamel - The enamel coating on the surface of a tooth consists of prisms, solid, rod-like structures extending from thedentin to the tooth's surface.

Occipital condyles - Two knobs at the base of the skull fit into the topmost neck vertebra; most tetrapods, in contrast, have only one such knob.

For paleontologists who define Mammalia phylogenetically, no limit can be set on the features used to distinguish the group. Any feature may be relevant to a fossil's phylogenetic position. Paleontologists defining Mammalia in terms of traits, on the other hand, need only consider those features that appear in the definition. The dentary-squamosal jaw joint is generally included.

[edit]Classification

Main article: Mammal classification

Over 70% of mammal species are in the orders Rodentia(blue), Chiroptera (red), and Soricomorpha (yellow)

George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" (AMNH Bulletin v. 85, 1945) was the original source for the taxonomy listed here. Simpson laid out a systematics of mammal origins and relationships that was universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, and the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself, partly through the new concept of cladistics. Though field work gradually made Simpson's classification outdated, it remained the closest thing to an official classification of mammals.

[edit]McKenna/Bell classification

In 1997, the mammals were comprehensively revised by Malcolm C. McKennaand Susan K. Bell, which has resulted in the McKenna/Bell classification. Their 1997 book, Classification of Mammals: Above the species level,^[5] is the most comprehensive work to date on the systematics, relationships, and occurrences of all mammal taxa, living and extinct, down through the rank of genus, though recent molecular genetic data challenge several of the higher level groupings. The authors worked together as paleontologists at the American Museum of Natural History, New York. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, covering living and extinct taxa that reflects the historical genealogy of Mammalia.

The McKenna/Bell hierarchical listing of many terms used for mammal groups above the species includes extinct mammals, as well as modern groups, and introduces some fine distinctions such as legions and sublegions (ranks which fall between classes and orders) that are likely to be glossed over by the nonprofessionals.

The published reclassification forms both a comprehensive and authoritative record of approved names and classifications and a list of invalid names.

Extinct groups are represented by a dagger (†).

Class Mammalia

Subclass Prototheria: monotremes: echidnas and the platypus
Subclass Theriiformes: live-bearing mammals and their prehistoric relatives
- Infraclass †Allotheria: multituberculates
- Infraclass †Triconodonta: triconodonts
- Infraclass Holotheria: modern live-bearing mammals and their prehistoric relatives
  - Supercohort Theria: live-bearing mammals
    - Cohort Marsupialia: marsupials
      - Magnorder Australidelphia: Australian marsupials and the monito del monte
      - Magnorder Ameridelphia: New World marsupials
    - Cohort Placentalia: placentals
      - Magnorder Xenarthra: xenarthrans
      - Magnorder Epitheria: epitheres
        
        Grandorder Anagalida: lagomorphs, rodents, and elephant shrews
        
        Grandorder Ferae: carnivorans, pangolins, †creodonts, and relatives
        
        Grandorder Lipotyphla: insectivorans
        
        Grandorder Archonta: bats, primates, colugos, and treeshrews
        
        Grandorder Ungulata: ungulates
        
        Order Tubulidentata incertae sedis: aardvark
        
        Mirorder Eparctocyona: †condylarths, whales, and artiodactyls (even-toed ungulates)
        
        Mirorder †Meridiungulata: South American ungulates
        
        Mirorder Altungulata: perissodactyls (odd-toed ungulates), elephants, manatees, and hyraxes

[edit]Molecular classification of placentals

Molecular studies based on DNA analysis have suggested new relationships among mammal families over the last few years. Most of these findings have been independently validated by retrotransposon presence/absence data. The most recent classification systems based on molecular studies have proposed four groups or lineages of placental mammals. Molecular clocks suggest that these clades diverged from early common ancestors in the Cretaceous, but fossils have not yet been found to corroborate this hypothesis. These molecular findings are consistent with mammal zoogeography:

Following molecular DNA sequence analyses, the first divergence was that of the Afrotheria 110–100 million years ago (mya). The Afrotheria proceeded to evolve and diversify in the isolation of the African-Arabian continent. The Xenarthra, isolated in South America, diverged from the Boreoeutheria approximately 100–95 million years ago. According to an alternative view, the Xenarthra has the Afrotheria as closest allies, forming the Atlantogenata as sister group to Boreoeutheria. The Boreoeutheria split into the Laurasiatheriaand Euarchontoglires between 95 and 85 mya; both of these groups evolved on the northern continent of Laurasia. After tens of millions of years of relative isolation, Africa-Arabia collided with Eurasia, exchanging Afrotheria and Boreoeutheria. The formation of the Isthmus of Panama linked South America and North America, which facilitated the exchange of mammal species in the Great American Interchange. The traditional view that no placental mammals reached Australasia until about 5 million years ago, when bats and murinerodents arrived, has been challenged by recent evidence and may need to be reassessed. These molecular results are still controversial because they are not reflected by morphological data and therefore not accepted by many systematists. Further, there is some indication from retrotransposon presence/absence data that the traditional Epitheria hypothesis, suggesting Xenarthra as the first divergence, might be true. With the old order Insectivora shown to be polyphylectic and more properly subdivided (as Afrosoricida,Erinaceomorpha, and Soricomorpha), the following classification for placental mammals contains 21 orders:

Clade Atlantogenata
- Group I: Afrotheria
  - Clade Afroinsectiphilia
    - Order Macroscelidea: elephant shrews (Africa)
    - Order Afrosoricida: tenrecs and golden moles (Africa)
    - Order Tubulidentata: aardvark (Africa south of the Sahara)
  - Clade Paenungulata
    - Order Hyracoidea: hyraxes or dassies (Africa, Arabia)
    - Order Proboscidea: elephants (Africa, Southeast Asia)
    - Order Sirenia: dugong and manatees (cosmopolitan tropical)
- Group II: Xenarthra
  - Order Pilosa: sloths and anteaters (neotropical)
  - Order Cingulata: armadillos (Americas)
Clade Boreoeutheria
- Group III: Euarchontoglires (Supraprimates)
  - Superorder Euarchonta
    - Order Scandentia: treeshrews (Southeast Asia).
    - Order Dermoptera: flying lemurs or colugos (Southeast Asia)
    - Order Primates: lemurs, bushbabies, monkeys, apes, human (cosmopolitan)
  - Superorder Glires
    - Order Lagomorpha: pikas, rabbits, hares (Eurasia, Africa, Americas)
    - Order Rodentia: rodents (cosmopolitan)
- Group IV: Laurasiatheria
  - Order Erinaceomorpha: hedgehogs
  - Order Soricomorpha: moles, shrews, solenodons
  - Clade Ferungulata
    - Clade Cetartiodactyla
      - Order Cetacea: whales, dolphins and porpoises
      - Order Artiodactyla: even-toed ungulates, including pigs, hippopotamus, camels, giraffe, deer, antelope, cattle, sheep,goats
    - Clade Pegasoferae
      - Order Chiroptera: bats (cosmopolitan)
      - Clade Zooamata
        
        Order Perissodactyla: odd-toed ungulates, including horses, donkeys, zebras, tapirs, and rhinoceroses
        
        Clade Ferae
        
        Order Pholidota: pangolins or scaly anteaters (Africa, South Asia)
        
        Order Carnivora: carnivores (cosmopolitan), including cats and dogs

[edit]Evolutionary history

For more details on this topic, see Evolution of mammals.

Cladogram following,^[9] which takes Mammalia to be the crown group.

Tritylodontidae

Adelobasileus

Sinoconodon

Mammaliaformes

Morganucodontidae	Morganucodon

Docodonta

Haldanon

Mammalia

Monotremata

	Ornithorhychus (Platypus)

	Tachyglossidae (Echidna)

Theriiformes

	Triconodonts, Multituberculates, Marsupials, and Placentals

Synapsida, the group which contains mammals and their extinct relatives, originated during the Pennsylvanian subperiod, when they split from the lineage that led to reptiles and birds. Crown group mammals evolved from earlier mammaliaforms during the Early Jurassic.

[edit]Evolution from amniotes in the Paleozoic

The original synapsid skull structure contains one temporal opening behind theorbitals, in a fairly low position on the skull (lower right in this image). This might have assisted in the containing the jaw muscles of these organisms that could have increased their biting strength.

The first fully terrestrial vertebrates were amniotes. Like their amphibian predecessors, they have lungs and limbs. Amniotes' eggs, however, have internal membranes which allow the developing embryo to breathe but keep water in. Hence, amniotes can lay eggs on dry land, while amphibians generally need to lay their eggs in water.

The first amniotes apparently arose in the Late Carboniferous. They descended from earlierreptiliomorph amphibians,^[10] which lived on land already inhabited by insects and other invertebrates, and by ferns, mosses and other plants. Within a few million years, two important amniote lineages became distinct: the synapsids, which include mammals; and the sauropsids, which include turtles, lizards, snakes, crocodilians, dinosaurs and birds.^[11]Synapsids have a single hole (temporal fenestra) low on each side of the skull.

One synapsid group, the pelycosaurs, included the largest and fiercest animals of the earlyPermian.^[12]

Therapsids descended from pelycosaurs in the middle Permian, about 265 million years ago, and took over their position as the dominant land vertebrates.^[13] They differ from pelycosaurs in several features of the skull and jaws, including: larger temporal fenestrae and incisors which are equal in size.^[14] The therapsid lineage leading to mammals went through a series of stages, beginning with animals that were very like their pelycosaur ancestors and ending with probainognathian cynodonts, some of which could easily be mistaken for mammals. Those stages were characterized by:

gradual development of a bony secondary palate.^[15]
Progress was made towards an erect limb posture, which would increase the animals' stamina by avoiding Carrier's constraint. But this process was slow and erratic: for example, all herbivorous nonmammaliaform therapsids retained sprawling limbs (some late forms may have had semierect hind limbs); Permian carnivorous therapsids had sprawling forelimbs, and some late Permian ones also had semisprawling hindlimbs. In fact, modern monotremes still have semisprawling limbs.
The dentary gradually became the main bone of the lower jaw and, in the Triassic, progressed towards the fully mammalian jaw (the lower consisting only of the dentary) and middle ear (which is constructed by the bones that were previously used to construct the jaws of reptiles).
There is some evidence of hair in Triassic therapsids but none for Permian therapsids.
Some Triassic therapsids also show signs of lactation.

These nonmammalian synapsids are sometimes called "mammal-like reptiles",^[13]^[16].

[edit]The mammals appear

The Permian–Triassic extinction event, which was a prolonged event due to the accumulation of several extinction pulses, ended the dominance of the carnivores among the therapsids. In the Early Triassic, all the medium to large land carnivore niches were taken over by early archosaurs, which over an extended period of time (35 million years) evolved into crocodilians, pterosaurs, dinosaurs and birds. By the Jurassic, the dinosaurs had come to dominate the large terrestrial herbivore niches as well.

The first mammals (in the sense given to the term by Kielan-Jawarowska et al.)^[4] appeared in the Late Triassic epoch (about 210 million years ago), 60 million years after the first therapsids. They expanded out of their nocturnal insectivore niche from the mid-Jurassic onwards; Castorocauda, for example, had adaptations for swimming, digging and catching fish.^[17]

The majority of the mammal species that existed in the Mesozoic Era were multituberculates, triconodonts and spalacotheriids.^[18]

The earliest known monotreme is Teinolophos, which lived about 123 million years ago in Australia. Monotremes have some features which may be inherited from the original amniotes:

They use the same orifice to urinate, defecate and reproduce ("monotreme" means "one hole") – as lizards and birds also do.
They lay eggs which are leathery and uncalcified, like those of lizards, turtles and crocodilians.

Unlike other mammals, female monotremes do not have nipples and feed their young by "sweating" milk from patches on their bellies.

The earliest known metatherian is Sinodelphys, found in 125 million-year-old Early Cretaceous shale in China's northeastern Liaoning Province. The fossil is nearly complete and includes tufts of fur and imprints of soft tissues.^[19]

The oldest known fossil among the Eutheria ("true beasts") is the small shrewlike Juramaia sinensis, or "Jurassic mother from China," dated to 160 million years ago in the Upper Jurassic.^[6] A later eutherian, Eomaia, dated to 125 million years ago in the Lower Cretaceous, possessed some features in common with the marsupials but not with the placentals, evidence that these features were present in the last common ancestor of the two groups but were later lost in the placental lineage.^[20] In particular:

Epipubic bones extend forwards from the pelvis. These are not found in any modern placental, but they are found in marsupials, monotremes, and nontherian mammals like the multituberculates as well as in Ukhaatherium, an Upper Cretaceous animal in the eutherian order Asioryctitheria.^[21] They are apparently an ancestral feature which subsequently disappeared in the placental lineage. These epipubic bones seem to function by stiffening the muscles of these animals during locomotion, reducing the amount of space being presented, which placentals require to contain their fetus during gestation periods.

A narrow pelvic outlet indicates that the young were very small at birth and therefore pregnancy was short, as in modern marsupials. This suggests that the placenta was a later development.

When true placental mammals evolved is uncertain – the earliest undisputed fossils of placentals come from the early Paleocene, after the extinction of the dinosaurs.^[22]

[edit]Rise to dominance in the Cenozoic

Mammals took over the medium- to large-sized ecological niches in the Cenozoic, after the Cretaceous–Paleogene extinction eventemptied ecological space once filled by reptiles.^[23] Then mammals diversified very quickly; both birds and mammals show an exponential rise in diversity.^[23] For example, the earliest known bat dates from about 50 million years ago, only 15 million years after the extinction of the dinosaurs.^[24]

Recent molecular phylogenetic studies suggest that most placental orders diverged about 100 to 85 million years ago and that modernfamilies appeared in the period from the late Eocene through the Miocene.^[25] But paleontologists object that no placental fossils have been found from before the end of the Cretaceous.^[22]

During the Cenozoic, several groups of mammals appeared which were much larger than their nearest modern equivalents, but none was even close to the size of the largest dinosaurs with similar feeding habits.

[edit]Earliest appearances of features

Hadrocodium, whose fossils date from the early Jurassic (approximately 195 million years ago, in the Lower Jurassic), provides the first clear evidence of a jaw joint formed solely by the squamosal and dentary bones; there is no space in the jaw for the articular, a bone involved in the jaws of all early synapsids.

It has been suggested that the original function of lactation (milk production) was to keep eggs moist. Much of the argument is based onmonotremes (egg-laying mammals).^[26]^[27]^[28]

The earliest clear evidence of hair or fur is in fossils of Castorocauda, from 164 million years ago in the Middle Jurassic. In the past, some scientists interpreted the foramina (passages) in the maxillae (upper jaws) and premaxillae (small bones in front of the maxillae) ofcynodonts as channels which supplied blood vessels and nerves to vibrissae (whiskers) and suggested that this was evidence of hair or fur.^[29]^[30] Foramina do not necessarily show that an animal had vibrissae, however; the modern lizard Tupinambis has foramina which are almost identical to those found in the nonmammalian cynodont Thrinaxodon.^[16]^[31]

The evolution of erect limbs in mammals is incomplete — living and fossil monotremes have sprawling limbs. Some scientists think that the parasagittal (nonsprawling) limb posture is a synapomorphy (distinguishing characteristic) of the Boreosphenida, a group which contains the Theria and therefore includes all eutherians (including the placentals).^[32] Since Juramaia, the earliest known eutherian, lived about 160 million years ago in the Jurassic, this implies that erect limbs must have evolved before then.

When endothermy first appeared in the evolution of mammals is uncertain. Modern monotremes have lower body temperatures and more variable metabolic rates than marsupials and placentals,^[33] but there is evidence that some of their ancestors, perhaps including ancestors of the therians, may have had body temperatures like those of modern therians.^[34] Some of the evidence found so far suggests that Triassic cynodonts had fairly high metabolic rates, but it is not conclusive. For small animals, an insulative covering like fur is necessary for the maintenance of a high and stable body temperature.

[edit]Anatomy and morphology

[edit]Skeletal system

The majority of mammals have seven cervical vertebrae (bones in the neck), including bats, giraffes, whales, and humans. The exceptions are the manatee and the two-toed sloth, which have only six cervical vertebrae, and the three-toed sloth with nine cervical vertebrae.^[35]

[edit]Respiratory system

The lungs of mammals have a spongy texture and are honeycombed with epithelium having a much larger surface area in total than the outer surface area of the lung itself. The lungs of humans are typical of this type of lung.

Breathing is largely driven by the muscular diaphragm, which divides the thorax from the abdominal cavity, forming a dome with its convexity towards the thorax. Contraction of the diaphragm flattens the dome, increasing the volume of the cavity in which the lung is enclosed. Air enters through the oral and nasal cavities; it flows through the larynx, trachea and bronchi and expands the alveoli. Relaxation of the diaphragm has the opposite effect, passively recoiling during normal breathing. During exercise, the abdominal wallcontracts, increasing visceral pressure on the diaphragm, thus forcing the air out more quickly and forcefully. The rib cage itself also is able to expand and contract the thoracic cavity to some degree, through the action of other respiratory and accessory respiratory muscles. As a result, air is sucked into or expelled out of the lungs, always moving down its pressure gradient. This type of lung is known as a bellows lung as it resembles a blacksmith's bellows. Mammals take oxygen into their lungs, and discard carbon dioxide.

[edit]Nervous system

All mammalian brains possess a neocortex, a brain region unique to mammals. Placental mammals have a corpus callosum, unlike monotremes and marsupials. The size and number of cortical areas (Brodmann's areas) is least in monotremes (about 8-10) and most in placentals (up to 50).

[edit]Integumentary system

The integumentary system is made up of three layers: the outermost epidermis, the dermis, and the hypodermis.

The epidermis is typically 10 to 30 cells thick; its main function is to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is 15 to 40 times thicker than the epidermis. The dermis is made up of many components, such as bony structures and blood vessels. The hypodermis is made up ofadipose tissue. Its job is to store lipids, and to provide cushioning and insulation. The thickness of this layer varies widely from species to species.

Although other animals have features such as feathers, whiskers, setae, or cilia that superficially resemble it, no animals other than mammals have hair. It is a definitive characteristic of the class. Though some mammals have very little, careful examination reveals the characteristic, often in obscure parts of their bodies.

Some primates and marsupials have shades of violet, green, or blue skin on parts of their bodies.^[36] The two-toed sloth and the polar bear sometimes appear to have green fur, but this color is caused by algae growths.

[edit]Reproductive system

See also: Sexual reproduction#Mammals and Animal sexual behavior#Mammals

Goat kids will stay with their mother until they are weaned.

Most mammals are viviparous, giving birth to live young. However, the five species ofmonotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals.^[37] In particular, the sexchromosomes of a platypus are more like those of a chicken than those of a therian mammal.^[38]

The mammary glands of mammals are specialized to produce milk, a liquid used by newborns as their primary source of nutrition. The monotremes branched early from other mammals and do not have the nipples seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly.

Viviparous mammals are in the subclass Theria; those living today are in the marsupial andplacental infraclasses. A marsupial has a short gestation period, typically shorter than itsestrous cycle, and gives birth to an undeveloped newborn that then undergoes further development; in many species, this takes place within a pouch-like sac, the marsupium, located in the front of the mother's abdomen. The placentals give birth to complete and fully developed young, usually after long gestation periods.

[edit]Physiology

[edit]Endothermy

Nearly all mammals are endothermic ("warm-blooded"). Most mammals also have hair to help keep them warm. Like birds, mammals can forage or hunt in weather and climates too cold for nonavian reptiles and large insects.

Endothermy requires plenty of food energy, so mammals eat more food per unit of body weight than most reptiles. Small insectivorous mammals eat prodigious amounts for their size.

A rare exception, the naked mole rat, produces little metabolic heat, so it is considered an operational poikilotherm. Birds are also endothermic, so endothermy is not a defining mammalian feature.

[edit]Intelligence

In intelligent mammals, such as primates, the cerebrum is larger relative to the rest of the brain. Intelligence itself is not easy to define, but indications of intelligence include the ability to learn, matched with behavioral flexibility. Rats, for example, are considered to be highly intelligent, as they can learn and perform new tasks, an ability that may be important when they first colonize a fresh habitat. In some mammals, food gathering appears to be related to intelligence: a deer feeding on plants has a brain smaller than a cat, which must think to outwit its prey.

Image Credits:
http://www.lpi.usra.edu/education/timeline/gallery/slide_55.html

Fossils of the earliest mammals are more than 200 million years old. These small, shrew-like animals probably lived in caves or burrows and hunted insects and small reptiles at night.

But perhaps the biggest changes came with the evolution of
dinosaurs and the first mammals in the late Triassic, starting
around 230 million years ago. ...