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Mammal

From Wikipedia, the free encyclopedia

 

Mammals

Fossil range: Late Triassic - Recent

Madras Treeshrew (Anathana ellioti)

Madras Treeshrew (Anathana ellioti)

Scientific classification

Kingdom:

Animalia

Phylum:

Chordata

Subphylum:

Vertebrata

(unranked)

Mammaliaformes

Class:

Mammalia
Linnaeus, 1758

Subclasses
  • Allotheria*
    • Order Multituberculata (extinct)
    • Order Volaticotheria (extinct)
    • Order Palaeoryctoides (extinct)
    • Order Triconodonta (extinct)
  • Prototheria
    • Order Monotremata
  • Theria
    • Infraclass Marsupialia
    • Infraclass Eutheria

The mammals are the class of vertebrate animals characterized by the production of milk in females for the nourishment of young, from mammary glands present on most species and specialized skin glands in monotremes that seep or ooze milk; the presence of hair or fur; specialized teeth; three minute bones within the ear; the presence of a neocortex region in the brain; and endothermic or "warm-blooded" bodies, and, in most cases, the existence of a placenta in the ontogeny. The brain regulates endothermic and circulatory systems, including a four-chambered heart. Mammals encompass some 5,500 species (including humans), distributed in about 1,200 genera, 152 families and up to 46 orders, though this varies with the classification scheme.

Phylogenetically, Mammalia is defined as all descendants of the most recent common ancestor of monotremes (e.g., echidnas and platypuses) and therian mammals (marsupials and placentals).

 

Distinguishing features

Living mammal species are easily identified by the fact that the females have mammary glands which produce milk.

But other features are required when classsifying fossils, because mammary glands and most of the other features listed above are not visible in fossils. The evolution of mammals from reptiles was a gradual process which took about 70 million years, from the beginning of the mid-Permian to the mid-Jurassic, and by the beginning of the mid-Triassic there were many species which looked very like mammals.

Paleontologists therefore use as the distinguishing feature one that is shared by all living mammals (including monotremes) but is not present in any of the early Triassic mammal-like reptiles - mammals use for hearing two bones which all reptiles use for eating. The earliest reptiles had a jaw joint composed 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). All non-mammalian reptiles use this system, including lizards, crocodilians, dinosaurs (and their descendants the birds) and mammal-like reptiles. But mammals have a different jaw joint, composed only of the dentary (the lower jaw bone which carries the teeth) and the squamosal (another small skull bone) - the word "only" is important here, as some late mammal-like reptiles had both types of jaw joint. And in mammals the old quadrate and articular bones have become the incus and malleus bones in the middle ear. Note: "non-mammalian reptiles" above implies that mammals are a sub-group of reptiles, and that is exactly what cladistics says they are.

Mammals also have a double occipital condyle, i.e. they have two knobs at the base of the skull which fit into the topmost neck vertebra, and other vertebrates have a single occipital condyle. But paleontologists use only the jaw joint and middle ear as criteria for identifying fossil mammals, because it would be very confusing if they found a fossil which had one feature but not the other.

 

Mammal anatomy

Skeletal system

The vast majority of mammals have seven cervical vertebrae (bones in the neck); this includes bats, giraffes, whales, and humans. The few exceptions include the manatee and the two-toed sloth, which each have only six cervical vertebrae, and the three-toed sloth with nine cervical vertebrae.

 

Respiratory system

See the section Mammalian lungs in the Lung article.

 

Circulatory system

The mammalian heart has four chambers: the right atrium, right ventricle, left atrium, and left ventricle. Atria are for receiving blood; ventricles are for pumping blood to the lungs and body. The ventricles are larger than the atria and their walls are thick, because muscular walls are needed to forcefully pump the blood from the heart to the body and lungs. Deoxygenated blood from the body enters the right atrium, which pumps it to the right ventricle. The right ventricle pumps blood to the lungs, where carbon dioxide diffuses out, and oxygen diffuses in. From the lungs, oxygenated blood enters the left atrium, where it is pumped to the left ventricle (the largest and strongest of the 4 chambers), which pumps it out to the rest of the body, including the heart's own blood supply.

 

Head and brain

All mammalian brains possess a neocortex which is a brain region that is unique to mammals.

 

Skin

Mammals have integumentary systems made up of three layers: the outermost epidermis, the dermis, and the hypodermis. This characteristic is not unique to mammals, since it is found in all vertebrates.

The epidermis is typically ten to thirty cells thick, its main function being 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 fifteen to forty 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 of adipose tissue. Its job is to store lipids, and to provide cushioning and insulation. The thickness of this layer varies widely from species to species.

No mammals are known to have hair that is naturally blue or green in color. Some cetaceans, along with the mandrills appear to have shades of blue skin. Many mammals are indicated as having blue hair or fur, but in all known cases, it has been found to be a shade of grey. The two-toed sloth can seem to have green fur, but this color is caused by algae growths.

 

Reproduction

Most mammals give birth to live young (vivipary), but a few (the monotremes) lay eggs. Live birth also occurs in some non-mammalian species, such as guppies and hammerhead sharks; thus it is not a distinguishing characteristic of mammals. Although all mammals are endothermic, so are birds, and so this too is not a defining feature.

A characteristic of mammals is that they have mammary glands, a defining feature present only in mammals. The monotremes branched from other mammals early on, and do not have nipples, but they do have mammary glands. Most mammals are terrestrial, but some are aquatic, including sirenia (manatees and dugongs) and the cetaceans (dolphins and whales). Whales are the largest of all animals. There are semi-aquatic species such as seals which come to land to breed but spend most of the time in water.

 

Flight

The only mammals for which true flight has been observed are bats; mammals such as flying squirrels and flying lemurs are more accurately classified as gliding mammals.

 

Evolution

Origins

Mammals belong to a group of amniotes called the synapsids, which are distinguished by having a single hole (temporal fenestra) low on each side of the skull on each side where jaw muscles attach. In comparison, dinosaurs, birds, and most living reptiles are diapsids, with two temporal fenestrae on each side of the skull; and turtles, with no temporal fenestra, are anapsids. The synapsids diverged from the other reptile lineages very early, in the late Carboniferous, and one of the earliest examples is Archaeothyris.

The synapsid pelycosaurs were the dominant land vertebrates of the early Permian. The therapsids probably evolved from pelycosaur ancestors, and consist of a sequence of groups which became increasingly mammal-like, especially the Triassic cynodonts ("dog-teeth"). The main article on synapsids presents a probable phylogeny ("family tree") which shows how mammals evolved from early synapsids.

Some cynodonts such as the early to mid Triassic Cynognathus had erect limbs and possibly hair or fur, and therefore may have been at least fairly warm-blooded.

Several groups of mammaliformes ("almost mammals") arose from the mid Triassic onwards - these had non-mammalian jaw joints but almost certainly had hair and were warm-blooded. Megazostrodon is a good example. Some of them were very successful in their own right, for example the multituberculates appeared in the mid-Jurassic and became extinct about 100M years later, in the Oligocene.

The first true mammals appeared in the early Jurassic, over 70M years after the first therapsids and about 30M years after the first mammaliformes. Hadrocodium is an example of the transition to true mammal status - it had a mammalian jaw joint but there is some debate about whether its middle ear was fully mammalian. The triconodonts may have been the earliest true mammals known so far.

Like their predecessors, and like modern monotremes, the first mammals laid eggs. The earliest known fossils of marsupial and placental mammals date from the mid to late Cretaceous, about 170M years after the first therapsids.

Lactation is the distinguishing feature of living mammals, but no-one is sure about when it evolved. Since all living mammals (including monotremes) produce milk, we can be confident that their last common ancestor (whatever and whenever that was) produced milk. Some scientists have suggested that the cynodonts produced milk, but this idea is a plausible speculation with little hard evidence to support it.

 

In the Mesozoic

Throughout the Mesozoic therapsids, mammaliformes and mammals lived in the shadow of archosaurs - first the "thecodonts" and following, the dinosaurs were the dominant land vertebrates.

From the mid-Triassic onwards, mammaliformes and mammals were mostly about between shrew-sized and rat-sized. But in January 2005, the journal Nature reported the discovery of two 130 million year old fossils of Repenomamus, one more than a meter in length, the other having remains of a baby dinosaur in its stomach (Nature, Jan. 15, 2005 [1]). And the 2004 discovery in China of a 164 million year old 50 cm long aquatic mammal-like fossil of a thus far unknown species, dubbed Castorocauda, by a team led by Dr. Ji Qiang of Nanjing University and the Chinese Academy of Geological Sciences, was reported in February 2006 in the journal Science (Science, Feb. 24, 2006 [2]).

Some orders of placental mammals appear to have arisen before the end of the Cretaceous - insectivores, rodents, ungulates, and possibly primates. But they were not able to expand into their modern ecological niches until after the extinction of the dinosaurs.

 

In the Cenozoic

Shortly after the start of the Paleocene, mammals exploded into the ecological niches left by the extinction of the dinosaurs and also found some which the dinosaurs apparently never occupied:

Some orders of mammals only appeared after the extinction of the dinosaurs:

Some Cenozoic mammals were considerably larger than their nearest modern equivalents - but none approached the size of the large dinosaurs.

 

Classification

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.

 

Standardized textbook classification

A somewhat standardized classification system has been adopted by most current mammalogy classroom textbooks. The following taxonomy of extant and recently extinct mammals is from Vaughan et al. (2000).

Class Mammalia

 

McKenna/Bell classification

In 1997, the mammals were comprehensively revised by Malcolm C. McKenna and Susan K. Bell, which has resulted in the "McKenna/Bell classification".

McKenna and Bell, Classification of Mammals: Above the species level, (1997) 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. The new McKenna/Bell classification was quickly accepted by paleontologists. The authors work 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 all of the 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 layman.

The published re-classification 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

 

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 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 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 mya. According to an alternative view, the Xenarthra has the Afrotheria as closest allies, forming the Atlantogenata as sistergroup to Boreoeutheria. The Boreoeutheria split into the Laurasiatheria and 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 murine rodents 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 thus 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.

Classification system used in related articles

In light of all the options available, the following classification system has been adopted for use in related articles.

Class Mammalia

 

References

 

 

Wikipedia

 

http://en.wikipedia.org/w/index.php?title=Mammal&action=history

 

http://www.gnu.org/copyleft/fdl.html