Difference between revisions of "Mammal" - New World Encyclopedia

From New World Encyclopedia
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*Subclass Palaeoryctoides (extinct)
 
*Subclass Palaeoryctoides (extinct)
 
*Subclass Triconodonta (extinct)
 
*Subclass Triconodonta (extinct)
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*Subclass Eutheria (includes extinct ancestors)/[[Placentalia]] (excludes extinct ancestors)
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*Subclass Eutheria/[[Placentalia]]  
 
**[[Afrosoricida]]
 
**[[Afrosoricida]]
 
**[[Artiodactyla]]
 
**[[Artiodactyla]]
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{{Taxobox_end}}
 
{{Taxobox_end}}
  
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The '''mammals''' are the class of [[vertebrate]] [[animal]]s characterized by the presence of [[mammary gland]]s, which in females produce [[milk]] for the nourishment of young; the presence of [[hair]] or [[fur]]; and endothermic or "[[warm-blooded]]" bodies. The brain regulates endothermic and [[Circulatory system|circulatory]] systems, including a four-chambered [[heart]]. Mammals encompass some 5,500 [[species]], distributed in about 1,200 [[genus|genera]], 152 [[Family (biology)|families]] and up to 46 [[Order (biology)|order]]s, though this varies with the classification scheme.
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'''Mammals''' are the class of [[vertebrate]] [[animal]]s characterized by the presence of [[hair]] (or [[fur]]) and [[mammary gland]]s, which in females produce [[milk]] for the nourishment of young. The other extant (living) classes of vertebrates (animals with backbones) include [[fish]] (with a few recognized classes), [[amphibian]]s, [[reptile]]s, and [[bird]].  
  
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[[Phylogenetics|Phylogenetically]], '''Mammalia''' is defined as all descendants of the most recent common ancestor of [[monotreme]]s (e.g., [[echidna]]s and [[platypus]]es) and [[Theria|therian]] mammals ([[marsupial]]s and [[placental]]s).
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Like birds, mammals have endothermic or "[[warm-blooded]]" bodies, and four-chambered hearts. Mammals also have a [[diaphragm]], a [[muscle]] below the rib cage, that aids breathing. Some other vertebrates have a diaphragm, but mammals are the only vertebrates with a prehepatic diaphragm, that is, in front of the liver.
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There are three major subdivisions of mammals: monotremes, marsupials, and placentals. Monotremes are mammals that lay eggs, and include the platypuses and echidnas (spiny anteaters). Marsupials are generally characterized by the female having a pouch in which it rears its young through early infancy, as well as various reproductive traits that distinguish them from other mammals. Oppossums, wombats, kangaroos, and wallabies are examples of marsupials. Placentals are distinguished from other mammals in that the fetus is nourished during gestation via a placenta while, in general, this is not the case with other mammals (Bandicoots are a conspicuous exception to this rule). About 5,500 living species of mammals have been identified.
 +
 
 +
Phylogenetically (classification based on evolutionary relatedness), '''Mammalia''' is defined as all descendants of the most recent common ancestor of [[monotreme]]s and the therian mammals. (Theria is a taxon that includes the closely affiliated [[marsupial]]s and [[placental]]s).
  
 
==Characteristics==
 
==Characteristics==
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===Molecular classification of placentals===
 
===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]] [[Retrotransposon_Marker|presence/absence data]]. The most recent classification systems based on molecular studies have proposed four groups or lineages of placental mammals. [[Molecular clock]]s 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]]:
 
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]] [[Retrotransposon_Marker|presence/absence data]]. The most recent classification systems based on molecular studies have proposed four groups or lineages of placental mammals. [[Molecular clock]]s 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]]:
 +
 +
[[Image:Northern right whale dolphin.jpg|thumb|right|300px|Northern right whale dolphin (''L. borealis'')]]
  
 
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. 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 [[Murinae|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 [[Morphology (biology)|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.  
 
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. 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 [[Murinae|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 [[Morphology (biology)|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.  
 +
 +
Added from article of that name: Cetartiodactyla is a hypothesized order to which whales (including dolphins) and even-toed ungulates are proposed to belong. The term was coined by merging the name for the two orders, Cetacea and Artiodactyla, into a single word. The term Cetartiodactyla is most commonly used to describe the hypothesis that whales evolved from within the artiodactyls. Under this definition their closest living land relative is thought to be the hippopotamus. The clade formed by uniting whales and hippos is called Cetancodonta. Alternatively, the term Cetartiodactlya is used to denote a clade where Cetacea and Artiodactyla are sister groups, but where Cetacea did not actually evolve from within the Artiodactyla. Under this definition, all artiodactyls, including hippos, are more closely related to one another than any are to the whales.
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* '''Group I: [[Afrotheria]]'''
 
* '''Group I: [[Afrotheria]]'''
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{{credit2|Mammal|47375460|Eutheria|43995227}}
 
[[Category:Life sciences]]
 
[[Category:Life sciences]]

Revision as of 17:41, 7 April 2006

Mammals
Mountain-Goat.jpg
Rocky Mountain Goat (Oreamnos americanus)
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertabrata
Class: Mammalia
Linnaeus, 1758
Orders

Mammals are the class of vertebrate animals characterized by the presence of hair (or fur) and mammary glands, which in females produce milk for the nourishment of young. The other extant (living) classes of vertebrates (animals with backbones) include fish (with a few recognized classes), amphibians, reptiles, and bird.

Like birds, mammals have endothermic or "warm-blooded" bodies, and four-chambered hearts. Mammals also have a diaphragm, a muscle below the rib cage, that aids breathing. Some other vertebrates have a diaphragm, but mammals are the only vertebrates with a prehepatic diaphragm, that is, in front of the liver.

There are three major subdivisions of mammals: monotremes, marsupials, and placentals. Monotremes are mammals that lay eggs, and include the platypuses and echidnas (spiny anteaters). Marsupials are generally characterized by the female having a pouch in which it rears its young through early infancy, as well as various reproductive traits that distinguish them from other mammals. Oppossums, wombats, kangaroos, and wallabies are examples of marsupials. Placentals are distinguished from other mammals in that the fetus is nourished during gestation via a placenta while, in general, this is not the case with other mammals (Bandicoots are a conspicuous exception to this rule). About 5,500 living species of mammals have been identified.

Phylogenetically (classification based on evolutionary relatedness), Mammalia is defined as all descendants of the most recent common ancestor of monotremes and the therian mammals. (Theria is a taxon that includes the closely affiliated marsupials and placentals).

Characteristics

Most mammals give birth to live young, 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.

Mammals have three bones in each ear and one (the dentary) on each side of the lower jaw; all other vertebrates with ears have one bone (the stapes) in the ear and at least three on each side of the jaw. A group of therapsids called cynodonts had three bones in the jaw, but the main jaw joint was the dentary and the other bones conducted sound. The extra jaw bones of other vertebrates are thought to be homologous with the malleus and incus of the mammal ear.

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

Mammals have integumentary systems made up of three layers: the outermost epidermis, the dermis, and the hypodermis. This characteristic is not unique to mammals, but 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.

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.

True flight has evolved only once in mammals, the bats; mammals such as flying squirrels and flying lemurs are actually gliding animals.

No mammals 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 cases, it will be 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.

Origins

Mammals belong among the amniotes, and in particular to a group called the synapsids, distinguished by the shape of their skulls, in particular the presence of a single hole on each side where jaw muscles attach, called temporal fenestra. In comparison, dinosaurs, birds, and most reptiles are diapsids, with two temporal fenestrae on each side of the skull; and turtles, with no temporal fenestra, are anapsids.

From early synapsids came the first mammal precursors, therapsids, and more specifically the eucynodonts, 220 million years ago (mya) during the Triassic period.

Pre-mammalian ears began evolving in the late Permian to early Triassic to their current state, as three tiny bones (incus, malleus, and stapes) inside the skull; accompanied by the transformation of the lower jaw into a single bone. Other animals, including reptiles and pre-mammalian synapsids and therapsids, have several bones in the lower jaw, some of which are used for hearing; and a single ear-bone in the skull, the stapes. This transition is evidence of mammalian evolution from reptilian beginnings: from a single ear bone, and several lower jaw bones (for example the sailback pelycosaur, Dimetrodon) to progressively smaller "hearing jaw bones" (for example the cynodont, Probainognathus), and finally (possibly with Morganucodon, but definitely with Hadrocodium), true mammals with three ear bones in the skull and a single lower jaw bone. Hence pelycosaurs and cynodonts are sometimes called "mammal-like reptiles", but this is strictly incorrect as these two are not reptiles but synapsids.

During the Mesozoic Period, mammals diversified into four main groups: multituberculates (Allotherium), monotremes, marsupials, and placentals. Multituberculates went extinct during the Oligocene, about 30 million years ago, but the three other mammal groups are all represented today. Most early mammals remained small and shrew-like throughout the Mesozoic, but rapidly developed into larger more diverse forms following the Cretaceous-Tertiary extinction event 65 mya.

The names "Prototheria", "Metatheria" and "Eutheria" expressed the theory that Placentalia were descendants of Marsupialia, which were in turn descendants of Monotremata, but this theory has been refuted. However, Eutheria and Metatheria are often used in paleontology, especially with regards to mammals of the Mesozoic.

Mammal evolutionary progression is below:

In the Mesozoic

Most early mammals were tiny shrew-like animals that fed on insects. However, 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]).

The earliest mammals include:

  • Eozostrodon: Triassic and Jurassic
  • Deltatheridium: Cretaceous
  • Jeholodens: mid-Cretaceous
  • Megazostrodon: late Triassic and early Jurassic
  • Triconodont: Triassic to Cretaceous
  • Zalambdalestes: late Cretaceous

Although mammals existed alongside the dinosaurs, mammals only began to dominate after the mass extinction of the dinosaurs 65 mya, in the Cenozoic.

In the Paleocene

During the next 8 million years, the Paleocene period (64–58 mya), mammals exploded into the ecological niches left by the extinction of the dinosaurs. Small rodent-like mammals still dominated, but medium and larger-sized mammals evolved.

  • Ptilodus: multituberculate
  • Pucadelphys andinus: an opposum-like marsupial
  • Purgatorius: a primate-like mammal, placental
  • Ectoconus: an early hoofed mammal, placental

Classification

Mammals encompass some 5,500 species, distributed in about 1,200 genera, 152 families and up to 46 orders, though this varies with the classification scheme.


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

  • Subclass Prototheria - monotremes: platypus and echidnas
  • Subclass Theria - live-bearing mammals
    • Infraclass Metatheria - marsupials
    • Infraclass Eutheria - placentals

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.

Click on the highlighted link for a table comparing the traditional and the new McKenna/Bell classifications of mammals

Extinct groups are represented by a dagger (†).

Class Mammalia

  • Subclass Prototheria: monotremes: platypuses and echidnas
  • 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

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:

Northern right whale dolphin (L. borealis)

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. 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.

Added from article of that name: Cetartiodactyla is a hypothesized order to which whales (including dolphins) and even-toed ungulates are proposed to belong. The term was coined by merging the name for the two orders, Cetacea and Artiodactyla, into a single word. The term Cetartiodactyla is most commonly used to describe the hypothesis that whales evolved from within the artiodactyls. Under this definition their closest living land relative is thought to be the hippopotamus. The clade formed by uniting whales and hippos is called Cetancodonta. Alternatively, the term Cetartiodactlya is used to denote a clade where Cetacea and Artiodactyla are sister groups, but where Cetacea did not actually evolve from within the Artiodactyla. Under this definition, all artiodactyls, including hippos, are more closely related to one another than any are to the whales.


  • Group I: Afrotheria
    • Clade Afro-insectiphillia
    • 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 Xenarthra: sloths and anteaters (Neotropical) and armadillos (Neotropical and Nearctic)
  • 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 (cosmopolitan).
      • Superorder Glires
        • Order Lagomorpha: pikas, rabbits, hares (Eurasia, Africa, Americas).
        • Order Rodentia: rodents (cosmopolitan)
    • Group IV: Laurasiatheria
      • Order Insectivora: moles, hedgehogs, shrews, solenodons
      • Order Chiroptera: bats (cosmopolitan)
      • Order Cetartiodactyla: cosmopolitan; includes former orders Cetacea (whales, dolphins and porpoises) and Artiodactyla (even-toed ungulates, including pigs, hippopotamus, camels, giraffe, deer, antelope, cattle, sheep, goats).
      • Order Perissodactyla: odd-toed ungulates
      • Clade Ferae
        • Order Pholidota: pangolins or scaly anteaters (Africa, South Asia).
        • Order Carnivora: carnivores (cosmopolitan)

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
ISBN links support NWE through referral fees

  • Bergsten, Johannes. February 2005. "A review of long-branch attraction". Cladistics 21:163–193. (pdf version)
  • McKenna, Malcolm C., and Bell, Susan K. 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp. ISBN 0-231-11013-8
  • Nowak, Ronald M. 1999. Walker's Mammals of the World, 6th edition. Johns Hopkins University Press, 1936 pp. ISBN 0-801-85789-9
  • Simpson, George Gaylord. 1945. "The principles of classification and a classification of mammals". Bulletin of the American Museum of Natural History, 85:1–350.
  • William J. Murphy, Eduardo Eizirik, Mark S. Springer et al., Resolution of the Early Placental Mammal Radiation Using Bayesian Phylogenetics,Science, Vol 294, Issue 5550, 2348-2351 , 14 December 2001.
  • Springer, Mark S., Michael J. Stanhope, Ole Madsen, and Wilfried W. de Jong. 2004. "Molecules consolidate the placental mammal tree". Trends in Ecology and Evolution, 19:430–438. (pdf version)
  • Vaughan, Terry A., James M. Ryan, and Nicholas J. Capzaplewski. 2000. Mammalogy: Fourth Edition. Saunders College Publishing, 565 pp. ISBN 0-030-25034-X (Brooks Cole, 1999)
  • Wilson, Don E., and Deeann M. Reeder (eds). 1993. Mammal Species of the World, 2nd edition. Smithsonian Institution Press, 1206 pp. ISBN 1-560-98217-9
  • Jan Ole Kriegs, Gennady Churakov, Martin Kiefmann, Ursula Jordan, Juergen Brosius, Juergen Schmitz. (2006) Retroposed Elements as Archives for the Evolutionary History of Placental Mammals. PLoS Biol 4(4): e91.[3]


See also

  • List of mammals
  • List of regional mammals lists
  • List of prehistoric mammals
  • Mammal classification

External links

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