Information AboutMammal |
|
| color = pink | name = Mammals | image = Lions snoozing in the sun.jpg | image_width = 300px | image_caption = Lion (''Panthera leo'') | regnum = Animal ia | phylum = Chordata | subphylum = Vertebrata | unranked_classis = Mammaliaformes | classis = Mammalia | classis_authority = Linnaeus , 1758 | subdivision_ranks = Orders | subdivision =
}} 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 systems, including a four-chambered Heart . Mammals encompass some 5,500 Species , distributed in about 1,200 Genera , 152 Families and up to 46 Order s, though this varies with the Classification Scheme . Phylogenetically , Mammalia is defined as all descendants of the most recent common ancestor of Monotreme s (e.g., Echidna s and Platypus es) and Therian mammals ( Marsupial s and Placental s). Characteristics Most mammals give birth to live young, but a few (the Monotreme s) lay Egg s. Live birth also occurs in some non-mammalian species, such as Guppies and Hammerhead Shark s; thus it is not a distinguishing characteristic of mammals. Although all mammals are endothermic, so are Bird s, and so this too is not a defining feature. A characteristic of mammals is that they have Mammary Gland s, 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 Bone s 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 Therapsid s called Cynodont s 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. The vast majority of mammals have seven Cervical Vertebrae (neck bones), including 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. All mammalian brains possess a Neocortex . This brain region is unique to mammals. Mammals have , 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 ( Manatee s and Dugong s) and the Cetaceans ( Dolphin s and Whale s). Whales are the Largest Of All Animals . There are semi-aquatic species such as Seal s which come to land to breed but spend most of the time in water. True Flight has evolved only once in mammals, the Bat s; mammals such as Flying Squirrel s and Flying Lemur s are actually gliding animals. No mammals have hair that is naturally blue or green in color. Some cetaceans, along with the Mandrill s 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 Amniote s, and in particular to a group called the Synapsid s, 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, Dinosaur s, birds, and most reptiles are Diapsid s, with two temporal fenestrae on each side of the skull; and turtles, with no temporal fenestra, are Anapsid s. From early synapsids came the first mammal precursors, Therapsid s, and more specifically the Eucynodont s, 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 (, '' 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 Reptile s", but this is strictly incorrect as these two are not reptiles but Synapsid s. During the s (Allotherium), Monotreme s, Marsupial s, and Placental s. 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 Evolutionary biology has long held that 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 {Link without Title} ). 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 {Link without Title} ). The earliest mammals include:
Although mammals existed alongside the Dinosaur s, 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 Niche s left by the extinction of the dinosaurs. Small Rodent -like mammals still dominated, but medium and larger-sized mammals evolved.
Classification ''Main article: Mammal 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". 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
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 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 : 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.
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
See also External links
|