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Mammalia is a class of animal within the Phylum Chordata. Mammal classification has been through several iterations since Carolus Linnaeus initially defined the class. Many earlier ideas have been completely abandoned by Linnaeus and modern taxonomists, among these are the idea that bats are related to birds or that humans represent a group outside of other living things. Competing ideas about the relationships of mammal orders do persist and are currently in development. Most significantly in recent years, cladistic thinking has led to an effort to ensure that all taxonomic designations represent monophyletic groups. The field has also seen a recent surge in interest and modification due to the results of molecular phylogenetics. George Gaylord Simpson's classic "Principles of Classification and a Classification of Mammals" (Simpson, 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 1945 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 Though field work gradually made Simpson's classification outdated, it remained the closest thing to an official classification of mammals. See list of placental mammals and list of monotremes and marsupials for more detailed information on mammal genera and species. ==Molecular classification of placentals== Molecular studies by molecular systematists, based on DNA analysis, in the early 21st century have revealed new relationships among mammal families. Classification systems based on molecular studies reveal three major groups or lineages of placental mammals- Afrotheria, Xenarthra, and Boreotheria- which diverged from early common ancestors in the Cretaceous. The relationships between these three lineages is contentious, and all three have been proposed as basal in different hypotheses.〔 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 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, and the formation of the Isthmus of Panama linked South America and North America, facilitating the distribution of mammals seen today. With the exception of bats and murine rodents, no placental land mammals reached Australasia until the first human settlers arrived approximately 50,000 years ago. It should however be noted that these molecular results are still controversial mainly because they are not reflected by morphological data and thus not accepted by many systematists. It is also important to note that fossil taxa are not and, in most cases cannot, be included. Although there are instances of DNA being recovered from prehistoric mammals such as the ground sloth ''Mylodon'' and Neanderthal humans, ''Homo neanderthalensis'', fossils can generally only be incorporated in morphological analyses. The following taxonomy only includes living placentals (infraclass Eutheria): 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Mammal classification」の詳細全文を読む スポンサード リンク
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