Mammalia is a class of animal within the phylum Chordata. Mammal classification has been through several iterations since Carl Linnaeus initially defined the class. No classification system is universally accepted; McKenna & Bell (1997) and Wilson & Reader (2005) provide useful recent compendiums. Many earlier ideas from Linnaeus et al. have been completely abandoned by 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) taxonomy text 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 Though field work gradually made Simpson's classification outdated, it remained the closest thing to an official classification of mammals.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 are contentious, and all three have been proposed as basal in different hypotheses. The following taxonomy only includes living placentals (infraclass Eutheria): Atlantogenata Afrotheria Class Afroinsectiphilia Order Macroscelidea Family Macroscelididae: (20 species), sengis or elephant shrews (Africa) Order Afrosoricida Family Tenrecidae: (31 species), tenrecs (Madagascar) Family Potamogalidae: (3 species), otter-shrews (West and Central Africa) Family Chrysochloridae: (21 species), golden moles (Africa south of the Sahara) Order Tubulidentata Family Orycteropodidae: (1 species), aardvark (Africa south of the Sahara) Class Paenungulata Order Proboscidea Family Elephantidae: (3 species), elephants (Africa, Southeast Asia) Order Hyracoidea Family Procaviidae: (4 species), hyraxes, dassies (Africa,