The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes , monkeys and lemurs ; the Cetartiodactyla including whales and even-toed ungulates ; and the Carnivora which includes cats , dogs , weasels , bears , seals and allies. Kemp has provided a more traditional definition: Class Mammalia Subclass Prototheria: In an influential paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes echidnas and platypuses and therian mammals marsupials and placentals and all descendants of that ancestor. These were grouped into 1, genera , families and 29 orders. 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. Their book, Classification of Mammals above the Species Level,  is a comprehensive work on the systematics, relationships and occurrences of all mammal taxa, living and extinct, down through the rank of genus, though molecular genetic data challenge several of the higher level groupings. Though field work gradually made Simpson's classification outdated, it remains the closest thing to an official classification of mammals. Ambondro is more closely related to monotremes than to therian mammals while Amphilestes and Amphitherium are more closely related to the therians; as fossils of all three genera are dated about million years ago in the Middle Jurassic , this is a reasonable estimate for the appearance of the crown group. Proboscidea Mammal classification has been through several iterations since Carl Linnaeus initially defined the class. McKenna and Susan K. 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.