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Work of the past fifty years, or so, has shed much additional light on the phylogeny and history of the Order Rodentia. Classical views have been challenged, and new concepts invoked, or older views rediscovered. Of these new concepts, hystricomorphy, rather than protrogomorphy, as a primitive state for rodents seems difficult to accept, and is not really new. Hystricognathy versus sciurognathy as the fundamental division of the Rodentia seems perilous if pardlelism is as important a phenomenon as is frequently suggested. The argument for multiserial incisor enamel, rather than pauciserial, as the primitive incisor kind is very persuasive, but perhaps more work is needed on Eocene rodent enamel. Punctuated equilibrium, if really a new idea, seems promising in explaining the obscure origin of most rodent groups, but gradualism is evident in many specific lines of descent in rodents. Virtually excluding temporal consideration from phylogenetic studies seems extreme, as does cladistic analysis when it excludes parallelisms and paraphyletic groups. In spite of recent work, the gap between Eocene groups such as the Paramyidae and Ctenodactyloidea, and the Oligocene and later families remains considerable, and largely unexplained. Extraterrestrial collision events in this case can hardly be regarded as pertinent for rodents.

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