Varying Definitions, Varying Dates
In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group 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. A broader phylogenetic definition was provided in a 2004 book by Kielan-Jaworowska, Cifelli, and Luo, who defined Mammalia as the clade originating with the most recent common ancestor, not only of the monotremes and the therians, but also of Sinoconodon, the morganucodonts, and the docodonts. The morganucodonts and the docodonts, included by Rowe in the unranked clade Mammaliaformes, had a widespread distribution in the northern continents and had many of the characteristics that traditionally would have classified them as mammals. In particular, some docodonts were furry. Finally, many paleontologists define Mammalia based on skeletal characteristics rather than ancestral relations; Adelobasileus is included on this basis, though this animal satisfies neither Rowe's definition nor that of Kielan-Jaworowska et al.
Mammalia, considered as the crown group, appeared in the Pliensbachian age of the early Jurassic period. In the broader sense given to the term by Kielan-Jaworowska et al., the group arose in the Carnian age at the beginning of the Late Triassic. Mammalia is no older if defined by skeletal characteristics; Adelobasileus, the earliest animal that is included on this basis, is also dated to the Carnian. In any case, the temporal range of the group extends to the present day.
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