Modern mammals are quite distinctive. They have hair, they generally have large brains, they feed their young on milk from mammary glands (hence the name'mammals') and they care for their young over extended periods of time. In the Late Triassic, when mammals arose, the boundary line between mammals and non-mammals was much less clear. Indeed, a succession of Triassic carnivorous synapsids, the cynodonts, successively acquired 'mammalian' characters over a time span of 30-40 Myr, and the exact point at which these synapsids became mammals can be established only by an arbitrary decision.

There is a debate about the true extent of the clade Mammalia. Traditional views place the base of the clade at a node from which Morganucodon and the other morganucodontids branched off the mammal line. This node is marked by a 'key' mammalian character, the possession of a single dominant jaw joint, between the dentary and squamosal. Supporters of the 'crown-

group' concept, such as Rowe (1988), define mammals as all of the descendants of the latest common ancestor of the monotremes and therians, the living forms, and they call the traditional Mammalia the Mammali-aformes. Proponents of both approaches argue that they have right on their side: the traditional view is said to be stable and based on a specific set of character transitions: the crown-group view offers a clear clade definition based on extant mammals only. I retain the traditional definition of Mammalia (and of other clades) in this book.

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