Mammals From The Age Of Dinosaurs

The first 155 million years (Ma) of mammalian history occurred during the Mesozoic. During this vast time span, mammals diversified into many lineages and underwent enormous anatomical evolution. Humans are primates; primates are placentals; placentals are eutherians; and stem eutherians have a long history, which extends well back into the Early Cretaceous. Mesozoic mammals include the trunk and a bewildering bush of basal branches for the entire mammalian family tree. Their fossil records are indispensable for our understanding of the deep history that gave rise to extant mammals, including our own lineage of placentals.

Mammals have thrived in the world's biota for 65 Ma, since the beginning of the Cenozoic, and they are represented today in a spectacular diversity of more than 4,600 modern species (Nowak, 1991; Wilson and Reeder, 1993; MacDonald, 2002). These diverse Cenozoic and modern mammals are descendants of three major evolutionary lineages: eutherians (including placentals), metatherians (including marsupials), and australosphenidans (including monotremes). The surviving members of these lineages, however, represent only a tiny fraction of a dazzling array of some 25 distinctive mammalian clades (at the order or family level of the traditional Linnaean taxonomy) that existed in the Mesozoic. Modern mammals represent merely the tips of three twigs of a vast evolutionary bush, most of which was pruned by extinction in the Mesozoic (Clemens et al., 1979; McKenna and Bell, 1997; Cifelli, 2001; Luo et al.,2002).

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