Abstract
Since the early XIXth century mammals were known to have existed during the Mesozoic Era. Only relatively recently, however, have major discoveries of these rare fossils opened new insights into the long history of mammals that preceded the appearance of modern forms during the Tertiary. The two purposes of my review are to place these discoveries in the context of a brief outline of Mesozoic mammalian evolution, and to provide a synopsis of the biology of monotremes. With the evidence now available that monotremes were derived from pre-tribosphenic therians during the mid-Mesozoic, various aspects of monotreme anatomy and physiology may be compared to those in other living mammals as a basis for estimating which adaptations were already established in their common ancestors during the Jurassic. My essay, therefore, explores the biology of Mesozoic mammals.

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