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Updated: October 8, 2026
All these remarkable reptilian characters must have been possessed by the stem-form of the whole mammal class, the Promammal of the Triassic period, and have been inherited from the Proreptiles. During the Triassic and Jurassic periods the sub-class of the Monotremes was represented by a number of different stem-mammals.
But, in any case, the common descent of man and all the other mammals from one stem-form is beyond question. This long-extinct Promammal was probably evolved from Proreptiles during the Triassic period, and must certainly be regarded as the monotreme and oviparous ancestor of ALL the mammals.