Estemmenosuchus. Computer artwork of Estemmenosuchus mirabilis standing by a lake in what is now the Perm region of Russia near the Ural Mountains. Resembling a modern day hippopotamus Estemmenosuchus were about the size of a modern adult bull. They were herbivorous early ancestors of today's mammals and lived during the middle part of the Middle Permian period (approx. 270-260 million years ago). This one is standing among several varieties of fern while cycads (Bjuvia simplex) populate the far side of the lake.