Of the three great hadrosaur groups of the Oligocene, the hipposaurids were undoubtedly the most diverse and arguably the most successful.  While the paramegahadrines were giants and the megacaudisaurids were probably more numerous, Hipposauridae occupied a much wider range of forms, from tiny, hoglike browsers to giant grazers rather like modern hmungos.

    Like the megacaudine hadrosaurs, hipposaurs retained only the middle three digits of their forelimbs, but these sported large nails that probably extended past the hoof-pad.  Most sported enlarged neural crests over the shoulders to anchor powerful grazers' neck muscles, and many later species possessed a very deep lower jaw, perhaps to aid in cropping early grasses. Hipposaur tails tended to be rather narrow and stubby, possibly because their function as balancing devises were irrelevant to the quadripedal herbivores.

(Picture by Ville Sinkkonen)

    Hipposaurs probably evolved some time during the Eocene, but their fossil record begins in the early Oligocene with such genera as Pygmihadrus, a meter-long herbivore with short, stumpy legs and tail.  Fossils similar to this one appear throughout the Oligocene and Miocene.

(Picture by Ville Sinkkonen)

    The last of the hipposaurs was the mighty Hipposaurus, a ten-meter-long monster with powerful  limbs and a bizzare skull.  The head of Hipposaurus was deep, but very narrow until half-way down the snout, when the beak flared out into a spatula shape.  Meanwhile, the lower jaw curves upward as it flares, forming a cutting edge with the upper mandible.  The jaw is thus constructed in such a way that the dinosaur could only have been able to feed by holding its head perpendicular to the ground and dragging its beak along the turf like a giant spoon.  Most paleontologists agree that these strange features were an adaptation for grazing upon early grasses.

    As the Oligocene gave way to the Miocene, hipposaur diversity began to wane.  By the Pliocene only the large and specialized Hipposaurus remained until it, too, succumbed to the changing climate and competition from other herbivores.