Brontosaurus
"Brontosaurus" was a large, long-necked quadrupedal animal with a long, whip-like tail, and forelimbs that were slightly shorter than its hindlimbs. The largest species, "B. excelsus", weighed up to and measured up to long from head to tail. For comparison, the current largest land animal, the average male African bush elephant is 3.20 m (10.5 ft) tall at the shoulder and has a body mass of 6,000 kg (13,228 lb) or 6.6 short tons.
Brontotheriidae
Brontotheres, over time, evolved massive body sizes, although some small species, such as "Nanotitanops", did persist through the Eocene. Some genera, such as "Dolichorhinus", evolved highly elongated skulls. Later brontotheres were massive in size, up to in height with bizarre horn-like skull appendages. For instance the North American brontothere "Megacerops" evolved large sexually dimorphic paired horns above their noses. The sexually dimorphic horns suggest that brontotheres were highly gregarious (social) and males may have performed some sort of head-clashing behavior in competition for mates. However, unlike rhinos, the horns of brontotheres are composed of bone, the frontal bone and nasal bone, and were placed side-to-side rather than front-to-back.