Superficially, they looked rather like rhinos with some developing bony nose horns, and were some of the earliest mammals to have evolved large body sizes of several tonnes.

[1] Brontotheres also shared an elongated postorbital cranium, meaning that their skulls are lengthened between their eyes and ears.

[1] The evolutionary history of this group is well known due to an excellent fossil record in North America.

Brontotheres evolved massive bodies, with some species standing over 2.5 meters (7 feet) tall,[3] with body masses of over a tonne, perhaps exceeding 4,000 kilograms (8,800 lb), in large individuals of Megacerops,[5] although some small species such as Nanotitanops did persist through the Eocene.

The North American brontothere Megacerops, for example, evolved large sexually dimorphic paired horns above their noses.

[2] Joseph Leidy was the first researcher to scientifically describe brontothere fossils, followed by Cope and Marsh, who studied skulls and entire skeletons.

[11] Eotitanopinae Palaeosyopinae Rhadinorhinina Telmatheriina Brontotheriita Embolotheriita Two classification systems for Brontotheriidae are presented below.

The first contains 43 genera and 8 subfamilies, and although it is based on a 1997 publication by McKenna and Bell, it summarizes research that was conducted before 1920 and is badly outdated.