Brontotheriidae

Brontotheres (lit.'thunder beasts'), also called titanotheres (lit.'titanic beasts'), are an extinct group of perissodactyl (odd-toed ungulate) mammals belonging to the familyBrontotheriidae. Brontotheres are traditionally interpreted as relatives of horses, though high-level perissodactyl relationships are not well understood. Brontotheres lived during the Eocene epoch and their fossils have mainly been found in Asia and North America, with some fossils also found in Eastern Europe. Early brontotheres were small animals, but later brontotheres grew to become the largest land mammals of the Eocene, and the largest land mammals to have evolved up until their time.

The first brontothere fossils known to science were found in South Dakota in the mid-20th century. The name "brontothere" was derived from Lakota legends of wakíŋyaŋ, which can be translated to "thunder beasts". Over forty brontothere genera have since been recognized. The rapid diversification of the brontotheres, and their sudden extinction at the end of the Eocene after roughly twenty million years of existence, has been described as a "live fast, die young" pattern of evolutionary radiation. The rapid increase in size from early forms such as Eotitanops, weighing less than 100 kilograms (220 lb), to later four-tonne (4.4 short-tonne) animals, such as Megacerops and Embolotherium, is one of the most extreme size increases in all of mammalian evolution. At their peak in the Middle Eocene, brontotheres were the most diverse clade of large mammals in both Asia and North America.

Most brontotheres had relatively robust skeletons, especially later and larger forms. On the whole, the postcranial brontothere skeleton was conservative, lacking family-specific synapomorphic traits. Brontothere skulls were characterized by the eyes being positioned relatively far forward, resulting in short faces. Dentally, brontotheres were characterized by a unique combination of features in their upper molars.

Various taxonomic frameworks have been used for brontotheres historically, often diverging in what subgroups are recognized. Current brontothere taxonomy, supported by phylogenetic analyses, recognizes several increasingly derived (advanced) clades leading up to the three derived subtribes Rhadinorhinina, Telmatheriina, and Brontotheriina. Brontotheres are also sometimes informally divided into hornless brontotheres and the superficially rhinoceros-like horned brontotheres. The subtribe Brontotheriina contains all horned brontotheres, and about half of all known brontothere genera. Brontothere horns were made of bone and occurred in a variety of shapes and sizes.