Mounted specimen on display at the British Natural History Museum, London.
When: Late Eocene - Early Oligocene (36-30 million years ago)
Where: Northern Africa
What: Arsinoitherium is the animal with the largest known horns relative to body size. These gigantic horns were composed entirely of greatly expanded nasal bones. It also had a pair of smaller horns behind these enormous protuberances. This stocky beast was about 6 feet (2 meters) tall at the shoulder, and 10 feet (3 meters) long. It lived in Northern Africa, when this region was covered with tropical rainforests and mangrove swamps, eating most manners of vegetation with its large crushing molars. Arsinoitherium was fairly unspecialized in general, with the exception of its gigantic horns - the function of which is not well understood.
Though Arsinoitheriumsuperficially resembles rhinos, it is not closely related to them at all (and its large horns have a bone core, unlike the horn of the rhino which has no bony component). It is a member of the extinct order Embrithopoda, which is in turn within the Paenungulata (almost ungulates). Living paenugulates are elephants, manatees, and hyraxes. It is not well known how Arsinoitheriumfits into this group, as there is extremely little fossil record of basal embrithopods. Arsinoitherium was named based on the site of the discovery of the first fossils: they were near the palace of Arsinoë, a Ptolemaic Egyptian queen.