Pyrotherium ('fire beast') is an extinct genus of South American ungulate in the order Pyrotheria, that lived in what is now Argentina and Bolivia during the Late Oligocene.

Because the remains of this animal originally appeared in the volcanic ash beds of the Deseado Formation, they named it Pyrotherium, which means "fire beast".

[9] The name of the species P. romeroi comes from the captain of the Argentine army, Antonio Romero, who sent Ameghino the first known remains of the animal, although in several texts the erroneous spellings P. romeri or P. romerii has been used.

[9][10] The first relatively complete skull did not appear until the 20th century, being discovered by Frederic Brewster Loomis during the Amherst College expedition in 1911-1912, and listed as specimen ACM 3207.

It reaches 72 centimeters (2.36 ft) in length from its front teeth to its occipital condyle, and has an elongated, relatively narrow snout when seen from above, with retracted nostrils, a large nasal opening located between the eye sockets in the middle of the front bone in parallel to the back of the skull, with thick bone walls for muscle support; inside there are cavities filled with air.

The occipital region, in particular the condyles, was particularly high, as a consequence of the flexion of the posterior part of the skull with respect to the plane of the base, which formed an obtuse angle with that of the palatine bone; in this and other characteristics, Pyrotherium resembled proboscideans.

There is a small ridge that emerges from the premaxilla and reaches the nasal bone, which appears to be broken and surrounded by a rough texture, which could be the result of erosion.

[11] Later analysis by Bryan Patterson in 1977, after some additional preparation work on the only known skull, indicated some errors in earlier interpretations, and that the brain would be somewhat larger, 80 millimeters wide, more similar in size to that of notoungulates such as Homalodotherium and Nesodon.

The tarsus of Pyrotherium was characteristic: the calcaneus tubercle was compressed dorsoventrally, as was the trochlea of the astragalus; in addition, it presents an extreme reduction in the contact between the heel and the cuboid.

[11] These derived characteristics, which involve a type of graviportal and plantigrade locomotion, are not found in any other known mammal, with the significant exception of the African embrithopod Arsinoitherium.

[25] The most recent analyses published, like the work of Billet (2010), suggests that pyrotheres such as Pyrotherium are a group of specialized notoungulates, related to Notostylops,[15][26] although this is still a controversial idea.

This type of prism in the enamel is characteristic of pyrotheres and is not known in the other orders of native South American ungulates (xenungulates, astrapotheres, litopterns, and notoungulates).

[17] Pyrotherium fossils recovered from Salla, Deseado and Quebrada Fiera correspond to relatively dry environments, with xerophytic vegetation and periods of drought;[30] this would contradict the hypothesis that they were semiaquatic animals, similar to hippopotamuses, while the remains of astrapotheres (another group of large, tusked native ungulates) are in fact found in areas associated with bodies of water, implying that they lived in humid environments and were able to spend some time in the water.

[31] Pyrotherium would have used its incisors and trunk in order to collect food such as leaves and branches of the trees, in a similar way to black rhinos and African forest elephants.

This includes predatory sparassodonts such as Pharsophorus, Notogale and the enormous Proborhyaena, and other ungulates (primarily notoungulates) such as Trachytherus, Leontinia, Rhynchippus, Propachyrucos, Argyrohyrax, Archaeohyrax, and Prohegetotherium.