332 Evolution? The Fossils Say YES!
then in the Oligocene (about 28 million years ago) they independently evolved two differ-
ent types of paired horns on the tip of the nose (Diceratherium and Menoceras). After these
lineages disappeared, there was a long period dominated by hornless rhinos, except for a
single nose horn in the Teleoceras lineage. Asian rhinos also had a single nose horn, while the
two African rhino genera, the black rhino and white rhino, have two horns in tandem. The
elephant-sized Eurasian ice age elasmotherine rhinos had a huge horn (1.5 meters or 5 feet
long) on their foreheads, but none on their noses!
Closely related to the rhinos are the tapirs, which look vaguely piglike, except that they
have an odd number of toes and a flexible snout or proboscis. Most people have seen them
only in zoos (if at all) because all the living species are restricted to the tropics, including
Southeast Asia (the Malayan tapir) and three species in Central and South America. But tapirs
FIGURE 14.7. The different rhinocerotoid families can be distinguished largely through the modifications
of their teeth. (A) Primitive forms such as Hyrachyus (a) have the tapir-like condition, while hyracodonts
like Ardynia (b) and Hyracodon (c) have reduced their canines and modified their incisors. The gigantic
indricothere Paraceratherium (d) has lost all its front teeth except for huge conical incisor tusks. The amynodont
Metamynodon (e) has reduced its incisors and developed heavy chisel-like upper and lower canines. The true
rhinoceros Trigonias (f), on the other hand, has almost all of its incisors and canines, with a chisel-like upper
incisor and a tusk-like lower incisor. (Drawing by Carl Buell). (B) Evolutionary transformation of the cheek
teeth of Hyracodon, showing the gradual transformation of the premolars into more molarized crests. (After
Prothero 2005)
(A)
(a) (b) (c)(d) (e) (f)(B)
I.III.IV.V.VII.VIII.IX.X.