Evolution What the Fossils Say and Why it Matters

Bossies and Blowholes 331

FIGURE 14.6. The evolutionary history of North American rhinoceroses. In the Eocene, they branched into
three families, the hippo-like amynodonts, the long-legged running hyracodonts, and the living family
Rhinocerotidae. During their evolution, they varied not only in body size and limb and skeletal proportions
but also in the number and position of horns (or lack of horns), the details of their teeth, and many other
features. (Drawing by C. R. Prothero; after Prothero 2005)

Aceratheriines Teleoceratins

AphelopsAphelops Teleoceras

Peraceras

Floridceras

Galushaceras

Menoceras

GulfocerasGulfoceras

Diceratherium

Skinneroceras Woodoceras

Amphicaenopus

Subhyracodon

Penetrigonias

Trigonias

UintacerasUintaceras

Hyracodon

Triplopides

Epitriplopus

Triplopus

Hyrachyus

Amynodon

Amynodontopsis

Metamynodon

Forster-

cooperia

from

Asia

from

Eurasia

from Europe

from

Eurasia

AMYNODONTIDAE HYRACODONTIDAE

RHINOCEROTIDAE

5

24

34

56

EOCENE

OLIGOCENE

MIOCENE

It comes as a shock to many people that most fossil rhinos were hornless. Modern rhino
horns are made of tightly compacted hairs and have no bony support (unlike the horns
of cattle or antelopes), so they seldom fossilize. We can usually determine the presence of
a horn by a patch of spongy roughened bone on the snout or forehead that served as the
attachment point. On this basis, rhinos were hornless during most of their evolution, and

http://www.ebook3000.com