Miocene

(began 24 million years ago)

The abundance of mammals reached a peak in the Miocene Epoch. The time was marked by refinements in life forms, and many animals and plants developed features recognizable in some species today. Forests and savannas persisted in some parts of North America; treeless plains expanded where cool, dry conditions prevailed. Many mammals adapted for life on the prairie by becoming grazers, runners, and burrowers. Large and small carnivores evolved to prey on these plains-dwellers. Great intercontinental migrations occurred throughout the Miocene, with various animals entering and leaving North America.

Moropus - A distant relative of today's horse

One of the strangest of all mammals was Moropus, a distant relative of the horse, but with great claws on its feet. Agate Fossil Beds NM

Menoceras - Miocene Version of a Rhino

Paleocastor - left many trace fossils with its burrows

Lacking other defenses, some larger rodents, like the dry-land beaver Paleocastor, lived in colonies beneath the High Plains of North America. Their burrows remain as trace fossils. Agate Fossil Beds NM

Daphoenodon - Early Relative of a Dog

The earliest true dogs of the Oligocene Epoch evolved into carnivores such as Daphoenodon, a hunter with characteristics common to both dogs and bears. Agate Fossil Beds NM

Miocene Dinohyus

Dinohyus, "the terrible hog," had bone-crushing teeth, which allowed it to scavenge the remains of other grassland animals.
Agate Fossil Beds NM

Stenomylus - similar to a gazelle and a camel

The tiny gazelle-camel Stenomylus probably grazed in herds for protection against predators. Agate Fossil Beds NM