No noteworthy fossil spiders are known; the best-preserved are in amber of Oligocene age.
The next epoch, the Oligocene, was similar but somewhat cooler.
During the upper Oligocene time very nearly the whole island was undoubtedly submerged.
Heteromyids are known from the Chadron formation, of early Oligocene age, in which a single tooth was found.
It is probable that the folding of the Oligocene strata noted in the vicinity of Havana and Matanzas took place during this time.
Specially interesting in the Eocene and Oligocene are the mammalian remains.
In the Oligocene we find primitive squirrels, beavers, rabbits, and mice.
They both date from the Eocene, and became extinct in the succeeding Oligocene.
It is these latter forms which come nearest to modern Carnivora, most of them being of Oligocene age.
The shells which have been found in them indicate that they belong for the most part to the Oligocene period.
The third epoch of the Tertiary Period, from about 37 to 24 million years ago. During this time there was an increase in volcanic activity, and Australia and South America separated from Antarctica. The climate started to cool and a glacier started to form in Antarctica. Modern mammalian groups continued to develop, and the first cats, dogs, horses, and related mammals appeared. Artiodactyls (even-toed ungulates) took over from the perissodactyls (uneven-toed ungulates) as the dominant medium-sized herbivores. Many types of grass also first appeared at this time. See Chart at geologic time.