How Old Are They?
How old are they Irvington mammals? The answer depends on which system of
animal classification you use. In Figure 24, only the genera of mammals are
listed , except for Canis (wolf), which will be explained later. Also, only land
mammals are on the list, which excludes the seals.
Species are subdivisions of genera. Genera are older than species,
families are older than genera, and orders are older than families. The
Irvington peccaries, deer, camels, antelopes, and oxlike animals belong to an
older that is more than 50 million years old. This order (Artiodactyla)
originated at the beginning of the Eocene epoch. Families that evolved from the
order of Artiodactyla are given below, with the approximate times of origin,
from the most recent to the earliest.
Miocene, Lower Middle
Oligocene, Lower Middle
Throughout this book, the Irvington fauna is assigned to the Irvingtonian
Mammal Age dated from about 1.3 million years ago. But in Figure 24, four
different Mammal Ages are indicated for the twenty-three genera listed. This
means that some of the genera found at Irvington survived through more than one
Mammal Age. For example, three Irvington rodent genera (Citellus, Perognathus,
and Peromyscus) are assigned to the Barstovian Mammal Age lasting from about 16
million to 11 million years ago.
A word about the only species noted in the tabulation, Canis dirus and
Canis irvingtonensis: C. dirus has been found in great numbers in the Rancho la
Brea pits of the Rancholabrean Mammal Age. Why is C. dirus assigned to the
older, Irvingtonian Mammal Age rather than to be Rancholabrean? And C.
irvingtonensis has been found only at Irvington. If this coyote should be found
in even older deposits in the Hemphillian, for example would you reassign him?
A final comment on the tabulated data: Perhaps you noticed that three
genera of land mammals are assigned to the Blancan Mammal Age but to neither the
Early nor the Late Blancan. Paleontologists were unable to date these genera
more precisely because the data were not sufficient.