Biology Reference
In-Depth Information
ing mammals (Janis et al. 2002). Grasslands were prominent in the plains
by at least 13 Ma (Leopold and Denton 1987; Fox 2000), refl ected in part
in growth increments in
Gomphotherium
tusks—the gomphotheriums were
ancestors to mammoths and elephants. In response to the climatic changes
of the Miocene, the Equinae (horse subfamily) underwent rapid speciation
by dispersal, rather than vicariance, from one species (
Parahippus leonen-
sis
) to seventy species (Maguire and Stigall 2008). The grassland/savanna
became progressively more grassy with the gradual loss of trees and shrubs
in later Miocene and Pliocene times circa 8 Ma. Both the fl oras and faunas
tell of increasing cold conditions toward the mid to late Pliocene. For ex-
ample, around 10 Ma, a volcanic eruption in southwestern Idaho deposited
3 m of ash in western Nebraska, preserving the Ash Bed Fauna. It includes
camel, fi ve species of horses, zebra, and other browsing and grazing mam-
mals. At Big Springs, Nebraska, around 2 Ma, the fauna includes lemming,
shrew, and mastodon. During this interval, tooth enamel of the ungulates
shows a shift from primarily C3 to C4 grasses (Cerling et al. 1997) involv-
ing the complex response to increasing seasonality and decreasing temper-
atures and humidity. Rodent communities near the Pliocene-Pleistocene
transition (2.06-1.95 Ma) in southwestern Kansas change from one with a
strong subtropical southern component to one of temperate aspect that
also coincides with expansion of C4 grasses (Martin et al. 2008). The data
also suggest a more precise date for the Blancan/Rancholabrean NALMA at
this time.
The dramatic changes in northern North American Tertiary biotas, fi rst
from tropical to temperate, then, in the midcontinent region to savanna
and grassland, and in the west from temperate to lower altitude shrubland
to desert, are familiar to those who have visited localities with fossil plants
or have seen dioramas depicting the ancient life. For example, a collector
in southeastern Oregon, sitting today in a desert of sagebrush, will be re-
trieving from rocks in which the sagebrush is rooted fossils of ash, bass-
wood, beech, birch, dogwood, elm, hickory,
Magnolia
, maple,
Sassafras
,
sweetgum, sycamore, and walnut, together with several present-day Asian
plants such as
Ginkgo
,
Glyptostrobus
,
Metasequoia
,
Eucommia
,
Platycarya
,
and
Pterocarya
. The trend from deciduous forest to shrubland/chaparral-
woodland-savanna to desert is consistently evident in mid- to low-elevation
mid-Tertiary fl oras between the Rocky Mountains and the rising coastal
mountains. To the east of the Rocky Mountains, the trend was toward grass-
land, with deciduous forest confi ned mostly to lake and river margins. The
altitudinal cline in vegetation revealed by the fossil assemblages is from
grassy woodland-savanna at low elevations, upward into a piñon pine and
