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149 miles north of the northernmost extant population on the Río Yaqui indicates that
the climates were once more tropical than at the site today. Teeth of a large extinct bison,
which only immigrated to North America from Siberia in the late Pleistocene between
170,000 and 150,000 years ago,
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were also found. The coexistence of bison and tropical
species indicates that sediments near Rancho La Brisca were deposited about 80,000 years
ago in the last interglacial (the Sangamon), which was considerably more tropical than the
Holocene.
6.6 Ice Age Mammals and Grassland Dynamics
Sediments in playas, springs, and caves provide insights into the ice age environments of
the highlands along
La Frontera
between the Chihuahuan and Sonoran deserts. Rich bone
beds in sites from the Great Plains to southeastern Arizona reveal a late Pleistocene fauna
rivaled today only by African savannas, like the Serengeti Plains of Tanzania.
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However,
Wisconsin-aged sediments scattered in sites from the Great Plains to Arizona were
dominated by pollen forest trees instead of grasses, illustrating the peril in inferring the
existence of grassland biomes from the presence of large vertebrates. Although conifers
are such prodigious pollen producers that ice age communities may have been relatively
open pine parklands rather than closed forests, typical grassland assemblages were not
encountered until the Holocene.
Sediments in U-Bar Cave, a desert grassland area in southwestern-most New Mexico
(ca. 6 miles north of the international boundary) yielded bones and teeth of a very diverse
fauna.
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Extinct large mammals included dire wolf giant short-faced bear, horses, mountain
deer, pronghorns, and shrub ox. The Willcox Playa and Murray Springs (50 and 8 miles
north of the international boundary, respectively) are desert grassland sites in southeastern
Arizona where the ice age portions of the pollen profiles were dominated by pine pollen.
Murray Springs is best known because elegant flint spear points were imbedded in
mammoth bones provided glimpses of Clovis hunters. Other large mammals in this rich
Pleistocene fauna included American lion, bison, camel, horses, llama, and tapir.
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6.6.1 Pleistocene Overkill
About 11,000 years ago, approximately two thirds of the large mammal species of North
America became extinct.
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Common, widespread grazers including horses and mam-
moths seem to have disappeared at the very time spruce and pine retreated and grass-
lands expanded from Canada to Arizona. Martin
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forcefully presents the case that big
game hunters, rather than changing climate, caused widespread extinctions within a few
hundred years after their entry into North America from Siberia via the Bering Strait.
Whether the “overkill” model is accepted or not, there is no paleobotanical evidence of cli-
matic changes severe enough to cause extinction in biotic regimes ranging from the boreal
forests and conifer parklands to the deserts of the Southwest, the Mediterranean chaparral
of southern California, and throughout the New World tropics.
Changes in climate and vegetation at the time of extinction in the southwestern United
States were not greater than similar fluctuations in other interglacials.
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The tropical nature
of the Rancho La Brisca fossil fauna suggests that the magnitude of climate change in last
interglacial was greater than in the Holocene.
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Moreover, plant remains in ancient packrat
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