Biology Reference
In-Depth Information
they did not arrive in South America until about 5.8-5.7 Ma. These early
immigrants would be examples of small mammals that may have drifted
over before closure of the Panama land bridge (Verzi and Montalva 2008),
but some believe the age of the deposits is not certain (Prevosti and Pardi-
ñas 2009).
In the Southern Andes, there was a period of rapid plate subduction be-
tween 25 and 10 Ma (Suárez et al. 2000) that created much of the high
elevation in these mountains. There are tillites interbedded with basalts of
Miocene to Quaternary age indicating that glaciation is beginning in Pata-
gonia in the Mio-Pliocene.
A rich vertebrate fauna is known from the early Miocene Santa Cruz
Formation of southeastern Patagonia. According to Tauber (1997), from the
bottom to the top of the formation the fauna shows a decrease in diversity;
an increase in species with euhypsodont teeth and a decrease in those with
brachydont teeth; glyptodontids and toxodontids become more diverse;
megathere sloth average body size is reduced; and protheroteid diversity
decreases. This has been interpreted to mean a climate that is becoming
cooler and seasonally drier. Paleoceanographic studies of the late Miocene
reveal sea levels are falling, consistent with the early onset of widespread
glaciation. Pollen of grasses, composites, and the arid Ephedra increase, fur-
ther indicating dryness, seasonality, and steppe conditions developing in
Patagonia (Palazzesi and Barreda 2004).
The ecosystems from pole to pole at the end of the middle Miocene re-
fl ect communities poised on the verge of major modernization through
mountain uplift, lowering temperatures, falling sea levels, increasing sea-
sonality, newly evolving forms, and migrations facilitated by continuing
connections between North America and Asia, and new ones being forged
between North and South America.
TWO MILLION YEARS AGO
A walk south from northernmost North America at 2 Ma will be diffi cult
because increasing expanses of land and the adjacent seas are covered by
ice. There are two centers of continental ice accumulation. One is a dome
over Hudson Bay called the Laurentide ice sheet. The other, called the Cor-
dilleran ice sheet, is in the northern Rocky Mountains of British Colombia.
Until about 735 kyr, these ice sheets waxed and waned on the tilt variation
of about 41,000 years of the Milankovitch cycle, and since that time they
have followed the eccentricity variation of 100,000 years. The cause for the
change is not fully known, although it may have involved a tipping point
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