North
America hosted a large and diverse megafaunal assemblage during the
Pleistocene, including Mammoths, Mastodons, Horses, Camels, Giant
Armadillos and Ground Sloths, Sabre-toothed Cats, Dire Wolves and a
variety of other large herbivorous and Carnivorous Mammals. However
at the end of the period the majority of these animals became extinct
in a very short period, an event that has fascinated palaeontologists
for over a hundred years. The extinctions coincided with two other
distinctive events, the arrival of the first Humans in many areas of
North America, and the climatic reversal known as the Younger Dryas,
in which the warming climate of the Late Pleistocene suddenly
underwent an abrupt cooling, with full glacial conditions returning
for a period of several centuries. Of the two events the arrival of
Humans is generally considered the more likely explanation for the
extinction, as the Megafauna had previously encountered a number of
previous cold spells, whereas Humans were a completely novel
phenomena with which the animals would not have been familiar.
However in the Pacific Northwest of North America Humans are known to
have co-existed with Pleistocene Megafuana for about 14 000 years
before the End Pleistocene Extinction, which occurred at roughly the
same time as elsewhere.
In a paper
published in the journal Quaternary Research on 22 October 2014,
Daniel Gilmour of Willamette Cultural Resources Associates Ltd and
the Department of Anthropology at Portland State University, Virginia Butler, also of the Department of Anthropology at Portland State
University, Jim O'Connor of the United States Geological Survey,
Edward Davis of the Museum of Natural and Cultural History and
Department of Geological Sciences at the University of Oregon,
Brendan Culleton and Douglas Kennett of the Department of Anthropology at Pennsylvania State University and Gregory Hodgins of
the Department of Physics and School of Anthropology at the
University of Arizona describe the results of a study of the Late
Pleistocene Megafaunal Extinction in the Willamette Valley of Oregon.
The deposits
of the Willamette Valley show a distinctive series of beds associated
with the Missoula Floods, in which the failure of an ice dam on a
huge lake in northwestern Montana triggered flooding across much of
the northwestern United States about 15 000 years ago. In the
Willamette Valley these deposits form a series of beds up to 35 m
thick, making them a useful and hard-to-miss geological marker. The
valley is also noted for producing extensive Late Pleistocene
Megafauna fossils, which have been collected since the 1840s.
Map of
Willamette Valley and larger regional context, showing locations of
palaeontological, archaeological and palaeoenvironmental sites noted
in study. Gilmour et al. (2015).
Gilmour et
al. examined records and
catalogues of finds at the University of Oregon Museum of Natural and
Cultural History as well as contacting numerous amateur and
professional collectors and associations known to have worked sites
in the Willamette Valley during the last 40 years, in order to
identify Megafuana specimens from bellow the Missoula Flood Beds.
None of these specimens showed any signs of cutting with tools or
other Human activity, nor of reworking by natural events (such as
flooding), suggesting that the specimens were found at or close to
the sites where they died.
They
were able to locate eleven such specimens, which were then dated
using radiocarbon methods, in order to determine when each individual
had died; a twelfth specimen which had been dated as part of a
previous study was also included. The sample included five Bison, two
Mammoths, two Horses, two Giant Ground Sloths and one Mastodon. All
of these except the Mastodon are thought to have been grazing species
favouring open grassland with a sparse tree canopy, while Mastodons
are thought to have favoured a more wooded environment.
Posterior
view of cranium, Bison. Daniel Gilmour in Gilmour et
al. (2015).
Radiocarbon dating of the specimens yielded dates of between 15 590
and 12 560 years old, with both the oldest and youngest specimens
being Bison. The Younger Dryas cooling event is generally reckoned to
have begun in about 12 900 years ago and lasted about 1300 years, but
this is a global average date, and the effect of climate events will
be felt at different times in different parts of the world.
Reconstructions of vegetation at Battle Ground Lake in Oregon, which
is about 30 km north of the Portland Basin and which forms an
extension of the same depression which forms the Wilamette Valley,
suggests that this area had a covering of tudra or open grassland
until about 14 300 years ago, when it was replaced by a mixture of
grassland and open woodland. This mixed environment was itself
replaced about 13 100 years ago by a more closed forest environment,
which persisted till about 10 800 years ago. This wooded environment
is considered to be associated with a period of cooler conditions
across western Oregon, which corresponds to the global Younger Dryas
Cooling Event.
Only one of the Willamette Valley specimens post-dates the
replacement of warmer open terrain with cooler wooded terrain, and
that only by a few hundred years, consistent with the area being
somewhat to the south. This supports the idea that the cooling
associated with the Younger Dryas, and subsequent vegetational
changes could have been the cause of the decline of the Late
Pleistocene Megafuana in the Willamette Valley. However the small
size of this study is not sufficient to state this with absolute
confidence, and changes in human behavior associated with a cooling
climate may also have played a role in the Megafaunal Extinction.
Only a very limited amount of archaeological material has been found
in the Willamette Valley; four Clovis points and two Western Stemmed
points, both spear tips associated with hunting large animals, though
the cultures which produced them are thought to have had a broad diet
including small game and a variety of herbaceous plants. Elsewhere in
the Pacific Northwest archaeological sites have yielded more evidence
of Human activity, and interaction with Late Pleistocene Megafauna.
The Manis on the Olympic Peninsula, which is dated to about 13 800
years ago, has yielded a Mastodon with what appears to be an embedded
bone tool, while the Paisley 5 Mile Point Caves locality in Central
Oregon has produced a variety of tools as well as Human coprolites,
produced between 14 720 and 14 000 years ago, indicating Humans and
Pleistocene Megafauna were co-existing in the area shortly after the
Missoula Floods.
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