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bugs, leafhoppers, beetles, flies, ants, and
wasps. Scorpions, millipedes, and several
spiders are also known. Many of these
were terrestrial and are rarely found
fossilized. Some of the beetles and flies
would have become trapped while
feeding on carrion. Others were blown
onto the sticky asphalt or became stuck
when crawling over it.
sloths, camels, pronghorns, and
mammoths, with occasional visits by
peccaries, deer, tapirs, and mastodons.
These numerous herbivores in turn
supported a diverse population of
cursorial carnivores, including the various
species of dogs, cats, and bears. There is
no need to infer a radically different
climate from that of the present to
account for the variety of Pleistocene
mammals in this region. In fact the
similarity of the smaller mammals to the
present day fauna (rodents, rabbits,
shrews), plus the absence of mammals that
are elsewhere associated with very cold
climates (e.g. musk oxen), suggest that
conditions were not significantly colder
during the Pleistocene.
In all, more than 600 species have been
recorded from La Brea, comprising
approximately 160 plants and 440 animals.
These include 59 species of mammals
(represented by over one million fossils)
and 135 different birds (represented by
100,000 specimens). The most significant
statistic, however, is the disproportionate
percentage of carnivores over herbivores,
which does not conform to the normal
pyramidal structure of a biological
ecosystem in which carnivores form the
top of the pyramid, outnumbered by
herbivores.
The Rancho La Brea assemblage clearly
does not represent an accurate cross
section of the ecosystem. The most
obvious explanation is that a single
herbivore, on becoming trapped in the
asphalt, might be pursued by a whole pack
of carnivores. When these also became
trapped, both would attract additional
scavengers to their carcasses. The dogs,
cats and bears, plus the smaller
carnivores, such as skunks, weasels, and
badgers, constitute 90% of the mammal
fauna, while the herbivores make up only
10%. Similarly the carnivorous birds
(condors, vultures, teratorns, eagles,
hawks, falcons, and owls) comprise
approximately 70% of the bird fauna.
A further bias in the La Brea fauna is
the preponderance for obvious reasons
Plants
Fossil plants from La Brea include wood,
leaves, pine cones, seeds, and microscopic
pollen and diatoms.
P ALEOECOLOGY OF THE R ANCHO
L A B REA BIOTA
The fauna and flora of the Rancho La Brea
'tar pits' represent a terrestrial ecosystem
situated on the western coastal plains of
the North American continent, between
30° and 35°N, as at present, but with a
cooler, glacial climate. The flora includes
many plants that no longer occur in this
region and indicates that while glacial
winters may have resembled modern
winters, the summers were not only cooler,
but were also more moist (Johnson, 1977a,
b). Annual rainfall was probably twice that
of today; freshwater ponds and streams
covered the plains, supporting fish, turtles,
frogs, toads, mollusks, and aquatic insects,
and a rich vegetation included four distinct
assemblages.
The slopes of the Santa Monica
Mountains were covered with chaparral -
tall, densely packed woody bushes of lilac,
scrub oak, walnut, and elderberry - while
the deep, protected canyons were home
to redwood, dogwood, and bay. The rivers
were lined with sycamore, willow, alder,
raspberry, box elder, and live oak, but the
plains were covered with coastal sage
scrub (drought-tolerant woody bushes)
interspersed with wide areas of grass and
herbs, with occasional groves of closed-
cone pine, valley oak, juniper, and cypress
(Harris and Jefferson, 1985).
These wide plains supported large
herds of hoofed mammals feeding on the
rich vegetation - bison, horse, ground
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