Deep History and Biogeography of La Frontera - Design with the Desert: Conservation and Sustainable Development - page 126

6.5 Tropical Interglacials
Although the general environmental history of La Frontera involves the transition from
tropical deciduous forests in the early Tertiary to more temperate ice age woodlands and
interglacial deserts in the Pleistocene, the vertebrate fossil record suggests that there were
more tropical interglacials than the Holocene.
6.5.1 El Golfo, Sonora
The first fossil record of the giant anteater in North America was in early Pleistocene
sediments from El Golfo de Santa Clara in northwestern Sonora (47 miles southwest of
the international boundary 45,46 ). The nearest populations of this large tropical mammal are
1864 miles to the southeast in the humid, tropical lowlands of Central America. As for many
large mammals, the modern distribution may not accurately reflect their physiological
range limits because of human predation in the last 11,000 years. 47 Other large extinct
animals in the El Golfo fauna include antelope, a bear, camels, cats, horses, proboscidians,
and a tapir. Fossils of extant species in the fauna included Sonoran Desert toad, slider, boa
constrictor, California beaver, and jaguar. The Sonoran Desert toad is a regional endemic
while the slider and boa constrictor occur today in Sonora in wetter, more tropical areas to
the southeast. The California beaver was a larger species than the extant beaver but much
smaller than the bear-sized giant beaver.
El Golfo is at the head of the Gulf of California in the Lower Colorado River Valley sub-
division of the Sonoran Desert. 48 Today, hyperarid desert at El Golfo is too dry to support
any of these animals, although historically the delta of the Colorado River was a very wet
area; extensive cottonwood ( Populus fremontii ) gallery forests supported abundant beaver.
There is an account of a large spotted cat (likely jaguar) that entered James Ohio Pattie's
camp on the Colorado south of Yuma in December of 1827 to feed on drying beaver skins. 49
Although the Sonoran Desert is the most “tropical” of the North American deserts, peri-
odic catastrophic freezes are the most important climatic factor setting the northern limits
of tropical species and communities. 50,51 Hastings and Turner 52 stated that there is prob-
ably no part of the Sonoran Desert that is free from freezing temperatures.
The El Golfo fossil fauna was deposited during the Irvingtonian Land Mammal Age
that began 1.8 mya. This fauna reflects an early Pleistocene interglacial when the climate
was much more tropical than it is today, i.e., frost free, much greater rainfall in the warm
season, and higher humidity. Greater summer rainfall would have been coupled with
intensified summer monsoonal circulation patterns and warmer sea surface temperatures,
unlike most of the Pleistocene when colder water in the northern Pacific intensified the
Aleutian Low, augmenting winter rainfall. 53 The giant anteater, capybara, and ground
sloths in the fauna were members of ten families of mammals that immigrated into North
America in the late Pliocene or early Pleistocene after the opening of the Panamanian land
bridge during the Great American Interchange. 54 In contrast, the imperial mammoth and
a hyena in the fauna were Eurasian immigrants.
6.5.2 Rancho La Brisca, Sonora
The Rancho La Brisca fossil locality is located in a canyon north of Cucurpe, Sonora (56
miles south of the international boundary). 55 The fauna was dominated by Sonoran mud-
turtle and fish, reflecting a wet ciénega paleoenvironment. The presence of the sabinal frog