Deep History and Biogeography of La Frontera - Design with the Desert: Conservation and Sustainable Development - page 123

6.3.3 Pliocene Climates
During the late Miocene and early Pliocene, sea level rose enough that the Gulf of California
expanded north into the Salton Trough of southeastern California to deposit the marine
sediments of the Imperial Formation. 2 7, 2 8 For part of this time, a marine embayment in the
Los Angeles Basin almost connected with the Gulf of California, again effectively isolating
Baja California. The extensive sediments of the roughly contemporaneous Bouse Formation
in the lower Colorado River basin have been interpreted as either estuarine based on
invertebrate fossils, including barnacles, or freshwater lakes on strontium isotopic analyses.
In Anza Borrego Desert State Park in southern California, the Pliocene-early Pleistocene
terrestrial sediments of the Palm Springs Formation overlie the marine Imperial Formation.
A fossil lizard skull from the Pliocene (ca. 4.3-2.5 mya) in the Vallecito Creek local fauna
25 miles north of the international boundary was described, although it could have been
easily placed in the extant genus Iguana . 29 It was associated with the extant desert iguana.
The green iguana is a tropical lizard that today occurs no farther north than southern
Sinaloa, ca. 932 miles to the southeast. Tropical species in desert at high latitudes and much
higher sea levels indicate warmer global temperatures and oceans, enhanced monsoonal
summer rainfall, and the northward expansions of tropical thornscrub and deciduous
forests in Sonora and the Sonoran Desert in La Frontera .
6.3.4 Historical Biogeography
The distributions of a number of plant and animal species or closely related species
pairs suggest past connections between the Chihuahuan and Mohave\Sonoran deserts.
This region was called Mojavia 30 and modified for herpetofauna. 31 Different distribution
patterns in the vicariant species pairs likely reflect different separation times and
evolutionary mechanisms. One type of east-west species pairs reflects the evolution of
similar species from common subtropical ancestors. For example, the Big Bend gecko in
the Chihuahuan Desert and the barefoot gecko in the Sonoran Desert were both derived
from a tropical ancestor (very close to Coleonyx mitratus 32 ; living in tropical deciduous
forest or thornscrub). The arborescent yuccas, Yucca filifera, , in the southern Chihuahuan
Desert and Y. v ali d a in Baja California are a similar example in plants. Presumably the
early-middle Miocene orogeny restricted the ancestor's ranges into a northerly “u”-shape
straddling the Sierra Madre Occidental; the living descendants occur at the northern tips
of the “u.” In these particular cases, the Sonoran Desert species were further isolated as
Baja California separated from the mainland.
Other species pairs likely reflect simple range splits that evolved into eastern and
western species with the uplift of the Continental Divide and the initiation of glacial
climates about 2 mya. Examples in reptiles of east-west species pairs are banded geckos,
horned lizards, 33 and rat snakes; justifications for not using Herndon Dowling's generic
name Bogertophis for these two species or Senticollis for Elaphe triaspis are in Van Devender
and Bradley. 34 In plants, the relationship between the Chihuahuan and Mohave deserts is
particularly strong, in part because of the physical connection across central Arizona along
the Mogollon Rim. Closely related (vicariant) species in Texas-Coahuila-Chihuahua and
Arizona-California include Torrey and Mohave yuccas ( Yucca torreyi/Y. schidigera ), Joshua
tree/Whipple and Thompson yuccas ( Yucca brevifolia , Yucca whipplei/Yucca thompsoniana
and relatives), heath and Burro Creek cliff roses ( Cowania ericaefolia/Cowania subintegra ),
canotias ( Canotia wendtii/Canotia holacantha ), crucifixion thorns ( Castela emoryi/Castela
holacantha ), and Texas and blue paloverdes ( Cercidium texanum/Cercidium floridum). ).