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of fossil assemblages from these arid lands, its history is not fully recorded. This
would be part of the Southwestern Chaparral element Axelrod (
1950
) recognized,
and presumed remnants of this persist in the summer rainfall chaparral of Arizona
and northeastern Mexico.
By the late Miocene (7 Ma), chaparral had migrated into southern California
(Axelrod
1982
). At this time the seasonal distribution of rainfall had changed
substantially and climates were progressively looking more like a MTC (
Fig. 10.2
).
As drought shifted toward summer, the combination of low rainfall and high
temperatures increased the portion of landscape favoring fire-prone sclerophyl-
lous vegetation. Also, as the inland sea in California receded and the coastline
moved westward to its present location, more extreme winter temperatures and
lower rainfall likely combined to reduce chaparral in the interior parts of the
southwest, leaving it largely a Californian vegetation.
One feature of the fossil record is that the many arid-adapted woody taxa are
unknown prior to the formation of the MTC in the late Pliocene. For example, in
the Mediterranean Basin the very widespread Cistaceae genera,
Cistus
,
Fumana
and
Helianthemum
, and Fabaceae
Cytisus
and
Genista
are unknown from these
fossil assemblages and likewise for the Californian taxa that occupy the most
arid end of the moisture gradient, such as
Salvia
spp. (Lamiaceae),
Hesperoyucca
(
Yucca
)
whipplei
(Liliaceae) and
Adenostoma fasciculatum
(Rosaceae). This obser-
vation has been used to infer a relatively recent origin for these taxa in response
to the MTC (Axelrod
1973
; Herrera
1992
; Valiente-Banuet
et al.
2006
). How-
ever, not only are these arid fire-prone sites unlikely places for fossil deposition,
but other evidence calls this generalization into question. In the Mediterranean
Basin, some fossil records and molecular clock estimates place the origin of
Cistus
,
Fumana
and
Helianthemum
in the Miocene or earlier (Guzma´ n&Vargas
2005
,
2009a
).
Illustrative of the inadequacy of the fossil record to address questions of origin
for semi-arid fire-prone vegetation is
Adenostoma fasciculatum
.Itisthemost
ubiquitous shrub in California chaparral and it not only has no Tertiary history
it has almost no Quaternary fossil record; it is only known from the late Pleisto-
cene, in a packrat midden (Wells
2000
), pollen in marine sediments (Heusser
1978
) and in the La Brea tarpits (Warter
1976
). The genus is morphologically
quite distinct from its nearest relative,
Chamaebatiaria
, a desert shrub from the
Great Basin and recorded (or a close relative) from Oligocene fossils (Wolfe &
Schorn
1989
). Morphologically the only other
Adenostoma
,
A
.
sparsifolium
,
appears more closely aligned with
Chamaebatiaria
than
A. fasciculatum
.Both
Chamaebatiaria
and
A. sparsifolium
are obligate resprouters and
A
.
fasciculatum
is a facultative seeder. When the switch from obligate resprouter to facultative
seeder occurred is unknown, but a Tertiary origin for
A
.
fasciculatum
cannot be
ruled out. Conceivably it was part of the interior Tertiary Southwestern Chapar-
ral (Axelrod
1950
) and is absent from Arizona and northeastern Mexican chap-
arral today due to its apparent intolerance of low winter temperatures (Keeley &
Davis
2007
).
