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was common in the fossil record through the Oligocene and early Miocene (de
Leo´ n & Cevallos-Ferriz
2000
). Contributing to its success were its light plumose
wind-dispersed fruits (only taxon in contemporary chaparral with well-
developed wind dispersal), which would have had significant advantages in
finding suitable habitat when such sites were dispersed as arid islands within a
landscape of more mesic woodlands.
On early Miocene sites in the interior (e.g. present-day Nevada) most chaparral
genera were represented, often by taxa indistinguishable from modern species.
These included species of
Arctostaphylos
,
Ceanothus
,
Cercocarpus
,
Fremontoden-
dron
,
Garrya
,
Heteromeles
,
Mahonia
,
Malosma
,
Prunus
,
Quercus
,
Rhamnus
,
Rhus
and
Ribes
. Through the Miocene, as rainfall seasonality increased, chaparral
patches expanded, and increased patch size almost certainly contributed to
increased fire activity in these seasonally dry habitats with highly flammable
sclerophyll foliage.
Throughout the early to middle Miocene (
17 Ma), associations with affinities
to contemporary chaparral were best developed away from the coast in more
arid and less equable interior parts of North America, such as the western edge
of the Great Basin (Axelrod
1939
,
1973
,
1985
; Wolfe
1964
). Middle Miocene
deposits from northern parts of the Great Basin reveal that
Artemisia
shrub and
grassland dominated these landscapes much like they do today (Davis & Ellis
2010
). Chaparral elements likely developed further south, but we have little infor-
mation since there is a lack of Miocene fossil floras for most of southwestern
North America. The present-day chaparral patches that persist in bimodal-rainfall
Arizona or the winter drought, summer rain climate of northeastern mainland
Mexico (see
Fig. 5.1
) may be relicts of a broad Tertiary distribution of chaparral
taxa in southwestern North America, even though chaparral was not dominant in
coastal California at that time.
Arctostaphylos
(Ericaceae) and
Ceanothus
(Rhamnaceae) are two of the largest
woody genera in North America and their center of radiation is in California
chaparral; species are also present in Arizona and northeastern Mexican chapar-
ral. The latter genus is known from the early Tertiary fossil record at high
latitudes; however,
Arctostaphylos
is not recorded until the Miocene, when chap-
arral elements had already moved into southern California (Axelrod
1939
). The
lack of an earlier history is something of an anomaly. The nearest relative appears
to be
Arbutus
, but not the present North American species, rather the Mediterra-
nean Basin taxa (Hileman
et al.
2001
). This is consistent with an earlier origin
for
Arctostaphylos
, perhaps prior to the Eocene splitting up of the continent
of Laurasia, which included the present-day continents of North America,
Europe and Asia. Alternatively, allies of the Mediterranean Basin
Arbutus
could have persisted in North America after the breakup of Laurasia; however,
the several
Arbutus
taxa known from Tertiary deposits have all been allied with
North American species (e.g. Axelrod
1939
; Wolfe
1964
). We hypothesize that
Arctostaphylos
originated early in the Tertiary and persisted for much of the
Tertiary in arid shrublands of southwestern North America, but due to the lack
∼
