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reflected in subtle differences between populations (Brown et al.,
1974
).Whitmore and
Prance (
1987
) have since identified overlapping centres of endemism of butterflies,
plants and birds in South American rainforests, which seem to reflect the existence of
refugia during times of drier and/or colder climate.
Based on pollen analysis from a somewhat limited number of sites, Anhuf et al.
(
2006
) concluded that the area now covered by humid rainforest in the Amazon was
probably reduced by around 54 per cent as a result of a 20-40 per cent decrease in
precipitation accompanied by a temperature drop of 4.5-5
C during the LGM. The
presence of now vegetated sand dunes within the Amazon Basin is also indicative of
previously drier conditions that were conducive to the formation of source-bordering
dunes. This was also a time when the area of the rainforest in the Congo Basin possibly
shrank by 84 per cent as a result of a 30-40 per cent reduction in rainfall and a 5
°
C
drop in temperature during the LGM (Anhuf et al.,
2006
), as inferred qualitatively by
Moreau forty years earlier (Moreau,
1963
). As we shall see in Chapters
18
and
19
,
elsewhere in many parts of tropical Africa and Asia, the deserts expanded, woodland
areas contracted and lake levels fell during the LGM. However, there was considerable
regional diversity during the LGM in Africa (Gasse et al.,
2008
), and some workers
disagree with the LGM glacial aridity scenario for the Amazon Basin (Colinvaux
et al.,
1996
; Colinvaux et al.,
2000
; Colinvaux,
2001
).
In Australia, Byrne et al. (
2008
) have sought to integrate evidence from phylogen-
etics, phylogeography and paleoenvironmental studies in order to reconstruct when
and how the present-day Australia arid zone biota arose. They concluded that aridity
first became evident in the plant record in the mid-Miocene some 15 million years ago.
Landforms consistent with full aridity (such as dunes and stony gibber plains) appear
in central Australia between 4 and 1 million years ago (Fujioka et al.,
2005
; Fujioka
et al.,
2009
). Dated molecular phylogenies indicate that some large, vagile taxa show
patterns of recent expansion and migration throughout the arid zone, while other taxa
appear to have persisted in multiple localised refugia during cold, dry glacial times
(Byrne,
2008a
; Byrne,
2008b
; Byrne et al.,
2008
). No similar studies have yet been
published for other major desert regions, such as the Sahara, the Gobi or the deserts
of central Asia, but this seems a potentially fruitful approach, drawing on insights
from both molecular ecology and the earth sciences. Indeed, Hewitt (
2000
) argued
that the use of DNA technology can show how different organisms have responded to
the climatic vicissitudes of the Quaternary ice ages and concluded that 'the present
genetic structure of populations, species and communities' has been mainly formed
by the environmental changes associated with Quaternary glaciations.
°
16.3 Vertebrate fossils: life and death assemblages
With rare exceptions, vertebrate remains are at best sporadic in most of the arid zone.
When they do occur, they tend to be found in association with prehistoric occupation
