Plant and animal fossils in deserts - Climatic Change in Deserts - page 300

China indicates an abrupt increase in summer rainfall at 12.5-11 ka that lasted until
8-7 ka, with maximum aridity at all five lake sites from 4.5 to 3.5 ka (Fan et al., 1996 ;
Gasse et al., 1996 ,VanCampoetal., 1996 ; Wei and Gasse, 1999 ).
16.6.3 Other plant and animal microfossils: phytoliths,
charophytes and sponge spicules
A number of other plant and animal microfossils can provide supplementary inform-
ation about past desert environments. Phytoliths are the microscopic remains of the
particular parts of certain plants that are composed of silica, and they can be used to
identify certain terrestrial plants. Charophytes are green algae. The reproductive part
of the organism is called the oogonium, and calcareous charophyte oogonia have been
used to date late Pleistocene lake lunettes in semi-arid New South Wales (Williams
et al., 1991a ).
Williams et al. ( 1986 ) and Adamson and Williams ( 1987 ) used the presence or
absence of diatoms, phytoliths and sponge spicules in Pliocene fluvio-lacustrine
deposits in the Middle Awash Valley of the Afar Desert to distinguish between fluvial
and lacustrine beds. Barboni et al. ( 1999 ) analysed the phytoliths within these Pliocene
deposits in far greater detail, as an adjunct to the pollen analysis by Bonnefille et al.
( 2004 ), which showed a mosaic vegetation pattern of woodland, grassland and dense
riparian forest during the time that the Pliocene hominid Australopithecus afarensis
roamed this area.
Sponge spicules are microscopic cylinders composed of silica. The discovery of
sponge spicules that were used to temper pottery in the White Nile Valley 2,000 years
ago came from a site situated around 350 km north of the nearest present-day swamps
(Adamson et al., 1987a ), and confirmed the account of these swamps given to the
Emperor Nero after his centurions found their attempt to trace the source of the White
Nile blocked by impenetrable swamps (Mawson and Williams, 1984 ).
16.7 Conclusion
The evidence from fossil plants and animals has been an invaluable supplement to
other lines of evidence relating to climatic change in deserts, and in many instances it
is the only evidence available. The fossils of large and small vertebrates afford reli-
able insights into past habitats, but care is needed in distinguishing between remains
that have been transported by running water, humans or other predators and remains
that are indeed in primary context. Invertebrate fossils such as freshwater mollusca,
ostracods, cladocera and chironomids are valuable indicators of water depth, temper-
ature and salinity or alkalinity, and if they are used together, they can form a powerful
tool to reconstruct past changes in desert lakes and swamps. Among plant fossils, pol-
len and spores have been most widely used to reconstruct past changes in vegetation