Terrestrial Palaeoecology and Global Change

Chapter 7. Climate change
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are evolutionary conservative organs, the more leaf characters are indiscriminately used
as past climate indicators, the greater a potential there is for an error to occur as a result
of such misleading similarities. This consideration casts some doubt on the alleged advantage
of using many poorly understood characters instead of few better known ones.
Reproductive characters. Some reproductive features, such as volume-regulating
(harmomegathy) structures in pollen grains, are commonly interpreted as climatic adaptations.
However, an importance of harmomegathy primarily depends on pollination mode,
with air-borne pollen grains normally exposed to desiccation for a shorter time than those
carried by insects. Generally, over the evolutionary history of reproductive structures,
the climatic effects are indirect, related to reproductive strategies. Thus, wind pollination
prevails in temperate climate because of relatively low biological diversity, high population
densities and the seasonal leaf shedding (canopies in full leaf screen the pollen rain).
Entomophily is more frequent in tropical climates for the opposite reasons.
Fossil evidence of pollination ecology is an important, although as yet little explored,
source of palaeoclimatological inference. For example, the giant male strobili of Cycandra,
a Jurassic cycadophyte (Fig. 88), produced thick-walled sporangia with narrow
apical beaks, from which pollen grains, several hundreds per sporangium, were released
in small portions. In contrast to instantaneous dispersal of wind-born pollen, this peculiar
mechanism of parsimonious exposure offered a persistent pollen source for diachronously
developing ovulate cones implying an extended pollination period that in turn implies
a weak seasonality or none at all, with pollinators available round the year (Krassilov,
2000a).
More than one pollen type in gut compressions of fossil insects is evidence of overlapping
pollination periods in the forage species. Such mixed pollen loads are rather
typical of the mid-Permian insects of Cisuralia (Krassilov & Rasnitsyn, 1997; Krassilov
et al., 1999d). The pollen types of their gut assemblages are also abundant in contemporaneous
dispersed assemblages indicating the dominant pollen producers. This type plant
– insect interactions indicates seasonality, with synchronous pollination probably confined
to the drier season. In contrast, all hitherto studied Early Cretaceous xyelids from
Baisa, Transbaikalia, had single-species pollen loads, with pollen types produced by relatively
rare plant species. Thus the feeding habits were highly selective and probably
adapted to diachronous pollen sources, as in the case of Ceroxyella dolichocera that
fed either on saccate pollen produced by proangiospermous Preflosella or on cryptosaccate
pollen of an unknown relative of the latter (VIII.3.3). Since xyelids are early
spring insects, the succession of flowering should have been rapid as in ephemeroids of
temperate climates.
Like fossil pollen, fossil disseminules are as yet a poorly explored source of palaeoclimatic
inference. The seed/fruit size is a function of (1) growth rates, with larger disseminules
facilitating a faster seedling growth; (2) seed dormancy period correlated with
small endosperm and thick coats; (3) seral stage, with disseminules typically smaller in
pioneer species and colonizers (Wilson, 1993), (4) dispersal mode, in particular, the body