Terrestrial Palaeoecology and Global Change

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Chapter 7. Climate change 201 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
202 Valentin A. Krassilov. Terrestrial Palaeoecology Fig. 88. Cycandra, a giant male cone, producing thick-walled sporangia with apical beaks (bottom left), through which the pollen grains were released in small portions providing a persistent pollen source. The Middle Jurassic of Georgia, Caucasus (Krassilov et al., 1996a).
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Terrestrial Palaeoecology and Global Change

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