Terrestrial Palaeoecology and Global Change

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Chapter 9. Crises 381 sented by the single-species leaf mats of a considerable extent. A decrease in diversity was apparently accompanied by an increase in population densities. At the same time, the survival of Cretaceous relicts and “Lazarus” species as well as evolutionary innovations (e.g., in hamamelid angiosperms: Maslova, 2001) both contributed to a high morphological disparity of the Palaeocene communities. Occasional patches of the riparian mosaics still bore a residual Cretaceous aspect. The β diversity remained relatively high owing to persistent seral communities of the Coryletum or Teileiaephylletum types. Taxonomic heterogeneity of the Early Palaeocene regional taphofloras of Amur Basin, Sikhote Alin and Sakhalin – Lesser Kuril Islands systematically exceeded that of the respective Cretaceous taphofloras. The subsequent Arcto-Tertiary flora might have evolved from several Early Palaeocene florogenic centres (Krassilov, 1976a). Allochthonous plant material suggests the relatively high innovation rates over the highlands of the end-Cretaceous orogeny, with an incipient altititudinal differentiation of the conifer forest – birch woodland belts. The climate-driven downslope migrations in the Early Palaeocene might have introduced the advanced elements to the lowland vegetation. Altogether, the regional vegetation developments over the KTB reflect a turnover that would have been more likely inflicted by long-term environmental instability rather than by an instantaneous impact. An increase in taxonomic diversity before the KTB, and a sharp drop off after it comply with actualistic ecological evidence of the highest pre-climax diversity as well as with the model of climax cut-off through a truncation of seral sequences in unstable environments. The low diversity – high disparity plant communities prevailing at the beginning of the Early Palaeocene testify to a change in the population adaptive strategies driven toward the fine-grain mode. The deciduousness of the mid-latitudinal Palaeocene biomes (perhaps enhanced by preferential survival of seral stages) is the result of a climatic temperization that started in the Maastrichtian already. The subsequent build-up of species richness over the Late Palaeocene reflects a gradual transition from the non-coherent post-crisis developments to the coherent evolution. VIII.9.3. Macropolymorphism and morphological innovation The taxonomic diversity of Early Palaeocene floras has been greatly exaggerated by assigning the remains of conspecific polymorphous leaves to different species or even genera. In the Tsagajan flora of Amur Province, two species, Trochodendroides arctica and Platanus raynoldsii, account for about 40 binominals published by Kryshtofovich & Baikovskaya (1966). For comparison, in the contemporaneous flora of Wyoming, four species encompass about 95% of all leaf morphotypes (Gemmill & Johnson, 1997). Such leaf species as Cercidiphyllum crenatum, Tetracentron amurensis, Trochodendroides speciosa, T. smilacifolia, T. richardsonii, T. genetrix, etc. (see synonymy in Krassilov, 1976a) are the more or less distinct leaf variants of T. arctica. They are found in each of the adequately sampled Early Palaeocene leaf populations (Figs. 153, 154).
382 Valentin A. Krassilov. Terrestrial Palaeoecology Fig. 153. Leaf variants of Trochodendroides arctica, a polymorphous species from the Early Palaeocene of Tsagajan, Amur Province. Such repeated co-occurrences suggest a stable system of leaf polymorphism including at least ten distinct leaf allelomorphs. For comparisons at the population level, the epithets of synonymized binominals like “speciosa,” “smilacifolia,” “richardsonii,” “genetrix,” etc. can be used as non-taxonomic descriptive designations (as in the population genetics of Drosophila). For comparison, the precursory Cretaceous species, Trochodendroides sachalinensis, is represented by no more than three stable leaf allelomorphs. Another dominant species, “Platanus” raynoldsii, is likewise polymorphous, with different leaf (leaflet) morphotypes previously described as Alnites, Betulites, Quercus, Pterospermites, Ficus, Grewiopsis, etc., allegedly belonging to different angiosperm families (Kryshtofovich & Baikovskaya, 1966; synonymized in Krassilov, 1976a). The cuticular characters unequivocally indicate polymorphous leaf population of a single species.
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Terrestrial Palaeoecology and Global Change

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