Terrestrial Palaeoecology and Global Change

42 Valentin A. Krassilov. Terrestrial Palaeoecology
In terms of life-form evolution, scleromorphy is considered as a preadaptation to dry
or cool climates (Hill, 1990). Sclerophyllous vegetation might have appeared as a marginal
derivate or a seral stage of laurophyllous woodlands adapted to a low fertility soil
(Axelrod, 1975). Such seral sclerophylls might have given rise to the chapparal–macchia
type xeromorphic communities initially appearing as undershrubs of sclerophyllous woodlands
(Axelrod, 1975). Sclerophyllous communities seem also to have been engaged in
the altitudinal differentiation of frost-resistant Alpine vegetation belt.
Taken literally, xeromorphy as indicator of aridity is a prolific source of erroneous
climatic inference (e.g., in the case of a climatological interpretation of the Late Permian
peltasperm–conifer assemblages: Krassilov, 2000b). Even in the commonly recognized
xeromorphs, such as the Permian callipterids, a scrutiny of epidermal features reveals a
helophytic adaptation to excessive waterlogging (Busche et al., 1978). A disambiguating
piece of evidence comes primarily from the lithofacies. In particular, a constant association
with coal beds suggests a scleromorphous helophytic, rather than xerophytic, life
form.
Thus, because of its functional ambiguity, morphological evidence should be supplemented
by taphonomic data. The modes of dead mass accumulation can even be a
better guide to life forms than morphology. Some leaf shapes, in particular those with a
long petiole and peltate wind-rough blade, imply deciduousness but, in addition to or
irrespective of morphology, seasonal leaf shedding is verified by leaf-mat type taphonomy.
Disseminules occur either in the leaf-mats of their source plants or separately. Notably,
not all deciduous trees accumulate leaf litter in their stands. Maple stands produce
thick leaf mats but lime stands do not. These distinctions in littering correlate with dispersal
strategies and sprouting under the canopy of parental plants or lack of such (in
lime). In the case of plants that flower before leaves, shed floral parts tend to associate
with leaves of other plants. Such associations sometimes lead to erroneous assignments
but, when properly recognized, are evidence of deciduousness and early flowering.
III.1.2. Life form inference: examples
The following examples illustrate the significance of taphonomic inferences in extinct
life form reconstructions.
Pleuromeia is a peculiar Triassic lycopsid with a typically unbranched (or forked)
stem, bulbous 4-lobed rhyzome, long bifacial leaves and a terminal heterosporous cone.
It was originally interpreted as xeromorphic and perhaps psammophytic, a reconstruction
that fitted a traditional notion of arid Triassic landscape (Mägdefrau, 1956). I have
studied Pleuromeia from three Early Triassic localities, the Ryabinsk on the Volga River,
the Russian Island near Vladivostok, and the Olenek River in northern Siberia (Fig. 22).
The facial occurrences are both terrestrial and marginal marine. The abundant remains
of rhyzomes and aerial stems indicate autochthonous assemblages, in which Pleuromeia