Terrestrial Palaeoecology and Global Change

Chapter 3. Palaeoecology
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Fig. 22. Pleuromeia, a dominant form of the lycopsid wetlands
spreading worldwide in the Early Triassic: a cone in the Olenekian
marine sandstones of Russian Island, Primorye, Russian Far
East (Krassilov & Zakharov, 1975).
was the only macrophyte. It obviously formed pure stands. The most common lithologies
are organic-rich shales with plant compressions and occasional marine fossils indicating
anoxic estuarine environments. Occasional finds in sandstones with chaotic cross-bedding
represent a stand buried by hurricane wave deposits (tempestites). Dispersed sporophylls
and spores are also found as allochthonous material in nodules with small ammonoids
at the core. Since the ammonoid shells and megasporophylls are similar in shape
and dimensions, they might have been transported together by sea currents. In agreement
with this suggestion, the sporophylls are boat-shaped, with a solitary sporangium
sunken in a spoon-like depression of the upper surface. Both the morphology and taphonomy
suggest dispersal by floating sporophylls as a regular feature of reproducitve strategy.
A reconstruction emerging from these considerations is that of a vast reed-like
coastal wetland extending into river mouths and further inland (Krassilov, 1972a; Krassilov
& Zakharov, 1975; compare Retallack, 1975; Wang & Wang, 1982). The dryland
Early Triassic vegetation is poorly known because of a strong filtering effect of the
ubiquitous Pleuromeia stands.
Nilssonia is a widespread Mesozoic gymnosperm that was conventionally assigned
to cycads. The genus was based on fossil leaves that are simple, elongate, shortly petiolate,
irregularly to regularly lobed, with entire or, rarely, dentate margins. Nilssonia
leaves are common in the levee and muddy flood-plains facies where they are scattered