Two Mesozoic Bennettiales
& Williamsoniella coronata
During much of the Mesozoic Era, gymnosperms were a major component of many Mesozoic floras, with the Cycadopsida (true cycads) and the Cycadeoideopsida the Bennettitales, being a conspicuous and significant component of low level and understory vegetation.
Once globally diverse, only the true cycads are extant today, their current distribution confined to a limited band of tropical and subtropical climates.
Yet some cycad families, the Stangeriacae and Zamiaceae have a good fossil record recognised from early in the Mesozoic era, with several extant genera, Dioon, Encephalartos and Stangeria, closely resembling their prehistoric counterparts.
The Bennettitales, however, are now completely extinct, and unfortunately, due to the often poor and fragmentary preservation of their remains, have presented a problem to confidently reconstruct. It is generally agreed, however, that they should be divided into three families :- the Cycadeoidaceae with stout trunks and Cycadolepsis or Zamites type foliage; The Williamsoniaceae with Ptilophyllum or Otozamites type foliage; and the Wielandiellacea, with leaves of an Anomozamites or Nilssonipoteris type.
Current research indicates that Williamsonians were generally small to medium, dense, busy plants exhibiting dichasial, multi-branching growth morphology with some species possessing leaves in the forks of branches and others limited to their terminal apex.
Wielandiella angustifolia presented here is a Williamsonian bennettitale with dichasial branching morphology, possessing a small cone at the branching fork and a spiral of leaves at the terminus of each branch. The leaves of Wielandiella are assigned to the Anomozamites leaf type. Examples of Wielandiella have been found in Russia, Turkmenistan, Ukraine, Mongolia and Sweden.
Williamsoniella coronata is another Bennettitale with dichasial branching. The leaves of Williamsoniella, are of the simple Nilssoniopteris type and sprout from the trunk and terminal ends of branches, with small, spherical cone-like flowers emerging from below each leaf axil.
Ink and graphite on paper
For further reading please see:-
Divaricate growth habit in Williamsoniaceae (Bennettitales): Unravelling the ecology of a key mesozoic plant group. Palaeobiodiversity and palaeoenvironments. June 2014 Christian Pott & Stephen McLoughlin.
A revision of Wielandiella angustifolia, a shrub-sized bennettitite from the Rhaetian-Hettangian of Scania, Sweden, and Jameson Land, Greenland. Int. J. Plant Sci. 2014.
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