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Earliest Triassic origin of Isoetes and quillwort evolutionary radiation
Published online by Cambridge University Press: 14 July 2015
Abstract
Isoetes beestonii new species is the most ancient known species of this living genus. In earliest Triassic shales of the Sydney and Bowen Basins of Australia it is locally abundant as circlets of transversely wrinkled leaves. It was heterosporous with megaspores of Maiturisporites rewanensis and microspores of Lundbladispora sp. cf. L. springsurensis.
Isoetes thus predates Pleuromeia from which it has been thought to have evolved. Australian Pleuromeia-like subarborescent lycopsids are here reviewed as whole plants, with names based on fertile structures, and include Cylostrobus sydneyensis Helby and Martin from the Sydney Basin, Pleuromeia dubia (Seward) Retallack from the Sydney and Canning Basins, and Cylostrobus indicus (Lele) new combination and Pleuromeia sternbergii (Münster) Corda for Germar, newly recorded from the Canning Basin.
There are in addition an array of cormose lycopsids that formed compact conelike plants when fertile, intermediate in stature between Isoetes and Pleuromeia. One of these is Tomiostrobus australis (Ash) Sadovnikov, formerly regarded as a cone, but here reinterpreted as a small pioneering plant of oligotrophic lakes and ponds, like Isoetes. Its megaspores are Horstisporites and its microspores are the stratigraphically important Aratrisporites tenuispinosus. Other similar forms are Tomiostrobus polaris (Lundblad) new combination from the early Triassic of Greenland, T. mirabilis (Snigirevskaya) new combination from the early Triassic of the Tunguska Basin of Siberia, T. taimyrica (Sadovnikov) new combination from the Early Triassic of the Taimyr region of Siberia, Lepacyclotes ermayinensis (Wang) new combination from the middle Triassic of China, L. convexus (Brik) new combination from the middle-late Triassic of Kazachstan, and L. zeilleri (Fliche) new combination from the middle Triassic of France and Germany.
The diversity of isoetaleans in early Triassic floras and the weak vascular system of permineralized Tomiostrobus and Pleuromeia contradict the traditional view that Isoetes evolved by reduction in size from Pleuromeia and that its opportunistic life style allowed it to avoid plant competition. It is now more likely that Isoetaceae were weedy survivors of Permian-Triassic extinctions. The adaptive radiation and decline of Triassic quillworts matches the recovery from near-extinction, then decline of therapsid reptiles, for which these plants may have been an important food.
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