Caught between two mass extinctions – the rise and fall of Dicroidium

In the aftermath of Earth’s greatest biotic crisis 251.9 million years ago – the end-Permian mass extinction – a group of plants arose that would come to dominate the flora of the Southern Hemisphere. Recovery of the vegetation from the end-Permian crisis was slow; but steadily, one group of seed plants, typified by the leaf fossil Dicroidium, began to diversify and fill the dominant canopy-plant niches left vacant by the demise of the Permian glossopterid forests (Fielding et al., 2019). Eventually, Dicroidium re-established a rich peat-forming vegetation across Gondwana through the Late Triassic, dominating the flora between 30°S and the South Pole (Kustatscher et al., 2018). Indeed, few fossil plant assemblages of this age can be found in Gondwana that do not contain this plant.

The importance of Dicroidium is not just its role in showing biogeographic and tectonic linkages between southern lands or its value in determining the age of continental strata. Dicroidium and its associated plant groups were so successful that they were major contributors to the development of thick coal seams in the Late Triassic that are now mined to produce electricity.

Fig. 1. The same species of Dicroidium (D. odontopteroides) from three southern continents: (A) Birds River,
South Africa; (B) Ipswich, Australia; (C) Transantarctic Mountains, Antarctica – this frond features both the
typical rounded leaflets, but also elongated forms (arrow), which may indicate hybridisation.

Although Dicroidium is generally envisaged as a plant of cool temperate climates, the very first fossils that might belong to this group are from the Permian-Triassic transition of Jordan, located near the palaeoequator (Blomenkemper et al., 2018). Nevertheless, the distribution of Dicroidium soon shifted to high southern latitudes in the Early Triassic and they overwhelmingly dominated the southern vegetation by the Middle and Late Triassic.

The Dicroidium plant had leaves that were superficially fern-like (Fig. 1). However, unlike ferns, these plants produced pollen and seeds, and are commonly lumped into an ill-defined group called the ‘seed-ferns’. Dicroidium leaves are characterised by the presence of a distinctive fork near the base of the frond. Numerous types of Dicroidium are recognised, but one of the difficulties involved in separating species is their considerable variation in leaf form. Many of the species established during the twentieth century have intergradational features. Moreover, some species may have hybridised, resulting in progeny that had different leaflet morphologies even on separate parts of the same frond (Fig. 1C). The world experts on this group, Heidi Anderson-Holmes and John Anderson, who have collected some 40,000 Triassic plant fossils during their careers, have recognised 23 species and 16 sub-species categories or ‘forma’ of Dicroidium (Anderson and Anderson, 1983).

Fig. 2. Map of Triassic Gondwana showing the distribution of Dicroidium.

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