Dicroidium Zuberi 2

''Dicroidium'' is an extinct genus of fork-leaved seed plants. It is the archetypal genus of the corystosperms, an extinct group of seed plants, often called "seed ferns", assigned to the order Corystospermales or Umkomasiales. Species of ''Dicroidium,'' which grew as large trees, were widely distributed and dominant over Gondwana during the Triassic (). Their fossils are known from South Africa, the Arabian Peninsula, Australia, New Zealand, South America, Madagascar, the Indian subcontinent and Antarctica.

Description

Within the form genus classification system used in paleobotany, the genus ''Dicroidium'' refers specifically to the leaves. Some authors have suggested dividing ''Dicroidium'' up into several genera, including ''Dicroidiopsis, Diplasiophyllum, Zuberia'', ''Xylopteris'', ''Johnstonia'' and ''Tetraptilon,'' but this is rejected by other authors. The leaves of ''Dicroidium'' bifurcate (fork) at their base, which is characteristic of all species. The leaves are highly variable in size and morphology, ranging from simple to tripinnate, with the individual leaflets having varying morphologies, including dissected, lobed, needle-like and entire. Some leaf specimens have more than one type of leaflet morphology, which may have been the result of hybridisation between different species. The venation of the leaves is also highly variable, encompassing taeniopteroid, odontopteroid, alethopteroid and simple morphologies.

Whole plant

''Dicroidium'' plants grew as medium-large sized trees, with some preserved trunk sections tall and over wide, with the wood assigned to the genera ''Kykloxylon'' and ''Rhexoxylon.'' The ovulate reproductive structures are usually assigned to the genus ''Umkomasia,'' while the pollen-producing organs are assigned to the genus ''Pteruchus,'' with pollen attributable to the genus '' Falcisporites.'' These structures are almost never found in organic connection, and their placement as part of the same plant primarily relies on their repeated co-occurrence with each other, and the similarities in the morphology of their cuticles. The leaves grew on short protuberances attached to the stem in a similar way to living ''Ginkgo biloba''. The leaves of ''Dicroidium'' are suggested to have been deciduous, and shed along with the reproductive organs during the winter. The seeds and pollen are suggested to have been wind dispersed. At least some of the Antarctic species are suggested to have engaged in prolific bark shedding.

Possible whole plant associations include:

* ''Dicroidium odontopteroides'' may have been produced by the same plant as '' Umkomasia macleanii'' (ovulate structures) and ''Pteruchus africanus'' (pollen organs).
* ''Dicroidium zuberi'' may have been produced by the same plant as ''Umkomasia feistmantelii'' (ovulate structures) and ''Pteruchus barrealensis'' (pollen organs)

Evolution

The earliest ''Dicroidium'' species are known from the Late Permian aged Umm Irna Formation of Jordan, which inhabited equatorial humid tropical environments, as well as equivalently aged more southerly subtropical deposits on the Indian subcontinent. Following the end-Permian mass extinction, ''Dicroidium'' expanded its range southwards across Gondwana, including South Africa, Antarctica, Australia, New Zealand and South America, with a record also reported from the Middle Triassic of China. Later Triassic ''Dicroidium''-bearing plants were dominant large canopy forming trees in temperate wetland and forested habitats at mid-high latitudes, extending to the South Pole. ''Dicroidium''-dominated ecosystems in Gondwana collapsed during the end-Triassic mass extinction, with ''Dicroidium'' surviving in parts of East Antarctica into the Early Jurassic (Sinemurian).

References

* Bomfleur, B. and Kerp, H. (2010). ''Dicroidium diversity in the Upper Triassic of north Victoria Land, East Antarctica.''

{{Taxonbar, from=Q5273916

Triassic plants
Fossil taxa described in 1912
Pteridospermatophyta