Corystosperms †

Successful seed ferns in Triassic

The Corystosperms are a major group of seed plants from the Triassic Period. They were so abundant that they are used as index fossils for this period. During the Early Triassic they replace the Glossopterids on the landscape. They had pinnately-compound leaves (Dicroidium) that were bifurcate emerging from distinctive short shoots. The wood (Rhexoxylon) was produced in wedges due to cambial growth that was staggered and not synchonized. Reproductively, they had helmet-like cupules that were stalked and recurved (Umkomasia).

Ecology & Form

Major plant in Triassic ecosystem; index fossil
Replace glossopterids in the flora
Probable affinities with Medullosa and cycads based on wood anatomy (Artebe & Brea 2003)
Possible ancestor-descendant relationship with Ginkgo (Shi et al. 2016)
Once thought to have a possible link to the angiosperms based on ovule, but recent evidence doesn't support this (Klavins et al. 2002; Soltis et al. 2005)

Stem

Permineralized woody stem (Rhexoxylon) up to 70 cm in diameter
- Primary xylem bundles produce wedges of wood on both sides.
- Cambium growth was staggered and not synchronized
Wood morphotaxa: Jeffersonioxylon and Kykloxylon
Distinctive short shoots
These were small to large palm-like, but deciduous trees living in seasonally flooded (monsoon) environments

Leaves

Pinnate leaves (Dicroidium)
Several types of foliage
All pinnate
Open dichotomous venation

Pachypteris

Hypostomatic, bipinnate or unipinnate leaves, with alethopteridian venation (midvein and secondary veins divided once or twice before reaching the pinnule margin), pinnules with entire margins and rounded apices
The stomata are haplocheilic, monocyclic or dicyclic, usually depressed, with the guard cells occurring in the lowermost part of the stoma.

Komlopteris

Leaves pinnate, pinnules in opposite or alternate position, elliptical-elongated or elongated in outline, apex acute or subacute to rounded, margins entire, occasionally slightly undulate
Venation alethopteroid, the midrib more or less symmetrically situated in the middle of the pinnules.
Secondary veins simple or irregularly once or twice forked
Leaves stiff, coriaceous, strongly cutinized. Cuticle thick; cuticle of lower leaf surface usually thinner than that of upper leaf surface
Pinnules amphistomatic with only a few stomata on the upper leaf surface
On the lower leaf surface stomata are situated in intercostal bands between well-marked veins
Stomatal complexes amphicyclic, consisting of two guard cells situated near the surface of the leaf and 5-7 subsidiary cells which are divided by a thick line into 2-3 narrow concentrical rings around the stomatal pit
Sometimes stomata are surrounded by a more or less complete ring of encircling cells.