Cordaitales: Characteristics, Examples and Affinities

Cordaitales is an extinct group of primitive gymnosperms that thrived predominantly during the late Paleozoic era, particularly in the Carboniferous and Permian periods. They are among the earliest members of the class Coniferopsida and are considered ancestral to both Coniferales and Ginkgoales. These plants were characterized by tall, woody trees with simple, strap-like leaves and compound strobili. Fossil evidence suggests that Cordaitales played a significant role in the formation of the first extensive forest ecosystems on Earth. In this post we discuss about the characteristics of Cordaitales with its Examples, Distribution and Affinties.

Overview of Cordaitales

Ø  Cordaitales are an extinct group of primitive gymnosperms.

Ø  They are considered among the most ancient and primitive members of the class Coniferopsida.

Ø  Cordaitales first appeared during the Devonian Period.

Ø  They were dominant during the Devonian and Carboniferous periods.

Fossil Specimens of Cordaitales

Ø  During the Carboniferous period, Cordaitales were the sole representatives of the class Coniferopsida.

Ø  Cordaitales are believed to have given rise to both Coniferales and Ginkgoales.

Ø  Among these descendant groups, Ginkgoales underwent evolutionary decline and are now represented by a single extant species, Ginkgo biloba.

Ø  In contrast, the Coniferales diversified extensively and are now the dominant gymnosperms in the modern flora.

Ø  The evolutionary relationship linking Coniferales and Ginkgoales through Cordaitales is well established and supported by paleobotanical evidence.

Ginkgo and Pinus

Examples of Cordaitales

Ø  Complete Fossil Reconstruct: Cordaites, Dorycordaites

Ø  Root: Ameylon

Ø  Stem: Mesoxylon, Metacordaites

Ø  Leaves: Cordaites

Ø  Fructifications (Reproductive Structures)

o   Male Fructification: Cordaitanthus

o   Female Fructification: Cordaitanthus williamsonii

Ø  Seed: Cardiocarpus

Morphology of Cordaitales

Characteristics of Cordaitales

Ø  The order Cordaitales is named after the 19th-century mycologist August Carl Joseph Corda.

Ø  Members of Cordaitales are believed to have formed the Earth’s first extensive forest ecosystems.

Ø  These plants were tall trees, reaching heights of up to 30 meters or more.

A Fossil Reconstruct of Cordaites

Ø  The stem of Cordaitales was typically monopodial, exhibiting a dominant central axis.

Ø  The stem could attain a diameter of approximately 65 cm.

Ø  Despite their height, the stem diameter was relatively narrow, giving the trees a slender appearance.

Ø  The branching pattern was predominantly lateral.

Ø  Branches were generally confined to the upper portion of the stem.

Slender Appearance of Cordaitales

Leaves

Ø  Leaves were simple in structure.

Ø  They were petiolate (possessing a leaf stalk).

Ø  Leaves were arranged spirally along the stem.

Ø  The leaf shape was either spatulate or grass-like.

Leaves of Cordaitales

Ø  Leaf margins were entire (smooth, without lobes or serrations).

Ø  Leaf apices were either obtuse or acute.

Ø  Venation was parallel.

Ø  The specific type of stomata in Cordaitales remains unclear.

Stem: Morphology and Anatomy

Ø  The stem possessed a wide cortex with patches of sclerenchyma tissue.

Ø  Secretory canals were present in the cortex, a feature resembling that of cycads (cycad-like character).

Ø  The pith contained large air chambers separated by transverse diaphragms.

Stem Anatomy of Cordaitales

Ø  Secondary growth was evident, and the wood was pycnoxylic (dense and compact).

Ø  Medullary rays were uniseriate (one cell wide).

Ø  The wood anatomy showed similarities to that of the modern conifer family Araucariaceae.

Ø  Leaf traces were either mesarch or endarch in nature.

Ø  Leaves exhibited both palisade and spongy mesophyll tissues.

Ø  Leaf vascular bundles were mesarch (with protoxylem located in the middle).

Ø  Vascular bundles in leaves were enclosed by a well-defined sclerenchymatous bundle sheath.

Ø  The precise nature of stomatal development is not clearly understood.

Ø  Stomata were arranged in distinct longitudinal rows, similar to those seen in grasses.

Ø  Each stoma was associated with two polar subsidiary cells and two lateral subsidiary cells

Structure of Stomata in Cordaitales

Reproduction in Cordaitales

Ø  Fructification in Cordaitales occurred among the leaves.

Ø  The strobili were unisexual and compound in nature.

Ø  Male strobili consisted of a central cone axis bearing numerous spirally arranged scales.

Ø  The lower scales in the male strobilus were sterile.

Ø  Fertile scales were present towards the upper region of the strobilus.

Male Reproductive Structures in Cordaitales

Ø  Each fertile scale bore 4 to 6 microsporangia (pollen sacs).

Ø  Microsporangia were elongate and finger-like in appearance.

Ø  Male gametes were motile, resembling those of cycads.

Ø  The organization of microsporangiate strobili was similar to that of ovulate (female) strobili, with a central axis and spirally arranged scales.

Ø  In the female strobili, the lower scales were sterile and lacked ovules.

Ø  The upper scales bore two or more terminal ovules.

Ø  Ovules were pendulous and recurved.

Ø  Ovules exhibited bilateral symmetry.

Cordaitales: Female Strobilii

Ø  Each ovule had a single integument, which was distinctly two-lobed.

Ø  The integument was completely free from the nucellus.

Ø  A well-defined pollen chamber was present at the apex of the nucellus.

Ø  A nucellar beak or endosperm beak was present.

Ø  The nucellar beak functioned as a tent pole, similar to that found in Ginkgo.

Ø  Archegonia were formed within the ovule.

Ø  The archegonia opened into a distinct archegonial chamber.

Ø  Seeds were bilaterally symmetrical in structure.

Tent Pole of Ginkgo biloba

Ø  Tentpole: Tent-pole is a unique structure of the female gametophyte in Ginkgo biloba Its exact functions in the reproductive process are unclear. Tentpole in G. biloba is commonly situated between two or more archegonia of the female gametophyte. It protrudes toward the archegonial chamber. Researchers have speculated that the tentpole functions as a secretory structure, producing a liquid favorable for the spermatozoid to swim via its flagella toward the archegonium just before fertilization. The cells of the tentpole accumulated abundant starch grains as the female gametophyte developed. Therefore, the tentpole may play an important role in the fertilization process in G. biloba. However, till date, there has been little convincing evidence about the morphology of the tentpole and its specific function in the sexual reproduction process.

Affinities of Cordaitales

Ø  Cordaitales is a unique group of fossil Gymnosperms with similarities towards other groups of Gymnosperms.

Resemblance of Cordaitales with Cycads

Ø  Both exhibit a xerophytic habit, adapted to dry environments.

Ø  The stem in both groups contains a large, well-developed pith.

Ø  Leaves show xerophytic features, such as thick cuticles and sunken stomata.

Ø  The nucellus in both possesses a single integument and contains both a pollen chamber and an archegonial chamber.

Ø  The vascular architecture of the ovules is similar in both Cordaitales and cycads.

Ø  Male gametes are motile in both groups.

Ø  Both produce large seeds, typical of early gymnosperms.

Difference from Cycads

Ø  Cordaitales possess pycnoxylic wood (dense wood with narrow rays and small pith), whereas cycads have manoxylic wood (less dense with wide rays and large pith).

Ø  Leaves of Cordaitales are simple, while cycads typically have pinnately compound leaves.

Ø  Venation in Cordaitales is parallel, in contrast to the open dichotomous or net-like venation seen in cycads.

Ø  Leaf traces in Cordaitales are straight and do not form girdles, unlike the girdling leaf traces commonly observed in cycads.

Ø  Cordaitales exhibit compound strobili, while cycad strobili are generally simple.

Ø  Female strobili are present in Cordaitales, whereas cycads typically bear megasporophylls in loose arrangements rather than organized female cones.

Resemblance of Cordaitales with Ginkgoales

Ø  Both Cordaitales and Ginkgoales possess double leaf traces.

Ø  Motile sperm cells are present in both groups, a primitive characteristic among gymnosperms.

Ø  An endosperm beak (also known as a tent-pole structure) is present in the ovules of both groups.

Resemblance of Cordaitales with Coniferales

Ø  Both groups include tall, branched trees with a dominant central axis.

Ø  Leaves are simple in structure in both Cordaitales and Coniferales.

Ø  A sclerenchymatous hypodermis is present in the leaves of both groups, contributing to their xerophytic adaptations.

Ø  Both exhibit pycnoxylic wood, characterized by dense, compact secondary xylem.

Ø  Resin ducts are absent in both, particularly resembling the condition found in the modern conifer family Araucariaceae.

Reference

Ø  Bhatnagar, S.P. and Moitra, A., 1996. Gymnosperms. New Age International.

Ø  Jin, B., Wang, D., Lu, Y., Zhang, M. and Wang, L., 2012. Structure and function of the tentpole in the reproductive process of Ginkgo biloba L. Plant signaling & behavior, 7(10), pp.1330-1336.

Ø  Vashishta, P.C., 1995. Gymnosperms: Botany for Degree Students, vol. 5. Chand S and Company LTD, New Delhi.

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