Fossil pteridophytes are one of the important groups of ancient vascular plants that provide valuable insights into the evolution of plant life on Earth.
Some of the important fossil pteridophytes are mentioned below-
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Genus Rhynia
Division: Pteridophyta
Class: Rhyniopsida
Order: Rhyniales
Family: Rhyniaceae
Genus: Rhynia
The genus Rhynia is named after the locality Rhynie in Scotland. This genus consists of two species- R. major and R. gwynne-vaughani. The genus was discovered in 1917 by Kidston and Lang from Rhynie.
They are believed to be of Devonian era (about 390-374 million years ago).
R. gwynne-vaughani
Morphology of R. gwynne-vaughani
It was a small herbaceous plant possibly about 18 cm in height, with cylindrical stems and branches arising from a basal rhizome-like portion that was buried in peat.
There were no traces of roots.
The stem was dichotomously branched and bears hemispherical outgrowths. The stem tapers towards its apex and may bear oval sporangia.
The hemispherical growths were sometimes the seats of the formation of additional branches.
Anatomy of R. gwynne-vaughani
The anatomical structure reveals a distinct epidermal layer with a cuticle layer and stomata.
Next to the epidermis lies the cortical region differentiated into the inner and outer cortex. The inner cortex possesses intercellular spaces that are connected to stomata through spaces between cells of the outer cortex.
The center of the stem is occupied by a vascular strand of simpler organization. The xylem is present at the center and the phloem is present in between the xylem and the inner cortex. The xylem is of endarch type i.e. tracheids in the center are smaller than that of the surroundings. Phloem lacks sieve plates.
In the rhizome, the outer and inner cortex is indistinct.
Sporangia of R. gwynne-vaughani
They are club-shaped or cylindrical structures borne terminally on fine aerial branches. The spores were homosporous and trilete.
R. major
It was a wholly leafless plant similar to R. gwynne-vaughani but had some different features. Its height is approx. 20 cm. or more and a diameter of about 6mm. The sporangia are borne at the ends of the branches. It differs from R. gwynne-vaughani as it does not possess hemispherical growths on the stem and adventitious branches. The stem is dichotomously branched and smooth in outline.
The internal structure is similar to that of R. gwynne-vaughani. Stele is protostele.
Sporangia are present on the terminal position of the stem.
Genus Cooksonia
Division– Pteridophyta
Class- Rhyniopsida
Order- Rhyniales
Family- Cooksoniaceae
Genus- Cooksonia
It is an extinct grouping of primitive land plants, ranging in age from middle-upper Silurian to lower Devonian. Cooksonia fossils are distributed globally and described from places like Ireland, South Wales, Scotland, Germany, Czechoslovakia, Russia, N. Africa and N. America. Cooksonia is the oldest known plant to have a stem with vascular tissue. Cooksonia pertoni, Cooksonia paranensis, Cooksonia acuminate and Cooksonia cambrensis are the species.
Morphology of Cooksonia
Only the sporophyte phase of Cooksonia is currently known. Individuals were small, a few centimeters tall, and had a simple structure. They had a simple stalk that branched dichotomously a few times.
Each branch ended in a sporangium. Sporangia were more-or-less trumpet-shaped with a ‘lid’ or operculum which disintegrates to release the spores. The existence of four different types of spores in C. pertoni has been proven from smooth to ornamented ones.
Much less has been known regarding the anatomy of the stem and internal structure of the sporangium.
Genus Yarravia
Division: Pteridophyta
Class: Rhyniopsida
Order: Rhyniales
Family: Rhyniaceae
Genus: Yarravia
This plant belongs to the middle Silurian period. There are two species, they are Y. subspherica and Y. oblonga.
Morphology of Yarravia
Stem is simple and dichotomously branched, stems similar to other early vascular plants. The stem is smooth without any leaves or scales.
Terminal sporangia located at the ends of branches.
Sporangia were cylindrical to spherical, containing spores for reproduction.
Sporangia opened to release spores, facilitating reproduction and dispersal. They also had rhizoids or root-like structures to anchor the plant and assist in water and nutrient absorption.
Anatomy of Yarravia
The internal structure is almost similar to Rhynia. It consists of a central strand of vascular tissue (xylem), vital for water and nutrient transport along with other tissues. The xylem was more advanced than that of earlier plants like Cooksonia, indicating evolutionary progression.
Genus Hicklingia
Division: Pteridophyta
Class: Rhyniopsida
Order: Rhyniales
Family: Rhyniaceae
Genus: Hicklingia
It was discovered in Caithness in Scotland. They are believed to be of Devonian era.
Morphology of Hicklingia
The plant was simple and leafless; having dichotomously branched stems similar to early vascular plants like Rhynia and Cooksonia. Stems were narrow, indicating a primitive vascular structure. The surface of the stems was smooth, without any leaves or scales.
Anatomy of Hicklingia
It contained a central strand of vascular tissue (xylem), essential for the transport of water and nutrients. The type of stele found was protostele. The xylem showed a primitive structure, with tracheids having simple thickenings.
Sporangia of Hicklingia
Sporangia were terminal, located at the tips of the branches. They were oval and like a coil in shape and contained spores.
Genus Protolepidodendron
Division: Pteridophyta
Class: Lycopodiopsida
Order: Protolepidodendrales
Family: Protolepidodendraceae
Genus: Protolepidodendron
Protolepidodendron were believed to be of the Middle Devonian to early Carboniferous era.
Morphology of Protolepidodendron
The plants were about 30 meters in height and 1 cm in diameter.
The plant body consists of both prostrate and aerial axes. Prostrate branches are dichotomously branched giving rise to aerial axes.
The whole plant body is covered with small leaves which are bifurcated at their apices; it is the unique feature of Protolepidodendron.
Small, needle-like, or scale-like leaves are called microphylls. These leaves were arranged spirally along the stem. Some of the leaves have radially elongated sporangia on their upper surface.
They possess true roots, essential for anchoring the plant and absorbing water and nutrients from the soil.
Anatomy of Protolepidodendron
Internal anatomy is unknown. However, it is noteworthy that the vascular bundle is a more advanced type. Triangular protostele is found in the stem which is uncommon among modern lycopods.
Sporangia of Protolepidodendron
Sporangia are found in association with leaves present in certain areas of the stem representing the cone of the plant.
Genus Lepidodendron
Division: Pteridophyta
Class: Lycopodiopsida
Order: Lepidodendrales
Family: Lepidodendraceae
Genus: Lepidodendron
It is the name assigned to ancient Lycopods known as the “scale tree.” It was one of the most common trees during the Carboniferous era.
Morphology of Lepidodendron
Lepidodendron trees could reach heights of up to 30 meters (98 feet) or more, making them among the tallest plants of their time.
The trunk was almost 6 feet or even more in diameter. The trunk was tall, straight, and covered with a spirally arranged distinctive diamond-shaped pattern of leaf scars. These scars are remnants of the fallen leaves and are known as leaf cushions, giving the trunk a characteristic textured appearance.
The leaves, known as microphylls, were long, narrow, and spirally arranged around the stem. They were typically up to 1 meter (3.3 feet) in length.
Anatomy of Lepidodendron
Stem anatomy showed the proportion of xylem wood is relatively small because secondary growth is mainly confined to the cortical zone.
Sporangia of Lepidodendron
Sporangia were borne on specialized leaves called sporophylls which were arranged in large, cone-like structures called strobili, which were located at the tips of branches. Lepidodendron exhibited heterospory, producing two types of spores i.e. microspores and megaspores.
References
- N, M. (2016, October 17). Notes on Protolepidodendrales | Plants. Biology Discussion. https://www.biologydiscussion.com/plants/notes-on-protolepidodendrales-plants/58753
- Cooksonia. (n.d.). https://steurh.home.xs4all.nl/engcook/ecooks.html
- Sam Noble Museum. (2018, January 30). Fossil lycophytes – Sam Noble Museum. Sam Noble Museum – the Sam Noble Museum at the University of Oklahoma Inspires Minds to Understand the World Through Collection-based Research, Interpretation, and Education. https://samnoblemuseum.ou.edu/common-fossils-of-oklahoma/plant-fossils/fossils-by-plant-group/fossil-lycophytes/
- Hait, G., Bhattacharya, K., & Ghosh, A. K. (2012). A textbook of Botany, Volume I.
- Pteridophyta by B.R. Vashishta, A.K. Sinha, Adarsh Kumar (S.Chand & Company ltd)
- Bose, M.N. and Roy, S.K. (1963). Paleobotanist. 12 : 226-228
- Arens, N. C. (1998, July 20). Lepidodendron | carboniferous, lycopodiophyta, tree-like. Encyclopedia Britannica. https://www.britannica.com/plant/Lepidodendron
- Gerrienne, P., Dilcher, D. L., Bergamaschi, S., Milagres, I., Pereira, E., & Rodrigues, M. a. C. (2006). An exceptional specimen of the early land plant Cooksonia paranensis, and a hypothesis on the life cycle of the earliest eutracheophytes. Review of Palaeobotany and Palynology, 142(3–4), 123–130. https://doi.org/10.1016/j.revpalbo.2006.05.005
- Fossil flora | School of Geosciences | The University of Aberdeen. (n.d.). https://www.abdn.ac.uk/geosciences/departments/geology/fossil-flora-1901.php