Lycopods are a primitive group of vascular plants which was dominant on Earth in the mid to late Palaeozoic. Once including a diversity from small herbs (herbaceous) to great trees, their living relatives are reduced to only a few herbaceous genera: Isoetes, Lycopodium, Huperzia and Selaginella. Lycopods and their ancestors are grouped in Division Lycophyta, that is the sister group to all other vascular plants (1).
Lycopods bear their leaves in a helix on the stem. Their leaves are uniquely simple and primitive, being fed only by a single vascular bundle, and are termed microphylls (2).
Lycopods were one of the most abundantly preserved plant groups in the Devonian fossil record. In South Africa they are typically known from casts of stems preserved in sandstone. Aside from the patterns of leaf scars on the stems, the anatomical details are usually obscure. In the Witpoort Formation however, some well-preserved specimens are providing a new look at lycopod diversity in the South African Late Devonian. These include Kowieria alveoformis, Leptophloeum rhombicum, Colpodexylon pullumpedes and Colpodexylon mergae as well as a number of as yet undescribed species.
Lycopods bear their leaves in a helix on the stem. Their leaves are uniquely simple and primitive, being fed only by a single vascular bundle, and are termed microphylls (2).
Lycopods were one of the most abundantly preserved plant groups in the Devonian fossil record. In South Africa they are typically known from casts of stems preserved in sandstone. Aside from the patterns of leaf scars on the stems, the anatomical details are usually obscure. In the Witpoort Formation however, some well-preserved specimens are providing a new look at lycopod diversity in the South African Late Devonian. These include Kowieria alveoformis, Leptophloeum rhombicum, Colpodexylon pullumpedes and Colpodexylon mergae as well as a number of as yet undescribed species.
Kowieria alveoformis
Locality: Waterloo Farm Age: Latest Devonian This new genus and species was described from the leafy twigs, cones and scattered leaves of what may have been a small tree lycopod (3). It would have grown on the shores of the Waterloo lagoon. Its leaves and strobili were regularly dropped and then buried in the muddy sediments at the bottom of the lagoon. Kowieria, like all lycopods, has leaves inserted helically about the stem, and its sporangia are held on the leaves. The fertile leaves are modified with respect to the vegetative leaves to have a spoon-like base which holds the male and female sporangia, and are densely packed on the ends of branches to form cone-like structures. Kowieria is the first fertile lycopod to have been described from Devonian of South Africa. This is surprising, since lycopods were relatively common in the Devonian, but it shows how rare the type of preservation necessary to preserve fertile structures is. Kowieria has two kinds of spores, microspores and megaspores (heterosporous). As such, it is most likely allied with the arborescent (tree-like) heterosporous lycopods, which had their heyday in the subsequent Carboniferous Period, having first appeared in the Mid-to- Late Devonian. |
Leptophloeum rhombicum
Locality: Waterloo Farm and upper Witpoort Formation
Age: Latest Devonian
Leptophloeum is an extinct species of arborescent (tree-like) lycopod which had a worldwide distribution in the Late Devonian. It was especially abundant in Gondwana and China (4). It is identified by its characteristic rhombic leaf bases, each with a central leaf scar, which would have borne a leaf. The leaves were shed as the plant grew, and as yet no leaf has been found attached to a Leptophloeum stem in South Africa. The stems are commonly preserved in quartzites of the Witpoort Formation, and their characteristic leaf scars have led farmers in the Cape to name them 'the Devil's bicycle tracks'.
Trunks of this small tree, up to around 5cm thick, are commonly preserved in the sandstones interfingering with the black lagoonal shales at Waterloo Farm (5). The stems are interpreted to have been washed out during flooding and bank collapse and buried on the sandbar deposits. Some of these specimens preserve roots, which despite its worldwide occurrence is quite unique for this plant. Outside of Waterloo Farm, only a single other example of the rooting system of Leptophloeum is known.
The rooting systems of lycopods are important for understanding their higher relationships. Based on poorly preserved Chinese fossil material, Leptophloeum was considered to have roots of the stigmarian type. This is contradicted by the Waterloo Farm specimens which show a cormose rooting system (5).
Based on specimens from China, Leptophloeum rhombicum has been reconstructed as a 25 metre tall tree with a trunk of 30-40 cm thick (4). The South African specimens are much smaller, up to 5cm thick, and are reconstructed as having had an unbranched trunk with a bristle of leaves at the tip
Locality: Waterloo Farm and upper Witpoort Formation
Age: Latest Devonian
Leptophloeum is an extinct species of arborescent (tree-like) lycopod which had a worldwide distribution in the Late Devonian. It was especially abundant in Gondwana and China (4). It is identified by its characteristic rhombic leaf bases, each with a central leaf scar, which would have borne a leaf. The leaves were shed as the plant grew, and as yet no leaf has been found attached to a Leptophloeum stem in South Africa. The stems are commonly preserved in quartzites of the Witpoort Formation, and their characteristic leaf scars have led farmers in the Cape to name them 'the Devil's bicycle tracks'.
Trunks of this small tree, up to around 5cm thick, are commonly preserved in the sandstones interfingering with the black lagoonal shales at Waterloo Farm (5). The stems are interpreted to have been washed out during flooding and bank collapse and buried on the sandbar deposits. Some of these specimens preserve roots, which despite its worldwide occurrence is quite unique for this plant. Outside of Waterloo Farm, only a single other example of the rooting system of Leptophloeum is known.
The rooting systems of lycopods are important for understanding their higher relationships. Based on poorly preserved Chinese fossil material, Leptophloeum was considered to have roots of the stigmarian type. This is contradicted by the Waterloo Farm specimens which show a cormose rooting system (5).
Based on specimens from China, Leptophloeum rhombicum has been reconstructed as a 25 metre tall tree with a trunk of 30-40 cm thick (4). The South African specimens are much smaller, up to 5cm thick, and are reconstructed as having had an unbranched trunk with a bristle of leaves at the tip
Colpodexylon pullumpedes
Locality: Waterloo Farm Age: Latest Devonian This herbaceous lycopod genus, which is characterized by its three-tipped leaves, was previously thought to have been extinct by the Late Devonian, but research on the Witpoort Formation shows that Colpodexylon lived on in sub-polar Gondwana for at least ten million years longer until the latest Devonian (360 million years ago), indicating that the genus had a high latitude refugium (6). C. pullumpedes is a 'short-tipped' species that is only known from Waterloo Farm. It had slender bifrucating axes about 1/2 to 1 cm wide bearing trident like leaves with trifurcate tips resembling chickens feet, hence the name pullumpedes. |
Colpodexylon mergae
Locality: Coombs Hill
Age: Late Devonian
Although at least three lycopod species occur at Coombs Hill, the only one formally described is Colpodexylon mergae (6). This is a herbaceous lycopod with long tipped trifurcate (with three tips) leaves. It is one of the more common plants fossilized at Coombs Hill. The name means 'a hayfork' due to the appearance of the leaves.
Locality: Coombs Hill
Age: Late Devonian
Although at least three lycopod species occur at Coombs Hill, the only one formally described is Colpodexylon mergae (6). This is a herbaceous lycopod with long tipped trifurcate (with three tips) leaves. It is one of the more common plants fossilized at Coombs Hill. The name means 'a hayfork' due to the appearance of the leaves.
References:
1) Gensel, P. G., & Berry, C. M. (2001). Early lycophyte evolution. American Fern Journal, 91(3), 74-99.
2) Taylor T. N., Taylor E. L., Krings M., (2009). Paleobotany: The biology and evolution of fossil
plants. Elsevier
3) Gess, R. W., & Prestianni, C. (2018). Kowieria alveoformis gen. nov. sp. nov., a new heterosporous lycophyte from the Latest Devonian of Southern Africa. Review of Palaeobotany and Palynology, 249, 1-8.
4) Wang, Q., Geng, B. Y., & Dilcher, D. L. (2005). New perspective on the architecture of the Late Devonian arborescent lycopsid Leptophloeum rhombicum (Leptophloeaceae). American journal of botany, 92(1), 83-91.
5) Prestianni, C., & Gess, R. W. (2014). The rooting system of Leptophloeum Dawson: new material from the upper Devonian, Famennian Witpoort Formation of South Africa. Review of palaeobotany and palynology, 209, 35-40.
6) Harris, C., Gess, R. W., Prestianni, C., & Bamford, M. K. (2021). A Late Devonian refugium for Colpodexylon (Lycopsida) at high latitude. Review of Palaeobotany and Palynology, 293, 104481.
1) Gensel, P. G., & Berry, C. M. (2001). Early lycophyte evolution. American Fern Journal, 91(3), 74-99.
2) Taylor T. N., Taylor E. L., Krings M., (2009). Paleobotany: The biology and evolution of fossil
plants. Elsevier
3) Gess, R. W., & Prestianni, C. (2018). Kowieria alveoformis gen. nov. sp. nov., a new heterosporous lycophyte from the Latest Devonian of Southern Africa. Review of Palaeobotany and Palynology, 249, 1-8.
4) Wang, Q., Geng, B. Y., & Dilcher, D. L. (2005). New perspective on the architecture of the Late Devonian arborescent lycopsid Leptophloeum rhombicum (Leptophloeaceae). American journal of botany, 92(1), 83-91.
5) Prestianni, C., & Gess, R. W. (2014). The rooting system of Leptophloeum Dawson: new material from the upper Devonian, Famennian Witpoort Formation of South Africa. Review of palaeobotany and palynology, 209, 35-40.
6) Harris, C., Gess, R. W., Prestianni, C., & Bamford, M. K. (2021). A Late Devonian refugium for Colpodexylon (Lycopsida) at high latitude. Review of Palaeobotany and Palynology, 293, 104481.