This page covers the stem group of the vascular plants (Phylum Tracheophyta, Superphylum Embryophyta). A summary of the phylogeny of this clade is shown below:
Figure 1. Summarized phylogenetic tree of the vascular plants
For ease of reference, the stem lines shown in the above tree are numbered as shown below:
Figure 2. Branch numbering scheme
The pages corresponding to the branches shown can be found by clicking the links in the following table:
Branch number |
Branch name |
Page link |
2 |
Stem-Tracheophyta |
This page |
2-1 |
Stem-Lycophyta |
|
2-2 |
Stem-Lycopodiales |
No stem-group fossils known |
2-3 |
Stem-(Isoetales+Selaginellales) |
|
2-4 |
Stem-Selaginellales |
No stem-group fossils known |
2-5 |
Stem-Isoetales |
|
2-6 |
Stem-Euphyllophyta |
|
2-7 |
Stem-Monilophyta |
No consensus on stem group |
2-8 |
Stem-(Ophioglossidae+Equisetales) |
No stem-group fossils known |
2-9 |
Stem-Equisetales |
|
2-10 |
Stem-Ophioglossidae |
No stem-group fossils known |
2-11 |
Stem-Psilotales |
No known fossil record |
2-12 |
Stem-Ophioglossales |
No published stem-group phylogeny |
2-13 |
Stem-(Polypodiidae+Marattiales) |
No stem-group fossils known |
2-14 |
Stem-Marattiales |
|
2-15 |
Stem-Polypodiidae |
No published stem-group phylogeny |
2-16 |
Stem-Osmundales |
|
2-17 |
Stem-(Hymenophyllales to Polypodiales) |
No stem-group fossils known |
2-18 |
Stem-(Gleicheniales+Hymenophyllales) |
No stem-group fossils known |
2-19 |
Stem-Hymenophyllales |
No published stem-group phylogeny |
2-20 |
Stem-Gleicheniales |
No stem-group fossils known |
2-21 |
Stem-(Schizaeales to Polypodiales) |
No stem-group fossils known |
2-22 |
Stem-Schizaeales |
No published stem-group phylogeny |
2-23 |
Stem-(Salviniales to Polypodiales) |
No stem-group fossils known |
2-24 |
Stem-(Salviniales+Cyatheales) |
No stem-group fossils known |
2-25 |
Stem-Salviniales |
No published stem-group phylogeny |
2-26 |
Stem-Cyatheales |
No stem-group fossils known |
2-27 |
Stem-Polypodiales |
No published stem-group phylogeny |
2-28 |
Stem-Spermatophyta |
This page thus covers Branch 2, along which are found the stem-group tracheophytes.
The primary synapomorphy of the tracheophyte crown group is the presence of vascular tissues (Morris et al, 2018). Furthermore, the walls of the water-conducting cells in the xylem have a thick, lignified, decay-resistant layer (De Queiroz et al, 2020). These synapomorphies represent evolutionary novelties that developed along the tracheophyte stem line after the split from the bryophyte clade.
The phylogeny of the stem-Tracheophyta is still contentious. Various proposals have been made in recent years (e.g. Cascales‐Miñana and Gerrienne, 2017; Cascales-Miñana et al, 2019; Niklas and Crepet, 2020). The version of Niklas and Crepet (2020) is generally representative of the consensus developed over the last few decades (following Kenrick and Crane, 1997); it is summarized in the following time tree:
The primary synapomorphy of the tracheophyte crown group is the presence of vascular tissues (Morris et al, 2018). Furthermore, the walls of the water-conducting cells in the xylem have a thick, lignified, decay-resistant layer (De Queiroz et al, 2020). These synapomorphies represent evolutionary novelties that developed along the tracheophyte stem line after the split from the bryophyte clade.
The phylogeny of the stem-Tracheophyta is still contentious. Various proposals have been made in recent years (e.g. Cascales‐Miñana and Gerrienne, 2017; Cascales-Miñana et al, 2019; Niklas and Crepet, 2020). The version of Niklas and Crepet (2020) is generally representative of the consensus developed over the last few decades (following Kenrick and Crane, 1997); it is summarized in the following time tree:
Figure 3. Time tree of the stem-Tracheophyta
The oldest known member of the tracheophyte total group is Cooksonia barrandei (Libertín et al, 2018a), found in the Motol Formation of mid-Silurian (Wenlock) age at the Špičatý vrch - Barrandovy jámy fossil site in the Czech Republic (Libertín et al, 2018). This is assumed here to be a stem tracheophyte based on its belonging to the same genus as the generally recognized stem-group tracheophyte Cooksonia pertoni (Edwards and Kenrick, 2015). Several species of the genus Cooksonia are illustrated below, together with other members of the stem group included in the above time tree (for a larger view, click on image):
Names in red indicate that the fossil is younger than the oldest known crown-group fossil.
Figure 4. Images of stem-Tracheophyta
The above images are numbered in order from the most basal to those closest to the crown group, but few obvious changes can be seen apart from a tendency to a greater degree of branching and the development of rhizomes later in the series. All of these plants were quite small, growing to a height of generally less than 20 cm.
The available fossil data indicate that the tracheophyte stem group developed during the Wenlock and Ludlow epochs of the Silurian, representing a stem-to-crown transition of only about 8 million years (see Figure 1). Most of the stem-group fossils shown above were found in NE Scotland in the Rhynie Chert lagerstätte of Early Devonian (Pragian – Emsian) age (Garwood et al, 2020; Kerp, 2018). These fossils are younger than the oldest known member of the crown tracheophyte (the stem lycophyte Zosterophyllum sp., of Ludlow age).
The available fossil data indicate that the tracheophyte stem group developed during the Wenlock and Ludlow epochs of the Silurian, representing a stem-to-crown transition of only about 8 million years (see Figure 1). Most of the stem-group fossils shown above were found in NE Scotland in the Rhynie Chert lagerstätte of Early Devonian (Pragian – Emsian) age (Garwood et al, 2020; Kerp, 2018). These fossils are younger than the oldest known member of the crown tracheophyte (the stem lycophyte Zosterophyllum sp., of Ludlow age).
References
Cascales‐Miñana, B., & Gerrienne, P. (2017). Teruelia diezii gen. et sp. nov.: an early polysporangiophyte from the Lower Devonian of the Iberian Peninsula. Palaeontology, 60(2), 199-212.
Cascales-Miñana, B., Steemans, P., Servais, T., Lepot, K., & Gerrienne, P. (2019). An alternative model for the earliest evolution of vascular plants. Lethaia, 52(4), 445-453.
De Queiroz, K., Cantino, P. D., & Gauthier, J. A. (Eds.). (2020). Phylonyms: a Companion to the PhyloCode. CRC Press.
Edwards, D., & Kenrick, P. (2015). The early evolution of land plants, from fossils to genomics: a commentary on Lang (1937)‘On the plant-remains from the Downtonian of England and Wales'. Philosophical Transactions of the Royal Society B: Biological Sciences, 370(1666), 20140343.
Garwood, R. J., Oliver, H., & Spencer, A. R. (2020). An introduction to the Rhynie chert. Geological Magazine, 157(1), 47-64.
Kenrick, P., & Crane, P. R. (1997). The origin and early evolution of plants on land. Nature, 389(6646), 33-39.
Kerp, H. (2018). Organs and tissues of Rhynie chert plants. Philosophical Transactions of the Royal Society B: Biological Sciences, 373(1739), 20160495.
Libertín, M., Kvaček, J., Bek, J., Žárský, V., & Štorch, P. (2018). Sporophytes of polysporangiate land plants from the early Silurian period may have been photosynthetically autonomous. Nature plants, 4(5), 269-271.
Morris, J. L., Puttick, M. N., Clark, J. W., Edwards, D., Kenrick, P., Pressel, S., ... & Donoghue, P. C. (2018). The timescale of early land plant evolution. Proceedings of the National Academy of Sciences, 115(10), E2274-E2283.
Niklas, K. J., & Crepet, W. L. (2020). Morphological (and not anatomical or reproductive) features define early vascular plant phylogenetic relationships. American Journal of Botany, 107(3), 477-488.
Cascales-Miñana, B., Steemans, P., Servais, T., Lepot, K., & Gerrienne, P. (2019). An alternative model for the earliest evolution of vascular plants. Lethaia, 52(4), 445-453.
De Queiroz, K., Cantino, P. D., & Gauthier, J. A. (Eds.). (2020). Phylonyms: a Companion to the PhyloCode. CRC Press.
Edwards, D., & Kenrick, P. (2015). The early evolution of land plants, from fossils to genomics: a commentary on Lang (1937)‘On the plant-remains from the Downtonian of England and Wales'. Philosophical Transactions of the Royal Society B: Biological Sciences, 370(1666), 20140343.
Garwood, R. J., Oliver, H., & Spencer, A. R. (2020). An introduction to the Rhynie chert. Geological Magazine, 157(1), 47-64.
Kenrick, P., & Crane, P. R. (1997). The origin and early evolution of plants on land. Nature, 389(6646), 33-39.
Kerp, H. (2018). Organs and tissues of Rhynie chert plants. Philosophical Transactions of the Royal Society B: Biological Sciences, 373(1739), 20160495.
Libertín, M., Kvaček, J., Bek, J., Žárský, V., & Štorch, P. (2018). Sporophytes of polysporangiate land plants from the early Silurian period may have been photosynthetically autonomous. Nature plants, 4(5), 269-271.
Morris, J. L., Puttick, M. N., Clark, J. W., Edwards, D., Kenrick, P., Pressel, S., ... & Donoghue, P. C. (2018). The timescale of early land plant evolution. Proceedings of the National Academy of Sciences, 115(10), E2274-E2283.
Niklas, K. J., & Crepet, W. L. (2020). Morphological (and not anatomical or reproductive) features define early vascular plant phylogenetic relationships. American Journal of Botany, 107(3), 477-488.
Image credits – stem-Tracheophtya
- Header (Woody Dicot Stem: Primary Phloem and Xylem in One Year Tilia): Berkshire Community College Bioscience Image Library, CC0, via Wikimedia Commons
- Figure 4 (Aglaophyton major): Open Access article Hetherington, A. J., & Dolan, L. (2018). Bilaterally symmetric axes with rhizoids composed the rooting structure of the common ancestor of vascular plants. Philosophical Transactions of the Royal Society B: Biological Sciences, 373(1739), 20170042.
- Figure 4 (Cooksonia barrandei): Open Access article Pšenička, J., Bek, J., Frýda, J., Žárský, V., Uhlířová, M., & Štorch, P. (2021). Dynamics of Silurian plants as response to climate changes. Life, 11(9), 906.
- Figure 4 (Cooksonia sp., fossil): Open Access article Pšenička, J., Bek, J., Frýda, J., Žárský, V., Uhlířová, M., & Štorch, P. (2021). Dynamics of Silurian plants as response to climate changes. Life, 11(9), 906.
- Figure 4 (Cooksonia sp. life restoration): MUSE, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 4 (Cooksonia cf. hemisphaerica): Open Access article Pšenička, J., Bek, J., Frýda, J., Žárský, V., Uhlířová, M., & Štorch, P. (2021). Dynamics of Silurian plants as response to climate changes. Life, 11(9), 906.
- Figure 4 (Nothia aphylla): Open Access article Hetherington, A. J., & Dolan, L. (2018). Bilaterally symmetric axes with rhizoids composed the rooting structure of the common ancestor of vascular plants. Philosophical Transactions of the Royal Society B: Biological Sciences, 373(1739), 20170042.
- Figure 4 (Rhynia gwynne-vaughani, fossil): Peter Coxhead, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 4 (Rhynia gwynne-vaughani, fossil and reconstructions): Open Access article Hetherington, A. J., & Dolan, L. (2018). Bilaterally symmetric axes with rhizoids composed the rooting structure of the common ancestor of vascular plants. Philosophical Transactions of the Royal Society B: Biological Sciences, 373(1739), 20170042.
- Figure 4 (Rhynia gwynne-vaughani, life restoration): MUSE, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 4 (Horneophyton lignieri, fossil with several corms or tubers): Peter Coxhead, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 4 (Horneophyton lignieri, fossil with magnified view of a single corm or tuber): Peter Coxhead, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 4 (Horneophyton lignieri, fossil and reconstructions): Open Access article Hetherington, A. J., & Dolan, L. (2018). Bilaterally symmetric axes with rhizoids composed the rooting structure of the common ancestor of vascular plants. Philosophical Transactions of the Royal Society B: Biological Sciences, 373(1739), 20170042.