Summary
The following figure is a summarized phylogenetic tree for the lycophytes, showing that the tree is well documented by transitional fossils. As will be seen in the following sections, each of the red stars shown below represents multiple transitional fossils.
Introduction
The lycophytes (Division Lycophyta in clade Tracheophyta) are spore-bearing plants that differ from bryophytes in having a sporophyte generation that is independent of, and much larger than, the gametophyte stage (Encyclopaedia Britannica). They share this characteristic with ferns, but they differ from ferns, and from all other vascular plants, in having microphylls – leaves that have just one vein, as opposed to the megaphylls of ferns and seed plants (University of Reading web page http://blogs.reading.ac.uk/tropical-biodiversity/2013/03/selaginellaceae-the-spike-moss-family). The lycophytes are represented at the present day by two classes, the Lycopodiopsida (the club mosses) and the Isoetopsida (the quillworts and the spike mosses). Their phylogeny (combined from Rothwell et al, 2014 and PPG I, 2016) is summarized in the above figure, in which the crown node of the lycophytes is represented by a black dot.
At the present day there are more than 1,200 species of lycophyte (Encyclopedia Britannica), divided into three families, the Lycopodiaceae (representing the crown-Lycopodiopsida), the Isoetaceae (crown-Isoetales) and the Selaginellaceae (crown-Selaginellales). The Lycopodiaceae are homosporous, which means that they produce only one kind of spore, while the Isoetaceae and the Selaginellaceae produce two kinds of spore and are thus heterosporous. The primary difference between the latter two families is that the Isoetaceae bear their different spore types on different plants, whereas Selaginellaceae bear both spore types on the same plant (University of Reading web page cited above).
The photograph shown above in the header shows a modern member of the crown-Lycopodiopsida, while representatives of the crown-Isoetales and the crown-Selaginellales are shown below (click on image for larger view):
At the present day there are more than 1,200 species of lycophyte (Encyclopedia Britannica), divided into three families, the Lycopodiaceae (representing the crown-Lycopodiopsida), the Isoetaceae (crown-Isoetales) and the Selaginellaceae (crown-Selaginellales). The Lycopodiaceae are homosporous, which means that they produce only one kind of spore, while the Isoetaceae and the Selaginellaceae produce two kinds of spore and are thus heterosporous. The primary difference between the latter two families is that the Isoetaceae bear their different spore types on different plants, whereas Selaginellaceae bear both spore types on the same plant (University of Reading web page cited above).
The photograph shown above in the header shows a modern member of the crown-Lycopodiopsida, while representatives of the crown-Isoetales and the crown-Selaginellales are shown below (click on image for larger view):
We will now consider the evolution of the stem lines of the tracheophytes and the lycophytes, shown as blue and red lines respectively in the following phylogenetic tree:
The above tree is summarized in the context of geological time in the figure below:
This time tree is constrained by fossils that will be discussed below. It indicates that the lycophyte stem line split from that of the bryophytes before the middle of the Ordovician and continued into the Devonian. This implies that the development of the lycophyte stem line could have occupied as many as 77 million years. It also shows that the vascular plants have a ghost lineage, of 37 million years, resulting in the difference in times of first appearance of the stem-Bryophyta and the stem-group vascular plants.
We will examine the evolution of the lycophytes by breaking the stem line into the following three stages:
We will examine the evolution of the lycophytes by breaking the stem line into the following three stages:
- The stem-Tracheophyta, or the stem group of the vascular plants.
- The stem-Lycophyta
- The crown-Lycophyta
The stem group of vascular plants
The phylogeny of the stem-Tracheophyta is suggested by the following tree, derived from combination of relationships presented in two recent publications:
The oldest known unequivocal vascular plant 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, 2018b). 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). No public-domain images of Cooksonia barrandei are available, but images of slightly younger Silurian species are shown below together with other members of the tracheophyte stem group (for a larger view, click on any image):
Names in red indicate that the fossil is younger than the oldest known crown-group fossil.
* after name indicates that the image represents a life restoration.
* after name indicates that the image represents a life restoration.
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 earliest crown-group tracheophyte is Zosterophyllum sp., a total-group lycophyte found in beds equivalent to the Bathurst Island and Stuart Bay Formations of Late Silurian (Ludlow) age, exposed 14 km north of Polar Bear Pass research station, eastern Bathurst Island, Arctic Canada (Clarke et al, 2011; Morris et al, 2018). No images of this fossil are available in the public domain, but an image of a Devonian example of the genus, and a reconstruction of a Late Silurian example, are shown below:
The earliest crown-group tracheophyte is Zosterophyllum sp., a total-group lycophyte found in beds equivalent to the Bathurst Island and Stuart Bay Formations of Late Silurian (Ludlow) age, exposed 14 km north of Polar Bear Pass research station, eastern Bathurst Island, Arctic Canada (Clarke et al, 2011; Morris et al, 2018). No images of this fossil are available in the public domain, but an image of a Devonian example of the genus, and a reconstruction of a Late Silurian example, are shown below:
Zosterophyllum species generally grew to a height of 15 cm or less.
The stem group of the vascular plants evolved from mid-Silurian to Early Devonian time, as shown in the time tree below:
The stem group of the vascular plants evolved from mid-Silurian to Early Devonian time, as shown in the time tree below:
The above tree indicated that most of the stem-group fossils post-date the appearance of the crown group; these fossils represent branches of the stem line that continued to evolve after the crown-group had appeared. In fact, the crown group appeared well before the end of the Silurian; according to the available fossil data, the stem-to-crown transition of the vascular plants lasted no more than 10 million years.
The stem group of the lycophytes
The following figure presents a recently-published phylogenetic tree of the stem-Lycophyta:
The oldest stem-group lycophyte shown in the above tree belongs to the genus Baragwanathia. This is based on the assumption that the unnamed Baragwanathia species analyzed by the authors of the above tree (Niklas and Crepet, 2020) is comparable to the oldest known species, which is Baragwanathia brevifolia. This species was found in the Late Silurian (Pridoli) Požáry Formation at Karlštejn – Ve Spáleném in the Czech Republic (Kraft and Kvaček, 2017). No image is available in the public domain, but Baragwanathia longifolia is shown below, together with other members of the stem group for which public-domain images are available (click on image for larger view):
* after name indicates that the image represents a life restoration.
The above images are numbered in order from the most basal to those closest to the crown group. The most obvious changes are the later development of sporangia in lateral positions on the stem (compare Zosterophyllum and Sawdonia), and the scaly leaf-like covering of the stem in Asteroxylon. Most of these plants were around the same size as the stem-Tracheophyta (i.e. less than 20 cm), but the last in the series, Asteroxylon, grew to a height of 50 cm.
The oldest known member of the lycophyte crown group is Leclercqia complexa, a stem-group isoetopsidan found in the Early Devonian (Emsian) Campbellton Formation on the south shore of the Restigouche River, between Dalhousie and Campbellton, New Brunswick, eastern Canada (Gensel and Albright, 2006; Morris et al, 2018). A poor-quality image of a fossil and a life restoration are shown below:
The oldest known member of the lycophyte crown group is Leclercqia complexa, a stem-group isoetopsidan found in the Early Devonian (Emsian) Campbellton Formation on the south shore of the Restigouche River, between Dalhousie and Campbellton, New Brunswick, eastern Canada (Gensel and Albright, 2006; Morris et al, 2018). A poor-quality image of a fossil and a life restoration are shown below:
Leclercqia complexa specimens reach lengths of 46 cm (Taylor et al, 2009).
The stem group of the lycophytes evolved from Late Silurian to Early Devonian time, as shown in the time tree below:
The stem group of the lycophytes evolved from Late Silurian to Early Devonian time, as shown in the time tree below:
The above tree indicates that the stem-to-crown transition for the Lycophyta took place over a period of between 11 and 30 million years.
The crown group of the lycophytes
The phylogeny of the crown-Lycophyta is illustrated in the following tree, which is derived from a number of publications:
Given the large number of branches on the above tree, we shall break it down into two parts: firstly the stem-Isoetopsida and secondly the stem-Isoetales. No stem-Lycopodiopsida or stem-Selaginellales have been recognized.
The earliest known member of the stem-Isoetopsida, also known as the Protolepidodendrales, is a species of the genus Estinnophyton. Again, the authors of the relevant part of the phylogenetic tree (Niklas and Crepet, 2020) do not specify the species of Estinnophyton that they used in their morphological analysis, but one of the oldest-known species is Estinnophyton yunnanense, described from the Early Devonian (Late Pragian) Posongchong Formation at Zhichang village, Gumu Town, Wenshan District, Yunnan Province, China (Hao et al, 2004). Images of this species and of others in the stem group for which public-domain images are available are shown below (for a larger view, click on any image):
The earliest known member of the stem-Isoetopsida, also known as the Protolepidodendrales, is a species of the genus Estinnophyton. Again, the authors of the relevant part of the phylogenetic tree (Niklas and Crepet, 2020) do not specify the species of Estinnophyton that they used in their morphological analysis, but one of the oldest-known species is Estinnophyton yunnanense, described from the Early Devonian (Late Pragian) Posongchong Formation at Zhichang village, Gumu Town, Wenshan District, Yunnan Province, China (Hao et al, 2004). Images of this species and of others in the stem group for which public-domain images are available are shown below (for a larger view, click on any image):
Names in red indicate that the fossil is younger than the oldest known crown-group fossil.
* after name indicates that the image represents a life restoration.
* after name indicates that the image represents a life restoration.
The above images are numbered in order from the most basal to those closest to the crown group, but no obvious trends can be seen. They are thought to have been mainly herbaceous (i.e. without woody stems) plants with a creeping habit (Liu et al, 2013).
The earliest known member of the crown-Isoetopsida is Lepidosigillaria whitei, found in the Middle Devonian (Givetian) Moscow Formation near Summit, New York, U.S.A. (Retallack, 2018; Pereira et al, 2017). In contrast with the stem-Isoetopsida, Lepidosigillaria whitei took the form of a tree that reached a height of around 5 m (Prestianni and Gess, 2014). Unfortunately, no images are available in the public domain.
An alternative member of the crown-Isoetopsida, besides Lepidosigillaria whitei, would be any species from the total group of the Selaginellales. However, no stem-Selaginellales have been recognized, and the oldest member of the crown-Selaginellales identified by the research for this website is Selaginella suissei (Arrigo et al, 2013), which is of Late Pennsylvanian age (Thomas, 1997) and thus much younger than Lepidosigillaria whitei.
The stem group of the Isoetopsida evolved from Early to Middle Devonian time, as illustrated in the time tree below:
The earliest known member of the crown-Isoetopsida is Lepidosigillaria whitei, found in the Middle Devonian (Givetian) Moscow Formation near Summit, New York, U.S.A. (Retallack, 2018; Pereira et al, 2017). In contrast with the stem-Isoetopsida, Lepidosigillaria whitei took the form of a tree that reached a height of around 5 m (Prestianni and Gess, 2014). Unfortunately, no images are available in the public domain.
An alternative member of the crown-Isoetopsida, besides Lepidosigillaria whitei, would be any species from the total group of the Selaginellales. However, no stem-Selaginellales have been recognized, and the oldest member of the crown-Selaginellales identified by the research for this website is Selaginella suissei (Arrigo et al, 2013), which is of Late Pennsylvanian age (Thomas, 1997) and thus much younger than Lepidosigillaria whitei.
The stem group of the Isoetopsida evolved from Early to Middle Devonian time, as illustrated in the time tree below:
This figure indicates that for the Isoetopsida the stem-to-crown transition took place over a period of between 20 and 28 million years.
Turning now to the stem-Isoetales, the oldest known member of the stem group is Yuguangia ordinate, found in the upper part of the Haikou Formation of the Middle Devonian (late Givetian) at Yuguang village, Zhanyi District of Yunnan Province, southwestern China (Hao et al, 2007; DiMichele et al, 2013). No public-domain image is available, but images of some of the other members of the stem-Isoetales are shown below (click on image for larger view):
Turning now to the stem-Isoetales, the oldest known member of the stem group is Yuguangia ordinate, found in the upper part of the Haikou Formation of the Middle Devonian (late Givetian) at Yuguang village, Zhanyi District of Yunnan Province, southwestern China (Hao et al, 2007; DiMichele et al, 2013). No public-domain image is available, but images of some of the other members of the stem-Isoetales are shown below (click on image for larger view):
* after name indicates that the image represents a life restoration.
The restorations in the above images show that many of these plants were in fact trees. Some of them (e.g. Sigillaria, Synchysidendron, Lepidodendron and Lepidophloios) reached heights of 30m or more (Wang et al, 2009; Johnston et al, 2017). However, some were much smaller: Chaloneria grew only to several meters (Boyce and DiMichele, 2016), Wuxia only to 1.5 m (Berry et al, 2003) and Hizemodendron to no more than 0.5m (Li and Johnston, 2000).
The above images are numbered in order from the most basal to those closest to the crown group, but no obvious trends can be seen.
The earliest known representative of the Isoetales crown group is Isoetes beestonii, described from the Early Triassic (Induan-Olenekian) basal Rewan Group in the Queensland Geological Survey borehole Woorona NS34 south of Blackwater, Queensland, Australia (Retallack, 1997). An image of the fossil and a life restoration are shown below:
The above images are numbered in order from the most basal to those closest to the crown group, but no obvious trends can be seen.
The earliest known representative of the Isoetales crown group is Isoetes beestonii, described from the Early Triassic (Induan-Olenekian) basal Rewan Group in the Queensland Geological Survey borehole Woorona NS34 south of Blackwater, Queensland, Australia (Retallack, 1997). An image of the fossil and a life restoration are shown below:
* after name indicates that the image represents a life restoration.
Note the small size of this crown-group isoetalean compared to that of the tree-sized stem-Isoetales.
The stem group of the Isoetales evolved from Middle Devonian to Early Triassic time, as illustrated in the time tree below:
The stem group of the Isoetales evolved from Middle Devonian to Early Triassic time, as illustrated in the time tree below:
The above time tree suggests that the isoetalean stem-to-crown transition lasted between 130 and 139 million years, but the lack of phylogenetically-analyzed fossils in the Permian allows the possibility that an older crown-group representative will be found in the future. In that case the stem-to-crown transition might have been as short as 90 million years, if a crown-group fossil were found in the Permian, or even shorter if one were found in the Carboniferous.
Image credits - Lycophytes
- Header: Huperzia serrata By Keisotyo / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/)
- Isoetes tegetiformans By Pattavina, Pete / Public domain
- Selaginella kraussiana By David J. Stang / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Aglaophyton major (1a) By Claire H. from New York City, USA / CC BY-SA (https://creativecommons.org/licenses/by-sa/2.0)
- Aglaophyton major (1a) By Falconaumanni / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Cooksonia pertoni From Open Access article by 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). (Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/)
- Cooksonia sp. By MUSE / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Stem of Rhynia gwynne-vaughani By Peter Coxhead / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Rhynia gwynne-vaughani By MUSE / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Stockmansella langii By Royal Belgian Institute of Natural Sciences
- Sporangium of Tortilicaulis transwalliensis By Falconaumanni / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Horneophyton lignieri (7a) From Open Access article by 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). (Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0)
- Horneophyton lignieri (7b) From Open Access article by 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). (Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0)
- Zosterophyllum sp. (8a) By Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Zosterophyllum sp. (8b and 9b) By MUSE / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Spore cones of Zosterophyllum myretonianum By permission of Hans Steur (https://steurh.home.xs4all.nl/engoudpl/ezost1.html)
- Sawdonia ornata (10a) By permission of Hans Steur (https://steurh.home.xs4all.nl/engoudpl/esawd1.html)
- Sawdonia ornata (10b) From Open Access article by Gensel, P. G., & Berry, C. M. (2016). Sporangial morphology of the Early Devonian zosterophyll Sawdonia ornata from the type locality (Gaspe). International Journal of Plant Sciences, 177(7), 618-632. (Creative Commons Attribution 4.0 International License (CC BY 4.0))
- Drepanophycus spinaeformis By Ghedoghedo / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Baragwanathia longifolia By Rodney Start, Museums Victoria / CC BY (https://creativecommons.org/licenses/by/4.0)
- Asteroxylon elberfeldense By Daderot / CC0
- Asteroxylon mackiei From Open Access article by Harrison, C. J., & Morris, J. L. (2018). The origin and early evolution of vascular plant shoots and leaves. Philosophical Transactions of the Royal Society B: Biological Sciences, 373(1739). (Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0)
- Leclercqia complexa (15a, 19a) By Claire H. / CC BY-SA (https://creativecommons.org/licenses/by-sa/2.0)
- Leclercqia complexa (15b, 19b) From Open Access article by Harrison, C. J., & Morris, J. L. (2018). The origin and early evolution of vascular plant shoots and leaves. Philosophical Transactions of the Royal Society B: Biological Sciences, 373(1739). (Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0)
- Estinnophyton yunnanense From Open Access article by Hao, S., & Xue, J. (2013). Earliest record of megaphylls and leafy structures, and their initial diversification. Chinese Science Bulletin, 58(23), 2784-2793.
- Clwydia microphylla From Open Access Doctoral dissertation by Rowe, N. P. (1986). The fossil flora of the Drybrook Sandstone (Lower Carboniferous) from The Forest of Dean, Gloucestershire (Doctoral dissertation, University of Bristol). (Creative Commons Attribution - NonCommercial-No Derivatives 4.0 International Public License).
- Colpodexylon deatsii By Daderot / CC0
- Lilingostrobus chaloneri From Open Access article by Gerrienne, P., Cascales-Minana, B., Prestianni, C., Steemans, P., & Cheng-Sen, L. (2018). Lilingostrobus chaloneri gen. et sp. nov., a Late Devonian woody lycopsid from Hunan, China. PloS one, 13(7).
- Sigillaria sp. By Falconaumanni / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Synchysidendron sp. By Falconaumanni / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Diaphorodendron sp. By Falconaumanni / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Stem of Lepidodendron sp. By Jstuby at en.wikipedia / Public domain
- Leaves of Lepidodendron sp. By Smith609 at English Wikipedia / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Lepidodendron sp. By Falconaumanni / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Lepidophloios sp. (25a) By Verisimilus / CC BY (https://creativecommons.org/licenses/by/3.0)
- Lepidophloios sp. (25b) Falconaumanni / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)
- Isoetes beestonii (26a) By Retallack / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Isoetes beestonii (26b) By Retallack / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)