Introduction
The land plants comprise the superphylum Embryophyta within the infrakingdom Streptophyta (Ruggiero et al., 2015). A recent phylogenetic tree of the modern land plants, based on extensive genetic analysis, is shown below:
All the nodes shown in the above figure, shown as black dots, represent crown nodes of the successive crown groups into which the embryophyte crown group has divided.
After reviewing the origin of the land plants in terms of their ancestors between the appearance of the crown eukaryotes and that of the crown-Embryophyta, this section of the website will examine the fossil evidence for the evolution of each of the terminal clades shown in the above figure.
For plant fossils, an important issue of nomenclature needs to be understood clearly. Fossils of plant materials often do not represent entire plants, but rather separate organs, such as leaves, stems, roots, cones or flowers. In many cases, it can be difficult to assign these organs to a known whole-plant species. The common practice has been to assign such fossils to what is known as a "form genus" (Faegri, 1963). These are commonly assigned to a whole-plant taxon at a later date, when more fossil data become available. A useful discussion of the use of form taxa and the related organ taxa can be found in Bateman and Hilton (2009). The plant fossils shown in this website are whole-plant taxa unless otherwise stated.
After reviewing the origin of the land plants in terms of their ancestors between the appearance of the crown eukaryotes and that of the crown-Embryophyta, this section of the website will examine the fossil evidence for the evolution of each of the terminal clades shown in the above figure.
For plant fossils, an important issue of nomenclature needs to be understood clearly. Fossils of plant materials often do not represent entire plants, but rather separate organs, such as leaves, stems, roots, cones or flowers. In many cases, it can be difficult to assign these organs to a known whole-plant species. The common practice has been to assign such fossils to what is known as a "form genus" (Faegri, 1963). These are commonly assigned to a whole-plant taxon at a later date, when more fossil data become available. A useful discussion of the use of form taxa and the related organ taxa can be found in Bateman and Hilton (2009). The plant fossils shown in this website are whole-plant taxa unless otherwise stated.
Origin of the land plants
The origin of the land plants may be traced by examining the phylogenetic relationships along the stem line that connects the land plants with the division of the eukaryotes into the bikonts and the unikonts, as shown below:
The above tree reflects the generally accepted view that the land plants evolved from the green algae (Wodniok et al, 2011). However, this stem line, which continued up to the first appearance of the crown-group land plants in the Middle Ordovician (the stem-bryophyte spore Tetrahedraletes cf. medinensis, Morris et al, 2018; Rubinstein et al, 2010), is very poorly documented by known fossils. The only clades for which fossils older than Tetrahedraletes cf. medinensis are known are the Rhodophyta and the Chlorophyta; their oldest known fossil representatives are the red alga Bangiomorpha pubescens (Butterfield, 2000) and the chlorophyte Proterocladus antiquus (Tang et al, 2020). No public-domain image is available for any of the above species, but close equivalents of Tetrahedraletes cf. medinensis (i.e. T. medinensis) and Proterocladus antiquus (i.e. Proterocladus sp.), together with a modern rhodophyte, are shown below:
The above plot shows that there is a period of at least 470 million years, from the early Neoproterozoic to the Middle Ordovician, in which there is no known fossil documentation of the transition from the land plant stem group to the crown group.
Image credits - Land Plants
- Header: Sunset in the woods in Tok, Alaska, United States. By Diego Delso [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Tetrahedraletes medinensis By PhilSteem / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)
- Proterocladus sp. From Cohen, P. A., & Macdonald, F. A. (2015). The Proterozoic record of eukaryotes. Paleobiology, 41(4), 610-632, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/)
- Bangia fuscopurpurea From Gabriele Kothe-Heinrich / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)