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Earliest true-vascular plants
Earliest true-vascular plants
Cooksonia is genus of early true vascular plants that were leafless and rootless plants, with spore-bearing upright forked stems. These plants may have been photosynthetic in the gametophyte phase, and non-photosynthetic in the upright, spore-producing (sporophyte) portions. Unlike the horneophytes, the cooksonioids had true vascular tissue with C-type tracheids. These xylem cells formed a small cylindrical strand through the center of the stems, moving water from rhizome to the aerial axes. There is increasing evidence that this genus represents many different, but superficially similar, genera from the Silurian and Devonian (e.g. Aberlemnia, Concavatheca)
Ecology & Form
Ecology & Form
Stems
Stems
Morphology
Stem photosynthetic ability unknown.
Originally assumed to have photosynthetic upright portions, although some have questioned whether the upright stems were photosynthetic in Cooksonia (Boyce 2008).
Some larger stems (diameter >1mm) were probably photosynthetic, but possibly not independent from the gametophyte
Smaller stems (diameter <1mm) were almost definitely non-photosynthetic, since they wouldn't be larger enough to have the requisite tissues for support and water conduction, and also photosynthesis.
This would have made them dependent on the gametophyte
Anatomy
True vascular tissue: haplostele with C-type tracheids
Assumed to be centrarch xylem maturation
Leaves
Leaves
Leafless
Roots
Roots
Rootless
Rhizoids anchor to substrate
Reproduction
Reproduction
Sporangia are spherical, bean-shaped, and funnel-shaped
Gametophyte
The gametophytes of cooksonioids are unknown, but assumed to be thalloid
Geologic Range
Geologic Range
Middle Silurian (Sheinwoodian Age) to Lower Devonian
Above: reconstruction of Cooksonia pertoni
Diversity
Diversity
Aberlemnia †
Aberlemnia †
Aberlemnia is positioned in a clade leading to the Lycophytina.
A. caledonica †
Formerly Cooksonia caledonica (Edwards, 1970; Libertin et al., 2023)
Early Devonian of Scotland, Wales, and Brazil
Smooth leafless stems (axes) up to 1.4 mm wide, decreasing in width at each branching.
Isotomous-branching up to five times, at angles between 25 and 55°, mainly dichotomously,
Basal-most segments are the longest, and each segment further distally is shorter
Some specimens from Brazil had some trichotomies.
Sporangia were borne at the tips of the ultimate axes.
The sporangium is circular, oval, rarely reniform in front view, vase-shaped in sagittal view, consisting of two equal valves with the line of dehiscence reaching the basal part of the sporangium
Individual sporangia varied in shape due to growth and maturation
To release their spores, the sporangia split into two valves along the border opposite to the stem on which they were attached (i.e. distally)
A. krizii †
A. krizii †
Pridoli, Silurian of the Barrandian Area, Czechia, in Central Europe
A. krizii is similar to A. caledonica in habit: isotomous branching and isovalvate sporangia.
It differs in having shorter subtending axes, sporangia of clearly reniform shape and markedly broader than high, an absence of radial striation of sporangia, and the line of dehiscence reaching as far as 1/3 of the way down the sporangium.
A. caledonica is more robust.
Above: Aberlemnia caledonica reconstruction (from Libertin et al. 2023, Figure 2
Amplectosporangium †
Amplectosporangium †
A. jiangyouense †
Emsian of Sichuan Province, China
Leafless plant, up to 7 cm with dichotomous axes and terminal fertile branches.
Short-stalked, oval-shaped sporangia are distributed in rows along the inner sides of the fertile branches.
The linear arrangement of the sporangia in a cup-like fashion along the inner sides of the terminal fertile branches has been hypothesized to represent an early stage in the evolution of the seed coat or integument
A. unilaterale †
Plants with naked parallel-sided unbranched stems terminating in fertile complexes
Stems with longitudinal lines, producing strap-shaped appearance which extends into the fertile complex
Complex comprises at least two consecutive dichotomously branching axes
Branching in daughters asynchronous, lateral sporangia borne on inside of each dichotomy
Sporangia sessile, hemispherical in outline, margin marked by unthickened narrow border
Sporangial bases contiguous or narrowly spaced
Above: Amplectosporangium unilaterale † (from Plate V, Fig 8, Edwards & Li 2018)
Cooksonia †
Cooksonia †
Lang, 1937
Middle Silurian (Sheinwoodian Age) to Lower Devonian
Narrow leafless stems (axes), which branched dichotomously, with terminal sporangia that were "short and wide".
There was a central vascular cylinder consisting of annular tracheids (water-conducting cells with thickened walls)
Sporangia formed by a widening of the end of a stem.
At maturity the sporangium is topped by a flattish disk (an operculum) and releases its spores when this breaks up
C. barrandei †
Sheinwoodian of the Czech Republic
C. paranensis †
Gerrienne et al., 2001
Similar to C. pertoni
C. pertonii †
Lang, 1937
Type species for Cooksonia
The earliest unequivocal evidence of stomata comes from this type specimen (Clark et al., 2022)
Several species are questionable due to poor preservation:
C. acuminata (Mussa et al., 2002)
C. bohemica (Schweitzer, 1980)
Although Libertin et al. (2022) provide stronger evidence for the retention of this species
C. cambrensis (Edwards, 1979)
C. downtonensis
C. rusanovii (Ananiev, 1960)
The following species have been removed from the genus
C. hemisphaerica (Lang, 1937)
C. crassiparietilis (Yurina, 1964)
C. caledonica renamed Aberlemnia caledonica (Gonez & Gerrienne, 2010);
C. banksii renamed Concavatheca banksii (Habgood et al., 2002; Morris et al., 2012)
Above/below: reconstruction of Cooksonia pertonii
Below: reconstruction of Cooksonia pertonii Credit: J. Boyer
Culullitheca †
Culullitheca †
Early Devonian of England
Known from charcoalified remains at the Brown Clee Hill locality.
Spores formed permanent dyads
Deuterophyton stockmansii †
Deuterophyton stockmansii †
Lower Devonian (Early Emsian) of Fooz-Wépion, Belgium
This new taxon is created for small fertile specimens, consisting of a main axis bearing laterals branched once distally; each segment is terminated by a sporangium
Sporangia range from rounded to longitudinally elongate and are borne by a well-defined stalk
The sporangia open into two valves of equal size
The fertile axes of Deuterophyton, Hsüa, Junggaria, Renalia, Sartilmania, Tirasophyton and Yunia show comparable organisation
Hsüa †
Hsüa †
Junggaria †
Junggaria †
Duo & Sun, 1983
Upper Silurian (Přídolí)
Isotomous or pseudomonopodially branching axes
Sporangia are complex in shape
Above: Sporangia of Junggaria
Renalia †
Renalia †
Gensel, 1976
Early Devonian from Battery Point Formation, Gaspé, Québec.
Leafless anisotomous branching stems
Lateral axes dichotomized into ultimate paired sporangia at their tips
Sporangia are kidney-shaped, splitting along the distal margin
R. hueberi †
Gensel, 1976
R. grabertii †
Schweitzer, 1980
R. major †
Schweitzer, 1980
Below: Renalia
Sartilmania jabachensis †
Sartilmania jabachensis †
Kräusel & Weyland; Fairon-Demaret, 1986
Emsian of Belgium
Narrow leafless axes, from 0.75 mm to a maximum of 2 mm in diameter, marked with irregular longitudinal striations.
Plant heavily and irregularly branched, with the lateral branches being arranged spirally.
Each lateral branch appears to have ended in a vertically-oriented sporangium, which caused a sharp upwards turn if the branch it terminated was near to the horizontal.
The sporangia were spatulate (spoon-shaped), most measuring 3.5–4 mm long by 2.5-3.5 mm wide, marked by the same longitudinal striations as were present on the stems.
The sporangia opened into two valves along their distal margins
Above: Sartilmania jabachensis † (from Plate I, Fairon-Demaret, 1986)
Tirasophyton †
Tirasophyton †
Istchenko, 1974
Uskiella †
Uskiella †
Shute & Edwards, 1989
Early Devonian of southern Wales
Naked, simple isotomous branching and ellipsoidal sporangia (Shute and Edwards, 1989)
The sporangial wall has several cell layers thick, with a longitudinal row of thin-walled cells along which the sporangium splits (Fanning et al., 1992).
The spores have been described as alete and range from 28-42 pm in diameter; they possess a two-layered sporoderm
U. spargens †
Shute & Edwards, 1989
U. reticulata †
Fanning et al., 1992
Above: Uskiella
Additional Resources
Additional Resources
How Green was Cooksonia? (Boyce, 2008)
Isabel Cookson: One Tough Cookie (Museums Victoria)
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