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Earliest true wood-producing plants
Earliest true wood-producing plants
The aneurophytes (Aneurophytales) are a group of progymnosperms, the most basal members of the lignophyte clade. This means they were among the first plants on Earth to produce robust wood from a cambium, similar to modern-day trees. These plants differed from modern woody plants in that they were spore-bearing plants, not seed-bearing (modern wood-producing plants are seed-bearing). Also, aneurophytes lacked leaves; they used small photosynthetic axes for photosynthesis.
Evidence from a preserved forest in Gilboa, NY, dating to the Middle Devonian indicates that some aneurophytes were large woody vines that grew among the Earth's earliest tree-like plants (i.e., Eospermatopteris/Watteiza). Other aneurophytes, like Tetraxylopteris, Triloboxylon, and Rellimia, may have been woody shrubs.
Above: Ancient Middle Devonian Gilboa Forest composed of cladoxylopsid trees with aneurophyte vines interspersed
Ecology & Form
Ecology & Form
Stem
Stem
Morphology
Three-dimensional branching systems
Lateral axes were helical or decussate
These plants were probably small, shrubby, or vine-like
Anatomy
Ribbed or lobed protostele in the center of the stem displaying mesarch maturation
Protoxylem surrounded by pycnoxylic wood formed from a cambium
Cauline protoxylem was a central strand
Leaves
Leaves
No laminate leaves
Ultimate appendages dichotomize were assumed to be photosynthetic
Roots
Roots
Adventitious from stems
Reproduction
Reproduction
Sporangia born on ultimate fertile appendages
Homosporous
Sporangia were elongated and pointed with lateral to tip & base dehiscence
Spores are radial, trilete, pseudosaccate
Geologic Age
Geologic Age
Classification
Classification
└Aneurophytales †
Above: A) The photosynthetic axes of Aneurophyton; B) the fertile, ultimate axes bearing sporangia of Aneurophyton.
Diversity
Diversity
Aneurophyton †
Aneurophyton †
Krausel & Weyland, 1923
Middle-Late Devonian of Europe, Russia, United States
Aneurophyton is similar to Tetraxylopteris in overall form, but its branches are arranged helically rather than decussately.
Dichotomous ultimate branches in Aneurophyton were photosynthetic but unwebbed, and sometimes interpreted as leaves
They were determinate in nature (Scheckler, 1976)
In the fertile axes, ultimate branches are replaced by dichotomies with multiple homosporous sporangia arising along the length of each fork of the branch.
Sporangia appear to recurve toward the center of the dichotomy, but this may be an artifact of preservation.
The main axis of Aneurophyton had a lobed protostele and produced a small amount of secondary xylem from meristematic regions between the lobes.
Axes are relatively slender, suggesting vine growth form, similar to aneuophytes found at Gilboa, NY
A. bohemicum
Kräusel & Weyland, 1933
Middle Devonian of Czech Republic
A. doui
At least four orders of axes and ultimate units (vegetative appendages/fertile organs)
Spines, 0.5–1.5 mm long, occur sparsely on the surface of all orders of axes and ultimate units.
First-order axes, up to 6 mm wide and at least 90 mm long, demonstrate only a slight distal taper.
Second-order axes, up to 4 mm wide, diverge helically from the first-order axis in closely inserted pairs.
From the second order, axes of subsequent orders are produced by bifurcation; third-order axes are 2.0–2.5 mm wide, and fourth-order axes are 1.5–2.0 mm wide.
Vegetative appendages, 7.0–13.0 mm long as a whole, diverge from the third- or fourth-order axes in helical pairs, as a one to three times dichotomizing system
of three-dimensionally-extended flattened branchlets.
Fertile organs, 3.7–8.0 mm long as a whole and borne on the third or fourth-order axes, are up to three times opposite/subopposite pinnate systems.
All three orders of fertile organ axes, in most cases recurved and rarely straight, bear sporangia or organ axes.
Sporangia, oppositely, suboppositely, or alternately, are inserted distichously to the usually adaxial side of the fertile organ axes; they are short-stalked, elliptical, 0.8–1.3 mm wide, and 2.2–3.5 mm long, and they sometimes have longitudinal dehiscence or a twisting configuration.
A. furcatum
Stockmans, 1968
Middle Devonian of Belgium
A. germanicum [type]
Krausel & Weyland, 1923, 1941
Middle Devonian of Germany
A. hallii
Arnold, 1940
Middle Devonian of New York
A. maroccanum
Termier & Termier, 1951
Eifelian of Marocco
A. olnense
Stockmans, 1948
Famennian of Belgium
A. pubescens
Krausel & Weyland, 1932
Middle Devonian of Germany
Above: Evidence of vine-like habit of an aneurophytalean (Gilboa, NY)
Above: Aneurophyton doui (Fig.6 from Jiang et al., 2013)
Above: Evidence of vine-like habit of an aneurophytalean (Gilboa, NY)
Above: Branches of Aneurophyton
Cairoa lamanekii †
Cairoa lamanekii †
Givetian (Middle Devonian) of New York
Fossils are preserved as petrifaction, showing three orders of branching, penultimate axes, and ultimate axes
The penultimate axis bearing the ultimate axes in an alternate manner (1/3 phyllotaxy)
Xylem in transverse section of penultimate axis three-lobed
Secondary xylem is present; protoxylem is circular and mesarch
Tip of xylem arm, just below separation of trace to penultimate branch, three-lobed with a protoxylem area in each lobe
Xylem in transverse section of penultimate branch commonly rounded to four-angled; traces to leaves terete and opposite
Above: Cross-section Cairoa lamanekii Rellimia † (from Fig. 14, Matten, 1973)
Proteokalon petryi †
Proteokalon petryi †
Early Frasnian of New York
Decussate axes, except ultimate axes that dichotomize several times in a single plane
Abaxial appendages appear with paired axes
Morphology similar to Tetraxylopteris, and anatomy similar to Aneurophyton, Rellimia, and Tetraxylopteris
Some specimens show a sparganum-type outer cortex, with anastomosing strands of sclerenchyma
Rellimia thomsonii †
Rellimia thomsonii †
Originally named Protopteridium and Milleria (Leclercq & Bonamo, 1971; 1973)
Shrubby plant with helically-arranged branches up to 5 orders
Ultimate axes had either leaves or sporangia
Anatomy: three-lobed mesarch primary xylem (actinostele) surrounded by pycnoxylic secondary xylem with a bifacial cambium
Stele shape was the same throughout all axes (i.e., 3-ribbed), but the ultimate appendage trace was terete
Above: The fertile portions of Rellimia †
Left: A reconstruction of Rellimia †
Stauroxylon beckii †
Stauroxylon beckii †
Galtier, 1970; Durieux et al., 2024
Mississippian: only aneurophyte found in the Carboniferous
Anatomically preserved axis
Opposite decussate [branches] second-order axes: the latter bear [apparently in a similar fashion, third-order axes] two opposite third-order axes in a perpendicular plane to the previous level of branching. The main axis has a cross-shaped protostele (actinostele).
Primary xylem has five mesarch strands of protoxylem: a central strand and one at the extremity of each rib. The metaxylem tracheids are larger in radial dimension and are larger at the center of the stele.
Each trace to a second-order axis is emitted by a single rib. Outgoing trace triangular proximally, produced after tangential division of trace’s protoxylem strand, becoming arc-shaped and adaxially concave before dividing into three mesarch strands, apparently circular and with a radial structure: two small traces (third-order axes) surround the strand of the second-order axis.
Secondary xylem is sometimes present [only] at the level of the central stele of the main axis. Protoxylem with parenchymatous elements. Inner [bark] cortex parenchymatous. [Middle and outer (?) bark] Outer cortex collenchymatous or sclerenchymatous.
Stauroxylon also displays structural fingerprints important for leaf and pith evolution.
Above: Main axis of Stauroxylon beckii † showing cross-shaped actinostele (Durieux et al., 2024)
Tetraxylopteris †
Tetraxylopteris †
Complex system of pseudomonopodial branches with three orders of decussate branches, probably giving the plant a bushy appearance
Laterals arranged in opposite pairs with successive pairs at right angles (decussate)
Primary, mesarch xylem strand was central in all axes, and cross-shaped in proximal axes
Ultimate axes exhibit terete primary xylem in cross-section.
Some specimens show a sparganum-type outer cortex, with anastomosing strands of sclerenchyma
Sporangia were borne on fertile axes.
Two successive dichotomies create four fertile axes, which were 3x pinnate.
Ultimate axes exhibit an elongated sporangium at the tip
T. schmidtii
Middle to Upper Devonian in age
T. schmidtii, all axes decreased in width, distally; final order axes bore appendages arranged oppositely and decussately
T. reposana †
Hammond & Berry, 2005
T. reposana, 1st and 2nd order axes show swelling at base of branches; decrease in width begins with 3rd order axes; final order axes were spirally or helically arranged
Above: reconstructions of Tetraxylopteris reposana †
Triloboxylon †
Triloboxylon †
This plant gets its name from the three-lobed vascular cylinder, and may represent one of the earliest lignophyte members
Middle Devonian - Late Devonian of New York
T. ashlandicum †
Early Frasnian (Late Devonian) of New York
In some specimens, secondary wood cells are found (Scheckler & Banks, 1971)
Spirally-arranged axes at all levels, and ultimate axes dichotomize several times in a single plane
Some specimens show a sparganum-type outer cortex, with anastomosing strands of sclerenchyma
T. arnoldii †
Matten, 1974; Stein & Beck, 1983
Givetian (Middle Devonian)
Originally Aneurophyton hallii (Arnold)
Three-ribbed primary xylem system with protoxylem strands near the tips and along midplanes of the ribs
Small transversely elliptical traces produced in pairs from successive primary xylem ribs
Extensive secondary tissues
Heterogeneous inner cortex containing solitary fibers and clusters of sclereids
An outer cortex, with peripheral bundles consisting of fibers and sclereids separated by thin-walled parenchyma cells in ordered arrays
Simple but extensively developed periderm
Above: Cross-section of T. ashlandicum †
Below: Cross-section of T. arnoldii †
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