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Earliest vascular plants
Earliest vascular plants
The rhyniophytes are a group of extinct spore-bearing vascular plants that were leafless and rootless plants, with upright branching stems. This group is the earliest known representatives of the tracheophyte clade, possessing true vascular tissue. This group gets its name from the locality from which they are found in Rhynie Scotland. The Rhynie Chert shows exquisitely preserved fossils, providing evidence of the entire life cycle of Rhynia. These plants had ground-creeping rhizomes, that gave rise to upright determinate stems. Unlike most plants of the Devonian, they had adventitious branching with latent "buds" or branches. In addition, this locality provides evidence of the gametophyte phase.
Features
Features
Sporophyte (spore-bearing phase)
Sporophyte (spore-bearing phase)
Stems/Axes
Similar to the early polysporangiophytes, such as Aglaophyton, these plants show increasing diversification of axis types. These early plants are constructed with indeterminate and determinate structures like modern plants.
Determinate aerial stems: above ground, upright axes (3 mm wide) which terminate in sporangia for dispersal (20 cm tall)
Branching is isotomous, anisotomous, and even pseudomonopodial
Indeterminate rhizomes: ground-running, or below ground portions, that creep similar to vegetative propagate the population
Vesicular arbuscular mycorrhizae found in the cortex of Rhynia rhizomes
Adventitious branching: latent "buds" or branches, which could elongate to make new axes, similar to modern plants.
Vascular tissue present, but not the same as eutracheophytes (see S-type tracheids below)
Haplostele with centrarch maturation and S-type tracheids (i.e., annular or helical thickenings made of spongy material)
Geologic Age
Geologic Age
There is no evidence of Rhyniales before the Devonian, but they probably existed in the mid- to late Silurian.
Left: Reconstruction of Rhynia gwynne-vaughanii
Below: Piece of Rhynie chert showing Rhynia axes
Right: Cross-section of a Rhynia axis showing haplostele
Leaves
Leafless; assumed by some to have photosynthetic upright axes
The rhizome may have been photosynthetic, but these organs lacked stomata which would limit such a process.
Boyce (2008) has questioned whether the upright axes of Cooksonia were photosynthetic due to their thin nature (< 1 mm).
This was probably not the case with Rhynia since this taxon had more robust axes that allowed for chlorophyllous tissues
Reproductive Structures
Spores in sporangia; Homosporous
Abscission layer at base of spornagia; attached to pad of tissue
Gametophytes (gamete-bearing phase)
Gametophytes (gamete-bearing phase)
Form genus Remyophyton delicatum
Dioicous*, bearing male and female gametangia (antheridia and archegonia) on different axes
*Note that in gametophytes, the term is "dioicous", not "dioecious" as in sporophytes
Gametophyte axes are vascular, unlike all gametophytes of modern pteridophytes (except for Psilotum)
Above (left): Reconstruction of a stand of Remyophyton delicatum; Above middle: Close-up of antheridia of Remyophyton; Above right: Close-up of archegonia of Remyophyton
Diversity
Diversity
Calyculiphyton blanai †
Calyculiphyton blanai †
Remy et al., 1991
Compression fossil with elongate axes that terminate in cup-like structures
May represent a gametophyte
Celatheca beckii †
Celatheca beckii †
Pragian of Posongchong Formation of eastern Yunnan, China
Leafless axes dividing anisotomously
Sterile lateral axes dichotomize two or three times, terminating in recurved tips
Fertile lateral axes dichotomize to produce a group of four sporangia
Each sporangium exhibits an outer leaf-like bract which folded around the sporangium
Celatheca resembles the Australian fossil Yarravia
Eddianna gaspiana †
Eddianna gaspiana †
Emsian of Gaspe, Canada
2mm in diameter with well-developed terete S-type xylem
Eocooksonia sphaerica †
Eocooksonia sphaerica †
Originally Cooksonella (Senkevitsch, 1978; Doweld, 2000)
Upper Silurian (Pridoli) of Xinjiang, Northwest China and Central Kazakhstan
Pseudomonopodial branching pattern, which forms an apparent main axis with lateral dichotomously branching systems.
Specimens demonstrate terminal sporangia with a central body and a border with four to eight elongate-triangular emergences
Eogaspesiea gracilis †
Eogaspesiea gracilis †
Daber, 1960
Tangled mess of branching axes that reached 10 cm in length, probably from a rhizome
Alete spores had thin walls
Eorhynia primigena †
Eorhynia primigena †
Ishchenko, 1975
Late Silurian (Pridoli) of Ukraine
Filiformorama simplexa †
Filiformorama simplexa †
Late Silurian (Pridoli) of north Xinjiang, China
Plant has naked axes and a dichotomous branching
Sporangia terminates one of the branches of a dichotomy, and the other daughter branch continues to grow and dichotomizes further.
Sporangia are circular, but some are reniform or tongue-shaped, with an asymmetric base.
There are no anatomical features that place this taxon in Rhyniales or tracheophytes in general
Right: Filiformorama compression (Yi et al., 2006, Figure 1, C & D)
Hedeia †
Hedeia †
Early Devonian from Victoria, Australia, Kazakhstan and China
Erect axes terminating in corymbose clusters of erect sporangia
H. corymbosa
Cookson, 1935
H. parvula
Jurina, 1969
H. sinica
Hao, 1998
Huvenia kleui †
Huvenia kleui †
Hass and Remy 1991
Pragian of the Rhenish Massif
Leafless axes that appear to be flattened and branch dichotomously
The strand of conducting tissue contains simple tracheids
The sporangia are borne on the ends of short branching stems (sporangiophores) rather than terminating main stems as in some other early land plants
The gametophyte of this plant may be Sciadophyton
Monnowella bennettii †
Monnowella bennettii †
Early Devonian (Lochkovian) of Welsh Borderland, UK
Axes with isotomous or weakly anisotomously branching
Single sporangia terminating unbranched laterals
Sporangia were terminal and ellipsoidal
Above: Monnowella bennettii † (from Plate II, Morris & Edwards 2014)
Rhynia gwynne-vaughanii †
Rhynia gwynne-vaughanii †
Kidston & Lang, 1917
From the Rhynie chert in Aberdeenshire, Scotland
Rhizomatous plant with upright isotomous and anisotomous axes
Upright axes were up to 3 mm wide and roughly 20 cm tall
Axes had a centrarch haplostele
Sporangia were spindle-shaped (fusiform) and terminal
Possible deciduous lateral branches were used to disperse laterally over the substrate and stands of the plant and may therefore have been clonal populations
Above: Reconstrcution of Rhynia gwynne-vaughanii †
Salopella †
Salopella †
S. australis †
Late Silurian (Ludlovian) - Early Devonian (Pragian) of Victoria, Australia
Axes with at least 2 dichotomies (0.9-2.4 mm wide)
Plants at least 145 mm tall
Sporangia 6.5-14.0 mm long and 1.3-2.0 mm wide
S. cf. marcensis †
Edwards et al., 1994
Above: Plate 32 (Tims & Chambers 1984) of Salopella australis
Sciadophyton †
Sciadophyton †
Remy et al., 1980
Early Devonian compression fossils
Gametophytes of rhyniophyte plant, bearing gametangia
Axes that radiate from a basal corm-like thallus that terminate in cup-like structures
Possibly the gametophytes of either Stockmansella or Huvenia (Kenrick et al. 1991, Remy et al. 1993), but it may be the gametophyte stage of several land plants
Sc. laxum ((Dawson 1871; Steinmann 1928)
Sc. palustre (Istchenko 1965)
Sc. steinmannii (Steinmann; Kräusel and Weyland 1930)
This species is thought to belong to the sporophyte Zosterophyllum rhenanum
Above: Compression fossil of Sciadophyton
Steganotheca striata †
Steganotheca striata †
Silurian (Ludlow) of Powys, Wales
Plant exhibiting elongate sporangium terminating a short length of parallel-sided, unbranched axis.
The diagnostic features of S. striata sporangia, the obliquely running striations and the terminal thicker lens-shaped region are usually clearly visible.
In the majority of specimens the distal region persists, the organic material of the main body of the sporangium has flaked off leaving dark strands adhering to the rock surface
Above: Plate 1 (Edwards & Rogerson 1979) showing Steganotheca striata sporangium showing surface striation
Stockmansella †
Stockmansella †
Fairon-Demaret, 1986
S. langii †
Ribbon-like axes bear lateral sporangia.
The sporangia are fusiform and attached to the axis by a pad of tissue, circular in outline, which most probably functioned as an abcission layer.
Sproangia dehisce along a specialized longitudinal slit which is clearly distinguishable from the more or less numerous splitting lines.
The conducting strand is composed of helically strengthened S-type, cylindrical elements which are of various diameters
S. remyi †
Schultka & Hass, 1997
Eifelian (youngest member of Rhyniaceae)
The gametophyte of this plant may be Sciadophyton
Yarravia †
Yarravia †
Lang & Cookson, 1935
Pragian of Victoria, Australia
Axes 2 to 2.5 mm in diameter, and up to 7.5 cm in length.
No examples of branching or anatomy known
The sporangia born radially in groups of 5-6 at the ends of axes.
Base of sporangia are fused together, but tips may be free; the entire structure can be described as a synangium
Y. oblonga
Lang & Cookson, 1935; McSweeny et al., 2021
In Y. oblonga, the synangium (the group of fused sporangia) was longer than wide, being about 7 mm long by 1.25 mm wide. The tips of the sporangia were free and appear to consist of tissue not containing spores.
Y. subsphaerica
Lang & Cookson, 1935
The synangium of Y. subsphaerica was significantly larger and as long as wide, being about 1 cm in length and width
The tips of the sporangia were less prominent
Y. minor
Danzé-Corsin, 1956
Y. gorelovii
Ananiev, 1960
Above: Yarravia oblonga Lang & Cookson, 1935 emend. McSweeny et al., 2021; Figure 3
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