Zosterophylls †
Ancestral lycophytes
The zosterophylllophytes are an extinct, leafless group of plants, which displayed prickle-like enations in some, and true roots in some derived forms, such as the Sawdoniales. The spore cases of this group are lateral and clam-shaped. Anatomically, they possess an exarch protostele, similar to lycopsids, therefore they are placed as a sister group to the clubmosses. The barinophytes, are heterosporous group that has similarities to the zosterophylls, and sometimes considered derived zosterophylls. Zostera is the taxonomic name of the extant eel grass, and the original interpretation of these fossil appeared to be similar in form.
Ecology & Form
Sporophyte (spore-bearing) phase
Stem
Anisotomous branching in proximal axes
Isotomous branching in the distal axes
Elliptical exarch protostele
Leaves
Leafless
Enations in some; smooth axes in others
Roots
Advanced members of this group exhibit roots
These represent some of the earliest known roots in the fossil record
Reproduction
Aggregated sporangia with clam-like dehiscence
Sporangia are lateral on the stems, not terminal like the "rhyniophytes" of the same time
The sporangia can appear cone-like in some members
Geologic Age
Classification
Diversity
Basal Zosterophyllophytes † (see below)
Sawdoniales †: derived zosterophylls, which possessed roots and fiddleheads
?Barinophytes †: heterosporous zosterophyll-like plants
Craswallia haegensis †
Early Devonian (Lochkovian) of the Welsh Borderland, UK
Possible zosterophyll, comprising bivalved elliptical sporangia, borne distally on stout unbranched stalks
Smooth stems in parallel alignment with rare lateral sporangia, all at approximately the same level in adjacent stems
The stems show very little change in diameter along their lengths, measuring 1.3 to 1.8 mm diameter, and no branching
Above: Craswallia haegensis † (from Plate IX of Morris & Edwards 2014)
Demersatheca contigua †
Li & Cai 1977; Li & Edwards 1996
Early Devonian of Posongchong Formation of Wenshan, southeastern Yunnan, China
Taxon originally described as Zosterophyllum contiguum (Li & Cai)
Strobili with sporangia, inserted decussately, form four vertical rows.
The sporangia have two valves, splicing around the convex margin, and are sunk into the strobilar axis such that only the abaxial valves, circular to elliptical in outline are visible.
Strobili similar to Zosterophyllum
Above: Demersatheca contigua † (from Fig 1, Li & Edwards 1996)
Above: Demersatheca contigua † (from Fig 1, Li & Edwards 1996)
Discalis longistipa †
Early Devonian (Pragian) of Yunnan, China
Leafless sporophyte consisted of creeping axes up to 5 mm in diameter
K- or H-shaped branches as well as upturned or trailing stems, slightly smaller in diameter, which also branched.
All stems had irregularly arranged multicellular spines up to 2.5 mm long with expanded tips.
The vegetative stems exhibit circinnate growth.
Fertile stems bore disc-shaped sporangia laterally on stalks up to 5 mm long, forming open spikes.
The sporangia, which were about 3.7 mm in diameter, had spines like the stems, and dehisced along their margin to release the trilete spores.
Vascular tissue was present in the stems, with tracheids having annular, spiral thickenings.
Above: Reconstruction of Discalis longistipa showing portion of fertile axis with spines (Taylor and Taylor 1993)
Distichophytum †
Early Devonian (Pragian or Siegenian to Emsian) from Beartooth Butte, Wyoming and Bathurst Island, Canada
D. ovatum † (Dorf 1933; Hueber 1972)
Formerly known as Bucheria
Leafless stems 1.5 to 2.0 mm in diameter, which branched dichotomously
Fertile stems had clusters of sporangia on distal ends of stems
Compact one-sided spikes of up to 20 laterally attached sporangia, more-or-less opposite
The sporangia were kidney-shaped (reniform) and had short stalks around 1.5 mm long which curved so that all the sporangia were on one side of the stem
The sporangia split (dehisced) distally into two equal parts in order to release the unornamented spores
D. mucronatum † (Mägdefrau 1938;Schweitzer 1979)
Formerly known as Rebuchia
This species had narrower, less branched stems and smaller, somewhat differently shaped sporangia than D. ovata.
Hueber considered the differences in sporangial shape were caused by compression and that the other differences were too small to warrant a different species (Hueber 1972)
Schweitzer retained genus, but as different species (Schweitzer 1979)
Ensivalia deblondii †
Gerrienne 1996
Early Devonian of Belgium
This plant has been combined, and is now called Sawdonia deblondii (Gensel et al. 2016)
This plant has spiny, anisovalvate sporangia terminating stout stalks
The abaxial sporangial valve is enlarged and spiny, inside of which the more delicate, smooth adaxial valve rests
Gosferia curvata †
Gerrienne 1991; Gerrienne 1999
Early Devonian from Belgium
Exhibited curved axes with reniform sporangia
Guangnania †
G. cuneata (Wang & Hao 2002)
Lower Devonian (Pragian) Posongchong Formation of southeast Yunnan and the (Pragian-early Emsian) Xujiachong Formation of east Yunnan, China
Pseudomonopodial axes; anatomy is unknown
Fertile region consists of lateral sporangia attached helically
Each sporangium is elongate cuneate and long-stalked dehiscing into two unequal valves, a large abaxial valve and a small adaxial one, which in transverse section are curved towards the axis
The dehiscence line is bordered by the thickening along the entire sporangial valve margin
G. minor (Edwards et al. 2016)
Distal and basal branching systems preserved separated by naked parallel-sided axes of a diameter that is remarkably uniform along their length (1.0–1.6 mm wide, n = 21, x = 1.3; majority are 1.2 and 1.4 m)
Infrequent isotomous branching with some overtopping; bifurcation may occur close to the fertile region
Bifurcations more common in the basal regions, where it is accompanied by K configurations and occasional, apparently short, unbranched laterals with rounded
Sporangia are lateral and arranged helically, although their exact disposition is unknown
Tips of sporangia are at approximately the same level, but sporangial insertion shows very small vertical spacing (up to three consecutive stalks observed) such that they are not opposite or verticillate
Slightly vertically extended mounds or depressions probably represent superficial insertion of sporangia which are not preserved
Strobili on the larger specimen show considerable relief, extending through some depth of sediment such that sporangia are fractured in different planes.
There is no evidence of pronounced curvature: the stalks are inserted at an acute angle, are decurrent and superimposed on the strobilar axis, appear as broad ridges or depressions
The abaxial part is continuous with most of the rest of the sporangium which is interpreted as the abaxial valve
Some specimens preserved dorsiventrally show a very narrow line around the distal margin, but this is not comparable to the marginal fractures noted in zosterophylls nor does it equate to the marginal split recorded above
Above/Right: Guangnania cuneata reconstruction and compression fossil
Below: Guangnania minor reconstruction and compression fossil
Huia †
H. gracilis † (Wang & Hao 2001)
K- or H-type branching in the rhizome
Isotomously branched erect system.
An axillary tubercle is sometimes present at the branching position.
Fertile axes terminate in loose spikes comprising terminal and lateral sporangia arranged helically.
Sporangia with long stalks are ovate or elongate–ovate and reflex adaxially.
The sporangium dehisces longitudinally in the radial plane of the fertile axis into two parts.
The xylem is probably centrarch.
Tracheids of G-type are characterised by annular secondary thickenings, between which is a sheet with irregular simple perforations.
There may be some ‘pores’ in the enclosing wall of the perforation
H. recurvata † (Geng 1985)
Type specimen
Pseudomonopodial erect branching
No axillary tubercle, like H. gracilis
Hicklingia †
Kidston and Lang 1923
Sporophyte had a tufted growth habit, with narrow leafless stems (axes) up to 17 cm high which branched dichotomously.
Sporangia were borne on short stalks (up to 3 mm), on all sides of the stem and also terminally.
There are oval scars on specimens where the stalks are presumed to have broken off.
The lateral sporangia were closely adpressed to the stem.
The effect is of a 'spike' of sporangia which terminates some stems.
The sporangia opened via slits, but these did not have the thickened borders which are a feature of some Zosterophyllum species.
The vascular system of the stem was not observed
H. edwardii † (Kidston and Lang 1923)
H. erecta † (Kidston and Lang 1929)
Above: Hicklingia edwardii (Figure 1, Kidston & Lang 1923)
Jugumella †
Senkevitch 1978
Early Devonian (Lochkovian) of Kazakhstan
J. burubaensis (Senkevitch 1978)
J. jugata (Senkevitch 1978)
? Juliphyton glazkini †
Stepanov 1975
Konioria †
Zdebska 1982
Macivera gracilis †
Kotyk et al. 2002
Late Silurian (Ludfordian) from Bathurst Island
Axes naked, thin; branching isotomous
Sporangia borne in small cluster at distal end of axis. Arrangement of sporangia not in rows.
Sporangia sessile, elliptical, longer than wide, oriented at an angle oblique to the axis.
Dehiscence zone distal. Homosporous
Above: Macivera gracilis
Odonax borealis †
Early Devonian from Marchin, Belgium
Spiny, mostly isotomous axes, with a spiny branch in a subaxillary position at some proximal branching points
Spines are triangular in outline
The axes end in compact strobili of sporangia
Because of the existence of two dichotomies just below them, the strobili are often borne in groups of four
The strobili are composed of two rows of sporangia, arranged in an opposite to alternate manner
The two rows are borne at 100–150° apart on the same side of the strobilus axis: the strobili thus show a bilateral rather than radial symmetry
Often, the tip of each strobilus is slightly curved
The reniform sporangia are borne erect by a strong spiny stalk bent upwards distally
Dehiscence occurs along their distal margin and opens the sporangia into two equal valves, the abaxial one being spiny
Ramoferis amalia †
Early Devonian (Pragian) of Guangnan County, Yunnan, in southwestern China
Isotomous (equal) branching forming angles of about 30–60°.
Axes 3–5 mm in width, and up to 12 cm in length. Anatomy unknown.
The longest axis branched four or five times, with the branching intervals decreasing towards the top.
Terminal axes were less than 1 mm wide and 6-10 mm long.
Sporangia were borne on short stalks both terminally and laterally.
Some were found below the first branching point of the main axis, others were gathered into a spike.
The sporangia were flattened, usually about 3–6 mm wide and 2.5–5 mm high.
Viewed from the front, the sporangia were ovoid or pear-shaped, narrowing towards the stalk.
They opened (dehisced) along a narrow rim at the edge opposite the stalk (the distal edge), producing two equal valves.
Only poorly preserved spores were found, 48–77 µm in diameter. Nothing is known about the internal anatomy of the axes
Above: Diagrams of specimens of Ramoferis amalia † (Fig 2, Hao & Xue 2011)
Trichopherophyton teuchansii †
Early Devonian (Pragian) plant from the Rhynie Chert
The aerial axes of the plant exhibit a maximum diameter of 2.5mm
The branching of Trichopherophyton is both dichotomous and pseudomonopodial
It is the only plant known in the Rhynie flora to display circinate vernation.
Generally, the cuticle of Trichopherophyton is poorly preserved, and as a result stomata have not been observed
The rest of the epidermis, however, is usually well preserved and very often exhibits unicellular spinose projections emerging perpendicular to the axis of the plant.
These spiny outgrowths are one of the most characteristic features of Trichopherophyton
The cortex can be divided into a distinct outer cortex comprising closely packed cells and an inner cortex or more loosely packed cells with an inter-cellular air-space network. Many cells of the inner cortex contain dark colored residues
The vascular tissue comprises a very distinct xylem strand. It is sub-terete, exarch and displays both annular and spiral thickening.
A narrow zone, uniform in thickness, of thin-walled cells surrounds the xylem strand and probably represents phloem
Sporangia appear to be reniform (kidney-shaped) with dimensions up to a maximum 3.7 mm by 2.5 mm.
They are attached laterally to the aerial axes with a vascularised sporangial stalk though their spatial distribution is uncertain.
The sporangia appear to be bi-valved with a marginal dehiscence mechanism.
Characteristically the sporangia display the same unicellular spinose projections emerging from the epidermis as seen in the aerial axes
This is the scarcest of the Rhynie plants having only been found in a few beds of chert.
Ventarura lyonii †
Pragian from Rhynie Chert
Stems of two kinds without clear connections between them.
Underground stems had single-celled hairs, presumably rhizoids, on all sides. They branched irregularly and more often compared to aerial stems
Aerial stems were leafless, smooth, and isotomously-branched, reaching 12 cm in length (7.2 mm max diameter)
They contained an exarch xylem strand containing G-type tracheids.
The cortex of the aerial axes displays an inner and outer cortical layer is separated by a dark middle cortex zone of sclerenchymatous cells
The outer cortex comprises closely-packed, thin-walled cells.
Evidence of sclerenchyma (found in the middle zone) is rare in Rhynie Chert plants
The inner cortex again comprises thin-walled cells, but with a well-developed inter-cellular air space network.
Sporangia were born on the sides of the stems, attached without clear stalks.
They consisted of two circular- to pear-shaped 'valves', one slightly narrower facing the stem, one away from it.
Sporangia may have formed a two-rowed spike or strobilus.
Spores were shed via a slit at the top of the sporangium between thickened valve borders
Wenshania zhichangensis †
Pragian of Yunnan, China
Plants with at least three orders of simple, naked, dichotomously and pseudomonopodially branching, non-circinate stems.
Bifurcation of main stem of equal width.
Sporangia lateral on main stem below and above dichotomies in a subopposite and decussate arrangement.
Sporangia borne upright on short stalks, subcircular to reniform, dorsiventrally flattened, and dehisce into two equal valves along a thickened distal margin.
Above: Wenshania zhichangensis † (from Fig 2 of Zhu & Kenrick 1999)
Xitunia spinitheca †
Xue 2009
Lower Devonian (Lochkovian) of Yunnan, China
Stalked sporangia lateral on axis and in a helical arrangement, roughly generating four vertical rows
Sporangia dorsoventrally flattened, composed of two unequal valves
Adaxial valve smaller round in face view
Abaxial valve larger triangular or wedge-shaped having long spiny appendages along distal margin and in a radial arrangement
Yunia †
Pragian or Emsian from the Posongchong Formation of Yunnan, China
Yunia was originally classified as a trimerophyte (Hao & BEck 1991) but Crane (2004) and Hao & Xue (2013) consider it a possible zosterophyll
Plant consisting of spiny axes which branch at wide angles called cruciate dichotomies.
Associated (but not attached) sporangia elongate-elliptical or elongate-ovoid, occasionally clustered in pairs and possibly borne terminally on dichotomous branches; dehiscence longitudinal along a narrow, marginal rim, splitting the sporangia into two equal valves.
Plant probably homosporous.
Primary xylem comprising a haplostele with a large, central, highly parenchymatous protoxylem strand with peripheral tracheids which divides prior to bifurcation of the stele to form a pair of protoxylem strands.
Protoxylem strands roughly circular in smaller axes, but often appearing lunate in transverse form in the stele of larger axes.
Y. dichotoma Hao & Beck, 1991
Y. guangnania Hao & Xue, 2013
Below: Yunia
Zosterophyllum †
Penhallow 1892
Z. australianum (Lang & Cookson 1930; Li et Cai 1977; Hao, 1992; Hao & Wang 2000)
Z. bifurcatum (Li & Cai 1977)
Z. fertile (Leclercq 1942)
Z. llanoveranum (Croft & Lang 1942)
Z. longhuashanense (Li & Cai 1977)
Z. longum (Høeg 1967)
Z. minifertillum (Xue 2009)
Z. minor (Ananiev 1960)
Z. minorstachyum (Xue, 2009)
Z. minutum (Hao & Xue 2013)
Z. myretonianum (Li & Cai 1977; Geng 1992)
Z. ovatum (Edwards & Li 2018)
Z. qujingense (Hao et al., 2007)
Z. ramosum (Hao et Wang, 2000)
Z. rhenanum (Kräusel & Weyland 1935)
Z. shengfengense (Hao et al., 2010)
*Z. sichuanensis (Geng, 1992)
Z. spathulatum (Li et Cai, 1977)
Z. spectabile (
Z. tenerum (Hao et Xue, 2013)
Z. xishanense (Hao et al., 2007)
Z. yunnanicum (Hsüe, 1966; Cai et Schweitzer, 1983, Hao, 1985)
