Ancestral euphyllophytes †
Trimerophytes
Early leafless plants with three-dimensional branching
Ancestral euphyllophytes, sometimes called "Trimerophytes", are an extinct group of mostly leafless plants, living during the Devonian Period. Trimerophytes get their name from the three-lobed shape of the xylem in stem cross-section. This shape is considered an advancement over the cylindrical shape of the rhyniophytes and cooksonioids, allowing for greater surface area to provide water conduction to lateral organs such as branches and leaves. Also, these plants had a pseudomonopodial branching pattern providing upright growth, accompanied by lateral branches. The specialization of branches allowed for structures that could be relegated to support, or photosynthesis, or reproduction, which was specialized for the early Devonian. This group indirectly gave rise to the cladoxylopsids, the horsetails, the ferns, as well as the progymnosperms & woody plants, making them the ancestors of most living plants.
Ecology & Form
Stems
Morphology
Anisotomous branching in proximal portions
Isotomous branching in distal axes
Anatomy
Actinostele pattern
Centrarch xylem development
Leaves
No leaves(?)
See Eophyllophyton bellum (below) from China which exhibits rudimentary megaphyllous leaves
Enations in some
Roots
Adventitious roots arising from a rhizome is assumed
Reproductive structures
Sporangia on bifurcating distal tips after many branching events
Some fertile tips are recurved, pointing back at the main stem
Right: Proposed relationships between early euphyllophytes ("trimerophytes"), Stenokoleales, and Aneurophytales (From Fig. 5, Toledo et al. 2021)
Classification
└Euphyllophytes
└"Trimerophytes" †
Most basal taxa of the euphyllophyte clade; probably not monophyletic, and represent an ancient grade of early euphyllophyte plants
Geologic Age
Silurian (Ludlow) - Late Devonian
Kotyk et al., 2002 may be earliest record
Diversity
Armoricaphyton chateaupannense †
late Pragian-early Emsian of the Chalonnes Formation in western France
Small, leafless, longitudinally-ribbed axes that branch pseudomonopodially.
Pairs of fusiform-shaped, twisted sporangia preserved as adpressions were found in association with the axes and may belong to this species.
Permineralized specimens reveal the oldest documented wood (secondary xylem) of any known fossil plant.
The water-conducting tissues or tracheids consisted of extinct P-type cell walls.
This type of cell wall consisted of scalariform bordered pits and perforated sheets that covered the openings (apertures) of the pits.
This plant is anatomically similar in some respects to other euphyllophytes, such as Psilophyton and Franhueberia.
Above: 3D and 2D specimens of Armoricaphyton chateaupannense showing xylem with secondary growth (from Strullu-Derrien et al. 2014)
Chaleuria cirrosa †
Early-Middle Devonian of Gaspe Bay, Quebec
Main axis bearing closely spiraled, monopodial lateral axes, which in turn bear spirally-arranged dichotomous ultimate axes
Pairs of sporangia terminate some ultimate axes
Sporangia bear spores of two sizes, which also differ from one another in shape and ornamentation.
Sporangia contained predominantly large, or predominantly small spores, or a mixture of both
This plant is evidence of euphyllophyte heterospory in the Early Devonian (Emsian)
Above: Chaleuria cirrosa specimen and reconstructions
Douaphyton levigata †
Middle Devonian sediments of Xinjiang China
Three orders of axis branching; each axis being less than 2 mm wide
The second-order axes are short, laterally and alternately attached to the main axis.
The third-order axes are paired and anisotomously divided, bearing the vegetative appendages or the fertile units.
The fertile unit consists of a short recurved axis giving off up to four short pedicels along one side, each of which bears one to four pairs of terminal sporangia
Above, left & center: Vegetative specimen of Douaphyton levigata; Above, right: Fertile specimen of Douaphyton levigata (Xu et al. 2017)
Hao & Beck, 1993; Hao et al., 2003
Pragian (Early Devonian) of Yunnan, China
Plant represents the earliest record of megaphyllous leaves
The leaves of Eophyllophyton bellum were borne laterally or terminally on the axis.
They are small and fan-shaped, with the laminae highly dissected; are either vegetative or fertile; and range in width from 1.7 to 4.8 mm, in length from 2.2 to 5.0 mm, and in thickness from 40 to 200 mm.
They seem to occur in clusters at the ends of small lateral branches. Their dichotomous pattern of division reflects an isotomous or anisotomous venation, with each subdivision of the lamina served by a single vein.
In some instances, the pattern of division of the lamina is almost pinnate
Franhueberia gerriennei †
Early Devonian (Emsian) sediments in the Battery Point Formation of the Gaspe Bay
Basal euphyllophyte with a centrarch protostele and metaxylem tracheids with circular and oval to scalariform bordered multiaperturate pits (P-type tracheids).
The outer layers of xylem, consisting of larger-diameter P-type tracheids, exhibit the features diagnostic of secondary xylem:
Radial files of tracheids
Multiplicative divisions
Combination of axial and radial components
Predate the evolution of stem-leaf-root organography and indicate that underlying mechanisms for secondary growth became part of the euphyllophyte developmental toolkit very early in the euphyllophyte evolution
Above: Anatomy of Franhueberia, one of the earliest plants to show secondary growth
Gmujij tetraxylopteroides †
Early Devonian (Emsian) of the Gaspe Peninsula
Small axes with four-lobed primary xylem and secondary xylem.
Primary xylem maturation mesarch, with several shallow protoxylem strands along midplanes of xylem lobes and one central protoxylem strand.
Primary and secondary xylem tracheids with Psilophyton-type secondary wall thickening.
Multiplicative divisions and rays present in secondary xylem.
Above: Stem anatomy of Gmujij tetraxylopteroides (From Fig. 1, Pfeiler & Tomescu 2020)
Hostinella †
Barrande 1882
Upper Silurian (Ludlow) of Bathurst Island, Canada, and Early Devonian Senni beds
Dichotomously branching stems (axes); stems are approximately 1.2 mm in diameter
Axes have a striated external appearance
Axes contain xylem with tracheids, 40 µm in diameter.
No known sporangia
It is known to co-occur with the barinophyte, Krithodeophyton
H. bohemica
Kräusel & Weyland 1932
H. crispa
Arnold 1939
H. globosa
Lang 1925
H. heardii
H. hercynica
Mägdefrau 1938
H. hostinensis
Barrande 1882
H. racemosa
Lang 1925
H. racheneuri
Ledoux-Marcelle 1927
H. silurica
Banks 1974
H. strictissima
Høeg 1942
H. wahnbachensis
Kräusel & Weyland 1935
Above: A-C) Hostinella; D) Psilophytites (From Fig. 4, Milagres et al, 2018)
Kunia venusta †
Late Middle Devonian (Givetian) Haikou Formation near Kunming City, Yunnan Province, China
This plant has three orders of naked axes that divide pseudomonopodially
The second-and third-order axes occur in a helix
Fertile appendages are distantly spaced and helically inserted to the third-order axis
They comprise equally dichotomous branches terminated by two clusters of paired and fusiform (spindle-shaped) sporangia
Sterile appendages are dichotomous and distally recurved
Above: Specimen PKU-XCH03 of Kunia from Xiong et al. 2012
Kenrickia bivena †
Emsian (late Early Devonian) of Gaspe Bay, Quebec
Plant with main axes bearing four-ranked, decussate, subopposite branches with dichotomous ultimate appendages
Main axes with central, deeply lobed, four-ribbed protostele
Primary xylem maturation mesarch
Protoxylem with radiate architecture: protoxylem strands present at center of stele, along xylem rib midplanes and at rib tips
Metaxylem tracheids with Psilophyton-type pitting
Secondary xylem present around primary xylem, tracheids with P-type pitting on radial and tangential walls
Inner cortex with sclereid nests forming discontinuous layer external to primary xylem
Outer cortex with alternating longitudinal bands of sclerenchyma and parenchyma
Pairs of Y-shaped bundles diverging tangentially from tips of primary xylem ribs form traces that supply branches
Protoxylem gaps present at the two tips of T bar formed by primary xylem rib proximal to trace divergence
Primary branches with bilateral symmetry of vascular tissues arranged in two Y-shaped bundles
Secondary xylem absent in branches
Discontinuous layer of sclereids in inner cortex, surrounding the two Y-shaped bundles.
Outer cortex primarily sclerenchymatous
Ultimate appendages are supplied by two terete bundles and with an anatomy similar to primary branches
Above: Five stages in the anatomy of trace divergence of Kenrickia bivena
Below:
Leptocentroxyla tetrarcha †
Bickner & Tomescu 2019 emend.; Tomescu & McQueen 2022
Small axes (~2 mm diameter) circular-subcircular in cross-section, with offset opposite decussate pseudo-whorled branching.
Architecture probably iterative (sensu Berry and Stein 2000).
Primary xylem ~1.1 mm across, a four-ribbed actinostele, with thick diamond-shaped central area and slender ribs.
Inconspicuous mesarch, shallow protoxylem strands at rib tips.
Metaxylem tracheids up to 60 μm wide, with angular, polygonal outlines and Psilophyton-type secondary wall thickenings, except for conspicuous central area of thinner-walled tracheids with scalariform thickenings derived from simplified Psilophyton-type thickenings
Xylem traces to lateral appendages radially elongated, lacking protoxylem, with central area occupied by well-defined core of thinner-walled tracheids derived from simplified Psilophyton-type thickenings
Inner cortex wide, parenchymatous, with groups of large sclerenchyma forming a discontinuous layer around the xylem
Outer cortex thin, primarily sclerenchymatous
Epidermis of radially flattened parenchyma
Nebuloxyla mikmaqiana †
Emsian (~400 Ma) from Quebec (Canada)
Plant with thin axes with protosteles
Central xylem strand with tracheids displaying Psilophyton-type wall thickenings
Xylem consists of mixed tracheids of variable size and lacks a distinguishable pattern of maturation
Protoxylem absent or randomly distributed within the xylem
Cortex differentiated into an inner parenchymatous layer and an outer sclerenchyma layer
Plant capable of periderm-type wound response
Oocampsa catheta †
Andrews et al. 1975
Plant from the early Middle Devonian of New Brunswick
Central axis with spirally arranged first-and second-order branches
Second-order branches dichotomize some what irregularly to produce ultimate fertile branchlets, located at varied distances along the second-order axes, so that the terminal, erect sporangia occur over a linear distance rather than in dense terminal clusters.
The sporangia are ovoid with pointed apices and contain spores that are trilete, zonate, and ornamented with cones and spines
Above: Oocampsa catheta specimen and reconstruction (from Andrews et al. 1975)
Pauthecophyton gracile †
Lower Devonian (Pragian) Posongchong Formation near Zhichang Village of Wenshan District, southeastern Yunnan, China.
This plant consists of pseudomonopodially and dichotomously branched main axes and lateral branches.
Some laterals are pseudomonopodially divided producing three to four times dichotomously branched systems terminating in sporangia; other branches are isotomously divided six to eight times terminating in sporangia.
Fusiform sporangia are grouped as fertile units; each unit, generally with two, three or four sporangia, is subtended by a branch which generally divides once or twice.
Sporangia lacking twisting have a longitudinal dehiscence and contain similarly sized spores with laevigate and folded exine.
Pauthecophyton probably occupies a basal position within the euphyllophytes, as one of the earliest divergent clades.
It is more similar to the genus Psilophyton than to others in branching, as well as sporangial shape, terminal position and longitudinal dehiscence
Pauthecophyton and Psilophyton are interpreted as "leafless", but have distal axes with lateral, three-dimensional, synchronously dichotomous trusses that have been called “incipient fronds”, “pinnules”, “dichotomous pinnule-like appendages”, and “proto-leaves” (Kenrick & Crane 1997; Beerling & Fleming 2007)
These were referred to as Branch-Leaf Complexes (BLC) by Hao & Xue (2013)
Above: Pauthecophyton gracile specimen and line drawing
Perplexa praestigians †
Mid-to-late Emsian, South shore of Gaspe Bay, Quebec, Canada
Axes with protostelic primary xylem, with or without surrounding layer of secondary xylem
Primary xylem terete, mesarch, with variable number of protoxylem strands distributed equidistantly close to the periphery of the metaxylem and decreasing in number distally
Metaxylem tracheids with scalariform thickenings, bordered pits and Psilophyton-type thickenings
Secondary xylem with multiplicative divisions and a radial component consisting of radially expanded tracheids and radially curved tracheid ends
Secondary xylem tracheids with Psilophyton-type thickenings
Inner cortex parenchymatous; outer cortex robust, sclerenchymatous, with gaps representing substomatal chambers
Early Devonian - Middle Devonian of USA and Canada
Pertica may represent the ancestral member of the Radiatopses and sister to the Lignophyte clade
Pe. dalhousii
Doran et al. 1978
Eifelian of North America; Canada; New Brunswick; Dalhousie Junction; Near Dalhousie Junction.
Pseudomonopodial main axis
Spiral-arrangement of laterals
Pe. quadrifaria
Eifelian of Trout Valley Formation in Baxter State Park near Mount Katahdin.
Tetrastichous lateral branching; may be a step in the evolution of whorled branching.
Pe. varia
Granoff et al. 1976
Emsian of Eastern Canada.
Taller than Pe. quadrifaria, reaching a height of nearly 3 m.
The sporangia were similar to those of Pe. quadrifaria, although there were fewer in each cluster.
Above: Fossils and recosntructions of Pertica quadrifaria
Planatophyton hujiersitense †
Late Early or Middle Devonian locality from the Hujiersite Formation in the Hujiersite area, Hoboksar County, North Xinjiang, North-West China
The plant is possibly rhizomatous and comprises main axes with alternately inserted lateral branching systems
All axis orders bear large triangular enations all over their surface
Lateral branching systems are up to six times dichotomous
Dichotomies in lateral branching systems occur in the same plane, which results in bilaterally symmetrical, two-dimensionally branched organs, with limited overlapping between any segments
Distal segments in sterile lateral branching systems look slightly flattened and/or webbed
The ultimate segments of fertile lateral branching systems bear Psilophyton-like pairs of pendulous twisted sporangia
Anatomy and spores of the plant are unknown
Above: Reconstruction of Planatophyton hujiersitense
Pleurorhizoxylon yixingense †
Three-lobed protostele with mesarch primary xylem, and single protoxylem lacuna in each rib
Thick aligned metaxylem with rays, appearing like secondary xylem, but no cambium or secondary phloem found
Adventitious roots found opposite each primary xylem rib
This is the oldest non-cladoxylopsid to exhibit adventitious root growth
Above: Cross section of the stem anatomy of Pleurorhizoxylon yixingense † (From Fig. 2, Zhang et al. 2018)
Polythecophyton demissum †
Pragian age in the Wenshan district of southeastern Yunnan
Major axes divide anisotomously to produce irregularly to helically arranged lateral branches
Lateral branches dichotomize once, producing a pair of curved axes; each may dichotomize again, and then produce a series of short, pendulous, lateral fertile systems via unequal bifurcations
Each fertile system may initially bifurcate, and then each component bears 3–4 alternately arranged, short axes terminated by numerous slender fusiform sporangia borne in pairs or in groups of three or four
Collectively, the fertile system is umbrella-like in outline and sometimes bilaterally symmetrical, with a convex upper contour and nearly even lower margin because all sporangia terminate at almost the same level
In some characteristics i.e. shape of the fertile system, type and attachment of sporangia, it shows similarities with Hedeia, but it differs in its more complex, mostly pinnate distal branching pattern and its pendulous form
Similar in some aspects of branching, sporangial morphology, and sporangial attachment to aneurophytalean progymnosperms
Above: Fertile portion of Polythecophyton
Above: Reconstruction of fertile portion of Polythecophyton
Psilophyton †
Dawson 1859; emend. Hueber & H.P.Banks 1967
Ps. bellum
Dou et al. 1983
May not be a valid taxon
Ps. charientos
Gensel 1979
Ps. coniculum
Trant & Gensel 1985
Ps. crenulatum
Doran 1980
Ps. dapsile
This species may represent one of the more ancestral members of this genus
Ps. dawsonii
Banks et al. 1975
late Emsian (Early Devonian)
This species exhibits more complexity in branching than other Psilophyton species
Ps. diakanthon
Fibers form discontinuous layer in inner cortex and two distinct types of spinescent emergences may be anti-herbivore defense.
Ps. forbesii
Andrews et al. 1968
Pseudomonopodial main axis
Spiral-arrangement of laterals
Ps. genseliae
Gerienne 1997
Ps. goldschmidtii
Dou et al. 1983
Ps. krauselii
Obrhel 1959
Ps. microspinosum
Kasper et al. 1974
Ps. parvulum
Gerienne 1995
Ps. primitivum
Hao & Gensel 1998
Ps. princeps
Hueber & Banks 1967
Ps. szaferi
Zdebska 1986
Above, left: stem xs showing 3-lobed xylem; Above, right: Reconstructions of Psilophyton
Below: Psilophyton diakanthon three-dimensional reconstruction of axis and spines as seen from five different angles. Asterisks mark the tips of spines of two distinct morphological types: shorter, sturdier, barb-shaped spines with short-tapering apically curved tips (white asterisks) and longer, straighter spines that diverge obliquely upwards from the axis at relatively sharp angles (orange asterisks); arrowheads mark ‘dimples’ in the axis surface interpreted as marking the locations of sunken stomata. Scale bar = 1.5 mm (Colston et al. 2022)
Shougangia bella †
Late Devonian (Famennian), Wutong Formation of Anhui and Jiangsu Provinces, South China
Fern-like euphyllophyte that evolved the earliest known laminate structures (leaves?)
Stems contain a dissected stele with a four-ribbed and an elongate curved primary xylem segments changing to three elongate and slightly curved segments.
Primary branches have a dissected stele with three similar primary xylem segments as in stems.
The primary xylem of the stems and primary branches is mesarch, and individual primary xylem segments bear peripheral protoxylem strands and are surrounded by secondary xylem.
Tenuisa frasniana †
Late Devonian (Frasnian) from Hunan Province of South China
Axes are naked and narrow
Reproductive:
Fertile stem is pseudomonopodial in branching and bears distichous laterals
Fertile laterals are subopposite and pseudomonopodial in the proximal part of stem
They become alternate and isotomous two to four times in the distal part.
Fertile laterals have tiny terminal sporangia that are paired and fusiform and that possess pedicels.
Vegetative:
Sterile ultimate units are dichotomous and terminate in tips recurving oppositely.
Tenuisa resembles Psilophyton in pseudomonopodial fertile stem with alternate and dichotomous lateral units terminated by paired and vertically elongate sporangia.
However, fertile lateral units are subopposite and pseudomonopodial on the basal part of stem, sporangial pedicels are present, and neither sporangial pairs nor sporangia themselves are twisted.
Above: Tenuisa frasniana axes exhibiting fertile unit (Wang 2007)
Tsaia †
Wang et al. 2004
late Middle Devonian (Givetian) of North Xinjiang, China
T. conica
T. ungulata
Trimerophyton robustius †
Dawson 1859
Wilhowia phocarum †
Early Devonian (Emsian) of Gaspe Bay, Canada
Plant with at least three levels of branching; perhaps up to 1 m tall
Main axes branch pseudomonopodially, bearing helically arranged, single or paired, long first-order axes
First-order axes produce helically arranged, cruciately dichotomous ultimate axes, terminating in acuminate, recurved tips or in clusters of paired, fusiform sporangia.
Sporangia in each pair curve or bend around each other near distal tips
All axis orders covered with small, sharply tapered, multicellular emergences.
Main axis a mesarch, three-ribbed actinostele composed of primary xylem with P-type and occasionally circular pitting
Protoxylem present at center and near the tips of each rib, sometimes represented by lacunae and evident as median bands or elongate regions in some ribs
Xylem parenchyma absent.
Trace formation via tangential elongation of rib tip, then separation of an oval trace with central elongate protoxylem.
Cortex two zoned, inner cortex not preserved, outer cortex with irregularly patterned, thin- to moderately thick-walled cells.
Paired first-order branches with cortex of thinner-walled parenchyma cells produce a vascular trace with central protoxylem dividing either before or after its separation from ribs, resulting in two and sometimes three (perhaps four?) oval traces
First-order traces elongate at one end to produce a smaller round-oval second order trace
Epidermis consisting of small, tightly packed, square to rectangular cells
Undoubted stomata not present, but gaps in epidermis/ outer cortex present.
Emergences multicellular, broader at base, and sharply tapered. Cortex composed of parenchyma cells of varying thickness, possible remnants of phloem present.
