Drepanophytes †
Lycophytes with early leaf-like structures
The Drepanophytes (Drepanophycales) are the most ancestral clubmosses, living during the Late Silurian and Devonian, and represent some of the earliest land plants to possess leaves. They represent a lineage that is sister to all other lycopsids. Baragwanathia †, from the Late Silurian, was one of the earliest plants with leaves (microphylls). Some members, such as Asteroxylon †, had leaf-like appendages, which appear transitional between microphylls and prickle-like enations.
Ecology & Form
Herbaceous plants growing in wetland environments
Stems
Morphology
Upright stems with isotomous & anisotomous branching; probably determinate shoots
Small lateral buds on aerial axes
Prostrate rhizomes that were presumably indeterminate
Anatomy
Vascular plectostele or actinostele
Exarch in maturation
Asteroxylon has four-lobed (star-shaped) vascular stele
Roots
Long-lived, below-ground root-bearing axes producing large clones that helped the plants survive frequent sediment burial in well-drained soils within a seasonal wet-dry climate zone
Asteroxylon possessed both root-bearing axes (rhizomes) and rooting axes (Hetterington et al. 2021)
Leaves
Leaf-like enations in some (e.g. Asteroxylon), microphylls in other taxa (e.g. Drepanophycus)
In Asteroxylon, enations, the vascular trace (xylem/phloem) stops at the base of the enation, and does not enter it
Reproduction
Reniform (kidney-shaped) sporangia
Not associated with a sporophyll as in derived lycopods
Not aggregated into cones
Classification
└Lycopsida
└Drepanophycales †
Diversity
Asteroxylon mackiei †
Early Devonian Rhynie Chert of Scotland
Genus name means "star-shaped xylem" for the transverse appearance of the primary xylem
Aerial, isotomously and anisotomously branching stems that reached 12 mm in diameter and 40 cm in length.
Asteroxylon possessed both root-bearing axes (rhizomes) and rooting axes (Hetterington et al. 2021)
The rooting axis was a transitional lycophyte organ between the rootless ancestral state and true roots
The rooting axis developed from root-bearing axes by anisotomous dichotomy
The rooting axes are similar to roots of extant lycopsids, but they lack root hairs and their meristems lack a root cap
The roots of extant lycopsids originate endogenously from shoots, rhizophores
The rhizomes reached a depth of up to 20 cm below the surface.
The tracheids are G-type
Sporangia, kidney-shaped with two valves, are interspersed among the leaflike appendages
Sporangia are curved on a short axis and appressed close to the axis
The form genus Hostimella is now believed to be the sporangia Asteroxylon (Fairon, 1967)
Leaf-like helical appendages (enations) up to 5 mm long; sometimes appear whorled
Enations, with a vascular trace (xylem/phloem) that stops at the base of the structure, and does not enter it, therefore, they lack true microphyllous leaves.
Enations and axes bore stomata, indicating that their tissues were capable of photosynthesis
Above: Reconstruction of Asteroxylon mackiei
Above: Reconstruction of Asteroxylon mackiei (Hetherington et al. 2021)
Left: Cladogram of lycophytes with rooting systems features mapped on (Hetherington et al. 2021)
Baragwanathia †
Late Silurian - Early Devonian of Australia, Canada, China and Czechia
One of the earliest plants with leaves (microphylls)
B. abitibiensis † (Hueber 1983)
B. longifolia † (Lang & Cookson 1935)
B. sp. † (Hao & Gensel)
Drepanophycus †
Göppert 1852
Isotomously or anisotomously branching stems with exarch actinostele
Microphylls are persistent, falcate, arranged in irregular to regular helix, with broad subtriangular bases
Roots adventitious from rhizomes
Sporangia cauline, stalked, dehiscent
D. crepini †
Stockmans, 1940
D. devonicus
Weyland & Berendt, 1968; Schweitzer & Giesen, 1980)
D. gaspianus †
Dawson; Kräusel & Weyland
D. qujingensis †
Li & Edwards, 1995
Lower Devonian of Yunnan Province, China
D. spinosus †
Krejči, 1880; Kräusel & Weyland, ????)
D. spinaeformis †
Göppert, 1852; Li, Hueber, & Hotton 2000
Found in Scotland, Russia, China and Egypt
Above: Reconstruction of Drepanophycus
Hestia eremosa †
Lower Carboniferous (Tournasian) from Oxroad Bay, East Lothian, Scotland
Axes of apparently herbaceous lycopsid, up to 5 mm in diameter.
Stele ranging from 40% of axial diameter in small axes to 20% in larger axes, radial, exarch, stellate
7–10 rounded lobes separated by moderate to deep troughs, each lobe expanding slightly to strongly radially before terminating in a single protoxylem strand
Metaxylem tracheids of broadly similar diameter except where approaching the protoxylem strands, 20–35 μm in diameter, scalariform pits partly infilled with perforated sheets;
Protoxylem tracheids are also scalariform but lacking perforated sheets, 5–12 μm in diameter
Dense inner cortex more or less contacts protoxylem strands, separated by narrow cylindrical void from wider (17–23% of axial diameter) outer cortex
Small, tangentially expanded leaf traces of ca 15 narrow tracheids emitted in alternating helices, probably penetrating microphyllous leaves
Above: Stem cross-sections of Hestia eremosa (from Plate I, Bateman et al., 2007)
Kaulangiophyton akantha †
Early Devonian of Maine from the Trout Valley Formation
Height axes of several decimeters; axes 5-9 mm diameter with trailing and up-right portions that divided by numerous wide dichotomies or H-branches
Sparsely distributed, stout spines of about 2 mm in length
Upright axes included occasional fertile zones with stalks, each of which bore a terminal sporangium
Gensel et al. (1969) claim that this taxon is similar to Asteroxylon
Schweitzer (1980) claimed that this taxon is conspecific with Drepanophycus spinaeformis, but the diagnostic feature of sporangium attachment in Drepanophycus is ambiguous (Gensel and Andrews 1984)
Above: Reconstruction of Kaulangiophyton
Sengelia radicans †
Early Devonian from Wyoming
Stems branched by K-branching; simple dichotomous branching sometimes present
K-branching produces stems and root-bearing axes
Stems with lobed protostele and helical phyllotaxis; leaves vascularized by one vein
Sporangia cauline, round to reniform
Root-bearing axes leafless or with reduced leaves only on axis base.
Roots borne laterally on root-bearing axes, dichotomously branched.
Sengelia had a rooting system of downward-growing root-bearing stems, formed dense monotypic mats of prostrate shoots in areas that experienced periodic flooding
It was characterized by a life-history strategy adapted for survival after floods, dominated by clonality, and featuring infrequent sexual reproduction.
Above: Reconstruction of Sengelia
Smeadia clevelandensis †
Late Devonian of Cleveland, Ohio
Herbaceous plant with a siphonostelic stem and helically arranged leaves. At the distal end was an erect strobilus that contained trilete spores 40-80 pm in diameter.
