Cladoxylopsids †
Ancient, fern-like plants lacking leaves
The Cladoxylopsids were a geologically short-lived, fern-like group of plants, which displayed complex morphological and anatomical patterns. Compared to the contemporary plants during the late Early Devonian (Emsian), these plants were significantly larger and more complex. They formed some of the first trees on Earth during the Middle Devonian, and some ancestral members of this group may have given rise to the ferns and horsetails. Some taxa exhibited small amounts of wood surrounding individual xylem bundles. None of the members of this group exhibited planated leaves. Instead the ultimate stem tips of the plants were assumed to be photosynthetic. This group was spore-bearing; reproducing through sporangium on ultimate tips. Little is known about the gametophyte phase of this group.
Ecology & Form
Early fern-like plants
Stems
Complex branching
Complex siphonstelic anatomy
Plates of xylem, sometimes single and sometimes united into "U", "W" or other complex arrangements
Ultimate appendages small and photosynthetic
Many taxa exhibit secondary xylem, but it is relatively limited compared to other vascular plants
Periderm has only been observed in Cladoxylon taeniatum
Bark may not preserve well in these plants, or it may have been truly absent and not needed if the cortex actively grew to keep pace with growth
Roots
Strap-like roots
Leaves
No laminate leaves
Reproductive Structures
Sporangia on recurved stalks
Ecology
First large tree-like plants
Lived in near-shore or aquatic environments
Geologic Range
Classification
Diversity
The Cladoxylopsida has a few groups that exhibit the above characteristics:
Although Meyer‐Berthaud et al. (2007) excluded Iridopteridales from the Cladoxylopsida
Non-pseudosporochnalean cladoxylosids
This group is represented below
Adelocladoxis praecox
Mid‐ to late Emsian
Axes, 1–1.5mm thick
Primary xylem is an actinostele with five or six lobes that protrude from the central area
Phloem layer thin around xylem, with thin‐walled fusiform cells
Epidermal cells with robust primary walls and secondary metabolite content fossilized inside as solid rods of dark, compact material
Xylem traces to ultimate appendages hourglass‐shaped, tangentially elongated
Ultimate appendages isotomously branched at least twice, some with recurved tips.
Sporangia erect, attached at tips of ultimate appendage segments, fusiform,
Cladoxylon †
Mississippian from Germany
Anatomically preserved stem axis with many stelar segments
Lateral axes are spirally-arranged
Primary xylem has scalariform tracheids
Secondary xylem abundant in some species, but nearly absent in others (e.g. Cladoxylon radiatum)
Periderm has been observed in some specimens of Cladoxylon (Decombeix & Galtier 2021)
Cladoxylon taeniatum might have had a creeping or climbing habit
Denglongia hubeiensis †
Xue and Hao 2008; Xue, Hao & Basinger 2010
Upper Devonian (Frasnian) of Hubei Province, China
Plant monopodial, with nodose and ridged main axis.
Dense aerial roots attached on basal nodes of main axis.
Whorls of four to six first-order branches, sterile or fertile, inserted at nodes of main axis, separated by conspicuous internodes.
First-order branches, with proximal alternate or subopposite sterile ultimate appendages and distal whorls of second-order branches, three or four at each node.
Sterile ultimate appendages planate or three-dimensional, composed of dichotomous branching systems, with three to six successive dichotomies.
One or two pairs of umbrella-shaped fertile structures, attached suboppositely on proximal portions of first-order branches
Fertile structure with an initial dichotomy forming two sister branches, along which lateral branches are alternately arranged in a pinnate pattern; each lateral branch further dichotomizing several times.
Small, fusiform sporangia in clusters of two to four, terminating short branchlets along inner side of lateral branches. Anisosporous.
Above: Reconstruction of Denglongia hubeiensis (Fig 2, Xue and Hao 2008)
Above: Anatomy of Denglongia hubeiensis † (Fig 2, Xue et al. 2010)
Below: Fertile unit of Denglongia hubeiensis † (Fig 13, Xue and Hao 2008
Foozia minuta †
Lower Devonian (early Emsian) of Belgium (Bois Collet quarry, at Fooz-Wépion, northern margin of the Dinant Synclinorium)
The main axis bears numerous lateral appendages, which are 1x dichotomous
The axes several longitudinal striations; each thought to be a single vascular strand
Fertile axes terminate in one or two pairs of erect sessile sporangia.
Sporangia are semicircular in outline and release spores identical with the sporae dispersae species Dibolisporites echinaceus
The vegetative axes display a flattened distal portions
Its flattened axes might illustrate an early stage of the evolution of the megaphyllous leaf,
Foozia might be a possible ancestor of the Pseudosporochnales
Above: Foozia minuta † reconstruction (Fig 3, Gerrienne 1992)
Panxia gabata †
Middle Devonian (Givetian) Haikou Formation, Yunnan Province southwestern China
Stout stem, two orders of branching, and recurved ultimate units forming more or less planate sterile branching structures laterally
The fertile unit is complex, with many stalked discoidal sporangia aggregated around the second order fertile branch
This plant shares few morphological characters with the only well known and well defined order of cladoxylopsid plants
The primary xylem in main axes consists of numerous primary xylem segments, most of which are radially directed and elongate.
Some xylem segments are connected toward the center to form V, Y, W, or more complex-shaped configurations
One permanent protoxylem strand occurs near the outer tip of each xylem segment of the main axes
A dissected primary xylem column occurs in first-order branches, but the xylem segments are fewer in number and smaller in size than those in main axes
Most xylem segments in first-order branches are unidirectional and parallel in orientation, some are connected to form Y, twice-dichotomous, or more complex-shaped configurations, and one protoxylem strand occurs near each of the two ends of individual xylem segment or a segment group
Metaxylem tracheids are characterized by multiseriate circular bordered pits
Vascular traces depart from the tip of primary xylem segments, produced by elongation and division of protoxylem strands
These traces are of various sizes and shapes, from circular, elliptical to elongate-elliptical in transverse outline, with one or two protoxylem strands
Cortex tissues are mainly composed of thin-walled, approximately isodiametric cells
Above/Below: Anatomy of Panxia
Polypetalophyton wufengensis †
Late Devonian of China
Branches were produced in an irregular helix, but ultimate appendages were borne in a pinnate arrangement on a frondlike system
Vegetative "pinnules" were dichotomous and planar (Sphenopteridium-type), whereas fertile "pinnules" were decussately branched and bore fusiform sporangia terminally
Anatomically, Polypetalophyton is unusual because it has a stele that consists of several primary vascular segments, each of which consists of tracheids and parenchyma surrounded by wood comprising radially aligned tracheids with ray-like slits
Similar in anatomy to Cladoxylon taeniatum
Probably aligned with the Pseudosporochnales.
Polypetalophyton differs from the handful of well-known members of this group in possessing radially aligned xylem and in lacking sclereid nests in the stem cortex.
It is also unique in its plagiotropic pinnate frond-like branching system, which we interpret to be the functional equivalent of a leaf or frond.
Polypetalophyton represents the first evidence of Sphenopteridium-type foliage occurring within Cladoxylopsids and demonstrates that this kind of foliage has evolved independently within at least three distinct plant clades: Zygopterids, Cladoxylopsids, and seed plants.
Above: Foliage of Polypetalophyton
Polyxylon †
Read and Campbell 1939
P. elegans (Read and Campbell 1939)
P. australe (Chambers and Regan 1986)
Late Devonian of Australia
Small stem axis with diameter of 2.7 cm.
Anatomy has 19 elongated arms with mesarch primary xylem
Each of the arms branching at the end of dichotomous and forms at least six traces running in the whorled standing side axes.
Above: Polyxylon australe line drawing of axis (Fig 2, Meyer-Berthaud et al. 2007)
Metacladophyton
M. ziguinum †
Wang & Geng 1997; Berry & Stein 2000; Wang & lin 2007
Eight stelar ribs arranged in four U-shaped strands
Each rib shows one (rarely, two) mesarch strands of protoxylem in peripheral position and produces a C-shaped trace at nodes.
Distribution of the traces within the two to seven branches described from compressions is unclear.
M. tetraxylum †
Wang & Geng 1997
Rotoxylon dawsonii †
Late Devonian of New York, USA
The primary xylem consists of 18 radially oriented and elongate primary xylem ribs.
Toward the center of the system, some xylem ribs are laterally continuous with adjacent ribs, forming elongate xylem segments, whereas others appear discrete.
Order of xylem maturation is mesarch, with one to three protoxylem strands located near the tip of each primary xylem rib.
Primary phloem immediately surrounds each primary xylem rib and consists of sieve cells.
To the outside of the primary phloem, a conspicuous ground tissue is comprised of roughly isodiametric parenchyma with dark contents in their lumina as well as parenchyma cells that are longitudinally elongate with clear lumina.
Beyond the vascular column, a fragmentary cortex contains numerous vascular traces embedded in parenchyma.
These latter cells are surrounded by conspicuous masses of longitudinally elongate cortical thick-walled parenchyma.
Vascular traces depart from the tips of the primary xylem ribs in a whorled phyllotaxis.
Most traces emerge along the midplane of the primary xylem ribs. However, some depart at an oblique angle.
Above: Fig 1 (left) and Fig 6 (right) of Cordi & Stein (2005) of the distal stem of Rotoxylon dawsonii
Serripteris feistii †
Serripteris is probably a transitional form between the ancestral Iridopteridales and the Cladoxylopsida s.s.
Branching system consisting of "main" axis, 3.5-4.5 mm wide with laterals (second-order axes) 2 mm wide, arranged in a complex spiral. Successive laterals 20-25 mm apart, branching angle 35°
Smaller appendages (third-order axes) with circular xylem strands born proximally. The four-ribbed xylem cylinder of the main axis is 1.5 mm wide with metaxylem tracheids & protoxylem elements. Xylem strand of laterals, V-shaped, broad tangentially.
Above: Figure 5 (Rowe & Galtier 1989) of Serripteris feistii LS26001 axis producing appendages I, I1 and III in complex spiral