Cladoxylopsida †

Ancient, fern-like plants lacking leaves
Cladoxylopsida

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.

Features
  • Biology
    • 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

    • Roots
      • Strap-like roots
    • Leaves
      • No laminate leaves
    • Reproductive Structures
      • sporangia on recurved stalks
Cladoxylopsid taxa - incertae sedis

Cladoxylon † (Unger 1856)
  • Mississippian from GermanyCladoxylon
  • Anatomically preserved stem axis with many stelar segments
  • Lateral axes are spirally-arranged
  • Only primary xylem with scalariform tracheids 
Denglongia hubeiensis (Xue and Hao 2008)


Foozia minuta (Gerrienne 1992)
  • 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 
Panxia gabata (Wang & Berry 2006Xue et al. 2015)
  • 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
Polypetalophyton wufengensis † (Hilton et al. 2003)
  • 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
  • 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.