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Rhacophytes †

Order Rhacophytales

The Rhacophytes are spore-bearing, fern-like plants that existed during the Middle and Late Devonian. They were probably tall shrubs that grew in monotypic stands in open floodplain environments (deltaic marshes and peat-accumulating backswamps). During the Late Devonian, they were co-dominant with Archaeopteris in drier portions of floodplains, growing in the understory, fueling low-intensity fires. They lacked laminate leaves, but their terminal appendages were flattened (no webbing) to collect light and photosynthesize. Internally, these lateral structures have fern-like primary anatomy, but they also exhibit wood, which is never found in fern leaves.

Ecology & Form

Stems

Rhizome

There are some indications that Rhacophyton employed clonal propagation to occupy and monopolize preferred habitats.
Dense mats of adventitious roots are commonly found along the main stem and on the laterals.
Instead of a cluster of separate and erect shrubs, Rhacophyton may have adopted a sprawling habit with the main axis and some laterals functioning like rhizomes

Upright shoots

Tall shrub with a central main stem 1-1.5 m in height (Cornet et al. 1976)
Branch system is semi-erect and frond-like
Laterals exhibit a quadriseriate branching pattern, which exhibit two vertical rows of alternating sets of paired branches
Laterals extended as much as 50 cm and exhibited a variety of morphologies.
- Vegetative laterals typically exhibited a biseriate arrangement (two lateral rows forming a plane) of side branches (pinnae) reminiscent of fern fronds
  - Laterals were true branches containing small amounts of secondary xylem (fern fronds are compound leaves)
  - Secondary growth may have been from unifacial cambium; only wood production and not secondary phloem
  - Secondary growth only in periclinal divisions; not anticlinal to add more cambial initials as stem increases in girth
- They can also exhibit a 3-dimensional quadriseriate side branching, often at the distal ends of otherwise biseriate laterals
- No lamina or blade on stems; photosynthetic terminal axes
  - The pinnae- and pinnule-like ultimate branches vary in their degree of flattening, but they are not webbed (no laminar tissues extending between veins).
Stem consists of a central, clepsydroid-shaped strand of primary xylem
Primary xylem surrounded by radially aligned rows of secondary xylem
Peripheral loops occur at the end of the primary xylem bar

Leaves

No distinct leaves since there is no lamina or blade on stems

Classification

└?Rhacophytales †

- The systematic position of Rhacophyton is uncertain.
- Andrews & Phillips (1968) connected this group to progymnosperms
- Cornet et al. (1976) concluded it was neither a progymnosperm nor a fern
- Non-fern traits: possess secondary xylem in their lateral branches.
- Fern-like traits: distinctive stelar morphology found only in ferns with permanent protoxylem poles
- Most place Rhacophyton within the monilophytes but outside the living fern lineages.

Geologic Age

Middle Devonian - Late Devonian
Fossils found in New York, Virginia, West Virginia, Maine, Belgium, Germany, Norway, and western Siberia.
By the Late Devonian (Famennian; 360 Ma), the pre-fern Rhacophyton was widespread in riparian river-margin and lake-margin marshes (Scheckler, 1986).
Urpicalis steemansii may represent an Emsian member of the rhacophytes, but lacks too many features to confirm

Reproduction

Three-dimensional structure that appears loosely ball-shaped
Main axis forks many times to produce both long sterile axes and highly branched fertile axes
Probably homosporous
Fertile lateral segments have a 3-dimensional quadriseriate arrangement, typically with two elongate sterile pinnae and two highly branched fertile pinnae. Fertile segments have been found on the distal ends of otherwise sterile laterals, but laterals bearing fertile structures have yet to be found attached to the main stem. Rhacophyton is probably homosprous. In other words, it produces only one type of spore.

Above: Compression of Rhacophyton †

Diversity

Cephalopteris

C. keilhaui †

Famennian of Sweden

C. mirabilis †

Famennian of Sweden

Chlidanophyton dublinensis †

Gensel, 1973
Late Devonian - Early Tournaisian

Eocladoxylon minutum †

Koidzumi, 1943; Berry & Wang, 2006
Middle Devonian of Yunnan province, China,
Demonstrate three orders of branching.
Sterile branching systems have a pinnate two-dimensional branching pattern with alternate insertion of lower orders.
Up to several times deeply dissected, overall wedge-shaped, flattened appendages are inserted alternately on the third order of branching.
Second and third orders of branching have an opposite pair of similar appendages inserted at the base, perpendicular to the plane of the remaining branching system.
Fertile appendages, consisting of a dichotomous axial system and terminal pairs of sporangia attached closely in groups of eight, replace the sterile appendages in fertile branches.
Anatomy of a probable second-order axis demonstrates a bipolar clepsydroid vascular system.
The sterile branching system and anatomy bear some similarity to the Late Devonian fernlike plant Rhacophyton.
The potential for an ancestor/descendant relationship between Iridopteridales and Rhacophyton

Above: Reconstruction of Eocladoxylon †

Melvillipteris quadriseriata †

Xue & Basinger, 2016
Late Devonian (Famennian) age, from Melville Island, Nunavut, Canadian Arctic Archipelago.
Main axes and two orders of lateral branches are recognized.
The main axes are monopodial in habit, with a zigzag and/or upright appearance and dense adventitious roots attached to some internodes.
First-order branches are inserted on the main axes in distichous pairs, showing a quadriseriate pattern (i.e., alternate pairs); they bear one or two sterile ultimate appendages on the proximal portions, and then distally bear alternate second-order branches.
Second-order branches bear alternate ultimate appendages, which may be fertile or sterile; the sterile ultimate appendages are composed of one or two successive dichotomies, terminating in oppositely recurved tips, and fertile appendages show an elaborate branching system.
Each fertile appendage has an initial dichotomy forming two sister branches, each of which further divides three or four times to produce ultimate divisions terminated by sporangia.
Two to four small fusiform sporangia are grouped in a truss, which is supplied by the penultimate division of the fertile appendage.
The sporangia have a pointed tip and a longitudinal dehiscence.

Protocephalopteris praecox †

Ananiev, 1960; Schweitzer, 1968
Originally named Cephalopteris praecox

Protopteridophyton devonicum †

Li & Hsu, 1987
Givetian of China

Rhacophyton ceratangium †

Crepin, 1875; Andrews and Phillips, 1968; Cornet et al., 1976
Eifelian - Late Devonian
R. condrusorum
- Crepin, 1875
R. incertum
- Kräusel & Weyland, 1941
R. mirabile
- Leclercq, 1950
R. zygopteroides
- Leclercq, 1950
- Late Devonian of Belgium

?Urpicalis steemansii †

Gerrienne, 1992
Early Devonian (Emsian) plant of Fooz-Wepion (northern margin of Dinant Synclinorium, Belgium)
Herbaceous plant made of smooth sporangiferous axis systems, isotomously branched, probably borne laterally on wider smooth axes. Distance between two successive isotomies, variable.
Sporangia fusiform to oval-elongated, one often borne at the axil of the isotomy and the others inserted in one row on the adaxial side of the isotomously branched axes.
Sporangium orientation parallel to the isotomy bisector.
Epidermal cells elongated with tapering ends.
Spores and plant anatomy unknown.
This incertae sedis taxon is reminiscent of the fertile axes of Aneurophyton and may represent one of the first steps leading to the progymnosperms

Additional Resources

Devonian Times

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