Iridopteridales †
Plants with fern-like anatomy, but lacking leaves
Iridopteridaleans were early fern-like/horsetail-like plants, which had complex anatomy, but no laminate leaves. The branches came off in a whorled pattern with recurved stem tips. The combination of morphological and anatomical features suggests that this group may have given rise to the ferns and horsetails. The plants of this group are potentially intermediate between Early Devonian basal euphyllophytes (Kenrick and Crane 1997) and the more complex Late Devonian/Mississippian major groups, for example, sphenopsids and ferns (Arnold 1940; Skog and Banks 1973; Stein et al. 1984; Berry and Stein 2000).
Ecology & Form
Most known Middle Devonian iridopteridaleans come from Laurentia (North America, Svalbard)
Stems
Anatomy has highly dissected, ribbed actinostele with permanent protoxylem strands and whorled trace departure (Berry and Stein 2000; Taylor and Taylor 2008)
Xylem is mesarch
Metaxylem tracheids large in the central part of primary xylem ribs; smaller at periphery
Vascular supplies to both the appendages and the lateral branches are provided by a single xylem rib and originate as a single trace
Proximal axes are pseudomonopodial
Lateral axes are iterative and whorled
Distal axes are isotomous and recurved
Leaves
Leafless
Roots
Unknown
Reproduction
Sporangia in pairs on recurved axes
Classification
Embryophytes
└Iridopteridales †
Although Meyer‐Berthaud et al. (2007) excluded Iridopteridales from the Cladoxylopsida
Geologic Age
Above: Vascular system of Iridopteridales and allies in transverse section. (A)(B) Iridopteris eriensis. (C)(D) Arachnoxylon minor. (E)(G) Metacladophyton tetraxylum. (H) Compsocradus laevigatus. (I) Asteropteris noveboracensis. (J)(L) Arachnoxylon kopfii. (M) Ibyka amphikoma. (N) Metacladophyton ziguinum. (O) Keraphyton mawsoniae. (P) Denglongia hubeiensis. (Q) Rotoxylon dawsonii. (R) Serripteris feistii. (S) Dixopodoxylon goense. Scale bar five mm. Secondary-type xylem in dark grey. (A)(D), (I) (M): adapted from Stein (1982); (E)(G): adapted from Wang & Geng (1997); (H): adapted from Berry & Stein (2000); (N): adapted from Wang & Lin (2007); (P): adapted from Xue, Hao & Basinger (2010); (Q): adapted from Cordi & Stein (2005); (R): adapted from Rowe & Galtier (1989); (S) adapted from Fairon-Demaret (1969).
Diversity
Anapaulia moodyi †
Berry & Edwards 1996
Arachnoxylon †
Read 1938
A. kopfii †
A. minor †
Asteropteris noveboracensis †
Dawson 1881
Compsocradus †
Morphology
Plants pseudomonopodial, with at least three orders of axes known. Lower orders of axes bear both higher-order axes and dichotomous appendages. Ultimate appendages consisting of narrow axes branched predominantly isotomously, successive dichotomies being approximately perpendicular
External surfaces lacking emergences
Fertile appendages terminate in erect or recurved paired sporangia. Sterile ultimate appendages terminate with oppositely recurved tips.
Both ultimate appendages and second-order axes are arranged in loose whorls on first-order axes, with attachments alternating in successive whorls; insertion patterns become less clear in higher-order axes.
Anatomy
Protostele ribbed, mesarch, with six primary xylem ribs; traces arising from every other rib at each whorl, successive whorls alternating; a similar pattern inferred from compression material, some branches and appendages in whorls may occur in neighboring ranks, up to 10 ranks on the largest axes. Protoxylem strands located permanently peripherally, one in each primary xylem rib; tracheids with uniseriate circular to oval bordered pits.
Species
C. laevigatus † (Berry & Stein 2000) from Venezuela
C. givetianus † (Fu et al 2011) from Xinjiang, China
Previously named Rhamophyton givetianum (Wang 2008),
Above, left (a): Compsocradus laevigatus † (Berry & Stein 2000); Above, Center (c) & Right (d): Compsocradus givetianus † (Fu et al 2011)
Dixopodoxylon goense †
Fairon-Demaret 1969
Ibyka †
Middle - Late Devonian
Known from anatomy and morphology
At least three orders of branching are known
Insertions of laterals are dominantly whorled, sometimes imperfectly, with distinct internodes
Within a whorl, branches may substitute for dichotomous appendages, with the latter more numerous
I. amphikoma †
Skog & Banks 1973; Berry et al. 2022
I. vogtii †
Hoeg 1942; Berry 2005
Above: Reconstruction of Ibyka amphikoma (Berry et al. 2022)
Iridopteris eriensis †
Arnold, 1940
Keraphyton mawsoniae †
Stem up to 20 mm in diameter, with primary tissues only.
Vascular system actinostelic, consisting of four fundamental ribs united to a central segment.
Fundamental ribs branching unequally, one branch dividing into two equal ultimate ribs, the other dividing in more ultimate ribs of distally decreasing dimensions.
Protoxylem strands exarch to mesarch, at tip of ultimate ribs. Metaxylem tracheids from 20 to 140 mm in diameter, the smallest ones arranged in 1-2 layers along the lateral edges of the ribs.
Tracheid walls showing scalariform to multiseriate bordered pit pairs with elliptical apertures.
Endodermis-like cells consisting of rectangular cells up to 200 mm high and 120 mm periclinally.
Inner cortical cells up to 160 mm in diameter, thin-walled, polygonal to circular in transverse section, with transverse to oblique endwalls in longitudinal section.
Outer cortex homogeneous, with cells becoming thicker-walled and narrower towards periphery.
