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Order Isoëtales
Order Isoëtales
The isoëtids are cousins to the lepidodendrids, retaining some of the features of this group (e.g., cormose axes, secondary wood). This order dates back to the Late Devonian, and taxa resembling Isoëtes date back to the Triassic Period.
Living isoetids are called quillworts, a group of grass-like lycophytes in the genus Isoëtes. Modern forms are small with linear leaves, living in nutrient-poor lakes and ponds. In order to identify species, spores (and a microscope) are needed. They have an underground corm, which produces tiny amounts of wood, and roots that can absorb carbon from the soil (unlike any other plant in the world). They are also able to switch between C3 photosynthesis and CAM photosynthesis, and may represent the first plants to create CAM photosynthesis.
Classification
Classification
└Isoëtales
└Chaloneriaceae †
└Nathorstianaceae †
└Pleuromeiaceae †
Geologic range
Geologic range
Late Devonian - present
The clade of rhizomorphic lycophytes, which includes isoetids and lepidodendrids, dates back to the Late Devonian.
Plants resembling the quillworts date back to the Triassic Period.
Chaloneria cormosa †
Chaloneria cormosa †
Late Pennsylvanian of Appalachian Basin
Unbranched, upright plants (~2m tall) with rounded base, bisporangiate fertile regions, and secondary growth
Leaf bases robust, parichnos present, leaf cushions absent
Exarch protostele, medullated above basal region, with radial plates of xylem parenchyma separating groups of tracheids
Leaves with blunt keel and two bands of sunken stomata on abaxial surface of lateral laminae
Thick-walled, trilete megaspores with auriculae, conforming to sporae dispersae genus Valvisisporites
Microspores trilete and bladdered often preserved in tetrads; conforming to sporae dispersae genus Endosporites
Above: Reconstruction of Chaloneria cormosa † (Fig. 1, Pigg & Rothwell 1983)
Changxingia longifolia †
Changxingia longifolia †
Late Devonian of northern Zhejiang Province, China.
It possesses dichotomous axes
Leaves are linear and smooth sterile which may have been persistent
Leaf cushions are rhomboidal with ligule pits and oval–oblanceolate leaf scars
Leaf cushions or bases bearing a ridge are helically arranged in parastichies on wide axes
Smooth megasporophylls with vertically expanded base borne in helices include a pedicel, a heel, and an upturned linear lamina
The pedicel consists of a distinct keel and horizontal alations and the lamina has the distal part reflexed abaxially
A single ellipsoidal and sessile megasporangium occurs on the adaxial side of the pedicel and produces Lagenicula-type megaspores with delicate spines
Changxingia may be assigned to the Dichostrobiles of the Isoёtales sensu lato and is compared with related taxa of the Late Palaeozoic
Fertile units (megasporophyll–sporangium complexes) are interpreted to have functioned as spore dispersal units
Heterosporous lycopsids with monosporangiate strobili are scarce in the Devonian, and Changxingia thus contributes to the understanding of their early evolutionary history and megaspore dispersal mechanisms
Above: Compression fossils of Changxingia † cones (Fig. 2, Wang et al., 2017)
Clevelandodendron ohioensis †
Clevelandodendron ohioensis †
Late Devonian (Famennian) of Ohio, USA
An unbranched, slender (2 cm wide) plant with a partially preserved plant base bearing thick appendages at the base; apically the plant bears a compact, terminal ovoid bisporangiate strobilus
Sporophyll/sporangium complexes attached to axis at 90 angle and extending downward at 60 angle to axis, with prominent distal laminae;
Clevelandodendron demonstrates that slender unbranched lycopsids with an isoetalean plant habit similar to the Carboniferous genus Chaloneria and the Triassic genus Pleuromeia were present as early as the Late Devonian.
The early occurrence of this unique habit suggests that diversification within the isoetalean clade sensu Rothwell and Erwin (including both Isoetales and Lepidodendrales) was well established prior to the Carboniferous
Above: Clevelandodendron ohioensis † (from Figs 1-4, Chitaley & Pigg 1996)
Cyclomeia †
Cyclomeia †
Early Triassic of Australia and Tasmania
Produced terminal bisporangiate, penduculate cones of the Skilliostrobus-type
It possessed helically-arranged, wedge-shaped sporophylls with an adaxial groove containing obovate sporangia (Taylor & Taylor 1997)
The cones were 8 cm wide, and about 4 cm long
C. longicaulis †
Originally named Pleuromeia longicaulis
C. undulata
Above: Reconstructions of Cylomeia undulata † and Cylomeia longicaulis † (Fig 6, White, 1981)
Cyclostigma kiltorkense †
Cyclostigma kiltorkense †
Chaloner, 1968; Schweitzer, 1969
Late Devonian (Famennian) of Kilkenny, Ireland, and Bear Island, Norway
Up to 300 mm in length
Sporophylls are linear, up to 15 cm long
Megasporangiate strobilus
Sporangia are ellipsoidal
Megaspores are 760–1520 µm in size
Ferganodendron †
Ferganodendron †
Dobruskina, 1974
Triassic
Resembles Pleuromeia and Sigillaria
20-30 cm in diameter and covered with numerous, elliptical-rhomdohedral leaf bases that are helically-arranged
Leaves are small and only found don distal portions
Cylostrobus †
Cylostrobus †
Retallack, 1995
Early Triassic of Australia
Similar to Pleuromeia, but with very compact and round cones
The genus Cylostrobus was erected for the compact cone only, in the paleobotanical system of morphotaxon, but these small plants are well enough understood that the name Cylostrobus is used for the whole plant
C. indicus †
Middle Triassic
C. ornatus †
Late Triassic of Argentina
Petrified cone specimens originally named Austrostrobus ornatum (Morbelli & Petriella 1973)
C. sydneyensis †
Middle Triassic
Above: Sporophylls of Cyclostrobus † species (Fig. 1, Retallack, 1997)
Cymastrobus irvingii †
Cymastrobus irvingii †
Large bisporangiate cone that may exceed 8 cm long and 5 cm wide
Cone axis narrow, about 10% the width of the cone, containing a ring of primary xylem showing a corrugated outline with an almost continuous band of exarch protoxylem; sporophyll traces departing from the bays of the primary xylem cylinder.
Sporophyll-sporangium units arranged-helically, about 8-10 per gyre
Sporophylls comprised of a long, narrow pedicel widening distally but without alations, and a delicate distal lamina oriented perpendicularly to the pedicel; pedicels showing an abaxial keel and a distal heel, the latter forming hexagonal shields protecting the sporangia externally
Megasporangia and microsporangia in distinct parts of the cones; megasporangia proximal, enclosing a large number of megaspores
Casts of megaspore central body up to 500 µm in diameter, showing numerous small circular pores arranged in several rows around the trilete mark, smooth elsewhere
Casts of microspore central body less than 100 µm in diameter, showing one small pore between the rays of the trilete mark.
Above: The cone of Cymastrobus irvingii † (Fig. 2, Evreïnoff et al. 2017)
Guangdedendron micrum †
Guangdedendron micrum †
Late Devonian of Xinhang, China
Small tree lycopsid, dioecious and monocarpic
Stigmaria-type rhizomorph with four axes, each of which equally divides at least once and bears helical rootlets nearly undivided.
Stems branch dichotomously into a simple crown with terminal and pendulous megasporangiate strobili
Vegetative leaves linear with entire margin.
[Leaf cushions and] leaf bases narrow-fusiform in shape, helically arranged in parastichies.
[Leaf cushion or leaf base bearing ligule pit and vascular bundle scar. Leaf scars oval.]
Megasporangiate strobili single, paired [or occasionally once-dichotomized], cylindrical with closely helically arranged megasporophylls.
[Each megasporophyll comprised a keeled pedicel and an upturned lamina with a downturned heel. Megasporangium long, ellipsoidal, sessile, and borne singly on the adaxial surface of the pedicel.
Each megasporangium contains multiple Lagenicula-type megaspores with trilete rays and a gula.
Above: Reconstruction of Guangdedendron: A juvenile plant (left) and a mature one (right), from Wang et al., 2019 Fig. 6
Left: Reconstruction of a long strobilus of Guangdedendron, from Goa et al., 2022, Fig. 6
Lepacyclotes †
Lepacyclotes †
L. cicrcularis †
L. convexus †
L. ermayinensis †
L. zeilleri †
Above: Sporophylls of Leptacyclotes † species (Fig 1, Retallack 1997)
Leptophloeum rhombicum †
Leptophloeum rhombicum †
Li et al. 1986; Wang et al. 2015
Late Devonian (Frasnian) Huangchiateng Formation of Hubei, China
300–400 mm in width; 10–25 m in length
This plant shows the lepidodendroid phyllotaxy
The leaf cushion inter-areas are absent.
Leaf cushions are rhombic, about 8–10 mm high and 10–12 mm wide, and the height-to-width ratios appear to be higher in the lower part of the trunk.
In a few instances, a tiny dent can be observed in the upper corner of leaf cushions, possibly representing a ligule pit.
There is an oval leaf scar about 3.0–3.5 mm long and 1.5–2.0 mm wide, approximately in the center of the leaf cushion.
Sporophylls are peltate in outline aggregated into a strobilus
Sporangia are elongate to ellipsoidal
Above: External surfaces of fossil trunk of Leptophloeum rhombicum † (Figs 2 & 3, Wang et al. 2005)
Above: Leptophloeum rhombicum † (Fig 15, Wang et al. 2005)
Lycaugea edieae †
Lycaugea edieae †
Late Devonian (Famennian) of Australia
Axis with helically arranged deciduous leaves
Leaf bases showing a parichnos below a small leaf vascular trace.
Leaf trace and parichnos are located in the upper part of the leaf base.
Parichnos in leaf bases is large, filled with contiguous parenchyma cells showing thicker walls towards the outer region of the leaf bases. Primary vascular tissues consist of a medullated stele.
Protoxylem is continuous, forming a thin ring with a smooth outer border around the metaxylem.
Primary cortex three-zoned, with a wide middle cortex and a homogeneous outer cortex bounded externally by a hypodermis consisting of narrow cells with heavily thickened walls
Leaf traces in the outer cortex are associated with the radially elongated abaxial cavity of the parichnos.
Above: Schematic view of stem cross-section of Lycaugea edieae † (Fig 1, Meyer-Berthaud et al. 2021)
Lycomeia rossica †
Lycomeia rossica †
Neuburg, 1960; Dobruskina, 1985
late Early Triassic or early Middle Triassic
Originally called Pleuromeia rossica
Lycostrobus †
Lycostrobus †
Late Triassic
Morphotaxon for isolated cones thought to be associated with Pleuromeiales
L. chinleanus
L. scottii
Minostrobus chaohuensis †
Minostrobus chaohuensis †
Wang, 2001; Wang et al., 2012
Late Devonian of Anhui, South China
Aerial axes multi-dichotomous
Leaf persistent, simple, linear, with single mid-vein.
Fertile portion with two separate monosporangiate strobili.
Megasporangiate and microsporangiate strobili monoecious, with six sporophylls per gyre.
Sporophyll with single vein, consisting of a lamina with a pointed apex and smooth margins, associated with a single adaxial sporangium.
Sporangia spherical to spherical-elliptical in outline. Each megasporangium with four megaspores, megaspore body with biform ornaments.
Solid exarch primary xylem.
Protoxylem confined to twelve ridges at the peripheral of primary xylem strands.
Metaxylem tracheids bearing Williamson’s striations
Pleuromeria †
Pleuromeria †
Triassic of Germany, France, Spain, Russia, China, and Australia
It was an opportunistic pioneer plant that grew on coastal mineral soils (halophytic) with little competition as monotypic stands (Retallack 1997)
Some specimens from China indicate that those species lived in more xeric, inland sites, near desert oases (Wang & Wang 1982)
Pleuromeia was a herbaceous plant that probably lacks secondary tissues, although there may be some evidence for secondary cortical tissues
and has an unbranched stem of 30 cm long and 2–3 cm wide in the earliest species, to around 2 meters long in later species
The stem supported microphylls that are discarded in the lower part of the stem
It had a 2-4 lobed bulbous base to which numerous adventive roots are attached
Pleuromeia produced a single large cone at the tip of the stem or in some species many smaller cones
The top of the cone carries microsporophylls, the lower part megasporophylls, and both types may be interspersed in the middle.
Sporophylls are shed from the bottom up
Both types are obovate, with a round to ovoid sporangium and a ligule nearer to the tip on the upper/inner side
The trilete microspores are hollow, round and 30–40 μm in diameter
Megaspores have a layered outer skin with a small trilete mark, are also hollow, round to ovoid and up to 300–400 μm in diameter
The anatomy of the spores in Pleuromeia is comparable to that of Isoëtes and substantiates the assumed close relationship between the Pleuromeiaceae and the Isoëtaceae.
P. dubia †
Seward; Retallack, 1995
Late Early Triassic
P. epicharis †
Wang & Wang, 1990
Triassic of Shiqianfeng Group of north China
P. hataii †
Kon'no, 1973
Late Early Triassic
P. jiaochengensis †
Wang & Wang, 1982
Early Triassic of Shanxi Province, China
Smallest species known, at only 50cm tall with 3mm long microphylls
P. rossica †
Neuburg, 1960
Transferred to the genus Lycomeia (Dobruskina 1985)
P. sternbergii †
Corda, 1839; Grauvogel-Stamm, 1990
Early Triassic of Germany
Covered in leaf bases to near the base of the plant
Exhibited 2 types of leaves in a lax position (subhorizontal)
Above: Reconstructions of three coexisting cormose lycopsids from the Early- to Middle- Triassic of Australia (Fig 9, Retallack 1997)
Sublepidodendron †
Sublepidodendron †
Nathorst, 1920; Hirmer, 1927
Early Carboniferous of Norway, China
impressions of very thin bark layers or entire tubes of bark that remained when the plant organs were macerated prior to fossilization
Arborescent, heterosporous lycopsid known from trunk, branches, and cones
Leaf bases are spirally arranged, fusiform in outline, with a vascular bundle scar and keel
The trunk has an intrastelar parenchyma concentration (pith), exarch primary xylem, and secondary xylem
The branch anatomy varies from exarch primary xylem with a small, centrally located pith, to a solid exarch primary xylem strand
S. grabaui
Famennian of Jiangsu, South China
S. mirabile
Nathorst; Hirmer, 1927
S. songziense
Chen 1977; Wang et al., 2003
Late Devonian of China
Monocaulous trunk growing from a stigmarian rhizomorph, the trunk bearing biseriate, sub-opposite to possibly alternate lateral branches that expand by means of isotomous to slightly anisotomous dichotomies, forming an excurrent canopy
Above: Reconstructions of Sublepidodendron songziense † (Plate I , Wang et al. 2003)
Tomiostrobus †
Tomiostrobus †
Early Triassic of Australia and Russia
This plant was especially widespread in the aftermath of the Permian Triassic mass extinctions
Tomiostrobus is preserved as whole plants closely spaced within bedding planes, and lived as an early successional weed in lake and pond sedimentary environments, like living Isoëtes
Unlike living Isoëtes, Tomiostrobus formed closed cones with sporophylls that were distinctly shouldered and woody
This may have been an adaptation to heavy grazing by herbivorous therapsids (Retallack, 1997)
T. australis
T. gorskyi
T. migayi
T. mirabilis
T. polaris
T. radiatus
T. taimyrica
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