| Aglaophyton | |
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| Reconstruction of the sporophyte ofAglaophyton, illustrating bifurcating axes with terminal sporangia, and rhizoids. Insets show a cross-section of a sporangium and the probable spores. | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Polysporangiophytes |
| Genus: | †Aglaophyton D.S.Edwards 1986[1] |
| Species | |
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| Synonyms | |
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Aglaophyton major (or more correctlyAglaophyton majus[2]) was thesporophyte generation of adiplohaplontic, pre-vascular, axial, free-sporing land plant of theLower Devonian (Pragian stage, around410 million years ago). It hadanatomical features intermediate between those of thebryophytes andvascular plants ortracheophytes.
A. major was first described by Kidston and Lang in 1920 as the new speciesRhynia major.[3] The species is known only from theRhynie chert inAberdeenshire,Scotland, where it grew in the vicinity of asilica-richhot spring, together with a number of associatedvascular plants such as a smaller speciesRhynia gwynne-vaughanii which may be interpreted as a representative of the ancestors of modern vascular plants andAsteroxylon mackei, which was an ancestor of modernclubmosses (Lycopsida).
The stems ofAglaophyton were round in cross-section, smooth, unornamented, and up to about 6mm in diameter. Kidston and Lang[3] interpreted the plant as growing upright, to about 50 cm in height, but Edwards[1] has re-interpreted it as having prostrate habit, with shorter aerial axes of about 15 cm height. The axes branched dichotomously, the aerial axes branching at a comparatively wide angle of up to 90°, and were terminated with elliptical, thick-walled sporangia, which when mature, opened by spiral slits, so that the sporangia appear to be spiral in form.[4] Sporangia contained many identical spores (isospores) bearing trilete marks. The spores may therefore be interpreted as meiospores, the product of meiotic divisions, and thus the plants described by Edwards and by Kidston and Lang were diploid, sporophytes. The plant was originally interpreted as a tracheophyte, because the stem has a simple central vascular cylinder orprotostele,[3] but more recent interpretations in the light of additional data indicated thatRhynia major had water-conducting tissue lacking the secondary thickening bars seen in the xylem ofRhynia gwynne-vaughanii, more like the water-conducting system (hydrome) ofmosssporophytes. Edwards[1] reinterpreted the species as non-vascular plant and renamed itAglaophyton major.
Aglaophyton is among the first plants known to have had amycorrhizal relationship with fungi,[5] which formedarbuscules in a well-defined zone in the cortex of its stems.Aglaophyton lackedroots, and like other rootless land plants of the Silurian and early Devonian may have relied on mycorrhizal fungi for acquisition of water and nutrients from the soil.
The malegametophyte of the species has been formally described,[6] which was assigned to a newform taxonLyonophyton rhyniensis, but is now properly referred to as anAglaophyton gametophyte. TheRhynie chert bears many examples of male and female gametophytes, which are loosely similar in their construction to the sporophyte phase, down to bearing rhizoids.[7]
Aglaophyton major was first described asRhynia major by Kidston and Lang in 1920.[3] In 1986 D.S. Edwards re-examined fossil specimens and reported that they did not contain truevascular tissue, but rather conducting tissue more similar to that ofbryophytes. As the diagnosis ofRhynia was that it was avascular plant, he created a new genus,Aglaophyton, for this species. (The other species ofRhynia,R. gwynne-vaughanii, was not affected.) AsRhynia major the species had been placed in therhyniophytes, but no alternative higher level classification was proposed for the new genus.[1]
In 2004, Crane et al. published acladogram for thepolysporangiophytes which placesAglaophyton as a sister of thevascular plants (tracheophytes), with theHorneophytopsida being sister to both.[8] The basis of the cladogram is thatAglaophyton has more developed conducting tissue than the Horneophytopsida, but does not have true vascular tissue.
