| Horneophyton | |
|---|---|
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| Schematic reconstruction ofHorneophyton lignieri to show its growth habit | |
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| Fossil corm (tuber) inRhynie chert | |
Scientific classification | |
| Kingdom: | Plantae |
| Class: | †Horneophytopsida |
| Order: | †Horneophytales |
| Family: | †Horneophytaceae Kenrick & Crane (1997)[2] |
| Genus: | †Horneophyton Bargh. & Darrah (1938)[1] |
| Species: | †H. lignieri |
| Binomial name | |
| †Horneophyton lignieri (Kidston & Lang 1920) Barghoorn & Darrah 1938 | |
| Synonyms | |
Genus
Species
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Horneophyton is an extinct early plant which may form a "missing link" between thehornworts and theRhyniopsida. It is a member of the classHorneophytopsida.Horneophyton is among the most abundant fossil organisms found in theRhynie chert, aDevonianLagerstätte inAberdeenshire, UK. A single species,Horneophyton lignieri, is known. Its probable femalegametophyte is theform taxonLangiophyton mackiei.
Thesporophyte had bare stems (axes) up to 20 cm high and about 2 mm in diameter with an undivided cortex. Stomata were present but rare. There was a thin central strand of conducting tissue, but this was not reinforced with spiral and reticulate thickenings and thus does not constitute truevascular tissue.[2] Early stages of development of the sporophytes ofHorneophyton (as of hornworts) may have been dependent on their parent gametophytes for nutrition, but mature specimens have expanded, corm-like bases to their stems, up to 6 mm in diameter, that bore rhizoids and appear to be anchored in soil, suggesting a capacity for independent existence after the gametophyte had degenerated.
Thesporangium (spore-forming organ) is unique among both living and fossil plants, consisting as it does of branched lobes at the apex of some of the branches of the stem. Each lobe contains a central collumella, analogous to the sporangia of hornworts; however, the sporangia of hornworts are not branched.[3][4] The number of lobes possessed by a sporangium varied; at least three orders ofdichotomous branching have been found, resulting in more than four lobes. The sporangia were much less regular than shown in most reconstructions (including that opposite), and they had 'bumps' or emergences on them. Spores were released through a slit at the top of each lobe.[5] The sporangia ofHorneophyton contained triletemeiospores, the surfaces of which were decorated with short conical protuberances.[3][6]
The femalegametophyte of the plant has been recognised and described as theform taxonLangiophyton mackiei.[7] It grew to a height of around 6 cm, and was free-living. The species wasdioicous (unisexual), since it produced male and femalegametes on separate gametophytes.[8]
Horneophyton grew on sandy, organic-rich soil in damp to wet locations. They usually grew as isolated individuals.[9]
First named by Kidston and Lang in 1920 fromEarly Devonian fossils in theRhynie chert,[10] the original generic nameHornea was later found to already refer to aflowering plant in the familySapindaceae,Hornea mauritiana, leading Barghoorn and Darrah to propose renaming the genusHorneophyton in 1938.[1] It was classified as arhyniophyte (subdivision Rhyniophytina) by Banks, but the absence of truevascular tissue led Kenrick and Crane in 1997 to create a new class,Horneophytopsida, for this and similar genera.[11] A single species,Horneophyton lignieri, has been described.[2]
A possible phylogeny forHorneophyton is shown below (based on Crane et al. for the polysporangiophytes[12] and Qiu et al. for the bryophytes.[13]
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With vascular tissue but bryophyte-like vascular tissue and sporangia, the organism has been considered a missing link between the hornworts and the vascular plants or tracheophytes (which molecular data suggest are sister groups).[13] Features suggesting a relationship with the hornworts include the general form of its sporangia; its corm also resembles the foot of some hornworts. The free living nature of its sporophytes, and the fact that they branching repeatedly, are marked differences which force it into the stem group of tracheophytes (along withAglaophyton).
