''Distichophytum'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of extinct

vascular plant Vascular plants (), also called tracheophytes () or collectively Tracheophyta (), form a large group of land plants ( accepted known species) that have lignified tissues (the xylem) for conducting water and minerals throughout the plant. They al ...

s of the

Late Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

(Ludfordian) to

Early Devonian The Early Devonian is the first of three Epoch (geology), epochs comprising the Devonian period, corresponding to the Lower Devonian Series (stratigraphy), series. It lasted from and began with the Lochkovian Stage , which was followed by the P ...

(Emsian), around . The genus has a tangled taxonomic history, also being known as ''Bucheria'' and ''Rebuchia'' (see below).

Description

The genus was first discovered as fossils of Early Devonian age (Pragian or Siegenian to Emsian, ), consisting of isolated spikes of

sporangia A sporangium (; from Late Latin, ) is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. Virtually all plants, fungi, and many other lineages form sporangia at some point in their life cyc ...

(spore-forming organs) found at Beartooth Butte, Wyoming, United States of America. Specimens of ''D. ovata'' with sporangia attached to stems were later found at the same location. The base of the plant remains unknown; the known part was about 8.5 cm high. The

sporophyte A sporophyte () is the diploid multicellular stage in the life cycle of a plant or alga which produces asexual spores. This stage alternates with a multicellular haploid gametophyte phase. Life cycle The sporophyte develops from the zygote pr ...

consisted of narrow leafless stems (axes) 1.5 to 2.0 mm in diameter, which branched dichotomously. Stems which did not bear sporangia ended in blunt points; fertile branches bore compact one-sided spikes of up to 20 laterally attached sporangia, more-or-less opposite. The sporangia were kidney-shaped (reniform) and had short stalks around 1.5 mm long which curved so that all the sporangia were on one side of the stem. The sporangia split (dehisced) distally into two equal parts in order to release the unornamented spores. Specimens from the Pragian flora of Bathurst Island, Nunavut, Canada, were later also assigned to this species, although their sporangia were smaller. A second possible species, ''D. mucronatum'', has narrower, less branched stems and smaller, somewhat differently shaped sporangia than ''D. ovata''. Hueber considered the differences in sporangial shape were caused by compression and that the other differences were too small to warrant a different species; Schweitzer put the two in the same genus but as different species. A third possible species was discovered in sediments from Bathurst Island, Nunavut, Canada, from the Late Silurian (Ludfordian, ); it was not assigned a species name as poor preservation obscured the sporangial shape.

Taxonomy

The genus has a somewhat tangled taxonomic history which has been clarified by Kotyk et al. The genus ''Bucheria'' Dorf was created in 1933 for what is now ''D. ovata''. Independently, ''Distichophytum'' Mägdefrau was created in 1938 for ''D. mucronatum''. Later, ''Bucheria'' Dorf was discovered to be homonym of ''Bucheria'' Heynhold of 1846, and the alternative ''Rebuchia'' (an anagram of ''Bucheria'') was suggested provisionally by Høeg in 1967, but only as a form genus for poorly preserved spikes., cited in ''Rebuchia'' was formally established by Hueber in 1970 as a replacement for the invalid ''Bucheria'' Dorf. However, Hueber regarded ''Bucheria ovata'' Dorf and ''Distichophytum mucronatum'' Mägdefrau as the same species; in which case the name ''Distichophytum'' had priority over his ''Rebuchia'' and should have been used as the genus name.

Phylogeny

On the basis of the shape of the sporangia (reniform), their lateral position on the stem, borne on short stalks, and their mode of dehiscence, Hueber placed the genus in the Zosterophyllophytina. A cladogram published in 2004 by Crane et al. agrees in placing ''Distichophytum'' (as ''Rebuchia'' in the original) in a

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

stem group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

of broadly defined "

zosterophyll The zosterophylls are a group of extinct land plants that first appeared in the Silurian period. The taxon was first established by Banks in 1968 as the subdivision Zosterophyllophytina; they have since also been treated as the division Zosteroph ...

s", basal to the lycopsids (living and extinct clubmosses and relatives). Hao and Xue in 2013 listed the genus as a zosterophyll.

References

External links

Cladogram
from {{Taxonbar, from=Q5282940 Early Devonian plants Silurian plants Ludlow first appearances Early Devonian genus extinctions Paleozoic life of Nunavut Zosterophylls Prehistoric lycophyte genera