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''Hexachara'' is a
In ''H. riniensis'' the laterals divide at about one-half of their length into smaller whorls each of which bears six oogonia and the internodes are uncorticated, and about 0.3 mm in diameter. The nodes bear whorls, about 8 mm in diameter, that consist of six radial laterals. Each lateral divides at about one-half of its length to give rise to a secondary hexaradial whorl each branchlet of which carries an oogonium. ''H. riniensis'' has a very small oogonia attached to the ends of branchlets of secondary whorls and surrounded by extremely fine hair-like tertiary branchlets. Each
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 fossil charophyte
Charophyta () is a group of freshwater green algae, called charophytes (), sometimes treated as a phylum, division, yet also as a superdivision or an unranked clade. The terrestrial plants, the Embryophyta emerged within Charophyta, possibly fro ...
(aquatic green alga
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mi ...
) that is likely to have formed meadows within sheltered oligohaline reaches of lakes.
Etymology
''Hexachara'' is derived from Greek "hex", meaning six, a reference to the hexaradial symmetry; chara, referring to membership of theCharales
Charales is an order of freshwater green algae in the division Charophyta, class Charophyceae, commonly known as stoneworts. Depending on the treatment of the genus ''Nitellopsis'', living (extant) species are placed into either one family ( Ch ...
. The specific name ''setacea'' is derived from Latin "seta", a bristle
A bristle is a stiff hair or feather (natural or artificial), either on an animal, such as a pig, a plant, or on a tool such as a brush or broom.
Synthetic types
Synthetic materials such as nylon are also used to make bristles in items such as br ...
, that is a reference to the thin pointed branchlet. The specific name ''riniensis'' is derived from the Xhosa word ''Rini'', which is a traditional name for Grahamstown/ Makhanda and the surrounding valley.
Description
Thewhorls
A whorl ( or ) is an individual circle, oval, volution or equivalent in a whorled pattern, which consists of a spiral or multiple concentric objects (including circles, ovals and arcs).
Whorls in nature
File:Photograph and axial plane floral ...
of branches are arranged in a hexaradial symmetrical manner. In ''Hexachara'' each node produces a whorl of six laterals, and oogonia
An oogonium (plural oogonia) is a small diploid cell which, upon maturation, forms a primordial follicle in a female fetus or the female (haploid or diploid) gametangium of certain thallophytes.
In the mammalian fetus
Oogonia are formed in larg ...
are produced on each lateral. The known species to date include ''Hexachara setacea'' and ''Hexachara riniensis'' that have been recovered from a carbonaceous shale near the top of the Late Devonian
The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wher ...
, Famennian, Witpoort Formation (Witteberg Group) exposed in a road cutting south of Grahamstown (Waterloo Farm lagerstätte
The Waterloo Farm lagerstätte is a Famennian lagerstätte in South Africa that constitutes the only known record of a near-polar Devonian coastal ecosystem.
History and discovery
The Waterloo Farm Lagerstätte is an approximately 360 millio ...
) in South Africa,. Together with ''Octochara'' species from the same locality these represent the only reconstructable Devonian charophytes with in situ oogonia.
In ''H. setacea'', internode parts of axis are outwardly uncorticated and about 3–4 mm long and 0.5 mm in diameter. The nodes in ''H. setacea'' bear whorls with diameters of about 6 mm and consisting of six radial branches and each branch encompasses a short stalk that gives rise to at least one slender pointed branchlet and carries an oogonium. The compact tips of ''H. setacea'' often formed small 'rollers', following fragmentation of axes. These are commonly enclosed by an organic 'halo', interpreted as a film of green algae utilizing the charophyte as an attachment substratum.
In ''H. riniensis'' the laterals divide at about one-half of their length into smaller whorls each of which bears six oogonia and the internodes are uncorticated, and about 0.3 mm in diameter. The nodes bear whorls, about 8 mm in diameter, that consist of six radial laterals. Each lateral divides at about one-half of its length to give rise to a secondary hexaradial whorl each branchlet of which carries an oogonium. ''H. riniensis'' has a very small oogonia attached to the ends of branchlets of secondary whorls and surrounded by extremely fine hair-like tertiary branchlets. Each oogonium
An oogonium (plural oogonia) is a small diploid cell which, upon maturation, forms a primordial follicle in a female fetus or the female (haploid or diploid) gametangium of certain thallophytes.
In the mammalian fetus
Oogonia are formed in larg ...
is ovoid, about 1.8 mm long and 1.0 mm wide at widest point, and tapers slightly towards point of attachment.
References
{{Taxonbar, from=Q104851205 Fossils Fossil algae Charophyta