''Asteroxylon'' ("star-shaped xylem") is an extinct genus of vascular plants of the Division

Lycopodiophyta The lycophytes, when broadly circumscribed, are a vascular plant (tracheophyte) subgroup of the kingdom Plantae. They are sometimes placed in a division Lycopodiophyta or Lycophyta or in a subdivision Lycopodiophytina. They are one of the oldes ...

known from anatomically preserved specimens described from the famous

Early Devonian The Early Devonian is the first of three epochs comprising the Devonian period, corresponding to the Lower Devonian series. It lasted from and began with the Lochkovian Stage , which was followed by the Pragian from and then by the Emsian, ...

Rhynie chert The Rhynie chert is a Lower Devonian sedimentary deposit exhibiting extraordinary fossil detail or completeness (a Lagerstätte). It is exposed near the village of Rhynie, Aberdeenshire, Scotland; a second unit, the Windyfield chert, is locate ...

and

Windyfield chert The Rhynie chert is a Lower Devonian sedimentary deposit exhibiting extraordinary fossil detail or completeness (a Lagerstätte). It is exposed near the village of Rhynie, Aberdeenshire, Scotland; a second unit, the Windyfield chert, is locate ...

in Aberdeenshire, Scotland. ''Asteroxylon'' is considered a basal member of the

Lycopsida Lycopodiopsida is a class of vascular plants known as lycopods, lycophytes or other terms including the component lyco-. Members of the class are also called clubmosses, firmosses, spikemosses and quillworts. They have dichotomously branching s ...

Description

''Asteroxylon'' is a genus of terrestrial

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 ...

which flourished in the Early Devonian period. This plant consisted of aerial, isotomously and anisotomously branching stems that reached 12 mm in diameter and 40 cm in length. The possibly procumbent aerial stems arose from a leaf-less rhizome which bore smaller-diameter, positively geotropic root-like branches. The rhizomes, which represent an independent origin of roots, reached a depth of up to 20 cm below the surface. A 407 million-year-old fossil from the

shows the roots formed through a modified version of a mechanism called “dichotomous branching”, where one of the branches that formed from a shoot-like axis buried into the soil. This method of root formation no longer exists. The xylem or conducting tissue at the center of the aerial stems is distinctly star-shaped in cross-section and has been considered an early actinostele or an "''Asteroxylon''-type"

protostele In a vascular plant, the stele is the central part of the root or stem containing the tissues derived from the procambium. These include vascular tissue, in some cases ground tissue (pith) and a pericycle, which, if present, defines the outermost b ...

. The

tracheid A tracheid is a long and tapered lignified cell in the xylem of vascular plants. It is a type of conductive cell called a tracheary element. Angiosperms use another type of tracheary element, called vessel elements, to transport water through th ...

s are of the primitive annular or helical type (so-called G-type). "Leaves" – not true leaves, but protrusions – were of the form of unbranched strap-shaped enations up to 5 mm long; a single vascular trace branched from the main bundle in the centre of the stem to terminate at the base of each enation. Enations and axes bore stomata, indicating that their tissues were capable of photosynthesis. "Sporangia, consisting of two kidney-shaped valves, are interspersed among the nonvascularized leaflike appendages and attached to the axis with a short pedicel. The sporangia are curved and lie close to the axis." Fertile regions of the axes alternate with sterile regions, suggesting periodic episodes of fertility. ''Asteroxylon'' differs from other similar Early Devonian lycopsids such as ''

Drepanophycus ''Drepanophycus'' is a genus of extinct plants of the division Lycopodiophyta of Early to Late Devonian age (around ), found in Eastern Canada and Northeast US, China, Russia, Egypt and various parts of Northern Europe and Britain. Description ...

'' and '' Baragwanathia'' in that the singular vascular leaf trace in these latter plants extends into the leaf. The leaves of ''

'' and '' Baragwanathia'' are therefore considered to be true

microphyll In plant anatomy and evolution a microphyll (or lycophyll) is a type of plant leaf with one single, unbranched leaf vein. Plants with microphyll leaves occur early in the fossil record, and few such plants exist today. In the classical concept of ...

s or, alternatively, small leaves.

Species

The

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specime ...

is ''Asteroxylon mackiei''. One other species has been described, ''Asteroxylon elberfeldense'', but this is now considered to be generically distinct and assigned to '' Thursophyton''. A fossil originally named as ''Asteroxylon setchellii'' is now considered to be a fern, and is known as ''Stenokoleos setchellii''.

Taxonomy

A 2021 reanalysis found that ''Asteroxylon'' was more closely related to crown lycophytes than the more basal

zosterophylls 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 Zostero ...

and '' Nothia''.

References

The Rhynie Chert and Asteroxylon
{{Taxonbar, from=Q135190 Early Devonian plants Silurian plants Drepanophycales Paleozoic life of Quebec Prehistoric lycophyte genera

Description

Species

Taxonomy

See also

References