''Asteroxylon'' ("star-shaped xylem") is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

genus of vascular plants of the Division Lycopodiophyta known from anatomically preserved specimens described from the famous

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 Pr ...

Rhynie chert The Rhynie chert is a Lower Devonian Sedimentary rock, 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 ...

and Windyfield chert in

Aberdeenshire Aberdeenshire (; ) is one of the 32 Subdivisions of Scotland#council areas of Scotland, council areas of Scotland. It takes its name from the Shires of Scotland, historic county of Aberdeenshire (historic), Aberdeenshire, which had substantial ...

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

Description

''Asteroxylon'' is a genus of terrestrial

vascular plant Vascular plants (), also called tracheophytes (, ) or collectively tracheophyta (; ), are plants that have lignin, lignified tissues (the xylem) for conducting water and minerals throughout the plant. They also have a specialized non-lignified Ti ...

which flourished in the Early

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

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. The

tracheid A tracheid is a long and tapered Lignin, lignified cell in the xylem of Tracheophyta, vascular plants. It is a type of conductive cell called a tracheary element. Angiosperms also use another type of conductive cell, called vessel elements, to t ...

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 In botany, a stoma (: stomata, from Greek ''στόμα'', "mouth"), also called a stomate (: stomates), is a pore found in the epidermis of leaves, stems, and other organs, that controls the rate of gas exchange between the internal air spa ...

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

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 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