''Hicklingia'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of extinct plants of the

Middle 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, w ...

(around ). Compressed specimens were first described in 1923 from the

Old Red Sandstone The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the northeastern seaboard of North America. It also exte ...

Scotland Scotland (, ) is a Countries of the United Kingdom, country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a Anglo-Scottish border, border with England to the southeast ...

., cited in Initially the genus was placed in the "

rhyniophyte The rhyniophytes are a group of extinct early vascular plants that are considered to be similar to the genus '' Rhynia'', found in the Early Devonian (around ). Sources vary in the name and rank used for this group, some treating it as the class ...

s", but this group is defined as having terminal

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), and later work showed that the sporangia of ''Hicklingia'' were lateral rather than strictly terminal, so that it is now regarded as having affinities with the

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 Zoste ...

Description

The sporophyte had a tufted growth habit, with narrow leafless stems (axes) up to 17 cm high which branched dichotomously. Sporangia were borne on short stalks (up to 3 mm), on all sides of the stem and also terminally. There are oval scars on specimens where the stalks are presumed to have broken off. The lateral sporangia were closely adpressed to the stem. The effect is of a 'spike' of sporangia which terminates some stems. The sporangia opened via slits, but these did not have the thickened borders which are a feature of some ''Zosterophyllum'' species. The vascular system of the stem was not observed. Spores are up to 50 µm in diameter and trilete., p. 254 The

gametophyte A gametophyte () is one of the two alternating multicellular phases in the life cycles of plants and algae. It is a haploid multicellular organism that develops from a haploid spore that has one set of chromosomes. The gametophyte is the ...

is not known.

Phylogeny

The affinity with zosterophylls is recognized in the

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...

published in 2004 by Crane et al. in which ''Hicklingia'' is placed as a sister to all the other

lycophytes 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 ...

(living and extinct clubmosses and relatives). Hao and Xue in 2013 listed the genus as a zosterophyll.

References

External links

Cladogram
from {{Taxonbar, from=Q16983152 Middle Devonian plants Zosterophylls Prehistoric lycophyte genera