Hicklingia

Hicklingia is a genus of extinct plants of the Middle Devonian (around 393 to 383 million years ago). Compressed specimens were first described in 1923 from the Old Red Sandstone of Scotland.[1] Initially the genus was placed in the "rhyniophytes", but this group is defined as having terminal sporangia (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 zosterophylls.[2]

Hicklingia
Temporal range: Middle Devonian
Scientific classification
Kingdom: Plantae
Clade: Tracheophytes
Clade: Lycophytes
Plesion: †Zosterophylls (?)
Genus: †Hicklingia
Kidst. & W.H.Lang (1923)[1]
Species

† H. edwardii Kidst. & W.H.Lang (1923)
† H. erecta Kräusel & Weyland (1929)

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.[2] Spores are up to 50 µm in diameter and trilete.[3] The gametophyte is not known.

Phylogeny

The affinity with zosterophylls is recognized in the cladogram published in 2004 by Crane et al. in which Hicklingia is placed as a sister to all the other lycophytes (living and extinct clubmosses and relatives).[4]

lycophytes
 

†Hicklingia

 
 
 

†basal groups (Adoketophyton, Discalis, Distichophytum (=Rebuchia), Gumuia, Huia, Zosterophyllum myretonianum, Z. llanoveranum, Z. fertile)

 
 

†'core' zosterophylls (Zosterophyllum divaricatum, Tarella, Oricilla, Gosslingia, Hsua, Thrinkophyton, Protobarinophyton, Barinophyton obscurum, B. citrulliforme, Sawdonia, Deheubarthia, Konioria, Anisophyton, Serrulacaulis, Crenaticaulis)

 
 
 

†basal groups (Nothia, Zosterophyllum deciduum)

 
 

lycopsids (extant and extinct members)

 
 
 
 

Hao and Xue in 2013 listed the genus as a zosterophyll.[5]

References
  1. Kidston, R. & Lang, W.H. (1923), "Notes on fossil plants from the Old Red Sandstone of Scotland, I. Hicklingia edwardii, K. and L.", Transactions of the Royal Society of Edinburgh, 53: 405–7, doi:10.1017/s0080456800004087, cited in Edwards 1976
  2. Edwards, D. (1976), "The systematic position of Hicklingia edwardii Kidston and Lang", New Phytol., 76: 173–181, doi:10.1111/j.1469-8137.1976.tb01449.x
  3. Taylor, T.N.; Taylor, E.L. & Krings, M. (2009), Paleobotany : The Biology and Evolution of Fossil Plants (2nd ed.), Amsterdam; Boston: Academic Press, ISBN 978-0-12-373972-8, p. 254
  4. Crane, P.R.; Herendeen, P.; Friis, E.M. (2004), "Fossils and plant phylogeny", American Journal of Botany, 91 (10): 1683–99, doi:10.3732/ajb.91.10.1683, PMID 21652317, retrieved 2011-01-27
  5. Hao, Shougang & Xue, Jinzhuang (2013), The early Devonian Posongchong flora of Yunnan: a contribution to an understanding of the evolution and early diversification of vascular plants, Beijing: Science Press, p. 329, ISBN 978-7-03-036616-0, retrieved 2019-10-25
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