Baragwanathia

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Baragwanathia
Temporal range: Early Ludlow (Gorstian, late Silurian)[1] - Early Devonian
Artificial Baragwanathia model
Artificial Baragwanathia model
Fossil of Baragwanathia
Fossil of Baragwanathia found in Victoria, Australia
Scientific classification
Kingdom: Plantae
Division: Lycopodiophyta
Class: Lycopodiopsida
Order: Drepanophycales
Family: Drepanophycaceae
Genus: Baragwanathia
Lang & Cookson, 1935
Species

Baragwanathia is a genus of extinct plants of the division Lycopodiophyta of Late Silurian to Early Devonian age (427 to 393 million years ago), fossils of which have been found in Australia, Canada and China. The name derives from William Baragwanath, director of the Victorian Geological Survey at the time of discovery.[2]

Description[edit]

Baragwanathia is an extinct terrestrial vascular plant interpreted as a primitive lycopod. It differed from such taxa as Asteroxylon in the presence of vascular tissue in its leaves—Asteroxylon had enations without vascular tissue. The sporangia were borne in the axils of the strap-shaped leaves, which were spirally arranged. By comparison, the closely related genus Drepanophycus of the same period (see Drepanophycaceae for more details) bore its sporangia on the upper surface of specialized leaves known as sporophylls. Baragwanathia had stems varying in diameter and length (up to a few cm for the diameter and a few metres for the length). They were erect or arched, dichotomized occasionally, and were furnished with adventitious roots arising directly from prostrate leafy stems. As in Asteroxylon the vascular bundle in the stems was an exarch actinostele, with a star-shaped arrangement of tracheids of a primitive annular or helical type (so-called G-type). Leaves were unbranched strap-shaped microphylls 1–2 cm long with a single prominent vascular thread, arranged spirally on the stem. The sporangia were borne in the axils of the leaves, broader than long, dehiscing by a transversely-orientated slit. Spores were trilete isospores. The gametophyte of Baragwanathia is currently unknown.

The age of Baragwanathia has been controversial, because the fossils described by Lang and Cookson (1935) at first appeared to be of Late Silurian age, associated as they were with the graptolite genus Monograptus.[3] This would make the species by far the most advanced known plant of the time. However, the Silurian (Ludlovian) dating of the deposit in Victoria, Australia which produced the type specimens of Baragwanathia longifolia was later disproved, as Monograptus (and in particular the species present at that site) was later proved to persist into the Early Devonian.[4] Since then, specimens from a different Victorian locality are claimed to occur with veritable Late Silurian graptolites.[5] The genus Baragwanathia persisted at least until the Emsian (Late Lower Devonian) and probably had a worldwide distribution.[6]

See also[edit]

Notes[edit]

  1. ^ Rickards (2000)
  2. ^ Lang and Cookson (1935) p.425.
  3. ^ Lang and Cookson (1935) p. 422.
  4. ^ Jaeger (1966). Since then, conodont studies have confirmed the Early Devonian age.
  5. ^ Rickards 2000, and earlier papers referred to therein. See Yea Flora Fossil Site.
  6. ^ Hueber (1983).

References[edit]

Lang, William H.; Cookson, Isabel C. (1935). "On a flora, including vascular land plants, associated with Monograptus, in rocks of Silurian age, from Victoria, Australia.". Philosophical Transactions of the Royal Society of London B 224 (517): 421–449. doi:10.1098/rstb.1935.0004. Retrieved 2012-01-31. 

Hueber, FM (1983) A new species of Baragwanathia from the Sextant Formation (Emsian) Northern Ontario, Canada. Journal of the Linnean Society (Botany) 86, 57-79.

Hao SG and Gensel PG (2001) The Posongchang Floral Assemblages of Southeastern Yunnan, China - Diversity and Disparity in Early Devonian Plant Assemblages. In Plants Invade the Land. Evolutionary and Environmental Perspectives, pp. 103–119. Eds PG Gensel and D Edwards. (Columbia University Press, New York).

Jaeger H (1966). Two late Monograptus species from Victoria, Australia, and their significance for dating the Baragwanathia flora. Proceedings of the Royal Society of Victoria 79, 393-413.

Rickards, R.B. (2000). "The age of the earliest club mosses: the Silurian Baragwanathia flora in Victoria, Australia" (abstract). Geological Magazine 137 (2): 207–209. doi:10.1017/S0016756800003800. Retrieved 2007-10-25.