Plenasium dowkeri
Plenasium dowkeri
Summary
Plenasium dowkeri was an extinct royal-fern relative known from anatomically preserved silicified stem or rhizome material. Like other osmundaceous ferns, its axis contained a central pith surrounded by a dissected vascular cylinder, with leaf traces departing into a mantle of persistent petiole-base and root tissues.
The fossil is especially informative because silicification can preserve internal tissues in fine detail. Species-level recognition therefore depends on the organisation of the stem anatomy rather than on a wood-like exterior alone. The species was originally described as Osmundites dowkeri, was later placed in Osmunda, and is recognised in modern fossil-Osmundales taxonomy as Plenasium dowkeri.
Quick Identification
Look for: a silicified fern stem or rhizome preserving internal vascular anatomy and a mantle of petiole-base or root tissues; species-level identification requires a transverse section or other anatomical exposure and should not be made from an unsectioned wood-like exterior alone.
Diagnostic Features
- Fossil is an anatomically preserved osmundaceous stem or rhizome rather than a leaf impression or ordinary woody axis.
- A central pith is surrounded by a dissected xylem cylinder divided into discrete strands in transverse section.
- Departing leaf traces preserve two protoxylem groups where their origin and early course can be observed.
- Petiole-base anatomy includes strongly curved xylem strands and scattered sclerenchymatous tissue when preservation is adequate.
- Species-level identification depends on the combined internal anatomy; weathered exterior fragments alone are not diagnostic.
Fossil Details
Recorded Fossil Sites
Reculver Sand Member
Synonyms & Previous Names
References
- Aldiss, D. T. (2014). The stratigraphical framework for the Palaeogene successions of the London Basin, UK. British Geological Survey Open Report, 95 pp.. British Geological Survey, Keyworth, Nottingham View reference
- Bomfleur, B.; Grimm, G.W.; McLoughlin, S. (2017). The fossil Osmundales (Royal Ferns)—a phylogenetic network analysis, revised taxonomy, and evolutionary classification of anatomically preserved trunks and rhizomes. PeerJ, 5, e3433 View reference
- Carruthers, W. (1870). On the structure of a Fern-stem from the Lower Eocene of Herne Bay, and on its Allies, Recent and Fossil. Quarterly Journal of the Geological Society of London, 26, 349–354 View reference
- Gardner, J.S. (1883). On the Lower Eocene section between Reculvers and Herne Bay, and on some modifications in the classification of the Lower London Tertiaries. Quarterly Journal of the Geological Society of London, 39, 197–210 View reference
- Holmes, S.C.A. (1981). Geology of the country around Faversham: Memoir for 1:50,000 geological sheet 273. Memoir of the Geological Survey of Great Britain, 117 pp.. Her Majesty's Stationery Office, London View reference
- Ward, D.J. (1978). The Lower London Tertiary (Palaeocene) succession of Herne Bay, Kent. Report of the Institute of Geological Sciences. Institute of Geological Sciences View reference